JP2010175964A - Apparatus and method for preparing relief printing form - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus for preparing a relief printing form from a photosensitive element. <P>SOLUTION: The present invention relates to a method and an apparatus for preparing a relief printing form in an environment, having a controlled oxygen concentration during exposure to chemical rays. The method includes steps of forming an in-situ mask on a photosensitive element, exposing the element to chemical rays via the in-situ mask in an environment, having an inert gas and an oxygen concentration ranging from 190,000 to 100 ppm, and processing the exposed element to prepare a relief printing form, having a pattern of an embossed surface segment. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method and apparatus for making a relief printing form. More particularly, the present invention describes a method and apparatus for making a relief printing form from a photosensitive element that is exposed to actinic radiation in an environment having a regulated oxygen concentration.

  Flexographic plates are widely used for printing packaging materials, including corrugated carton boxes, cardboard boxes, continuous webs of paper, and continuous webs of plastic film. A flexographic printing plate is a form of relief printing in which ink is conveyed from a raised image surface and transferred to a printing medium. Flexographic printing plates can be made from photopolymerizable compositions such as those described in US Pat. Generally, the photopolymerizable composition comprises an elastomeric binder, at least one monomer and a photoinitiator. Generally, the photosensitive element has a solid layer of photopolymerizable composition sandwiched between a support and a cover sheet or multilayer cover element.

  Flexographic forms have the property that they can be crosslinked or cured when exposed to actinic radiation. Typically, the printing form precursor was uniformly exposed from its backside to a certain amount of actinic radiation to form a floor, i.e. backflashed and used for backflash exposure. Imagewise exposure from the front with the same actinic radiation as the one. Imagewise exposure may be an image-bearing art-work such as a photographic negative or transparency (eg, a silver halide film) that is kept in close contact with the photopolymerizable layer under vacuum or Photo tools, so-called analog workflow can be used. Alternatively, imagewise exposure can be by means of a so-called digital workflow, called an in-situ mask with pre-formed radiopaque and radiolucent areas above the photopolymerizable layer, so-called digital workflow. The precursor is exposed to actinic radiation such as ultraviolet (UV) radiation to selectively cure the photopolymerizable layer. Actinic radiation penetrates the transmissive area and enters the photosensitive element and is prevented from entering the photopolymerizable layer by the black area or the opaque area of the transparency or in-situ mask. The areas of the photopolymerizable layer that are exposed to actinic radiation cure or solidify and crosslink. The non-exposed areas of the photopolymerizable layer that were under the opaque area of the transparency or in-situ mask upon exposure do not crosslink or cure (ie do not solidify). The uncured areas are soluble in the solvent used during washout development and / or can melt, soften or flow upon heating. The plate is then subjected to a processing step in which unexposed areas (i.e. uncured areas) are removed by treatment with washout liquid or heat, leaving a convex image suitable for printing. If treated with a washout solution, the plate is then dried to remove any solvent that can be absorbed by the plate. In addition, the printing plate can be exposed to UV radiation to ensure complete polymerization and remove surface tack. After all desired processing steps, the plate is then placed on a printing press in order to print the formed convex image on the substrate.

  Analog workflows require the creation of photo tools, which are complex, expensive and time consuming processes that require separate processing equipment and chemical developer solutions. Furthermore, the dimensions of the phototool may change slightly due to changes in temperature and humidity. This same photo tool may give different results when used at different times or in different environments. Because a phototool is created for each printing plate corresponding to the color of ink printed on a multicolor image, dimensional instability of the phototool can cause misalignment of the multicolor image during printing . Also, the use of a phototool requires special care and handling when making a flexographic printing form to ensure that the adhesion between the phototool and the plate is maintained. In particular, care must be taken when placing both the phototool and the plate in the exposure apparatus together with a special material in order to ensure tight contact with minimal air entrapment when generating the vacuum. In addition, care must be taken to ensure that the entire surface of the photopolymer plate and phototool is clean and free of dust and dirt. The presence of such foreign objects can cause a lack of adhesion between the phototool and the plate as well as image artifacts.

  An alternative to an analog workflow is called a digital workflow, which does not require the creation of a separate phototool. Photosensitive elements suitable for use as a precursor capable of forming an in-situ mask in a digital workflow are described in Patent Document 3, Patent Document 4, Patent Document 5, Patent Document 6, Patent Document 7, and Patent Document 8. , Patent Document 9, Patent Document 5, Patent Document 10 and Patent Document 11. The precursor or the combination material having the precursor includes a layer that is sensitive to infrared rays and impermeable to actinic radiation. The infrared photosensitive layer is imagewise exposed by laser radiation, whereby the infrared photosensitive material is removed from or transferred onto / from the overlay film of the combination material, An in-situ mask having a radiopaque area and a transparent area adjacent to the photopolymerizable layer is formed. The precursor is exposed to actinic radiation through an in-situ mask in the presence of atmospheric oxygen (since no vacuum is required). Furthermore, due to the presence of atmospheric oxygen during the main exposure, the flexographic printing form has a convex structure that is different from the convex structure formed in the analog workflow (both workflows have the same size mask). Based on aperture). The digital workflow results in a convex image with differently structured raised surface areas. In particular, the fine relief surface of the halftone dot (ie, the individual elements of the halftone image) is typically relatively small, with a rounded top and a curved sidewall cross-section, often this This is called a dot sharpening effect. The halftone dots generated by the analog workflow are typically conical and have a flat top. The convex structure formed by the digital workflow brings positive printing characteristics such as further miniaturization of printing highlight halftone dots where the color becomes lighter in white, an increase in printable tone range, and sharp line drawing. Thus, digital workflow is widely accepted as a desirable method for making flexographic printing forms because of its ease of use and desirable printing performance.

In the free radical photopolymerization process, the presence of oxygen (O ₂ ) during exposure induces side reactions, causing primary reactions between reactive monomer molecules and free radical molecules reacting with oxygen. Known to traders. This side reaction is known as inhibition (ie, oxygen inhibition) because it delays the polymerization or formation of cross-linked molecules. Many prior disclosures recognize that photopolymerization exposure to actinic radiation is desirably performed in air (for digital workflows), under vacuum (for analog workflows), or in an inert environment. By the way. Often nitrogen is said to be a suitable inert gas for an inert environment. This suggests that the nitrogen environment includes substantially from less than atmospheric oxygen concentration to a degree of total oxygen removal, or that oxygen is less than about 10 ppm. Nitrogen having an oxygen impurity concentration level of less than 10 ppm is readily available as a commercial product.

  An important substrate to be printed from the market point of view for graphic printing of packaging materials is corrugated paperboard. Corrugated paperboard includes a corrugated core, which is a layer of corrugated paperboard commonly referred to as a flute or a plurality of grooves, adjacent to a flat paper or paper-like layer called a liner. . An exemplary embodiment of corrugated paperboard includes a flute layer sandwiched between two liner layers. Other embodiments of corrugated paperboard can include multiple layers of flutes and liners. The fluted intermediate layer provides structural rigidity to the corrugated paperboard. Because corrugated paperboard is used as a packaging material and formed into boxes and containers, often the necessary identification information about the packaging is printed on the liner layer that forms the outer surface of the corrugated paperboard. Often, the outer liner layer may have some indentations due to uneven support by the underlying flute layer. A problem often encountered when printing on a corrugated board substrate is commonly referred to as fluting or banding, and is sometimes referred to as striping or washboarding. A possible printing effect is to occur. In general, fluting occurs when post printing is performed on a liner on the outer surface of the corrugated paperboard after the corrugated paperboard is assembled. The fluting effect appears as a dark print area, i.e. a relatively high density band, and a bright print area corresponding to the corrugated structure underneath the corrugated paperboard, i. It appears alternately. Printing darkening occurs where the uppermost portion of the corrugated interlayer structure supports the printing surface of the liner. The fluting effect can be achieved in areas of the printed image where the ink application area has a tone or tint value corresponding to a small portion of the entire area, as well as in the printed image where the ink coverage is complete, i.e. solid. May appear in the area. However, it has been found that this fluting effect becomes even more pronounced when printed using a relief printing form produced using a digital workflow.

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"Flexography: Principles and Practice" 1992 4th edition, the Foundation of Flexographic Technical Association (Ronkonkoma, NY) "Flexography: Principles and Practice" 1999 5th edition, the Foundation of Flexographic Technical Association (Ronkonkoma, NY)

  Therefore, there is a need for a modified process and apparatus for making a relief printing form from an easy-to-use photosensitive element (precursor), which is similar to or more like a printing form by an analog workflow process. It provides a printing form with an excellent convex structure and alleviates the drawbacks of conventional digital workflow processes, particularly those caused by the presence of oxygen during exposure to actinic radiation.

The present invention
(A) forming an in-situ mask adjacent to the photopolymerizable layer in the photosensitive element, wherein the photosensitive element comprises a binder, an ethylenically unsaturated compound and a photoinitiator. A step including a layer;
(B) sealing the photosensitive element in a sealed exposure chamber;
(C) adjusting the oxygen concentration in the hermetic exposure chamber within a range between 190,000 ppm and 100 ppm;
(D) exposing the photosensitive element to actinic radiation through an in-situ mask, and providing a method for making a relief printing form from the photosensitive element.

Furthermore, the present invention provides
(A) an inlet for introducing an inert gas or mixture of gases into the sealed exposure chamber and an outlet for removing oxygen from the sealed exposure chamber to seal the sealed exposure chamber from the external environment of the sealed exposure chamber; A sealed exposure chamber having means for:
(B) an actinic radiation source disposed adjacent to the exposure chamber;
(C) a source of inert gas or gas mixture coupled to the inlet;
(D) an apparatus for exposing a photosensitive element to actinic radiation in an environment having an oxygen concentration below atmospheric oxygen concentration, comprising: a means for measuring the oxygen concentration in a sealed exposure chamber provide.

1 is a schematic top plan view of one embodiment of an apparatus for exposing a photosensitive element in a sealed exposure chamber capable of providing an environment having an oxygen concentration less than atmospheric oxygen concentration. FIG. FIG. 2 is a schematic cross-sectional view of an embodiment of an apparatus having a sealed exposure chamber illustrated in FIG. 1 is a schematic cross-sectional view of a relief printing form having an raised surface in an optical device for determining the area of the raised surface. It is the schematic of the image of the raising surface on the relief printing form imaged with the optical device. It is an elevational view of a relief printing plate having a raised surface with a rounded halftone dot shoulder and pressed against a corrugated paperboard substrate. FIG. 5 is a schematic diagram of one halftone dot image of a raised surface having the rounded halftone dot shoulder of FIG. 4 onto a supported portion of a corrugated paperboard substrate. FIG. 5 is a schematic diagram of one halftone dot image of a raised surface having the rounded halftone dot shoulder of FIG. 4 on an unsupported portion of a corrugated paperboard substrate. FIG. 2 is an elevational view of a relief printing plate having a raised surface with a sharp halftone shoulder and pressed against a corrugated paperboard substrate. FIG. 6 is a schematic diagram of one halftone dot image of a raised surface having the sharp halftone dot shoulder of FIG. 5 onto a supported portion of a corrugated paperboard substrate. FIG. 6 is a schematic diagram of one halftone dot image of a raised surface having the sharp halftone dot shoulder of FIG. 5 on an unsupported portion of a corrugated paperboard substrate. Schematic of cumulative sum of area versus height of raised surface of convex structure, individually graphed for each of the three printed forms produced by various workflow processes, produced by analog workflow It is a figure of the cumulative sum curve of the area with respect to the height of the raised surface of the printing form. FIG. 3 is a schematic diagram of a cumulative sum curve of area versus height of a raised surface of a convex structure, individually graphed for each of three printing forms produced by various workflow processes, It is a figure of the cumulative sum curve of the area with respect to the height of the raised surface of the printing form produced by exposure in the presence of atmospheric oxygen. FIG. 3 is a schematic diagram of the cumulative sum curve of area versus height of a raised surface of a convex structure, individually graphed for each of the three printed forms produced by various workflow processes, with inert gas and Of the area relative to the height of the raised surface of a printing form made according to the invention using a modified digital workflow in which exposure is performed in an oxygen concentration environment between 190,000 and 100 ppm (parts per million) It is a figure of a cumulative sum curve. FIG. 4 is a series of wire frame images of an embodiment of a raised surface or halftone dot structure that provides a relief printing form depending on the oxygen concentration at the start of actinic exposure. FIG. 6 is a graph showing a plot of oxygen concentration in a sealed exposure chamber versus time for each test E1 through E6 where the photosensitive element was exposed to actinic radiation as described in Example 4. FIG. The microscope picture of Example 4 is shown.

  Throughout the following detailed description, like reference numerals refer to like elements in all figures of the drawings.

  The present invention is a method and apparatus for making a relief printing form from a photosensitive element (ie, a precursor) that provides an improved relief structure on the relief printing form. The present invention is a modified digital workflow process that adjusts the oxygen concentration during exposure of a photosensitive element to actinic radiation. The present invention has the ease and advantages of a conventional digital workflow process, particularly for the formation of in-situ masks for photosensitive elements, while the disadvantages of the conventional digital workflow process, particularly upon exposure to actinic radiation. Alleviates the drawbacks caused by the presence of atmospheric oxygen. The present invention also provides a printing form having a convex structure similar to or better than a printing form produced by an analog workflow process.

In the general method modified digital workflow process of the present invention , the relief printing form is composed of a photosensitive element having an in-situ mask. The photosensitive element is a photopolymerizable printing element having a layer of a photopolymerizable composition comprising a binder, an ethylenically unsaturated compound and a photoinitiator on a support. A relief printing form is formed by exposing a photosensitive element to actinic radiation imagewise in an environment having an inert gas and an oxygen concentration between 190,000 ppm (parts per million) and 100 ppm. Has a pattern. Imagewise exposure of the photosensitive element in an environment having an inert gas and a specific oxygen content results in the formation of a printing form having a plurality of raised surfaces, each raised surface carrying ink. A top surface area, a side wall surface area, and a shoulder surface area that is a transition between the top surface area and the side wall surface area, and the sum of the top surface area and the shoulder surface area Has a printing area.

  Surprisingly and unexpectedly, adjustment of the oxygen concentration can indirectly control the quantitative contribution of the shoulder surface area to the total printing area of the raised printing surface. There was found. This control has several process advantages. For example, in flexographic printing on a substrate such as corrugated paperboard, surprisingly and unexpectedly, the radius of the top printing surface area of the shoulder surface area is increased by about 10% or less. It has been found that the fluting or banding effect of images printed on corrugated board with a relief printing form is at least minimized. Surprisingly and unexpectedly, when the shoulder surface area increases the radius of the top printing surface area by about 10 microns or less (for a 155 micron diameter dot relief surface), the relief printing It has been found that the form reduces the fluting or banding effect of images printed on corrugated paperboard. Surprisingly and unexpectedly, the printing form made by the method of the present invention increases its radius of the top printing surface area of the shoulder surface area by about 2.5% or less. It has been found that it provides a print area on the raised surface that greatly reduces the fluting or banding effects of images printed on corrugated paperboard by relief printing forms. For halftone dots printed in the halftone area of the tone scale, if the shoulder surface area is less than 30%, preferably less than 10% and most preferably less than 2% of the total printed area, the relief It has been found that the fluting or banding effect of an image printed on corrugated paperboard by a printing form is at least minimized. Surprisingly, this shoulder area has an oxygen concentration below atmospheric concentration, but higher than a completely anoxic environment or higher than an environment consisting entirely of inert gas. It can be realized when the digital plate is generated in an environment that has.

  The present invention relates to a relief printing form capable of improving the quality of a printed image on a corrugated paperboard as compared with a relief printing form produced by a conventional method of a digital workflow and, more particularly, a conventional method of an analog workflow. I will provide a. The present invention provides a relief printing form having a convex structure with a raised surface element having a flat or substantially top printing surface and a sharp shoulder transition to the sidewalls of the raised element. The present invention also avoids the cost and manufacturing drawbacks associated with analog workflows and avoids the difficulty of establishing a completely inert environment while taking advantage of the efficiency of digital workflows. The present invention also provides dot form chipping, i.e., improved printing performance of a printing form for long-term printing operations by reducing the likelihood that the raised printing surface will wear out or break off from the printing form. Improvements can be realized.

A method for forming a relief printing form from an in-situ mask photosensitive element comprises the steps of forming an in-situ mask adjacent to a photopolymerizable layer, an inert gas and 190,000 ppm to 100 ppm (parts per million). ) Exposing the photopolymerizable layer to actinic radiation through the mask in an environment having an oxygen concentration between, and processing to form a relief printing form having a pattern of printing areas. An environment with an inert gas and an oxygen concentration of less than 100 ppm is feasible and can produce the desired results on the printing surface of the printing form, but purged with an inert gas and an oxygen concentration of less than 100 ppm. The manufacturing time may be significantly extended depending on the time required to generate the environment, so it is not practical from a market point of view. The atmospheric environment contains about 21% oxygen, about 78% nitrogen and about 1% other gases. In most embodiments, the atmospheric environment generally surrounding the photosensitive element is purged or substantially purged with an inert gas, resulting in an inert gas and 190,000 ppm to 100 ppm (parts per million). ) (For imagewise exposure of the photosensitive element) with an oxygen concentration between

  The photosensitive element includes a layer of photopolymerizable material composed of at least a binder, an ethylenically unsaturated compound, and a photoinitiator, the layer being on or adjacent to the support. The photosensitive element for use in the present invention is not limited as long as it can have an in-situ mask on or adjacent to the photopolymerizable layer. An in-situ mask is an image consisting of opaque and transmissive areas that are integral with or substantially integral with a photosensitive element for imagewise exposure to actinic radiation, and are photopolymerizable. No vacuum is required to ensure contact of the mask to the layer. In-situ masks have the disadvantages associated with generating a separate phototool and using a vacuum to ensure contact of the phototool to the photosensitive layer when producing a relief printing form. To avoid. As used herein, exposure of a photosensitive element having an in-situ mask in an environment having an inert gas and an oxygen concentration between 190,000 ppm and 100 ppm (parts per million) is referred to as “modified digital. Sometimes called “workflow”.

  The in-situ mask image is formed on or disposed on the surface of the photopolymerizable layer opposite the support. The mask is an image that includes impermeable areas and transmissive or “transparent” areas. The impermeable area of the mask prevents the underlying photopolymerizable material from being exposed to radiation, and thus the area of the photopolymerizable layer covered by this dark area does not polymerize. The “transparent” areas of the mask are polymerized or crosslinked by exposing the photopolymerizable layer to actinic radiation. Finally, the mask image of the photosensitive element produces a pattern of printing areas for the relief printing form. The in-situ mask can be generated by any suitable method and is referred to as a digital direct-to-plate method (often referred to as a digital method or digital workflow) using laser radiation. ) And ink jet coating performed prior to imagewise exposure of the photosensitive element to actinic radiation. In digital direct-to-plate image technology, laser radiation is used to form an in-situ mask image for the photosensitive element. In general, digital in-situ mask formation uses laser radiation to selectively remove the radiopaque layer from the side of the photosensitive element opposite the support, or of the support. A radiopaque layer is selectively transferred to the side of the opposite photosensitive element. In most embodiments, the in-situ mask present on the photosensitive element does not serve as a barrier to oxygen for the photopolymerizable layer. In one embodiment, the photosensitive element does not include a barrier layer against an oxygen environment.

  In one embodiment, the photosensitive element initially includes an actinic radiation opaque layer disposed on or over the surface of the photopolymerizable layer opposite the support, wherein the laser radiation is radiation-impermeable. The transmissive layer is imagewise removed, i.e. ablated or vaporized, to form an in-situ mask. Only those portions of the radiopaque layer that have not been removed from the photosensitive element remain on the element, creating a mask. In another embodiment, the photosensitive element initially does not include an actinic radiation opaque layer. The independent element carrying the radiopaque layer forms a combination with the photosensitive element such that the radiopaque layer is adjacent to the side of the photosensitive element opposite the support. This combination is imagewise exposed with laser radiation to selectively alter or selectively alter the adhesion balance of the radiopaque layer to form a mask on the photopolymerizable layer or to photopolymerize. A processed mask is formed over the conductive layer. In this embodiment, only the altered portion of the radiopaque layer is on the photosensitive element, forming an in-situ mask. In another embodiment, digital mask formation can be achieved by imagewise applying a radiopaque material in the form of an inkjet ink over the photosensitive element. Imagewise application of inkjet ink can be performed directly on the photopolymerizable layer, or can be performed by processing over the photopolymerizable layer on another layer of the photosensitive element. . Another intended way in which digital mask formation can be achieved is by generating a mask image of the radiopaque layer on a separate carrier. In some embodiments, a separate carrier includes a radiopaque layer that is imagewise exposed to laser radiation to selectively remove the radiopaque material to form an image. The mask image on the carrier is then transferred by applying heat and / or pressure to the surface of the photopolymerizable layer opposite the support.

  In some embodiments, the laser radiation used to form the mask is infrared laser radiation. Infrared laser exposure can be performed using various types of infrared lasers that emit within the range of 750 to 20,000 nm. Infrared lasers including diode lasers emitting in the range 780 to 2,000 nm and Nd: YAG lasers emitting at 1064 nm are preferred. A preferred apparatus and method for infrared laser exposure for imagewise removal of an actinic radiation opaque layer from a photosensitive element is disclosed by Fan et al. In US Pat. The in-situ mask image remains on the photosensitive element for the subsequent step of full exposure (and processing) to actinic radiation.

The next step in the method for making an oxygen conditioned environmental relief printing form is to fully expose the photosensitive element to actinic radiation through an in-situ mask, ie, imagewise exposure of the element. Imagewise exposure of the photosensitive element to actinic radiation is performed in an environment that includes the presence of an inert gas and an oxygen concentration between 190,000 ppm and 100 ppm (parts per million). An inert gas has a zero or low reaction rate to the photosensitive element (ie, is inert to the polymerization reaction) and replaces oxygen in the exposure environment (ie, a closed exposure chamber). It is possible gas. Suitable inert gases include, but are not limited to, argon, helium, neon, krypton, xenon, nitrogen, carbon dioxide and combinations thereof. Inert gases and combinations or mixtures of inert gases may contain small amounts of oxygen, but in the presence of small amounts of oxygen, the ability of the inert gas to replace the atmosphere in the chamber, or the desired oxygen concentration in the chamber Does not cause a significant change in performance. In one embodiment, the inert gas is nitrogen.

  Imagewise exposure of a photosensitive element in a specific environment consisting of an inert gas and an oxygen concentration between 190,000 ppm and 100 ppm produces a convex structure in the printing form consisting of multiple raised surfaces. Each of the raised surfaces has an ink-carrying top surface area that is structurally similar to the ink-carrying top surface area produced in a printing form made by an analog workflow. That is, the top area of the raised surface in the relief printing form produced by this method is flat or substantially flat, typical of a conventional digital workflow where elements are exposed in the presence of atmospheric oxygen. Is not rounded. Conventional digital workflow methods use photosensitive elements in air under normal atmospheric conditions consisting of 78% nitrogen, ˜21% oxygen, <1% argon and carbon dioxide, respectively, and traces of other gases. Expose to actinic radiation. In other words, when imagewise exposure is performed in air, the oxygen concentration is about 210,000 ppm. In one embodiment, the photosensitive element does not include any additional layers on the top surface of the in-situ mask that can serve as a barrier to the environment against the imagewise exposed surface.

In one embodiment of a sealed exposure chamber , the present invention is a device or apparatus for exposing a photosensitive element to actinic radiation in an environment having an oxygen concentration below atmospheric oxygen concentration. The device includes a sealed exposure chamber, an actinic radiation source, a gas source introduced into the sealed exposure chamber, and a means for measuring the oxygen concentration in the sealed exposure chamber.

  The sealed exposure chamber includes an inlet for introducing a gas or a mixture of gases into the sealed exposure chamber, an outlet for removing oxygen and / or gas from the sealed exposure chamber, and a sealed exposure chamber from an external environment of the sealed exposure chamber. Means for sealing. The actinic radiation source can apply actinic radiation to the surface of the photosensitive element having an in-situ mask when the photosensitive element is in a sealed exposure chamber. The source of actinic radiation can be placed adjacent to the photosensitive element. In some embodiments, the actinic radiation source is positioned adjacent to the sealed exposure chamber. The gas source can include a mixture of gases, but is coupled to the inlet of the hermetic exposure chamber. The means for measuring the oxygen concentration in the sealed exposure chamber includes at least one oxygen meter coupled to the outlet or the sealed exposure chamber.

  The photosensitive element is confined within the sealed exposure chamber during imagewise exposure of the photosensitive element, such that the sealed exposure chamber has an internal environment that differs from the external environment of the sealed exposure chamber during exposure, or Virtually confined. The internal environment within the hermetic exposure chamber is a specific gas or gas environments, ie, an inert gas and an oxygen concentration between 190,000 ppm and 100 ppm. A sealed exposure chamber seals the photosensitive element in the internal environment during exposure to adjust or maintain the oxygen concentration in the exposure chamber. The hermetic exposure chamber can be sealed from the external environment by sealing means such as gaskets and adhesive tape. However, sealing does not necessarily mean an airtight seal or a hermetic seal. Although hermetic seals and hermetic seals are included in embodiments of the present invention, means for sealing the hermetic exposure chamber include that the internal environment of the hermetic exposure chamber is oxygenated between 190,00 ppm and 100 ppm with an inert gas. The chamber is well sealed from the outside environment, provided that it is adjusted or maintained to have a concentration. The chamber also has at least one inlet for introducing a gas or mixture of gases into the chamber from a gas source, and at least for removing oxygen and air containing the introduced gas or gases from the chamber. One outlet. The hermetic exposure chamber can be a separate housing that is added to or installed in an existing exposure apparatus, or can be incorporated into the frame of the exposure apparatus, or can be incorporated into an exposure apparatus, such as a housing. It can be formed from existing structures. In one embodiment, the hermetic exposure chamber is an integral part of the exposure apparatus, so that the exposure apparatus is an all-workflow, ie analog workflow process using exposure under vacuum, conventional digital using exposure in air. A workflow and a modified digital workflow using exposure in an environment consisting of inert gas and oxygen concentration between 190,000 and 100 ppm can be accommodated.

  In one embodiment, the hermetic exposure chamber includes at least one wall and a roof mounted to the at least one wall. In another embodiment, the closed exposure chamber comprises four walls and one lower floor and one upper roof. In another embodiment, the hermetic exposure chamber includes four walls and an upper roof, the upper roof being a flat support in an exposure apparatus having an actinic radiation source that forms the floor or bottom of the chamber, i.e., an exposure plate. It can be combined with or placed on an exposure bed. The at least one wall has a height sufficient to accommodate the photosensitive element within the chamber such that the roof does not contact or substantially contact the surface of the photosensitive element with the in-situ mask. In some embodiments, the height of the wall creates an open space of the internal environment above the in-situ mask, and provides an in-situ covering of the gas (or gases) and the adjusted concentration of oxygen. Should be sufficient to produce adjacent to the mask. In some embodiments, the sealed exposure chamber is set to a size sufficient to accommodate all photosensitive printing elements of various sizes, shapes and thicknesses. In other embodiments, the sealed exposure chamber is sized to accommodate only one or several sizes, shapes or thicknesses of photosensitive printing elements. In some embodiments, the hermetic exposure chamber has a box-like shape for containing a photosensitive element that is flat or forms a printing plate. However, the shape of the sealed exposure chamber is not limited and includes other shapes such as a cylinder shape.

  Sealed exposure, depending on the size of the sealed exposure chamber required to accommodate the photosensitive element, particularly the flat shaped photosensitive element, and the type and thickness of the material used to form the roof of the sealed exposure chamber It may be necessary to ensure that the chamber roof does not bend towards the interior of the chamber. The roof of the exposure chamber may bend, i.e., bend or curve, toward the interior of the chamber due to its own weight. The bending of the roof towards the internal environment limits the open space above the in-situ mask on the element and prevents the flow of gas through and into the chamber, all or parts of the photosensitive element, In particular, it is anticipated that the oxygen concentration experienced by the portion adjacent to the in-situ mask can be changed in some cases. In some cases, the roof of the sealed exposure chamber can be bent, bent or curved to such an extent that the roof contacts a photosensitive element sealed within the chamber. In this case, the part / parts of the photosensitive element in contact with the roof cannot receive the same oxygen concentration from the internal environment as the part / parts of the photosensitive element not in contact with the roof. In addition, it is expected that the convex structure of printing becomes non-uniform. By any manner such as forming a vault above the photosensitive element to eliminate the possibility that the roof of the hermetic exposure chamber bends, curves or flexes into the inner environment of the hermetic exposure chamber, The roof can be bent in the opposite direction. The vaulted roof is not limited and can include one or more domes, arches, or even a camber bent above the plane of the photosensitive element. The shape of the vault is not limited and can include, for example, a circular dome, a cylindrical dome, an oval dome, one or more semi-cylindrical domes, or an arch. In some embodiments, the vaulted roof can have more than one dome, arch or camber, which is particularly effective when the roof is formed from a thin (sheet) material. is there. The bending degree of the vaulted roof is not limited. The vaulted roof may form one or more domes, for example, formed at the intersection of the roof with respect to at least one wall, and from about 0.125 inches to about 1 in a plane perpendicular to the at least one wall. It may be formed up to .5 inches (about 3.175 mm to about 3.81 cm). A hermetic exposure chamber having a vaulted roof increases the overall rigidity of the chamber, and its strength can be increased by increasing the bending of the vault. In most embodiments where the source of actinic radiation is outside the hermetic exposure chamber and the roof is sufficiently transparent to actinic radiation, the actinic radiation impinging on the photosensitive element sealed in the chamber will cause the roof to be vaulted. It is observed that it is not affected by being or not affected by bending the roof of the closed exposure chamber. In one embodiment, the hermetic exposure chamber forms a parallel side of the roof having a length that is longer than the length of the corresponding wall, as well as the wall of the roof to match the wall corresponding to the parallel side of the roof. By limiting the parallel sides, the roof can be manufactured to have a semi-cylindrical dome shape.

  The actinic radiation source can be placed inside or outside the hermetic exposure chamber, provided that it can be irradiated with actinic radiation on the surface of the photosensitive element having an in-situ mask. The actinic radiation source is or can be arranged adjacent to the hermetic exposure chamber, and in particular can be or can be arranged adjacent to the photosensitive element. In particular, the actinic radiation source is arranged such that the actinic radiation source can expose the photosensitive element through an in-situ mask when the photosensitive element is sealed in the chamber. Should be positioned or positionable. The source of actinic radiation can be located at a distance from the photosensitive element of 1.5 inches to 60 inches (3.81 cm to 152.4 cm).

  In one embodiment, the actinic radiation source is located inside a sealed exposure chamber. Thus, the walls / walls and roof of the hermetic exposure chamber need not be transmissive to actinic radiation. In other words, the source assembly, i.e. all or part of the actinic radiation source, as well as the relief printing form precursor (photosensitive element) are placed in a sealed exposure chamber. This particular embodiment adjusts the exposure time and the need to build up a sealed exposure chamber or parts thereof from a material or materials that are transparent or substantially transparent to actinic radiation, That is, it eliminates the need to compensate for attenuation or absorption of actinic radiation by the material forming the hermetic exposure chamber by extending or shortening. For example, in one embodiment, a housing or cover used in a laser exposure apparatus for digital imaging of an in-situ mask with laser radiation, as disclosed in US Pat. It can also serve as a sealed exposure chamber for imagewise exposure in an environment consisting of oxygen concentrations of 190,000 to 100 ppm. One example of a suitable laser exposure apparatus that is commercially available is the CYREL® Digital Imager. In this embodiment, the actinic radiation source can be placed in a housing and the photosensitive element can be placed on a rotatable shaft or drum. The actinic radiation source disposed in the housing can be stationary or movable with respect to the photosensitive element.

  In another embodiment, the actinic radiation source is located outside the sealed exposure chamber. In this embodiment, at least the wall or roof adjacent to the side of the photosensitive element with the in-situ mask penetrates toward the photosensitive element without significant diffusion or absorption of the actinic radiation. Must be transparent to, or substantially transparent to, actinic radiation. “Transparent” means that a sufficient amount of actinic radiation can be transmitted through the surface of the chamber, ie the wall or roof, to strike and expose the photosensitive element. In some embodiments, the material of the hermetic exposure chamber is transmissive or substantially transmissive if at least 4% of actinic radiation penetrates the surface of the chamber and strikes and exposes the photosensitive element. In other embodiments, the material forming the chamber is transmissive or substantially transmissive if at least 45 to 65% of actinic radiation penetrates the surface of the chamber and strikes and exposes the photosensitive element. is there. In yet other embodiments, the material forming the chamber is transmissive or substantially transmissive if at least 65 to 99% of actinic radiation penetrates the surface of the chamber and strikes and exposes the photosensitive element. It is. It should be noted that the exposure time may need to be adjusted, i.e. extended, to compensate for the attenuation or absorption of actinic radiation by the exposure chamber. The determination of the appropriate adjustment of the exposure time is well within the general skill of the person skilled in the art. In this embodiment, where the source of actinic radiation is located outside the hermetic exposure chamber, the roof of the hermetic exposure chamber is typically transmissive to actinic radiation. In some embodiments, both the upper and lower roofs of the hermetic exposure chamber are transparent to actinic radiation. Polycarbonates sold under the Lexan® trade name are suitable for use and are, for example, transmissive, substantially transmissive or partially transmissive to actinic radiation such as UV rays. Such as polycarbonate, acrylic resin such as acrylic resin sold under the trade name of Plexiglas® and Acrylite®, and fluoride such as fluorocarbon resin sold under the trade name of Teflon® FEP Examples include, but are not limited to, carbon resins and glass. The thickness of the material is not limited.

  The sealed exposure chamber also includes means for measuring the oxygen concentration in the sealed exposure chamber. The means for measuring the oxygen concentration can be at least one oxygen meter connected to the outlet or the enclosed exposure chamber. More than one oxygen meter may be required to monitor the oxygen concentration over the entire range from 190,000 ppm (parts per million) to 100 ppm. The oxygen meter may be capable of measuring only a portion of the possible range of oxygen concentrations suitable for the present invention. In another embodiment, an apparatus for exposing a photosensitive element in an environment having an oxygen concentration less than atmospheric oxygen concentration further includes a feedback controller between the oxygen meter and the gas source, whereby oxygen The concentration is automatically monitored and adjusted. It is expected that the measurement of oxygen concentration at the outlet of the oxygen chamber represents the oxygen concentration in the internal environment of the sealed exposure chamber.

  FIGS. 1 and 2 show that in order to expose the photosensitive element to actinic radiation, the photosensitive element is disposed in the sealed exposure chamber 100 of the apparatus 125 and the source of actinic radiation is outside the sealed exposure chamber 100. An embodiment is shown. The hermetic exposure chamber 100 has a roof 115 that is transparent to actinic radiation and is arranged on a plate of the exposure apparatus 125. FIG. 1 shows a top schematic view of one embodiment of a sealed exposure chamber 100, and FIG. 2 shows a cross-sectional schematic view of the sealed exposure chamber 100 installed on an exposure plate 120.

  As illustrated in FIGS. 1 and 2, the hermetic exposure chamber 100 includes a four-sided box 110 consisting of a wall having a top surface or roof 115, which is a standard flat-bed exposure frame. It can be installed on the exposure platen 120 of the apparatus 125. The exposure plate 120 completes the enclosure of the hermetic exposure chamber 100, thereby creating the desired internal environment for the photosensitive element that sits on and is contained within the chamber 100. Is possible. The roof 115 can be constructed from a transparent material, i.e., a material that is transparent to actinic radiation, and the walls can be constructed from any material suitable to support the roof 115. In one embodiment, the roof 115 is composed of FEP Teflon® and the walls are tubular metal. In another embodiment, the roof 115 and walls are composed of Lexan® polycarbonate. In another embodiment, the roof 115 is composed of Acrylite® OP-4 UV transmissive acrylic (Cyro Industries, Orange, CT) and the walls are stainless steel.

  The roof 115 through which actinic radiation 130 (not shown in FIG. 1 but perpendicular to the page) passes and strikes the photosensitive element 135 is transparent to the actinic radiation 130. At least 4% of the actinic radiation passes through the roof 115. A photosensitive element 135 that receives actinic radiation 130 is disposed in the hermetic exposure chamber 100 and on the exposure plate 120. The photosensitive element 135 is positioned on the platen 120 such that the surface of the element having the in-situ mask faces the actinic radiation 130 light source.

  Depending on the material suitable for forming the hermetic exposure chamber 100, the exposure time of the actinic radiation 130 may be extended (or shortened) between the various materials used to construct the hermetic exposure chamber 100. Differences in permeability can be compensated. For example, the exposure time is extended to compensate for the ultraviolet absorption characteristics of the Lexan® material. The sidewall 110 of the hermetic exposure chamber 100 can be composed of any material that provides structural stability to the hermetic exposure chamber 100 and that is easy to seal, such as plastic or metal or wood. Sidewall 110 can be transmissive to actinic radiation 130 but is not required. The hermetic exposure chamber 100 can be sealed from the external environment (ie, room conditions) by a sealing gasket 132 that is secured to the end of the wall adjacent to or in contact with the exposure platen 120. The sealed exposure chamber 100 includes an inlet port 140 for introducing a gas 145 through the gas supply manifold 150 into the sealed exposure chamber 100. In one embodiment, seal 132 is not a hermetic seal. In FIG. 1, the flow of gas 145 is represented by arrows.

  In one embodiment, gas 145 is an inert gas or a mixture of inert gases. The inert gas 145 is supplied from the inert gas source 155 into the gas supply manifold 150, from which the inert gas 145 enters the sealed exposure chamber 100 through the gas supply hole 160. The sealed exposure chamber 100 also provides some or most of the air initially present in the sealed exposure chamber 100 to provide an oxygen concentration between 190,000 and 100 ppm to the internal environment of the sealed exposure chamber 100. Has an outlet port 165 mounted to a gas exhaust manifold 170 for purging gas. In another embodiment, the gas 145 introduced into the internal environment of the sealed exposure chamber 100 is an inert gas or mixture of inert gases, and oxygen. In this embodiment, it may be desirable to maintain the oxygen concentration at a certain set point or steady state within the range of 190,000 to 100 ppm, which purges oxygen in the air with an inert gas. It can be achieved more easily by introducing oxygen with an inert gas / a plurality of inert gases rather than only by doing so. An oxygen meter 175 for measuring the oxygen concentration in the hermetic exposure chamber 100 can be disposed at the outlet port 165. The oxygen meter 175 presents the oxygen concentration in the gas 145 that exits the gas exhaust manifold 170 through the gas outlet hole 180 and proceeds into the outlet port 165. Based on the reading of the oxygen meter 175, the flow of the gas 145 can be monitored manually. Further, in order to adjust the oxygen concentration in the hermetic exposure chamber 100 to a desired concentration, the oxygen meter 175 can be looped to the gas inlet port 140 in an automatic feedback mechanism.

  In the preferred embodiment, the actinic radiation source 185 is located outside the hermetic exposure chamber 100. The photosensitive element 135 is exposed to radiation 130 that penetrates the transmissive roof 115. In FIG. 2, the arrow starting from the actinic radiation source 185 represents actinic radiation 130 that passes through the roof 115 and strikes the photosensitive element 135. In one embodiment, the actinic radiation source 185 is mounted in a sealed exposure chamber. In another embodiment, the actinic radiation source 185 is not attached to the sealed exposure chamber 100. In another embodiment, a second optional actinic radiation source 190 is located below the exposure plate 120 in the apparatus 125. This optional actinic radiation source 190 can be used for the generation of a floor in the backflush or photosensitive element 135. The optional actinic radiation source 190 can be attached to the exposure frame 125 or may not be attached to the exposure frame 125. When an optional actinic radiation source 190 is used, the exposure platen 120 is transmissive to actinic radiation.

  In some embodiments, the oxygen concentration in the internal environment of the sealed exposure chamber is adjusted with a fixed reduction rate in the oxygen concentration range between 190,000 ppm and 100 ppm. In other embodiments, the oxygen concentration in the internal environment of the hermetic exposure chamber is adjusted with a variable reduction rate within an oxygen concentration range between 190,000 ppm and 100 ppm. In most embodiments, air with a standard oxygen concentration of 210,000 ppm from the internal environment of the hermetic exposure chamber will be inert gas / multiple inert gases until the oxygen concentration in the chamber is about 190,000 ppm or less. It is first purged with active gas and requires adjustment of the oxygen concentration in the chamber. In one embodiment, the oxygen concentration is adjusted by reducing the oxygen concentration from a closed exposure chamber at a fixed or variable reduction rate, starting at about 190,000 ppm and up to 100 ppm. In another embodiment, the oxygen concentration from the closed exposure chamber starts at an oxygen concentration of 210,000 ppm and is maintained at a fixed or variable reduction rate to maintain an oxygen concentration between 190,000 ppm and about 100 ppm. By reducing the oxygen concentration, the oxygen concentration is adjusted. The oxygen concentration (in terms of the partial pressure of oxygen in the hermetic exposure chamber) is reduced by introducing an inert gas or mixture of inert gases, as explained above.

  In some embodiments, exposure of the photosensitive element to actinic radiation is initiated after a steady state level of oxygen concentration is reached in the sealed exposure chamber to crosslink or cure the exposed area of the photopolymerizable layer. Continue for a specific time. In other embodiments, exposure of the photosensitive element to actinic radiation is initiated before the steady state exposure chamber reaches a steady state level of oxygen concentration and is continued for a specified amount of time after reaching steady state. . The steady state oxygen concentration in the hermetic exposure chamber is maintained in the range of about 190,000 ppm to about 100 ppm. For example, the steady state oxygen concentration can be the following values measured in ppm (parts per million): Also, the steady state oxygen concentration can be within a range defined by any two of the following ppm values. That is, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, 10,000, 15,000, 20,000, 25,000, 30,000, 35,000, 40,000, 45,000, 50,000, 55,000, 60,000, 65,000, 70,000, 75,000, 80,000, 85, 000, 90,000, 95,000, 100,000, 105,000, 110,000, 115,000, 120,000, 125,000, 130,000, 135,000, 140,000, 145,000, 150,000, 155,000, 160,000, 165,000, 170,000, 1 Is a numerical value of 5,000,180,000,185,000 and 190,000. In another embodiment, the oxygen concentration is maintained at a steady state greater than 1 as defined by the above numerical values.

  In other embodiments of the present invention, the oxygen concentration in the interior environment of the exposure chamber is continuously reduced from about 190,000 ppm to about 100 ppm. Exposure of the photosensitive element to actinic radiation begins at any intermediate point in the oxygen concentration range from 190,000 ppm to 100 ppm and continues while reducing the oxygen concentration in the hermetic exposure chamber. Actinic radiation can be initiated at an oxygen concentration determined by any of the following numbers measured in ppm. That is, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, 10,000, 15,000, 20,000, 25,000, 30,000, 35,000, 40,000, 45,000, 50,000, 55,000, 60,000, 65,000, 70,000, 75,000, 80,000, 85, 000, 90,000, 95,000, 100,000, 105,000, 110,000, 115,000, 120,000, 125,000, 130,000, 135,000, 140,000, 145,000, 150,000, 155,000, 160,000, 165,000, 170,000, 1 Is a numerical value of 5,000,180,000,185,000 and 190,000.

  In photosensitive elements that are imagewise exposed in an internal environment where the oxygen concentration starts at 190,000 ppm and is continuously reduced, an improved effect on the shape of the raised surface elements, ie flat top and sharp shoulder More analog-like halftone dots are observed. However, the improved effect on the shape of the raised surface element is particularly noticeable in photosensitive elements that are imagewise exposed in an internal environment with an oxygen concentration of about 20,000 ppm to less than 100 ppm. In some embodiments, imagewise exposure is initiated when the internal environment has an oxygen concentration of less than about 20,000 ppm, and the oxygen concentration is continuously reduced or maintained at a steady state of less than about 20,000 ppm. It is possible to observe an improved effect on the shape of the raised surface element in the photosensitive element. In another embodiment, in a photosensitive element implemented in an internal environment where at least about 30% of the total image-wise exposure time has an oxygen concentration of less than 20,000 ppm, particularly less than 1000 ppm, the shape of the raised surface element It is possible to recognize an improved effect on. In still other embodiments, the improvement to the relief element shape is improved in a photosensitive element wherein at least about 65% of the total image-wise exposure time is performed in an internal environment having an oxygen concentration of less than 20,000 ppm. It is possible to recognize the effect. In yet another embodiment, a photosensitive element implemented in an internal environment where at least about 45% of the total image-wise exposure time has an oxygen concentration of 5000 ppm or less has an improved effect on the shape of the raised surface element. It is possible to admit.

  In some embodiments, the oxygen concentration is continuously introduced into the sealed exposure chamber during imagewise exposure after the oxygen concentration in the sealed exposure chamber reaches 190,000 ppm or less. Is continuously reduced. Imagewise exposure can be initiated when the oxygen concentration is at or below 190,000 ppm (19%) and continued while the oxygen concentration is reduced to 5000 ppm or less. In other embodiments, imagewise exposure can be initiated when the oxygen concentration is 1000 ppm and continued until the oxygen concentration reaches about 100 ppm.

  In some embodiments, the internal environment for the photosensitive element under exposure has an oxygen concentration that is an average of the oxygen concentration at the beginning of the imagewise exposure and the oxygen concentration at the end of the imagewise exposure.

  In other embodiments, the internal environment for the photosensitive element during imagewise exposure has an oxygen concentration that is a weighted average of the oxygen concentration based on a percentage of time from the total exposure time.

  In some embodiments, the internal environment has an average oxygen concentration of 80,000 ppm or less. In other embodiments, the environment has an average oxygen concentration of 30,000 ppm or less. In yet another embodiment, the average oxygen concentration is less than or equal to any of the following ppm values. That is, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, 10,000, 15,000, 20,000, With numbers of 25,000, 30,000, 35,000, 40,000, 45,000, 50,000, 55,000, 60,000, 65,000, 70,000, 75,000 and 80,000 is there.

  As previously described, in some embodiments, the oxygen concentration in the sealed exposure chamber closes the inlet and outlet ports after the oxygen concentration in the sealed exposure chamber reaches 190,000 ppm or less. Maintained or substantially maintained. The oxygen concentration can be adjusted by having two gas sources that feed and mix the gas in a manifold that feeds the mixed gas into the inlet port.

Actinic radiation The photosensitive element of the present invention is exposed to actinic radiation from a suitable source through a mask. Actinic exposure time can vary from seconds to minutes depending on the intensity and spectral energy distribution of the radiation, the distance from the photosensitive element, the desired image resolution, and the nature and amount of the photopolymerizable composition. Is possible. The exposure temperature is preferably ambient temperature or slightly higher, ie, about 20 ° C. to about 35 ° C. Exposure is performed for a time sufficient to crosslink the exposed area to the support or back exposed layer or floor. In general, imagewise exposure times are much longer than backflash exposure times, ranging from minutes to tens of minutes.

  The actinic radiation source includes an ultraviolet wavelength region and a visible wavelength region. The suitability of a particular source of actinic radiation is determined by the photosensitivity of the initiator and at least one monomer used in making the flexographic printing plate. The preferred photosensitivity of the most common flexographic plates is in the UV and far UV spectrum, because the spectrum provides relatively good room light stability. Examples of suitable visible and UV light sources include carbon arcs, mercury vapor arcs, content fluorescent lamps, electronic flash units, electron beam units, lasers and photographic floodlights. Examples of industry standard radiation sources include Sylvania 350 Blacklight fluorescent lamps (FR48T12 / 350 VL / VHO / 180, 115w) and Philips UV-A “TL” -series low pressure mercury vapor fluorescent lamps. In some embodiments, a mercury vapor arc or a sunlamp can be used. In other embodiments, a high UV content fluorescent lamp can be used at a distance of about 1 to about 10 inches (about 2.54 to about 25.4 cm) from the photosensitive element. These radiation sources generally emit long wave UV radiation between 310 and 400 nm.

  In some embodiments, a method for making a relief printing form includes a back exposure or backflush step. This is a blanket exposure to actinic radiation through the support. This is used to create a layer of polymerized material, i.e., a floor, on the support side of the photopolymerizable layer and to provide photosensitivity to the photopolymerizable layer. The floor is useful for improving the adhesion between the photopolymerizable layer and the support, enhancing the dot resolution, and defining the height of the plate protrusions. Backflash exposure can be performed before, after, or during other imaging steps. Any of the conventional radiation sources described above for the full (imagewise) actinic radiation exposure step can be used for the backflash exposure step. In general, the exposure time ranges from a few seconds to a few minutes. In some embodiments, when producing a photosensitive element, a floor may be included in the photosensitive element, thereby eliminating the need for a separate backflash exposure.

  After full exposure to UV radiation through the mask, the photosensitive printing element is processed to remove unpolymerized areas in the photopolymerizable layer, thereby forming a convex image. This processing step removes at least areas of the photopolymerizable layer that have not been exposed to actinic radiation, ie, unexposed or uncured areas of the photopolymerizable layer. With the exception of the elastomeric capping layer, typically additional layers that may be present on the photopolymerizable layer are removed or substantially removed from the polymerized layer of the photopolymerizable layer. In some embodiments of photosensitive elements having an in-situ mask, this processing step also removes the mask image (exposed to actinic radiation) and the unexposed areas of the underlying photopolymerizable layer. .

Development Process-Processing Photosensitive elements can be processed by (1) “wet” development in which the photopolymerizable layer is contacted with a suitable developer solution to wash out non-polymerized areas, and / or (2) development. Heating the photosensitive element to temperature includes “dry” development that melts, softens, or dissolves the non-polymerized areas of the photopolymerizable layer to remove the non-polymerized areas. Dry development may also be called thermal development. It is also contemplated that the protrusions can be formed using a combination of wet and dry processing.

  Wet development can be performed at room temperature, but is typically performed at about 80 to 100 ° F. (26.7 to 37.8 ° C.). The developer can be an organic solvent, an aqueous or semi-aqueous solution, and water. The choice of developer depends mainly on the chemical properties of the photopolymerizable material to be removed. Suitable organic solvent developers include aromatic hydrocarbons or aliphatic hydrocarbon solvents, and aliphatic halohydrocarbons or aromatic halohydrocarbon solvents, or mixtures of suitable alcohols and their solvents. Other organic solvent developers are disclosed in published US Pat. Typically, suitable semi-water soluble developers include water and water miscible organic solvents and alkaline materials. Usually, suitable water-soluble developers include water and alkaline materials. Other suitable water-soluble developer combinations are described in US Pat.

  Development time can vary based on the thickness and type of photopolymerizable material, the solvent used, and the equipment and its operating temperature, but is preferably in the range of about 2 to 25 minutes. The developer can be applied in any convenient manner, including dipping, spraying, and brushing or roller coating. Brushing aids can be used to remove non-polymerized portions of the element. Washout can be performed in an automated processing unit that utilizes developer and mechanical brushing motion to remove uncured portions of the plate, leaving the exposed image and the convexity that makes up the floor.

  In general, after processing by development in solution, the relief printing plate is blotted dry and wiped dry and then more fully dried in forced air or infrared oven . The drying time and temperature may vary based on equipment design, airflow, plate material, but typically the plates are dried at 60 ° C. for 60 to 120 minutes. High temperatures are not recommended because the support can shrink and this can cause alignment problems.

  Heat treating the element includes at least one photopolymerizable layer to a temperature sufficient to liquefy the non-cured portion of the photopolymerizable layer, ie, to soften, melt, or dissolve to remove the uncured portion. Heating the photosensitive element with (and additional layers / multiple additional layers). The layer of the photosensitive composition can be partially liquefied during thermal development. That is, during thermal development, the uncured composition should soften or melt at the appropriate processing or development temperature. Where the photosensitive element includes one or more additional layers over the photopolymerizable layer, the one or more additional layers are also within the acceptable development temperature range for the photopolymerizable layer. It is desirable (but necessary) to be removable. The polymerized zone (cured portion) of the photopolymerizable layer has a higher melting temperature than the non-polymerized zone (uncured portion) and therefore does not melt, soften or dissolve at the heat development temperature. The non-cured portion includes air flow or liquid flow under pressure described in Patent Document 16, vacuum described in Patent Document 17, and Patent Document 18, Patent Document 19, Patent Document 20, Patent Document 21, Patent Document 22, and can be removed from the cured portion of the composition layer by any means including contact with the absorber material described in US Pat. A preferred method for removal of the uncured portion is by contacting the outermost surface of the element with an absorbent surface, such as a development medium, to absorb or suck away or suck away the melted portion.

  The term “melt” is used to describe the behavior of the unirradiated (uncured) portion of a composition layer that has been subjected to elevated temperatures that soften and reduce viscosity to allow absorption by the absorbent material. However, throughout this specification, the terms "melting", "softening" and "liquefaction" are used regardless of whether the composition can have a well-defined transition temperature between the solid and liquid phases. It may be used to describe the behavior of the heated non-irradiated part of the composition layer. A wide temperature range can be used to “melt” the composition layer for the purposes of the present invention. Absorption can be slower at lower temperatures and faster at higher temperatures while the process is being performed successfully.

  The heat treatment step consisting of heating the photosensitive element and contacting the outermost surface of the element to the development medium is still soft or molten when the uncured portion of the photopolymerizable layer is contacted with the development medium. In such a condition, it can be carried out simultaneously or continuously. At least one photopolymerizable layer (and additional layer / several additional layers) is sufficient to melt the uncured portion by heat transfer, convection, radiation or other heating methods, but of the layer It is heated to a temperature that is not high enough to cause distortion in the cured part. Also, one or more additional layers disposed above the photopolymerizable layer may be absorbed by being softened, melted, or flowed by the development medium. The photosensitive element is heated to a surface temperature greater than about 40 ° C., preferably from about 40 ° C. to about 230 ° C. (104-446 ° F.), in order to melt or flow the uncured portion of the photopolymerizable layer. The By roughly maintaining the adhesion of the development medium to the photopolymerizable layer in which the uncured region is melted, movement of the non-cured photosensitive material from the photopolymerizable layer to the development medium occurs. While still in the heated state, the development medium is separated from the cured photopolymerizable layer in contact with the support layer to reveal a convex structure. The cycle of steps consisting of heating the photopolymerizable layer and contacting the molten (partial) layer to the development medium is the number of times necessary to sufficiently remove the uncured material and produce sufficient convex height. Is only possible to repeat. However, it is preferred to minimize the number of cycles for proper system performance, and typically the photopolymerizable element is heat treated for 5 to 15 cycles. The adhesion of the development medium to the photopolymerizable layer (while the uncured portion is melted) can be maintained by pressing the layer and the development medium together.

  An apparatus suitable for heat developing a photosensitive element is disclosed by Peterson et al. In US Pat. The photosensitive element may be placed on a drum or plane to perform the heat treatment.

  In all embodiments, the photosensitive element is in the form of a plate. However, it should be understood that one of ordinary skill in the art can modify each of the disclosed devices to accommodate the installation of photosensitive elements in the form of cylinders or sleeves.

  The development medium has a melting temperature that exceeds the melting temperature of the non-irradiated or uncured portion of the radiation curable composition, or the softening temperature, or the liquefaction temperature, as well as good tear resistance at the same operating temperature. Selected to have. The material selected is one that will withstand the temperatures required to process the photosensitive element during heating. In the present specification, the development medium may be referred to as a development material, an absorbent material, an absorbent web, and a web. The development medium is selected from non-woven materials, paper stocks, fiber woven materials, open cell foam materials, porous materials, where a substantial portion of the volume they have is included as void volume. The development medium can be in the form of a web or a sheet. The development medium should also have a high absorption for the molten elastomeric composition as measured by the number of milligrams of elastomeric composition that can be absorbed per square centimeter of the development medium. In some embodiments, the development medium is a nylon nonwoven web or a polyester nonwoven web.

  It is also anticipated that the photosensitive element may go through one or more processing steps to sufficiently remove the uncured portion and form the protrusion. The photosensitive element may undergo both wet development and dry development in any order to form a convex portion. If one or more additional layers disposed above the photopolymerizable layer are not removable by the washout solution and / or by heating, a pre-development processing step to remove these additional layers May be necessary.

  After the processing steps, the photosensitive element can be uniformly post-exposed to ensure that the photopolymerization process is complete and to ensure the stability during printing and storage of the flexographic printing plate thus formed It is. This post-exposure step can use the same radiation source as the imagewise main exposure. Furthermore, if the surface of the flexographic printing plate is still sticky, a detackification process can be adapted. Such methods, also referred to as “finishing”, are well known in the art. For example, tackiness can be removed by treating flexographic printing plates with bromine or chlorine solutions. Preferably, detackification is achieved by exposing to a UV radiation source having a wavelength not longer than 300 nm. This so-called “light-finishing” is disclosed in US Pat. Various finishing methods may be combined. Typically, post-exposure and finish exposure are performed simultaneously on the photosensitive element using an exposure device having two radiation sources.

Letterpress printing form The letterpress printing form forms a relief structure that includes a plurality of relief surfaces from the floor, each relief surface having an ink carrying top surface area. The relief printing form produced by a digital workflow modified in an environment having an inert gas and oxygen concentration between 190,000 ppm and 100 ppm is a traditional analog workflow, i.e. a phototool that is kept in contact by a vacuum. • Give the raised surface a shape similar to that of the raised surface produced by imagewise exposure through the film. This method of a modified digital workflow, in which the photosensitive element is imagewise exposed through an in-situ mask in the presence of an inert gas and an oxygen concentration of 190,000 ppm to 100 ppm, is an “analog-like” structure. That is, a shape that becomes a raised surface having a flat top and a sharp shoulder is generated on the raised surface. In some embodiments, the modified digital workflow has a raised surface with a shape that also minimizes the fluting effect when used to print on a substrate such as corrugated paperboard. give. The shape of the raised surface is determined by the ink carrying top surface area, the side wall surface area, and the shoulder surface area which is the transition between the top surface area and the side wall surface area. For each raised surface, the total printing area that can contact the substrate to transfer ink is the sum of the top surface area and the shoulder surface area, between the substrate and the relief printing form. Related to pressure.

  In the present invention, for a raised surface having a halftone dot area of about 30% of the total pixel area (based on the screen resolution of the image), the shape of the raised surface is under pressure by the shoulder area. Thus, the shape of the ink transferred to the printing medium is 30% or less of the total printing area. Since the shoulder area contribution to the total print area is reduced, the fluting effect is reduced. In some embodiments, the ink transferred to the substrate by the shoulder area is no more than 25% of the total print area. In other embodiments, the ink transferred to the substrate by the shoulder area is no more than 20% of the total print area. In some embodiments, the ink transferred to the substrate by the shoulder surface area is no more than 15% of the total printed area. In some embodiments, the ink transferred to the substrate by the shoulder area is no more than 10% of the total print area. In some embodiments, the ink transferred to the substrate by the shoulder surface area is no more than 5% of the total printed area.

  The shape of the raised surface is somewhat similar to the shape of the raised surface in letterpress printing forms produced in analog workflow, that is, relatively flat ink carrying that moves sharply through the shoulder to the side wall surface area While having a generally conical shape with a top surface area, the contribution of the shoulder surface area to the total printing area with the relief printing form of the present invention is significantly different in that a reduction in the fluting level in the printed image is observed. It can be a thing.

  Conventional imaging techniques such as a microscope or optical device are used to determine the observed dot area. An example of a suitable optical device is the Betaflex flex analyzer unit by Beta Industries, Inc. (Carlstedt, NJ), which defines the relief surface of a relief printing form with a dot area, a screen ruling, and a screen ruling. Record as an image for measurement and analysis of convex properties such as halftone dot quality. As shown in FIG. 3, when using the optical device described above, the plate 10 having the convex structure 12 of the raised surface 15 is illuminated from the bottom surface 18 of the plate using a light source 22, and the light Passes through the plate 10 and proceeds to the microscope objective or optical device detector 24. The raised surface 15 typically called a halftone dot structure includes a flat portion 15a, a shoulder portion 15b, and a side wall portion 15c. Due to the geometry of the raised surface 15, light travels through the flat portion 15 a of the raised surface and is collected by the detector 24 as shown as vector (or vectors) A. . Light striking the shoulder portion 15b and sidewall portion 15c of the raised surface 15 of the convex structure 12 is refracted away from the objective lens or detector 24, as shown as vector (or vectors) B. As a result, the halftone dot size of the raised surface 15 to be observed or measured correlates with the flat portion 15a of the halftone dot structure. Other optical devices may illuminate and detect in the opposite manner, i.e. illuminate from the top surface of the plate and detect light transmitted on the opposite side of the plate, but the measured dot size is still It is a flat part of a halftone dot structure. FIG. 3A represents an image captured by the optical device of the flat portion 15a of the raised surface 15, and shows the halftone dot area 15f observed. These types of optical measurements are accurate in determining the flat portion 15a of the relief printing surface, but do not accurately record the shoulder portion 15b of the raised surface.

  Common assignee's patent document 27 describes the effect of a modified digital workflow on a substrate such as corrugated paperboard. It should be noted that the present invention will be described with respect to a corrugated board substrate as an example only to demonstrate the results of using a digital workflow process in which the oxygen concentration is adjusted. The present invention can be easily used including printing on other printing materials.

  In the corrugated paperboard printed material, the printing surface of the raised surface 15 includes not only the flat portion 15a of the halftone dot but also at least a portion of the halftone shoulder 15b. Accordingly, the effective halftone dot area includes not only the halftone dot area to be observed, but also a part of the halftone dot shoulder that may come into contact with the substrate during printing. The shape of the halftone dot shoulder 15b, which is a transition from the flat portion 15a to the side wall portion 15c, affects the portion that may come into contact with the printing medium.

  Assuming the plate durometer hardness, impression pressure, and substrate compliance are the same, consider the increase in printing surface as the pressure applied to the printing form increases Printing using a relief printing form having a raised surface with a rounded shoulder onto a corrugated paperboard substrate, a relief printing form having a raised surface with a very sharp shoulder It is possible to make a comparative determination with respect to printing using the.

  FIG. 4 shows a relief printing plate 10 having a raised surface 15 with a rounded half-point shoulder 15b and pressed against a corrugated paperboard substrate 30 having a flute layer 31 between two liner layers 32,33. Indicates. At position X, the flute layer 31 contacts the upper liner layer 32 and provides additional support to the upper liner layer 32. This support of the flute layer 31 adds additional pressure to the printed structure, resulting in additional transfer of ink by more halftone shoulder areas, in areas wider than the flat area 15a of the halftone dots. An ink region is generated on the corrugated paper substrate. FIG. 4A shows an ink image 35 of the raised portion 15 where the flat portion 15a and the rounded shoulder portion 15b have transferred the ink portions 35a and 35b onto the corrugated paperboard substrate 30 at position X. . The observed ink image 35 has a halftone dot size of 15b. At a position Y where the upper liner layer 32 is not supported by the flute layer 31, a lower pressure is applied to the printing structure and the ink area 35a on the corrugated paperboard substrate 30 is more directly directed to the flat portion 15a of the printing surface. Correspond. FIG. 4B shows an ink image 35 of the raised portion 15 where the flat portion 15a has the ink portion 35a transferred onto the corrugated paperboard substrate (in the unsupported area) at position Y. In the case of printing on this corrugated paperboard, it is possible to observe and measure significant density differences correlated to areas supported and unsupported by the flute layer, and printing artifacts commonly referred to as fluting artifact).

  FIG. 5 shows pressing onto a substrate such as a corrugated paperboard substrate 30 having a raised surface 15 with a sharp half-tone shoulder 15b and a flute layer 31 between two liner layers 32, 33. 1 shows a relief printing plate 10. At position X where the flute layer 31 provides additional support to the upper liner layer 32, additional pressure is applied to the printed structure. The halftone shoulder 15b also transfers the ink, but the shoulder is much smaller and the ink area 35 is correspondingly smaller. The ink image 35 on the corrugated paperboard substrate is only slightly wider than the flat portion of the halftone dot surface 15a. FIG. 5A shows an ink image 35 of the raised portion 15 where the flat portion 15a and the sharp shoulder portion 15b have transferred the ink portions 35a and 35b onto the corrugated paperboard substrate 30 at position X. At a position Y where the upper liner layer 32 is not supported by the flute layer 31, lower pressure is applied and the ink area 35 on the corrugated paperboard substrate 30 corresponds more directly to the flat portion 15a of the printing surface. FIG. 5B represents the ink image 35 of the raised portion 15 with the flat portion 15a transferred at position Y to the ink portion 35a onto the corrugated paperboard substrate (in the unsupported region). Because the density difference between the supported and unsupported areas of the corrugated paperboard substrate (generated by the raised portion 15) is much smaller, fluting printing artifacts are minimized. Or in some cases.

  In general, fluting printing artifacts are derived from a 1% halftone dot, where the ink application area is part of the entire area relative to the solid printed area. Over the range scale, more or less can be observed on the corrugated paperboard substrate. However, in some embodiments, a printed mid-tone region that is a tonal scale with a printed halftone dot area of about 30 to about 70% is most effective for fluting. There is a tendency to show effectively. In other embodiments, a printed quarter-tone region having a printed dot area of about 10 to about 30% can effectively exhibit fluting. Grading percentages corresponding to such printed halftone areas and printed quartertone areas are generally acceptable to those skilled in the art, but to the extent they are not considered established or binding standards. I want you to understand.

  The shape of the raised surface of the printing plate formed during photopolymerization (ie, imagewise exposure within a constrained oxygen concentration) and subsequent processing steps depends on the process used for plate making of the printing plate. It is shown that they are different. In particular, the shape of the raised surface generated by the present process is different from the shape of the raised surface generated by the conventional analog process and the conventional digital process. To assess the shape of the raised surface of the printing plate, specifically to quantify the area (expressed as a percentage of the total area) of the printing surface that is in contact with the substrate during the printing process A method has been established.

  A more accurate method if the general imaging technique used to determine the observed halftone area as described above is not sufficient to show an effective printed halftone area of a convex structure. Is used to determine the appropriate dot structure for printing on corrugated paperboard. This method should record not only the observed dot area (flat part), but also the dot shoulder area and some dot side wall areas. This method for evaluating the shape of the raised surface 15 spans a given surface area of surface profilometry, optical interferometry, three-dimensional optical microscopy, or a representative sample of material. It involves the measurement of the printing plate surface by any one of a number of methods, including any method that allows the surface shape to be measured with sufficient resolution. In particular, this measurement process generates Z-axis data (height) for all X and Y positions in the region of interest. Using this information, it is possible to generate a cumulative sum of areas of the printing plate surface, such as raised portions, from the top region of the plate to the floor region.

  FIG. 6 shows a cumulative sum of percent area area versus height of the raised surface of the convex structure (z-axis position) for each of the three printing forms made by various workflow processes. Curve A shows the cumulative sum of raised portions generated in a plate made by an analog workflow process (imagewise exposure through a phototool under vacuum). Curve B shows the cumulative sum of raised portions produced in a plate made by a digital workflow process (imagewise exposure through an in-situ mask in the presence of atmospheric oxygen). Curve C shows the raised portion produced in a plate made by the present workflow process of imagewise exposure through an in-situ mask in an atmosphere of inert gas and oxygen between 190,000 and 100 ppm. Indicates the cumulative sum. 6 can be used to describe the printing element structure, i.e. the raised part, in particular the flat part of the printing surface, is a curved line whose area increases rapidly. The sidewall portion of the printing element, indicated by the region, is indicated by a linear portion of the curve where the cumulative area does not increase too rapidly. The transition area between these two areas shows the shoulder of the printing element. The final area shown is a non-printing area called the floor where the raised surface structure is attached to the bulk plate material. In FIG. 6, curve A and curve C are similar, but curve A is characterized by a more gradual transition from the flat portion to the sidewall portion of the printing element structure (the raised portion). Curve B represents a very different printing element structure (raised part) with an extended transition area between the flat part of the printing surface and the side wall part.

  This analysis can be used to evaluate the effect that press pressure has on the surface area of the printing element that is in contact with an anilox roller or substrate during the printing process. The surface area is affected by several variables including physical plate characteristics such as durometer hardness, plate mounting material, anilox characteristics and other printing conditions. Since these factors can be reduced or eliminated to a minimum, the present three-dimensional analysis of the raised surface provides a method for evaluating the impact of printing pressure on the effective printing surface area. At the minimum printing pressure, only the flat portion of the raised halftone dot surface contacts the substrate, and at excessive printing pressure, the raised portion is wider than the flat portion, including some or all of the shoulder portion. The surface area of the halftone dot surface contacts the substrate. Thus, for the same change in printing pressure, the cumulative surface area that is in contact during the printing process is significantly wider for the material characterized by curve B than for the material characterized by curve A. It will be a thing. Also, due to the same identical change in printing pressure, the cumulative surface area that is in contact during the printing process is the narrowest for the material characterized by curve C.

  The key elements for each of the raised portions of the convex structure can be described by a cumulative sum of area percentages, so that the raised surface and the resulting printed area is another descriptor. Can be determined by: The maximum slope point (when the first derivative reaches a maximum or when the second derivative exceeds zero) represents the point that represents the uppermost printing surface. Flat line portion of the curve (second derivative approaches zero-for this analysis, it was identified as the point where the second derivative is within zero to 0.0005 after maximization of the first derivative) Represents a shoulder having a halftone dot structure. The second derivative represents the effective dot area under pressure.

  Instead of representing the shoulder area as a percentage of the dot diameter as measured by the microscope, it is also possible to represent the shoulder area as the rate of increase in the dot radius (or diameter) from the top surface area of the dot. The diameter of the top surface area or the raised halftone dot surface can be determined by measurement using an optical microscope as described above. The effective print area area under halftone dot surface pressure (halftone dot area) measured by the surface shape measurement method, optical interference spectroscopy or three-dimensional optical microscopy as described above depends on the screen resolution. It is possible to convert to a halftone dot diameter including a shoulder portion based on the determined total pixel area. If the increase or increase under pressurization of the raised dot surface radius (due to the action of the shoulder portion during printing) is less than about 10 microns, the printing form will have acceptable printing performance, ie acceptable. It has been found to have a degree of fluting on corrugated paperboard. In some embodiments, if the increase or increase under pressurization of the radius of the raised halftone dot plane (due to the action of the shoulder portion during printing) is less than about 8 microns, the printing form is at a low level. An image with fluting was printed on corrugated paperboard. In some embodiments, the print form is very low if the increase or increase under pressurization of the radius of the raised halftone dot plane (by the action of the shoulder portion during printing) is less than about 5 microns. An image with a degree of fluting, or an image with no degree of fluting was printed on the cardboard. In some embodiments, if the increase or increase (due to the action of the shoulder portion during printing) under pressure of the raised dot surface radius is less than about 2 microns, the printing form is very An image with a low degree of fluting, or an image that does not have a degree of fluting, was printed on corrugated paper.

Printing method: installing a relief printing form on a printing press; applying ink to a printing area of the printing form (ie, a raised portion of a convex surface); and printing to transfer a pattern of ink onto the substrate. The step of contacting the body with the inked printing area is not limited and includes various conventional types for installation, inking and contacting, as known to those skilled in flexographic printing, and Includes unconventional practices. Non-Patent Document 1 and Non-Patent Document 2 are suitable information sources representing the knowledge field in many aspects of flexographic printing. In particular, the chapters on installation and proofing, printing presses, inks, flexographic printing plates and substrates are most applicable to the present invention.

Photosensitive element The photosensitive element used to make the flexographic printing form comprises at least one photopolymerizable composition layer. The term “photosensitive” includes any mechanism by which at least one photosensitive layer can initiate a reaction or reactions, particularly a photochemical reaction, upon exposure to actinic radiation. In some embodiments, the photosensitive element includes a support for the photopolymerizable layer. In some embodiments, the photopolymerizable layer is an elastomeric layer that includes a binder, at least one monomer, and a photoinitiator. In some embodiments, the photosensitive element includes a layer of actinic radiation opaque material adjacent to the photopolymerizable layer on the opposite side of the support. In other embodiments, the photosensitive element includes an image of an actinic radiation opaque material suitable for use as an in-situ mask adjacent to the photopolymerizable layer.

  Unless otherwise indicated, the term “photosensitive element” includes printing precursors that can be exposed and processed to actinic radiation to form a surface suitable for printing. Unless otherwise indicated, “photosensitive elements” and “printing forms” include any form of element or structure that becomes or is suitable for printing, including flat sheets, plates, seamless continuous Including, but not limited to, foam, cylindrical foam, plates-on-sleeves, and plates-on-carriers. Printing forms obtained from photosensitive elements are expected to have end use printing applications for letterpress printing such as flexographic printing and letterpress printing. Letterpress printing is a printing method in which printing is performed by the image area of the printing form, the image area of the printing form is raised and the non-image area is concave.

  The photosensitive element includes at least one photopolymerizable composition layer. As used herein, the term “photopolymerizable” includes mechanisms of photopolymerizable, photocrosslinkable, or both properties. The photopolymerizable layer is a solid elastomeric layer formed from a composition comprising a binder, at least one monomer, and a photoinitiator. The photoinitiator has photosensitivity to actinic radiation. Throughout this specification, actinic light includes ultraviolet radiation and / or visible light. The solid layer of photopolymerizable composition is treated with one or more solutions and / or heated to form protrusions suitable for flexographic printing. As used herein, the term “solid” refers to the physical state of a layer that has a constant volume and shape and resists forces that attempt to change its volume or shape. The layer of photopolymerizable composition is solid at room temperature, which is a temperature between about 5 ° C. and about 30 ° C. The solid layer of the photopolymerizable layer may be polymerized (photocured), may not be polymerized, or may be used in combination.

  The binder is not limited and can be a single polymer or a mixture of polymers. In some embodiments, the binder is an elastomeric binder. In other embodiments, the binder becomes an elastomer when exposed to actinic radiation. Binders include natural or synthetic polymers of conjugated diolefin hydrocarbons, including polyisoprene, 1,2-polybutadiene, 1,4-polybutadiene, butadiene / acrylonitrile and diene / styrene thermoplastic-elastomer block copolymers. . In some embodiments, the binder is an elastomeric block copolymer of an ABA type block copolymer, where A represents a non-elastomeric block and B represents an elastomeric block. The non-elastomeric block A can be a vinyl polymer such as polystyrene. Examples of the elastomer block B include polybutadiene and polyisoprene. In some embodiments, elastomeric binders include poly (styrene / isoprene / styrene) block copolymers and poly (styrene / butadiene / styrene) block copolymers. The ratio of non-elastomeric to elastomer of an ABA type block copolymer can be in the range of 10:90 to 35:65. The binder can be soluble, swellable or diffusible in aqueous, semi-aqueous, water or organic solvent washout solutions. Elastomer binders that can be washed out by processing in a water-soluble or semi-water-soluble developer are described in Patent Document 28 by Proskow, Patent Document 29 by Proskow, Patent Document 30 by Worns, Patent Document 31 by Suzuki et al., This is disclosed in US Pat. No. 6,057,097 by Suzuki et al. And US Pat. The block copolymers disclosed in Chen, U.S. Patent No. 5,057,049, Heinz et al., U.S. Patent No. 5,057,028 and Toda et al., U.S. Patent No. 5,057,078 can be washed out by treatment in an organic solvent solution. In general, elastomeric binders suitable for washout development are also suitable for use in heat treatments that soften, melt or dissolve the non-polymerized areas of the photopolymerizable layer simultaneously with heating. It is preferred that the binder be present in an amount of at least 50% by weight of the photosensitive composition.

  As used herein, the term binder refers to core-shell microgels, microgels and preformed, such as those disclosed in Flyd et al. US Pat. High polymer blends.

  The photopolymerizable composition comprises at least one compound capable of addition polymerization that is compatible with the binder to the extent that a clear, non-cloudy photosensitive layer is produced. At least one compound capable of addition polymerization is sometimes referred to as a monomer. Monomers that can be used in the photopolymerizable composition are well known in the art and include, but are not limited to, addition-polymerized ethylenically unsaturated compounds having at least one terminal ethylene group. Generally, the monomer has a relatively low molecular weight (less than about 30,000). In some embodiments, the monomer has a relatively low molecular weight of less than about 5000. Unless otherwise indicated, molecular weight is a weighted average molecular weight. The addition polymerization compound may be an oligomer and can be a single oligomer or a mixture of oligomers. Some embodiments include polyacrylol oligomers having a molecular weight greater than 1000. The composition can include a single monomer or a combination of monomers. The monomeric compound is present in at least 5% by weight of the composition, and in some embodiments is present in 10 to 20% by weight.

  Suitable monomers include, but are not limited to, alcohol and polyol acrylic monoesters, alcohol and polyol acrylic polyesters, alcohol and polyol methacrylate monoesters, and alcohol and polyol methacrylate polyesters. Alcohols and polyols include alkanols, alkylene glycols, trimethylolpropane, ethoxylated trimethylolpropane, pentaerythritol and polyacrylol oligomers. Other suitable monomers include acrylate and methacrylate derivatives such as isocyanates, esters, epoxides and the like. A combination of monofunctional acrylates, polyfunctional acrylates, monofunctional methacrylates, and / or polyfunctional methacrylates may be used. Other examples of suitable monomers include acrylate and methacrylate derivatives such as isocyanates, esters, epoxides, and the like. In some end use printing forms, it may be desirable to use monomers that impart elastomeric properties to the element. Examples of elastomeric monomers include acrylated liquid polyisoprene, acrylated liquid butadiene, high vinyl content liquid polyisoprene, and high vinyl content (ie, a content of 1-2 vinyl groups of about 20 weights). Liquid polybutadiene) (but not limited to).

  Other examples of monomers can be found in US Pat. No. 6,057,049, Chen, US Pat.

  A photoinitiator is any single compound or combination of compounds that is sensitive to actinic radiation and generates free radicals that initiate polymerization of the monomer or monomers without undue termination. It is possible. Any known type of photoinitiator, especially quinone, benzophenone, benzoin ether, aryl ketone, peroxide, biimidazole, benzyldimethyl ketal, hydroxylalkylphenylacetophenone, dialkoxyacetophenone, trimethylbenzoylphosphine oxide derivative, aminoketone, benzoyl Free radicals such as cyclohexanol, methylthiophenylmorpholinoketone, morpholinophenylaminoketone, alpha halogenoacetophenone, oxysulfonylketone, sulfonylketone, oxysulfonylketone, benzoyloxime ester, thioxanthrone, ketocoumarin and Michler's ketone Photoinitiators may be used. Alternatively, the photoinitiator may be a mixture of compounds that produces free radicals when one of the compounds is stimulated by a sensitizer activated by radiation. Preferably, the photoinitiator for main exposure (and post-exposure and backflash) is sensitive to visible radiation or ultraviolet radiation between 310 and 400 nm, preferably between 345 and 365 nm. A second photoinitiator that is sensitive to radiation between 220 and 300 nm, preferably between 245 and 265 nm may optionally be present in the photopolymerizable composition. After processing, the plate can be finished with radiation between 220 and 300 nm to detackify the convex surface. The second photoinitiator reduces the finish exposure time required for detackification of the plate. Generally, the photoinitiator is present in an amount from 0.001% to 10.0% based on the weight of the photopolymerizable composition.

  The photopolymerizable composition can contain other additives depending on the desired final properties. Other additives to the photopolymerizable composition include sensitizers, plasticizers, fluidity modifiers, thermal polymerization inhibitors, colorants, processing aids, antioxidants, ozone degradation inhibitors, dyes and A filler is included.

  Plasticizers are used to adjust the film forming properties of the elastomer. Examples of suitable plasticizers include aliphatic hydrocarbon oils such as naphthene oil and paraffin oil, liquid polydienes such as liquid polybutadiene, liquid polyisoprene, polystyrene, poly-alpha-methylstyrene, alpha-methylstyrene-vinyltoluene copolymer. , Hydrogenated rosin pentaerythritol esters, polyterpene resins and ester resins. Generally, a plasticizer is a liquid having a molecular weight of less than about 5000, but can have a molecular weight of up to about 30,000. A plasticizer having a low molecular weight includes a molecular weight of less than about 30,000.

  The thickness of the photopolymerizable layer can vary over a wide range depending on the type of printing form desired. In one embodiment, the photosensitive layer can have a thickness of about 0.015 inches to about 0.250 inches or more (about 0.038 to about 0.64 cm or more). In another embodiment, the photosensitive layer can have a thickness of about 0.107 inches to about 0.300 inches (about 0.27 to about 0.76 cm). In some embodiments, the photosensitive layer can have a thickness of about 0.020 to about 0.067 inches (about 0.5 mm to about 1.7 mm). In still other embodiments, the photosensitive layer can have a thickness of about 0.002 inches to 0.025 inches (0.051 to 0.64 mm).

  The photosensitive element may optionally include a support adjacent to the photopolymerizable composition layer. The support can be composed of any material or combination of materials conventionally used with photosensitive elements used to make printing forms. In some embodiments, the support is transparent to actinic radiation to accommodate “backflash” exposure through the support. Examples of suitable support materials include polymer films such as those formed by addition polymers and linear condensation polymers, permeable foams, and fibers such as fiberglass. Metal supports are opaque to radiation, but under certain end use conditions, metals such as aluminum, steel and nickel may be used as the support. In some embodiments, the support is a polyester film. In one embodiment, the support is a polyethylene terephthalate film. The support may be in the form of a seat or a cylinder such as a sleeve. The sleeve can be formed from any material or combination of materials conventionally used in the formation of printing sleeves. The sleeve can have a single layer structure, a multilayer structure, a composite structure, or a unitary structure. The sleeve is typically transparent to actinic radiation and can therefore be composed of a polymer film that is compatible with backflash exposure to build up a floor in a cylindrical printing element. The multilayer sleeve may include an adhesive layer or tape between layers of flexible material, as disclosed in US Pat. The sleeve can also be composed of an impermeable actinic radiation blocking material such as nickel or glass epoxy. The sleeve can be composed of one or more layers of a resin composition, which can be the same or different and can include excipients and / or fibers therein. Materials suitable as the resin composition are not limited, and examples include epoxy resins, polystyrene resins such as polyvinyl chloride and polyvinyl acetate, and polyvinyl resins, phenolic resins, and aromatic amine-cured epoxy resins. The fibers used in the resin composition are not limited and can include, for example, glass fibers, aramid fibers, carbon fibers, metal fibers, ceramic fibers, and the like. The fibers mixed into the sleeve can include continuous materials, woven materials, and / or wound materials. A support formed from a resin composition reinforced with fibers is an example of a composite sleeve. In some embodiments, the support has a thickness of 0.002 to 0.050 inches (0.0051 to 0.127 cm). The sleeve can have a wall thickness of about 0.01 to about 6.35 mm or greater. In some embodiments, the sleeve has a wall thickness between about 0.25 and 3 mm. In some embodiments, the sleeve is between about 10 to 80 mils (0.25 to 2.0 mm), and in other embodiments 10 to 40 mils (0.25 to 1.0 mm). Having a wall thickness between.

  Optionally, the element includes an adhesive layer between the support and the photopolymerizable layer, or the surface of the support adjacent to the photopolymerizable layer has an adhesion promoting surface, the support Provides strong adhesion between the photopolymerizable layer and the photopolymerizable layer.

  The photopolymerizable layer itself can be made in a number of ways by mixing binders, monomers, initiators and other materials. Preferably, the photopolymerizable mixture is formed into a hot melt and then calendered to the desired thickness. An extruder can be used to melt, mix, degas and filter the composition. In order to achieve a uniform thickness, a rolling step of rolling the heated mixture between two sheets, such as a support and a temporary cover sheet, or between a flat sheet and a release roll, It can be advantageous to combine extrusion steps. Alternately, the material can be extruded / rolled onto a temporary support and then laminated to the desired final support. The element can also be made by mixing the components in a suitable mixing device and then compression forming the material into the desired shape in a suitable mold. In general, the material is compressed between the support and the cover sheet. The forming step can involve compression and / or heating. The cover sheet may include one or more additional layers that are transferred to the photopolymerizable layer when forming the photosensitive element. The cylindrical photopolymerizable element can be made by any suitable method. In one embodiment, the cylinder-shaped element can be formed from a photopolymerizable printing plate, which is wound on a carrier or a cylindrical support, ie a sleeve, Both ends are matched to form a cylinder shape. The cylinder-shaped photopolymerizable element can also be produced by the method and apparatus disclosed by Cushner et al.

  The photosensitive element includes at least one photopolymerizable layer and can thus be a two-layer or multi-layer configuration. The photosensitive element may include one or more additional layers on top of or adjacent to the photosensitive layer. In most embodiments, the one or more additional layers are on the side of the photosensitive layer opposite the support. Examples of additional layers include, but are not limited to, a protective layer, a capping layer, an elastomer layer, a barrier layer, and combinations thereof. The one or more additional layers can be all or partly removable during processing. The one or more additional layers may cover the photosensitive composition layer or only partially.

  The protective layer can protect the surface of the composition layer and allow easy removal of the mask material used for imagewise exposure of the photosensitive element. The photosensitive element may include an elastomeric capping layer on at least one photopolymerizable layer. Typically, the elastomeric capping layer is part of a multilayer cover element that becomes part of the photosensitive printing element during the rolling process of the photopolymerizable layer. Multilayer cover elements and compositions suitable as an elastomeric capping layer are disclosed in Gruetzmacher et al. US Pat. In some embodiments, the composition of the elastomeric capping layer includes an elastomeric binder, and optionally monomers and photoinitiators and other additives, all of which are bulk photopolymerizable layers. ) Can be the same or different from those used in. The elastomeric capping layer does not necessarily include a photoreactive compound, but the layer eventually becomes photoreactive when in contact with the underlying bulk photopolymerizable layer. Thus, upon imagewise exposure to actinic radiation, the elastomeric capping layer has a cured portion that has undergone polymerization and crosslinking and a non-cured portion that remains unpolymerized, i.e., is not crosslinked. By processing, the non-polymerized portion of the elastomeric capping layer is removed along with the photopolymerizable layer to form a convex surface. The elastomeric capping layer exposed to actinic radiation remains on the surface of the polymerization area of the photopolymerizable layer and becomes the actual printing surface of the printing plate.

  Actinic radiation opaque layers are used in digital-to-plate direct imaging technology, which uses laser radiation, typically infrared laser radiation, to form photosensitive elements. An image mask for [as an alternative to conventional image transparency or photo tools]. The actinic radiation opaque layer is substantially opaque to actinic radiation corresponding to the photosensitivity of the photopolymerizable material. By digital methods, a mask is disposed in-situ on or above the photopolymerizable layer using laser radiation. Digital methods for generating mask images require one or more steps to produce a photosensitive element prior to imagewise exposure. In general, digital methods of in-situ mask formation selectively remove the radiopaque layer from the side of the photosensitive element opposite the support, or the photosensitive element on the opposite side of the support. Transfer the radiopaque layer to the surface. The actinic radiation opaque layer is also sensitive to laser radiation capable of selectively removing or transferring the opaque layer. In one embodiment, the actinic radiation opaque layer is sensitive to infrared laser radiation. The method of forming a mask with a radiopaque layer on the photosensitive element is not limited.

  In one embodiment, the photosensitive element is disposed over the photopolymerizable layer opposite the support and includes an actinic radiation opaque layer that covers or substantially covers the entire surface of the photopolymerizable layer. May include. In this embodiment, infrared is disclosed as disclosed in US Pat. Nos. 5,047,096 to Fan, US Pat. Laser radiation removes the radiopaque layer imagewise, i.e. ablated or vaporized, forming an in-situ mask. There is a material capture sheet adjacent to the radiopaque layer during laser exposure to capture the material as it is removed from the photosensitive element, as disclosed in US Pat. Good. Only those portions of the radiopaque layer that have not been removed from the photosensitive element remain on the element, forming an in-situ mask.

  In some embodiments, the actinic radiation opaque layer includes a radiopaque material, an infrared absorbing material, and an optional binder. In general, dark non-organic pigments such as carbon black and graphite, mixtures of pigments, metals and metal alloys serve as both infrared-sensitive and radiopaque materials. Optional binders include self-oxidizing polymers, non-self-oxidizing polymers, thermochemically decomposable polymers, polymers and copolymers of butadiene and isoprene with styrene and / or olefins, thermodegradable polymers, amphoteric copolymers (amphoteric copolymers). interpolymers), polyethylene wax, polyamides conventionally used as release layers, polyvinyl alcohol, hydroxyalkyl cellulose, copolymers of ethylene and vinyl acetate, and combinations thereof, and other polymeric materials. . The thickness of the actinic radiation opaque layer should be within a range that optimizes both photosensitivity and transmission, and generally this range is from about 20 angstroms to about 50 micrometers. The actinic radiation opaque layer should have a transmitted optical density greater than 2.0 to effectively prevent actinic radiation and polymerization of the underlying photopolymerizable layer.

  In another embodiment for digitally forming the in-situ mask, the photosensitive element does not initially include an actinic radiation opaque layer. A separate element carrying the actinic radiation opaque layer forms a combination with the photosensitive element, so that the radiopaque layer is usually a photopolymerizable layer on the opposite side of the support. Adjacent to the face. (If there is a cover sheet associated with the photosensitive element, this is usually removed prior to the formation of the combination). The independent element may include one or more layers, such as an emissive layer or a heat generating layer, to assist in the digital exposure process. In this regard, the radiopaque layer is also sensitive to infrared radiation. As disclosed in U.S. Pat. Nos. 5,047,049 by Fan et al., And U.S. Pat. Are selectively altered or selectively altered to form an image disposed on or over the photopolymerizable layer. By the imagewise transfer process, only the transferred portion of the radiopaque layer remains on the photosensitive element, forming an in-situ mask.

  In another embodiment, digital mask formation can be accomplished by imagewise applying a radiopaque material in the form of an inkjet ink over the photosensitive element. The imagewise application of the inkjet ink can be performed directly on the photopolymerizable layer or can be assigned above the photopolymerizable layer on another layer of the photosensitive element. Another anticipated way in which digital mask formation can be achieved is by generation of a mask image of a radiopaque layer on a separate carrier. In some embodiments, the independent carrier includes a radiopaque layer that is imagewise exposed to laser radiation to selectively remove radiopaque material and form an image. The mask image on the carrier is then transferred by applying heat and / or pressure to the surface of the photopolymerizable layer opposite the support. Generally, the photopolymerizable layer is tacky and retains the transferred image. The independent carrier is then removed from the element prior to imagewise exposure.

  The photosensitive printing element may also include a temporary cover sheet on the top surface of the top layer of the element, which is removed prior to making the printing form. One purpose of the cover sheet is to protect the top layer of the photosensitive printing element during storage and handling. Examples of suitable materials for the cover sheet include thin films made of polystyrene, polyethylene, polypropylene, polycarbonate, fluoropolymer, polyamide or polyester, which can be subbed to the release layer. is there. The cover sheet is preferably made from a polyester such as Mylar® polyethylene terephthalate film.

  The printing form has a durometer hardness of about 20 to about 85 Shore A after exposure (and processing) of the photosensitive element. Shore durometer hardness is a measure of the resistance of a material to press fit. Shore A durometer hardness is a commonly used measure for soft rubber or elastomeric materials, the higher the value, the greater the resistance to penetration. In one embodiment, the printing form has a Shore A durometer hardness of about 50 to less than about 20. In another embodiment, the printing form has a Shore A durometer hardness of about 40 to less than about 25. In another embodiment, the printing form has a Shore A durometer hardness of about 35 to less than about 30. Printing forms having a Shore A durometer hardness of less than about 40 are particularly suitable for printing on corrugated paperboard. The durometer hardness of the printing form can be measured according to standardized procedures described in DIN 53,505 or ASTM D2240-00. In some embodiments, the printing form can be placed on a carrier that has the same or different elasticity as the printing form. The elasticity of the carrier can affect the overall elasticity of the entire printing form package (ie, the carrier and the printing form), resulting in a durometer hardness for the package that is different from the printing form.

The terminology “mask opening” is the “transparent” area of the integral mask to allow exposure of the underlying photopolymerizable material to actinic radiation. (In some embodiments, the transparent area is created by removing actinic radiation opaque material from the element. In other embodiments, the transparent area is non-actinic radiation opaque material to the element. Generated by transfer.) The mask aperture is measured with a measuring microscope. The effective mask open area is calculated by measuring the open area and dividing by the total pixel area defined by the (LPI) screen resolution in lines per inch. The total pixel area is calculated using the equation (1 / LPI) ² and the effective mask aperture is defined as (open area area) / (1 / LPI) ² . Typically, the mask opening is expressed as a percentage (of the total pixel area).

  “Phototool aperture” is the area of an area that is transparent to actinic radiation and expressed as a percentage of the total pixel area, and is calculated in the same manner as the mask opening area described above.

“Optical Density” or simply “Density” is the darkness (absorbance or opacity) of the image and has the following relationship: density = log ₁₀ (1 / reflectance)
Where the reflectance is (reflected light intensity / incident light intensity).

  The “halftone dot size on plate” is a measurement of the diameter of a halftone dot which is generally evaluated using a calibrated microscope or a dedicated optical device. In general, the measured value accurately represents the flat portion of the dot structure on the plate.

“Plate dot area” or “plate dot area” is usually expressed as a percentage, generally converting the dot size to area (area = πr ² ), and the total pixel area defined by the screen resolution. Calculated by dividing by area.

“Effective printed halftone area” refers to density measurements of areas printed by a regular array of uniformly sized halftone dots, called tint areas, and complete ink coverage [100% coverage. Or a density measurement of a printed area having a solid coverage]. The formula used is called the Murray Davies formula and is expressed as follows:
Effective halftone dot area = (1-10 ^−Dt ) / (1-10 ^−Ds )
Here, Dt = tint density and Ds = solid density.

  “Effective printing area under pressure (halftone area)” is expressed as a percentage of the total area, when the plate and the substrate are in close contact with each other, and when the plate / substrate is pressed together Sometimes the halftone dot area on the plate that is in intimate contact with the substrate to be printed.

  “Plate to Print Dot Gain” is printed by the dot area on the plate expressed as a percentage of the total pixel area area to the effective print dot area area expressed as a percentage of coverage. This corresponds to an increase in the halftone dot area. This is simply the difference between the two.

  CYREL <(R)> photopolymerizable printing plate or printing element, CYREL <(R)> Digital Imager, CYREL <(R)> exposure unit, CYREL <(R)> ECLF exposure unit, CYREL (R) processor and CYREL (R) ) All Cyrosol developers are available from The Du Pont Company (Wilmington, Germany).

  All plates tested have a total thickness of 125 mils (including the photopolymerizable layer and support thickness) and are suitable for use as flexographic printing plates. It was a photopolymerizable printing element. The photopolymerizable printing element comprises a photopolymerizable composition comprising an elastomer binder, at least one monomer and a photoinitiator between a Mylar® (5 mil) support and a cover sheet (7 mil). Of layers. For analog plates, the cover sheet included a polyamide release layer adjacent to the photopolymerizable layer. For the digital plate, the cover sheet included an infrared sensitive actinic radiation opaque layer consisting of 33 wt% carbon black and 67 wt% polyamide adjacent to the photopolymerizable layer.

To form the floor, each element was backflash exposed to UV light (365 nm) for 85 seconds (17.6 mJ / cm ² / sec) with a CYREL® exposure unit. After imagewise exposure, CYREL® Cylosol Wash, an organic solvent, for 8.9 minutes on a CYREL® 1000P type processor to remove unexposed areas and form a relief printing plate Each element was developed in the out solution. The plates were then dried in a convection oven for 2 hours. After drying, the plate was further exposed to UV light (254 nm) with an exposure unit for 5 minutes in order to photofinish the plate to remove any residual stickiness.

  As described below, each element was imagewise exposed to UV light (365 nm) via a test target image. The test target image was designed to be substantially the same for the phototool used in the analog workflow and for the in-situ mask formed adjacent to the photopolymerizable layer used in the digital workflow. . The test target image includes a plurality of sufficiently wide patches of various percent printing areas corresponding to ink tint percentages ranging from 2% to 100%, on the plate regardless of the plate making workflow Ensured that patches of the same dot size were available and provided sufficient area to quantify fluting performance. In addition, the test target image includes a solid ink density bar that intersects with a leading edge to ensure printing uniformity on a press (printed) sheet, and exposure uniformity. Included multiple patches, a positive type and reverse type, and a line target.

Plate A—Analog Workflow For plate A, an analog workflow was used to expose and process a CYREL® photopolymer printing element, type TDR, as described above. For back exposure, 17.6 mjoules / cm ² / sec was set to 113 seconds instead of 85 seconds. For imagewise exposure, the cover sheet is removed and an image bearing negative (ie, a phototool with the test target described above) is placed on the face of the element opposite the support, evacuated and exposed. The unit was exposed to UV radiation for 15 minutes (17.6 mjoules / cm ² / sec).

Plate B—Digital workflow plate B was also a CYREL® photopolymer printing element, type TDR, which contained an infrared sensitive actinic radiation opaque layer (instead of an emissive layer) . The plate was exposed and processed as described above using a digital workflow. The cover sheet is removed and an infrared sensitive actinic radiation opaque using infrared laser radiation on a CYREL® Digital Imager, Model SPARK 4835 Optics 40 by ESKO Graphics Imaging GmbH (Itzeo, Germany) A test target in-situ mask was formed on the photosensitive element by imagewise abrading the layer. It is known to those skilled in the art of digital plate making that the digital file of the mask image can be adjusted to compensate for oxygen inhibition and halftone dot gain typically associated with digital workflow plates. . To generate a mask image on plate B, appropriate “bumps” and compensation curves were applied to the mask image digital file as such. For plate B, the digital file of the mask image was properly bumped by about 4%.

For imagewise exposure of plate B, the photosensitive element is placed in the exposure unit and in air, ie atmospheric oxygen concentration of 21% (210,000 ppm), for about 16 minutes (33.9 mjoules / cm ^2). / Second) and exposed to UV radiation through an in-situ mask.

Plate C—Modified Digital Workflow— Regulated Oxygen Environment Plate C is also a CYREL® photopolymer printing element, type TDR, with infrared sensitive actinic radiation opaque (instead of emitting layer) It included a sex layer. The plate was exposed and processed using a digital workflow. Remove the cover sheet and image the test target on the element by imagewise ablating the infrared-sensitive actinic radiation opaque layer using infrared laser radiation in a CYREL® Digital Imager. An in-situ mask was formed. Those skilled in the art of digital plate making can adjust the digital file of the mask image to compensate for oxygen inhibition and halftone dot gain typically associated with plates produced by a digital workflow. Are known. However, unlike plate B, because of the exposure environment, i.e. the modified digital workflow environment, plate C was not adjusted for the digital file of the mask image using bumps and compensation curves.

  For imagewise exposure of plate C, a CYREL® ECLF exposure unit is on the exposure bed of the unit to achieve the desired conditions within the housing for the element during imagewise exposure. Modified to include a UV transmissive housing that is sealed along the periphery of the housing. The inlet port introduced nitrogen (inert gas) through the tapered hose into the chamber and the outlet port purged the initial air from the chamber. The oxygen concentration present in the chamber was measured by an Alpha Omega Series 3000 Trace Oxygen Analyzer (Alpha Omega Instruments, Cumberland, Rhode Island) installed at or adjacent to the outlet port. During UV exposure, plate C was sealed during UV exposure in a chamber environment consisting mainly of nitrogen with some oxygen in the chamber and on the exposure platen.

  The sealed exposure chamber roof was constructed of FEP Teflon and the four walls were made of tubular metal. The photosensitive element was inserted into the chamber, nitrogen was sent out, and the air in the chamber was exhausted. This was done by placing the photosensitive element on the plate of the exposure unit, placing a sealed exposure chamber covering the element on the plate, and purging the atmosphere by introducing nitrogen gas. . Next, image exposure was performed while covering one surface with nitrogen in this way. In practice, it is possible to use a material with a lower UV absorbance and this does not require the same compensation.

In this example, in an environment consisting mainly of nitrogen and continuously reducing the oxygen concentration, the plate is irradiated with UV rays through an in-situ mask for about 20 minutes (33.9 mjoules / cm ² / sec). C was exposed. The exposure is started when the oxygen concentration in the sealed exposure chamber is about 1,000 ppm (oxygen 0.1%), and nitrogen is introduced into the sealed exposure chamber so as to reduce the oxygen concentration to less than about 100 ppm at the end of the exposure. Exposure was continued with continuous introduction.

  For each of Plate A to Plate C, measure the dot size in the patch area of the plate using a measuring microscope [Nikon Measurements, model MM-11, manufactured by NIKON (USA) (Melville, NY)] did. The convex structure of the printing plate includes a raised portion and a concave portion. Each patch corresponding to a specific percentage of the printed area in the mask contained a plurality of raised portions of the same size. The raised portion that carries the ink to the substrate to be printed is generally called a halftone dot. Microscopic analysis determined the patch closest to 30% area coverage based on a line screen line count of 100 lines per inch, ie, a patch that provided a flat raised surface with a diameter of about 157 microns for each dot. . For this example, a 30% plate dot area was selected as a comparison criterion for three plates A to C. This is because the tint scale midtone area tends to exhibit fluting very effectively. The comparative analysis results from plate A to plate C were expected to be substantially the same as in the plate dot area with other options.

Plates Printing plates A to C were mounted on polyester reinforced cushioned carrier [R / bak foam, type SF, 0.060 inch, manufactured by Rogers Corp (Rogers, Conn.) Using double-sided tape. . This polyester reinforced cushioned carrier is fitted with a leading edge strip for mounting on its leading edge. The carrier was then placed on top of the printing cylinder in a Bobst Flexo 160 printing machine by fixing the leading edge strip for mounting in a groove in the printing cylinder and fixing the trailing edge of the carrier to the cylinder. The plate was connected to an anilox roll, a 500 line screen having a volume of 3.0 virion cubic microns per square inch (BCMI) at 60 degrees. The ink used was a BCM ink based on water with a pH of 9.5 and a viscosity of 15.4 seconds in a 4DIN cup. The plate was printed on a corrugated paperboard coated with Kemiart B-flute 200 #, with ink printed with an ink pattern corresponding to the convex image. The plate was set to print during printing with a print setting of +0.010 inch and +0.020 inch minimum (kiss) contact.

For each of the evaluation and analysis plates A to C, a patch corresponding to 30% print tint area (i.e. the halftone dot area on the plate) and 0.40 +/- 0. Patches corresponding to a density equal to 03 were analyzed. These areas were then identified on the printed corrugated paperboard and X-RITE, Inc. A 3.5 inch square sample was cut from corrugated paperboard for densitometric analysis with the company [Grandville, Michigan] X-RITE Spectrodensitometer, model 528. Multiple measurements were made and spaced to ensure density readings across both fluted and non-fluted regions in the printed image. The density measurement was converted to% dot coverage using the Murray Davies equation so that the actual dot gain (printed dot area-plate dot area) could be calculated.

  The image printed on the corrugated paperboard alternates with a relatively low (or non-high coverage) coating band that results in a relatively low ink density reading, corresponding to the light print area, ie, underlying fluting. Thus, the fluting effect that the dark print area, that is, the relatively high ink reading that results in a relatively high density reading appears as a high band was evaluated. A group of 10 savvy observers is given an unlabeled sample and each person is assigned a flute performance on a scale of 1 to 10 (1 corresponds to the best flute performance, 10 Hierarchy was determined by having the hierarchy divided from highest to lowest (corresponding to the lowest flute performance). The average fluting hierarchy is based on patches corresponding to approximately equal plate dot areas (based on microscopic analysis of dot diameter) and patches corresponding to approximately equal density values (based on density measurements). Calculated. Plate to Print gain is the halftone dot effectively printed on the corrugated board substrate from the plate dot area (approximately 30% equivalent plate dot area in this example). Represents the increase in dot size to the area.

  The fluting that appears in the image printed by plate A is consistent with and representative of the level of fluting performance that is considered acceptable in the flexographic printing industry for printing on corrugated paperboard. Plate B has the same physical characteristics as plate A, whereas plate B is made using a digital workflow. Plate equal to plate B has been evaluated in the corrugated paperboard printing market, but has not been accepted due to its poor fluting performance. Thus, the fluting that appears in the image printed by plate B is not acceptable in the corrugated paperboard printing industry. Plate C is the same material as Plate B, but Plate C was made using the modified digital workflow of the present invention as described above, utilizing a specific environment during exposure. The fluting that appears in the image printed by plate C is the lowest fluting effect when compared to that of plate A and plate B, which have the lowest ink density difference between the support and non-support areas of the corrugated board. Significantly improved from the image printed by plate A, which is currently accepted in the industry with respect to the reduction of the fluting level, as well as from the image printed by plate B made by digital workflow It was done. Plate C exposed in a modified digital workflow is significantly lower than both Plate A made by a conventional analog workflow and Plate B, a traditional digital workflow (exposure in air) Press halftone gain was obtained.

  Three-dimensional (3D) structural analysis was performed on each of plate A to plate C using a Hirox microscope, model KH-770 from Hirox-USA (River Edge, NJ). A three-dimensional microscope produces an image of 1600 × 1200 microns (X × Y) with dimensions of 0.517 microns per pixel. In order to characterize the “printing area” of the halftone dots, Z-axis (halftone dot height) data was recorded for each XY position. This data was loaded into a spreadsheet software package to obtain a cumulative sum (height × area area) of Z-axis data that effectively sliced the continuous layer of the plate structure from the top to the minus Z direction.

  Three-dimensional analysis revealed that all three plates have relatively flat print areas, while plate A and plate C have a convex structure that is significantly different from plate B. The raised portions or halftone dots of plate A and plate C had a relatively flat top area and a shoulder area that transitioned sharply from the top area of the halftone dot to the support side of the halftone dot. The raised portion or halftone dot of plate B had a halftone dot top printing surface and a shoulder that migrated much more slowly to the side wall of the halftone dot.

  The surface area of the raised surface of the plate as a percentage of the total surface area was calculated as a function of depth (Z-axis position), which applied pressure between the substrate and the printing plate. When correlating, it was correlated with the surface area of the plate in contact with the substrate (effective halftone print area under pressure). Further, the second half derivative of the cumulative sum of the raised surface, i.e., the percent surface area of the halftone dots, for each of plates A to C was determined to determine the effective halftone print area under pressure. Due to the difference in the plate structure described above, the effective plate area that comes into contact with the substrate under pressure has changed dramatically. Plate A, commonly regarded as highly acceptable for fluting in the corrugated paperboard printing industry, has an effective print area area under pressure of 38%. This corresponded to an increase of 8.6% over the area of the area measured using a microscope, which corresponds to an increase of about 29% over the original size. Plate B, which is considered to have poor fluting performance, exhibits an effective print area of 60% under pressure. This corresponds to an increase of about 29% relative to the printed area measured using a microscope, i.e. an increase of about 96% relative to the original size. Plate C, which is much better stratified for fluting, exhibits an effective dot area of 32% under pressure. This corresponds to an increase of less than 2% (ie a rate of increase) relative to the printed area area measured using a microscope and to an increase of 5.6% relative to the original size.

  These results are consistent with the above-described hierarchization of printed images.

  Rather than representing the shoulder area as a percentage of the dot diameter measured by the microscope, the shoulder area is represented as an increase in the dot radius (or diameter) from the substantially flat top print area of the dot. Also good. The effective print area area under pressure (halftone area) measured by a Hirox microscope was converted to halftone dot diameter including the shoulder area (transition from the shoulder area to the side wall area using the curve of the cumulative surface area area) Part second derivative calculation). This example provides the best performance when the dot radius is increased by less than about 2 microns (plate C), ie, reduces fluting and increases the radius up to about 10 microns (plate A). Is acceptable, but for a dot size between 155 and 160 microns corresponding to 30% print area at 100 line / inch resolution, the radius increased above 30 microns (plate B), The performance is clearly unsatisfactory. Identical or substantially identical results were observed for other halftone dot sizes, each exhibiting a different percent printed area area of the tonal range.

  The modified digital workflow process described for plate C in Example 1 was also used for Example 2 with the following exceptions. Two oxygen meters, each measuring a different oxygen concentration range, were used: Alpha Omega Series 2000 and Alpha Omega Series 3000. Under various conditions, a plurality of plates are made with a modified ECLF exposure unit, i.e. exposed, and the oxygen concentration in a sealed exposure chamber (see FIGS. 1 and 2) is 190,000 ppm, 150,000 ppm. , 100,000 ppm, 20,000 ppm, and 10,000 ppm, exposure to actinic radiation was started. During the exposure, the chamber was continuously purged of oxygen. The oxygen concentration was recorded every minute during the purge and exposure of the photosensitive element to actinic radiation.

  Table 5 shows the oxygen concentration in the hermetic exposure chamber when chemical radiation was started, and recorded microscopic observations (Hirox 3D) of wireframe images of the plate at various concentrations. A quantitative indicator by visual observation was created, and observation number 0 was assigned to low dot structure performance and unacceptable dot structure. The dot structures ranging from very good to superior, which were considered acceptable for subsequent printing applications, were assigned observation numbers 4-5.

  FIG. 7 shows a wire frame image of a dot structure on a plate exposed under various oxygen concentrations by initiating actinic exposure at various starting points during the purge / introduction of nitrogen gas. The image was developed with a Hirox 3D system. The image showed that all plates with a starting point from 190,000 to 10,000 ppm show a marked improvement in the dot structure compared to plates made at atmospheric conditions. A halftone dot structure created in an oxygen conditioned environment was completely formed. On the other hand, a halftone dot structure under atmospheric conditions (21% oxygen) was not completely formed.

  Two photosensitive elements with IR laser radiation photosensitive layers (ie digital plates), CYREL® photopolymer printing element type DPC-155, were imaged with an Esko Spark plate-setter using the same pattern. Thus, an in-situ mask having a halftone dot area corresponding to a range of halftone dot areas including 2%, 10% and 30% area coverage was formed. One plate was exposed to ultraviolet light using a standard conventional digital workflow and processed as described above for Example 1 Plate B. The second plate was exposed to ultraviolet light using a modified (inert environment) workflow by placing the plate in a modified ECLF exposure unit. The modified ECLF exposure unit included a roof composed of Lexan placed on the exposure unit plate and a chamber of four walls. Seal the chamber from the external environment by applying tape along the periphery of the chamber where the wall contacts the platen, and allow a continuous flow of nitrogen (nitrogen gas containing less than 10 ppm oxygen) into the chamber through the inlet port. The atmosphere was purged from the chamber by feeding and monitoring the oxygen concentration of the gas extruded through the outlet port. Exposure began when the oxygen concentration monitored at the outlet port reached 1000 ppm. After exposure in a modified environment, the plate was subjected to the same standard processing and post-processing as that performed on the first plate, using the conditions described in Example 1.

  The plate is then inspected under the microscope and different features in the convex structure (corresponding to 2%, 10% and 30% dot area respectively) of the exposed plate in two different environments as follows: I found that it was shown.

  The conclusion from this example is that the complete purging of the oxygen environment during exposure (to less than 1000 ppm) makes it significantly clearer (as compared to exposure in the presence of atmospheric oxygen) that is clear to the savvy observer. The projecting halftone dot convex structure was provided.

  Imaging multiple CYREL® photopolymer printing elements, type DPC-155 plates, with an infrared laser emitting photosensitive layer for the formation of a mask, with an Esko Spark plate-setter using the same pattern A halftone dot area corresponding to a range of halftone dots including 2%, 20%, 30%, 40% and 50% area coverage was formed on the in-situ mask. After mask imaging, the plate is placed in a sealable exposure chamber as described in Example 1 [consisting of a tubular metal wall with a Teflon® FEP roof] and CYREL® ECLF 2000 exposure. Placed in the frame. A nitrogen supply (containing less than 10 ppm of oxygen) was attached to the inlet port of the chamber to allow mixing with atmospheric gas present in the chamber and purging of atmospheric gas. The gas exiting the chamber was monitored using an oxygen sensor and the oxygen concentration was recorded over time. When the oxygen concentration reached a predefined point, exposure to ultraviolet light was started. FIG. 8 shows a plot of oxygen concentration versus time during imagewise exposure with ultraviolet light for each of the designed tests. For test E1, plate exposure began when the oxygen concentration in the chamber reached 7500 ppm. For test E2, exposure of the plate to ultraviolet light was initiated when the oxygen concentration in the chamber reached 5000 ppm. For test E3, plate exposure began when the oxygen concentration in the chamber reached 2500 ppm. For test E4, plate exposure began when the oxygen concentration in the chamber reached 1000 ppm. For test E5, plate exposure began when the oxygen concentration in the chamber reached 500 ppm. For test E6, plate exposure began when the oxygen concentration in the chamber reached 100 ppm.

  After exposure, each of the test plates was processed according to standard processing conditions as described in Example 1, and the dot structure formed was visually evaluated. In all cases, the resulting halftone dot structure was not inferior to that formed using an analog plate making method and resulted in a plate exposed in an inert environment. The halftone dot shape was similar to an analog halftone dot with a flat top surface and a sharp shoulder.

  The result of this example is that it is possible to achieve a certain minimum amount of exposure at low oxygen concentrations and still achieve the desired convex structure with raised convex elements, ie shoulder transitions with sharp halftone dots. Insofar, it suggests that imagewise exposure can be carried out at the relatively high oxygen levels present during exposure. Test E1 of Example 3 revealed that a significant amount of “oxygen inhibition” is observed when atmospheric oxygen is present during imagewise exposure. In this example, the theoretical value for the halftone printing surface is calculated to be 141.3 microns, and the resulting halftone dots for tests E1 to E6 are substantially equal to the theoretical value. there were. The measured values obtained from the micrographs of the halftone dots formed on the plate of Example 4 show that the change in the halftone dot size (<4 microns) under various exposure conditions demonstrated by tests E2 to E6. This suggests that the magnitude of the change is significantly smaller compared to exposures performed in the presence of atmospheric oxygen for those in an inert environment having less than 7,500 ppm of oxygen.

  Also, microscopic inspections suggest signs of pixelation over all raised element (ie, halftone dot) structures made in Example 4 (Test E2 to Test E6), which are atmospheric oxygen In this environment (test E1), it did not appear on the exposed plate. In particular, pixelated artifacts were detected in the halftone dot images for tests E2 through E6, which began exposure at an oxygen concentration of 7,500 ppm or less. The signs of pixilation are characterized by the jagged nature of the halftone dot edges, suggesting the formation of a curved structure with regular square image elements. It should be noted that the structure created in the presence of oxygen has a tendency to form a gently curved halftone dot edge as described in Example 3.

  A second test was performed following the same procedure outlined in Example 4 with exposure starting at oxygen concentrations of 7,500, 5,000, 2,500, 1,000, and 100 ppm. . For each halftone dot size, optical microscopic measurements of the halftone dot printed surface over the size of the printed area coverage of 2%, 20%, 30%, 40%, 50%, and theoretical size (based on 100 LPI screen line number) Compared with.

  These results are shown in the above table and suggest that the halftone dot size is not substantially different from the predicted value regardless of the exposure density at the start of exposure.

  This example allows the formation of the desired dot structure in the photopolymerizable printing element when imagewise exposure is initiated at a relatively high initial oxygen concentration (ie, greater than 7,500 ppm to 190,000 ppm). It has been proved possible by the present invention.

  To implement this example, a second oxygen meter operating with a relatively high range of oxygen sensitivity was required. To achieve this, an Alpha Omega series 2000 oxygen meter was placed in series with an Alpha Omega series 3000 meter on the exit side of the exposure chamber. Both instruments were used to monitor the oxygen concentration every minute during the purge and exposure times. Under normal atmospheric conditions with an oxygen concentration measurement of 218,000 ppm (test plate D1), as well as purging the atmosphere from the exposure chamber, the following oxygen concentrations were obtained: 190,000 ppm, 170,000 ppm, 150,000 ppm, 100 Imagewise exposure was carried out in an inert environment (test plate MD1 to test plate MD9, respectively) starting imagewise exposure at 1,000, 50,000, 20,000, 10,000, 5,000 and 1,000 ppm. . A CYREL® photopolymer printing element, type 125 DPC (with an infrared laser emitting photosensitive layer for the production of a mask by digital workflow) was used for the test plate. Imagewise exposure to ultraviolet light was performed in a 2000 ECLF exposure unit. Inert environmental exposure was carried out in a closed exposure chamber placed on the plate of the exposure unit, consisting of the side of a metal frame with Teflon® FEP as the UV transmissive top surface. Standard solvent processing was performed after exposure as described in Example 1.

  The top printing surface dimension with 30% dot area was measured for three different dots in a single field of view. On the top plane, essentially perpendicular to each other, one orientation is horizontal to the east and west of the top plane, and the other orientation is two different orientations perpendicular to the north and south of the top plane. And the average was determined as reported in the table above. The theoretical dot size for a 30% coverage dot at 100 lines per inch is effectively 157 microns (156.98). Note that the plate exposed in atmospheric oxygen, ie the test plate D1 made by a conventional digital workflow, resulted in a reduction of 119.35 microns, ie from theoretical to over 37 microns. want. All other halftone dots made in inert gas, ie, test plate MD1 to test plate MD9, practically ranged in the range of +/- 5 microns of the halftone dot size.

  A photomicrograph of a plate (test plate D1) made using a conventional digital workflow (exposure in atmospheric oxygen) and a modified digital workflow (exposure in an inert environment) starting with an oxygen concentration of 190,000 ppm Photomicrographs of the plate (test plate MD1) produced in 1 were taken and the images were compared. Comparison of the images in each photo showed a significant difference in the size of the printed surface as evidenced by the data. In addition, the plate made by the conventional digital workflow, i.e. the test plate D1, lacks a clear pixelation, on the other hand, i.e. made by a modified digital workflow in an environment starting with an oxygen concentration of 190,000 ppm. The test plate MD1, which is a plate, had a clear pixelation. Although the pixelation described in these examples is not a functional problem of the printed image, the plate system is able to refine an accurate image to a very high degree of resolution as it exists on the mask. It is possible to copy to.

  Table 6A lists exposures as measured by one or two oxygen concentration meters (Alpha Omega Series 2000 or Series 3000 meters) for exposures initiated at various measured oxygen concentrations. FIG. 6 is a list of the percentage of total exposure time that each plate (test plate MD1 to test plate MD9) is present in the oxygen concentration. The designation "M2" represents the most sensitive Alpha Omega Series 2000 oxygen meter for oxygen concentrations above 10,000 ppm, and the designation "M3" is the most sensitive for oxygen concentrations below 10,000 ppm. A good Alpha Omega Series 3000 oximeter.

  The rightmost column of Table 6A is expressed as a percentage, at or below a given oxygen concentration, where an “analog-like” dot structure, ie, a dot with a flat top and a sharp shoulder, is produced. Indicates the minimum amount of exposure time to be performed. From the data in Table 6A, acceptable results were obtained when at least 33% (35%, 51%, 48%, etc.) of the total exposure was performed in an environment corresponding to less than 1000 ppm oxygen in the exposure chamber. Observed. Similarly, it is noted that acceptable results were achieved when at least 50% (52%, 60%) of the total exposure was performed in an environment corresponding to less than 2500 ppm oxygen in the exposure chamber. . Similarly, it is noted that acceptable results were achieved when at least 46% (52%, 61%) of the total exposure was performed in an environment corresponding to less than 5000 ppm oxygen in the exposure chamber. . It is further noted that acceptable results were achieved when at least 58% (67%) of the total exposure was performed in an environment corresponding to less than 10,000 ppm oxygen in the exposure chamber.

  The conclusions based on these results are that unexpectedly and surprisingly high enough polymerization to still provide an embossed printing element with an analog-like halftone dot shape, i.e. a flat top and a sharp shoulder transition. Occurs when a level of oxygen remains in the exposure chamber during a portion of the exposure process.

Claims (29)

(A) forming an in-situ mask adjacent to the photopolymerizable layer in the photosensitive element, the photopolymerizable layer comprising a binder, an ethylenically unsaturated compound and a photoinitiator; ,
(B) sealing the photosensitive element in a sealed exposure chamber;
(C) adjusting the oxygen concentration in the hermetic exposure chamber within a range between 190,000 ppm and 100 ppm;
(D) exposing the photosensitive element to actinic radiation through the in-situ mask, and producing a relief printing form from the photosensitive element. The adjusting step includes introducing an inert gas;
(I) starting with an oxygen concentration of 190,000 ppm, reducing the oxygen concentration in the hermetic exposure chamber at a fixed or variable reduction rate, or (ii) from 190,000 ppm to about 100 ppm The method of claim 1, wherein the method is performed by maintaining an oxygen concentration in between.   After step b), purging the atmosphere having an oxygen concentration of about 210,000 ppm from the internal environment with an inert gas until the internal environment of the sealed exposure chamber has an oxygen concentration of 190,000 ppm or less. The method of claim 1 further comprising:   4. The method of claim 3, wherein after the purging step, the exposing step is started, and the adjusting step continuously reduces the oxygen concentration.   The method of claim 1, wherein the exposing step begins when the oxygen concentration is 190,000 ppm or less.   The method of claim 1, further comprising the step of continuously reducing the oxygen concentration during the exposing step.   The method of claim 6, wherein the exposing step is completed when the oxygen concentration is 5000 ppm or less.   The method of claim 1, wherein the exposing step begins when the oxygen concentration is between 20,000 ppm and 100 ppm.   The step of exposing the photosensitive element is performed for a time sufficient to photopolymerize a portion of the photopolymerizable layer, wherein at least 30% of the exposure time is performed at an oxygen concentration of less than about 20,000 ppm. The method of claim 1, wherein:   The method of claim 1, wherein the sealed exposure chamber has an internal environment and the exposing step is performed in the internal environment having an average oxygen concentration of 80,000 ppm or less.   The method of claim 1, wherein the sealed exposure chamber has an internal environment, and the exposing step is performed in the internal environment having an average oxygen concentration of 30,000 ppm or less.   The method of claim 1, further comprising processing the photosensitive element obtained by the exposing step to form a convex surface having a pattern of raised surface elements.   The convex surface includes a plurality of the raised surface elements, each raised surface element having an ink carrying top surface area, a side wall surface area, and a shoulder surface area, wherein the shoulder surface area is the top surface. Each raised surface element has a total printing area which is the sum of the top surface area and the shoulder surface area, each shoulder surface being a transition area between the area and the side wall surface area. The method of claim 12, wherein the ink transferred by the area to the substrate provides no more than 30% of the total printed area.   14. The method of claim 13, wherein the ink transferred to the substrate by each shoulder surface area provides no more than 5% of the total printed area area. (A) An inlet for introducing an inert gas into the sealed exposure chamber and an outlet for removing at least oxygen from the sealed exposure chamber are provided, and the sealed exposure chamber is sealed from the external environment of the sealed exposure chamber. A sealed exposure chamber having means for:
(B) an actinic radiation source disposed adjacent to the hermetic exposure chamber;
(C) an inert gas source coupled to the inlet;
(D) an apparatus for exposing the photosensitive element to actinic radiation in an environment having an oxygen concentration less than atmospheric oxygen, comprising: means for measuring the oxygen concentration in the sealed exposure chamber.   The apparatus of claim 15, wherein the sealed exposure chamber comprises at least one wall and a roof mounted to the at least one wall.   The apparatus of claim 15, wherein the source of actinic radiation is disposed inside the hermetic exposure chamber.   The apparatus according to claim 15, wherein the actinic radiation source is disposed outside the hermetic exposure chamber.   The apparatus of claim 16, wherein the roof of the hermetic exposure chamber is transparent to actinic radiation.   The apparatus of claim 19, wherein the roof material is selected from the group consisting of polycarbonate, acrylic resin, fluorocarbon resin and glass.   16. The apparatus of claim 15, wherein the sealed exposure chamber is disposed on a flat support of an exposure unit having the actinic radiation source.   The apparatus of claim 21, wherein the flat support forms the bottom of the hermetic exposure chamber on the opposite side of the roof.   The apparatus of claim 21, wherein the flat support is transparent to actinic radiation.   16. The apparatus of claim 15, wherein the sealed exposure chamber is disposed on a frame of an exposure unit.   The apparatus of claim 15, wherein the means for measuring oxygen concentration comprises at least one oxygen meter coupled to the outlet or the sealed exposure chamber.   26. The apparatus of claim 25, further comprising a feedback control device between the oxygen meter and the gas source, wherein the oxygen concentration control is automatically adjusted.   The apparatus of claim 15, wherein the roof is vaulted.   28. The apparatus of claim 27, wherein the vaulted roof forms one or more domes, arches or cambers that are bent above the bottom support of the chamber.   The sealed exposure chamber comprises four walls, the roof being about 0.125 to about 1.5 inches (about 3 inches) of a plane formed perpendicular to the wall at the intersection of the roof with the wall. 16. The apparatus of claim 15, wherein a vault is formed above (.175 mm to about 3.81 cm).