CA2422132A1 - Screening method for preparing flexographic printing plates - Google Patents
Screening method for preparing flexographic printing plates Download PDFInfo
- Publication number
- CA2422132A1 CA2422132A1 CA 2422132 CA2422132A CA2422132A1 CA 2422132 A1 CA2422132 A1 CA 2422132A1 CA 2422132 CA2422132 CA 2422132 CA 2422132 A CA2422132 A CA 2422132A CA 2422132 A1 CA2422132 A1 CA 2422132A1
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- Prior art keywords
- dots
- printing
- halftone
- screening
- image
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- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
- B41N—PRINTING PLATES OR FOILS; MATERIALS FOR SURFACES USED IN PRINTING MACHINES FOR PRINTING, INKING, DAMPING, OR THE LIKE; PREPARING SUCH SURFACES FOR USE AND CONSERVING THEM
- B41N1/00—Printing plates or foils; Materials therefor
- B41N1/12—Printing plates or foils; Materials therefor non-metallic other than stone, e.g. printing plates or foils comprising inorganic materials in an organic matrix
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
- B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
- B41C1/00—Forme preparation
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
- B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
- B41C1/00—Forme preparation
- B41C1/02—Engraving; Heads therefor
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
- B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
- B41M1/00—Inking and printing with a printer's forme
- B41M1/02—Letterpress printing, e.g. book printing
- B41M1/04—Flexographic printing
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
- H04N1/40—Picture signal circuits
- H04N1/405—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels
- H04N1/4055—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a clustered dots or a size modulated halftone pattern
- H04N1/4057—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a clustered dots or a size modulated halftone pattern the pattern being a mixture of differently sized sub-patterns, e.g. spots having only a few different diameters
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- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
- G03F5/00—Screening processes; Screens therefor
- G03F5/02—Screening processes; Screens therefor by projection methods
- G03F5/12—Screening processes; Screens therefor by projection methods using other screens, e.g. granulated screen
Abstract
A method of screening a continuous tone image into a halftone representation for a flexographic printing operation compensates for characteristic printing problems in the highlight area by selectively placing non-printing dots proximate the highlight dots. The non-printing dots raise the printing relief floor in the highlight areas providing additional support for marginally printable dots.
Description
scREENING r~THOD FoR PREPARING PLExoGRAPHIC PRINTING PLATEs TECHNICAL E'IELD
The invention relates to the field of flex>ographic printing and more particularly to screening methods for representing an image or. a flexographic plate.
BACKGROUND
Flexographic printing is a method of directs rotary printing that uses a resilient relief image in a plate of rubber or photopolymer to print articles such as cartons, bags, labels or books. Flexographic printing has found particular application in packaging where it has displaced rotogravure and offset lithography printing techniques in many cases. While the quality of articles printed using flexographic plates has improved significantly as the technology has matured, physical limitations related to the process of creating a relief image in a plate remain.
In particular, it is very difficult to print small graphic.
elements such as fine dots, lines and even text using flexographic printing plates. In the lightest areas of the :image (commonly referred to as highlights) the density of the image is represented by the total area of dots in a halftone screen representation of a continuous tone image. For Amplitude Modulated (AM) screening this involves shrinking a plurality of halftone dots located on a fixed periodic grid to a very small size, the density of the highlight being represented by the area of the dots. For Frequency Modulated (FM) screening the size of the halftone dots is generally maintained at some fixed value, and the number of randomly or pseudo-randomly placed dots represents the density of the image. In both the aforementioned cases, it is necessary to print very small dot sizes to adequately represent the highlight areas.
Maintaining small dots on a flexographic plate is very difficult due to the nature of the plate making process. Digital flexographic plate precursors have an integral UV-opaque mask layer coated over the photopolymer. In a pre-imaging (or post-imaging) step the floor of the plate is set by area exposure to UV light from the back of the plate. This exposure hardens the photopolymer to the relief depth required for optimal printing. This step is followed by selective ablation of the mask layer with an imagewise addressable high power laser to form an image mask that is opaque to ultraviolet (UV) light in non-ablated areas. Flood UV exposure and chemical processing follow wherein the areas not exposed to UV are removed in a processing apparatus using solvents or by a heating and wicking process. The combination of the mask and UV exposure produces relief dots that have a generally conical shape. The smallest of these dots are prone to being removed during processing, which means no ink is transferred to these areas during printing (the dot is not "held" on plate and/or on press). Alternatively, if the dot survives processing they are susceptible to damage on press. For example small dots often fold ever and/or partially break off during printing causing either excess ink or no ink to be transferred.
Conventional or non-digital flexographic platemaking follows a similar process except that the integral mask is replaced by a separate mask or phototool that is imaged separately and placed in contact with the photopolymer plate precursor under a vacuum frame for the UV exposure.
In printing, it is well known that there is a limit to the minimum size of halftone dot that may be reliab__y represented on a plate and subsequently printed on press. The accrual minimum size will vary with a variety of factors including plate type, ink, imaging device characteristics etc. This creates a problem in the highlight areas when using conventional AM screening since once the minimum dot size is reached, further size reductions will generally have unpredictable results. If, for example, the minimum size dot that can be printed is a 50 ~m square dot, corresponding to a 5o tone at 114 lines per inch screen frequency, then it become~~ very difficult to faithfully reproduce tones between Oo and 50. A common workaround is to increase the highlight values in the original. file to ensure that after imaging and processing, that all the dots on plate are properly formed. However, the downside to this practice is the additional dot gain in the highlights, which causes a noticeable transition between inked and non-inked areas. Another well-known practical way of improving highlights is through the use of "Respi" or "double dot"
screening. One such technique is shown in FIG's. 1-A to 1-C. A
The invention relates to the field of flex>ographic printing and more particularly to screening methods for representing an image or. a flexographic plate.
BACKGROUND
Flexographic printing is a method of directs rotary printing that uses a resilient relief image in a plate of rubber or photopolymer to print articles such as cartons, bags, labels or books. Flexographic printing has found particular application in packaging where it has displaced rotogravure and offset lithography printing techniques in many cases. While the quality of articles printed using flexographic plates has improved significantly as the technology has matured, physical limitations related to the process of creating a relief image in a plate remain.
In particular, it is very difficult to print small graphic.
elements such as fine dots, lines and even text using flexographic printing plates. In the lightest areas of the :image (commonly referred to as highlights) the density of the image is represented by the total area of dots in a halftone screen representation of a continuous tone image. For Amplitude Modulated (AM) screening this involves shrinking a plurality of halftone dots located on a fixed periodic grid to a very small size, the density of the highlight being represented by the area of the dots. For Frequency Modulated (FM) screening the size of the halftone dots is generally maintained at some fixed value, and the number of randomly or pseudo-randomly placed dots represents the density of the image. In both the aforementioned cases, it is necessary to print very small dot sizes to adequately represent the highlight areas.
Maintaining small dots on a flexographic plate is very difficult due to the nature of the plate making process. Digital flexographic plate precursors have an integral UV-opaque mask layer coated over the photopolymer. In a pre-imaging (or post-imaging) step the floor of the plate is set by area exposure to UV light from the back of the plate. This exposure hardens the photopolymer to the relief depth required for optimal printing. This step is followed by selective ablation of the mask layer with an imagewise addressable high power laser to form an image mask that is opaque to ultraviolet (UV) light in non-ablated areas. Flood UV exposure and chemical processing follow wherein the areas not exposed to UV are removed in a processing apparatus using solvents or by a heating and wicking process. The combination of the mask and UV exposure produces relief dots that have a generally conical shape. The smallest of these dots are prone to being removed during processing, which means no ink is transferred to these areas during printing (the dot is not "held" on plate and/or on press). Alternatively, if the dot survives processing they are susceptible to damage on press. For example small dots often fold ever and/or partially break off during printing causing either excess ink or no ink to be transferred.
Conventional or non-digital flexographic platemaking follows a similar process except that the integral mask is replaced by a separate mask or phototool that is imaged separately and placed in contact with the photopolymer plate precursor under a vacuum frame for the UV exposure.
In printing, it is well known that there is a limit to the minimum size of halftone dot that may be reliab__y represented on a plate and subsequently printed on press. The accrual minimum size will vary with a variety of factors including plate type, ink, imaging device characteristics etc. This creates a problem in the highlight areas when using conventional AM screening since once the minimum dot size is reached, further size reductions will generally have unpredictable results. If, for example, the minimum size dot that can be printed is a 50 ~m square dot, corresponding to a 5o tone at 114 lines per inch screen frequency, then it become~~ very difficult to faithfully reproduce tones between Oo and 50. A common workaround is to increase the highlight values in the original. file to ensure that after imaging and processing, that all the dots on plate are properly formed. However, the downside to this practice is the additional dot gain in the highlights, which causes a noticeable transition between inked and non-inked areas. Another well-known practical way of improving highlights is through the use of "Respi" or "double dot"
screening. One such technique is shown in FIG's. 1-A to 1-C. A
screening grid 10 for conventional AM screen is shown in a simplified schematic format. The screening grid comprises a plurality of halftone cells 12. A halftone cell is an area wherein an AM halftone dot is grown from a low density, where only a small dot is placed at the centre of the cell, to a high density or so:Lid where the cell is completely filled. In FIG. 1-A dots 14 are placed only in every second halftone cell. Dots 14 have size corresponding to the minimum reliably reproducible dot. As the density of the screen is increased, dots 14 are increased in size as shown in FIG. 1-B at 16. At some point in increasing the density the previously empty halftone cells are populated with minimum size dots 18 as shown in FIG. 1-C. The dots 16 may be held at a fixed size with increasing screen density while allowing dots 18 to grow. When all dots are the same size as dots 16, conventional AM screening takes over.
The problem with this type of screening technique when applied to flexographic printing is that the size of dot that may be printed in isolation is actually quite large, typically 40 - 50 pm in diameter.
Even when using this technique, the highlights are difficult to reproduce without having a grainy appearance (which occurs when dots are spaced far apart to represent a very low density).
There remains a need to improve the representation of small dots iv flexographic printing processes.
SUMMARY OF THE INVENTI~N
In a first aspect of the present invention a method for representing a continuous tone image on a flexographic printing plate is presented. A halftone image representation is created by screening 5 the continuous tone image to create a plurality of halftone regions of varying density. Within these regions, one or rnore marginally printable regions that are too small to be reliably printed are identified. One or more non-printing dots are established proximate to but spaced apart from the marginally printable regions. Finally, the flexographic plate is exposed to image forming radiation in accordance with the halftone image representation.
In another aspect of the present invention a method for reproducing halftone image having areas of varying density on a flexographic printing plate is presented. Marginally printable regions that will be too small to be reliably printed are identified within the areas of varying density. One or moz-e non-printing dots are placed, proximate to but spaced apart from the marginally printable regions, the non-printing dots providing a base of support for the marginally printable surfaces.
In another aspect of the invention a method for preparing a flexographic printing plate is presented. The flexographic printing plate is back-exposed to set the relief floor. The halftone image data representing an image to be printed additionally defines a plurality of non-printing dots surrounding marginally printable regions. The flexographic printing plate is patterned in accordance E
with halftone image data provided, the relief floor being selectively raised in the areas containing non-printing dots. The flexographic printing plate is then processed to develop the relief image.
In yet another aspect of the present invention a flexographic printing plate has a non ink-accepting background area and an ink-accepting surface defined in relief to the background area and formed in accordance with an image to be printed. The ink-accepting surface has areas of varying density and within the areas of varying density, marginally printable regions that are too small to be reliably printed. One or more non-printing dots are dish>osed proximate to but spaced apart from the marginally printable regions, the non-printing dots providing a base of support for the marginally printable regions.
For an understanding of the invention, reference will now be made by way of example to a following detailed description in conjunction by accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In drawings which illustrate by way of example only preferred embodiments of the invention:
FIG. 1 is a schematic diagram of a prior art screening method.
FIG. 2 is a schematic diagram of an embodiment of a screening method in accordance with the present invention.
FIG. 3 is a schematic diagram of another embodiment of a screening method in accordance with the present invention.
FIG. 4 is a schematic diagram of a threshold arx:ay based screening method.
FIG. 5 is a schematic diagram of a tile based screening method.
FIG. 6-A - 6-C are a series of photographs of fl_exographic plate samples taken of plates prepared with and withoLa the use of the methods of the present invention.
DESCRIPTI~N
Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and the drawings are to be regarded in an illustrative rather than a restrictive sense.
The invention is described in relation to a screening technique wherein a marginally printable halftone dot that. is desired to print is surrounded by one or more smaller nan-printing halftone dots to provide an extra base of support for the small halftone dot.
An embodiment of the invention. is shown in FIG. 2. Lines 20 demarcate a plurality of halftone cells 22. It will be readily appreciated that each cell has a plurality of possible locations where the imaging device may place a picture element !pixel) to make up a halftone dot. These locations are shown as grid lines 24 in the upper left hand corner of FIG. 1 (these locations are omitted in the rest of FIG. 2 for sake of clarity). The pixel size is commonly a function of the imaging device configuraticn and may be fixed or variable. The halftone cell size is a function of the screening engine and is chosen to suit a particular screening job or a category of screening jobs.
A plurality of small halftone dots 26 representing a low density tone or highlight which it is desired to print in the final product are placed at the centres of a halftone cells 22.. In this case, only one quarter of the halftone cells have printing dots 26. Halftone dots 26 generally comprise a plurality of pixels>. While dot 26 is shown having circular shape, in practice the shape will depend on the shape of the pixels and how many pixels are used to make up the dot and the particular spot function chosen. Dot 2E> could therefore be irregularly shaped or even square. The size of Dot 26 is such that it may not print without the aforementioned problems. In other words, dot 26 is a marginally printable region, which may or may not print properly in a subsequent printing operation. The marginally printable dot size may be chosen by the printer from a knc>wledge of a particular printing process.
Dots 26 each have neighbouring halftone dots 28 which are specifically sized to be non-printing i.e. when the plate is processed dots 28 are small enough that they will be at least partially removed by the processing solvents. Non-printing dots 28 are spaced apart from the dots 26 by an amount sufficient to ensure that dots 28 and 26 do not join. Consequently, dots 28 af'~er processing do not present a relief point that will take up ink and print. Surprisingly, it was found that non-printing dots 28 surrounding printing dots 26, provides an extra base of support for the dot 26 that appears to prevent the dots 26 from being removed by processing. Furthermore, dots 26 were also prevented from breaking off during printing operations. The result is that a printer could now reliably hold smaller dot sizes on press than was previously possible. The inclusion of non-printing dots 28 has the added advantage that since the dots do not print but only contribute to the base of the relief, they do not change the printed density of the particular tone that it is desired to represent.
In another embodiment of the invention, the supporting dots need not necessarily be in adjacent halftone cells. FIG. 3 shows a portion of the screen from FIG. 2 where there is a printing halftone dot 26 in every halftone cell and the non-printing halftone dots 28 form a periphery around dot 26 in the same halftone cell.
There are several practical screening methods for placing non-printing dots. A preferred embodiment, that is also simple, involves using threshold array based AM screening wherein a few centre pixels of every halftone dot on the halftone grid are always turned on.
Threshold array screening is very well known in the art. This is the situation depicted in FIG. 4. A plurality of halftone dot cells are established as indicated by the darkened outlines 50. Each cell has a plurality of locations, in this case 25. The halftone cells are commonly located on what is known as the screen grid shown as dotted lines 52. The screen grid shown has a screen angle of 0°but it should be understood that the screen grid may also be rotated by some angle 5 to form the screen at commonly used screen angles as is well known in the art. In such a case the halftone cells may not be rectangular in shape but will be defined within the confines of the underlying imaging engine resolution grid. In operation, the threshold matrix as shown is tiled across the plane of the image to cover the entire area 10 thereof.
The values at each location determine whether a particular pixel will be on or off, depending on the density of the continuous tone (contone) image at that point. In FIG. 4 the contone image density for the situation and area shown is 3 units (assuming the continuous tone is broken up into 25 levels). Locations in the threshold matrix that have levels less than and equal to 3 will be on. Dot 54 is shown shaded black to indicate that it is on, as it comprises values between 1 and 3. In cells adjacent to the cell contain~_ng dot 54, non-printing dots 56 are formed. These dots are just below the determined printing threshold while dot 54 is on or above t=hat threshold. As the density increases dots 54 may increase in size and dots 56 may also be encompassed by growing other printing dots in these other regions.
Eventually all non-printing dots are obscured by printing dots; the non-printing dots being mainly of use in highlight areas. Once the non-printing dots have disappeared, the screening method returns to conventional AM screening.
The inclusion of non-printing dots has the effect of selectively altering the relief depth in only the areas where such a change is necessary to support marginally printable dots or features.
Advantageously areas where the halftone dot size is large enough to print without problem are unaffected.
In an alternative embodiment, threshold array based FM screening may be adapted to include the non-printing dots., An intermediate tone level is chosen in the FM screen where clusters of dots have formed and the clusters are in close proximity to one another. A non-printing dot is placed at the centre of the clu:~ter. The tone level is chosen having regard to the development of random or pseudo-random dots into clusters of dots according to the particular FM screening technique in use. For example, a 30o tone might be convenient since clusters will generally have formed at this stage. This serves to centre a substantial number of the non-printing dots in the clusters so that as the tone density increases ~~he non-printing dots are mostly overwritten by printing dots, thus rendering them redundant as the density increases.
In another embodiment where it is desired to have more control over the non-printing dots, threshold array screening may be found limiting since it is technically only possible to have the non-printing dot always on. For the situai~ion depicted in FIG. 3 there is no particular problem having all dots 28 on while dot 26 is still small. However, as the tonal density is increased and dot 26 is grown to fill more of the area of halftone cell 22, dot 26 will eventually join with dots 28. When joined to a larger area dot, dots 28 will add to the printing area thus causing undesirable tonal discontinuities.
Preferably, dots 28 should be switched off before reaching this point, or as soon as dots 26 no longer need support. Such control over the dots 28 is not easily achievable using threshold arrays but other known screening methods may be employed.
One particular method is shown in. FIG. 5. A series of tiles 30 -36 contain monotcnically increasing density screens. Tile 38 is at some intermediate density and tile 40 is a solid. There are n tiles in all corresponding to the desired number of tone levels in the image. It should be appreciated that the tiles in FIG. 5 are not physical objects, but rather manifestations of data structures in a computer device. For example, the set of tiles may be a 3 dimensional array of values, each individual tile being represented as a two dimensional array, there being n such individual tiles in the array.
The screening of an image from a contone representation into a halftone representation, involves, for each pixel of the contone image, determining the tone of the pixel and looking up the corresponding tile and applying that tile to the image pixel to achieve the halftone representation.
Tile 30 is the tile that will be used where no-density (Oo tone) is desired. This tile has non-printing dots 28 dispersed over the surface. At this tone level, none of the dots 28 will print on the final article. Tile 32 shows the lowest density step where a plurality of small dots 26 have been placed, each being peripherally surrounded by non-printing dots 28.
In tile 34, the dot 42 is the dot 26 from tile 32 that has now been increased in size. Note though that dot 42 is still surrounded by, but not touching non-printing dots 28. In tile 36 dots 44 are again. increased in size but now non-printing dogs are removed. Dot 44 is large enough not to reed support from the non-printing dots - this is now conventional AM screening. An intermediate tile 38 shows dots 46. If dots 28 were still on at this stage, they would be adding to the size of dots 46 causing undesirable increases or jumps in tone level. The final tile 48 is a solid tile wrerein all dots have grown into each other to completely cover the area of the tile.
I5 While the embodiment in FIG. 5 has been described in relation to AM screening, it is also adaptable to FM screening wherein each tile has a density represented by an FM screening technique and the non-printing dots are placed in close proximity to (but not adjacent to) small clusters of pixels in the FM screen that are desired to be printed. The size, number, and placement of these non-printing dots are allowed to vary for each tile (i.e. may not place any non-printing dots once all of the clusters in the FM screen have reached a sufficient size). This method has an advantage over the threshold array based approach in that the non-printing dots can be placed closer to the clusters (thus providing more support) while avoiding the potential tone jump caused by printing dots joining with non-printing dots. The advantage of using a tiled screen representation is that the non-printing dots can be confined to a few lower density tiles. This allows the highlights to be treated in isolation without affecting the intermediate and high-density tones. With a threshold matrix, it is generally speaking an all or noting proposition.
In yet another embodiment of the invention the .image may be conventionally screened without placing any non-printing dots. The screened image is then post processed to add a supporting infrastructure of non-printing halftone dots, in close proximity to any small halftone dots that meet some pre-determined criteria of needing support of non-printing dots. Likewise, the post processing may also identify thin lines or other delicate structure defining marginally printable regions that may not print properly. A support infrastructure may be placed around the periphery of lines and other small features in the same way as for dots.
Similarly sub-marginally printable dots may be also be removed from the screened image_ By "sub-marginally printable dot" is meant a dot that is smaller than the minimum dot size that is reliably printable with a supporting infrastructure. These sub-marginal dots, even with a supporting infrastructure may be susceptible to the problems previously described. It may be advar.~~ageous to remove these sub-marginal dots from the image after screening. As an example consider a situation where it is determined that dots of diameter smaller than 50 um are marginally printable but that dots smaller than 30 um will still be marginally printable even with the support of non-printing dots. Placing non-printing 20 um diameter dots around the 30 um dot may not be sufficient to guarantee that the dot will print reliably. In this situation, a step may be added to the process 5 whereby all dots smaller than 30 ~m are completely removed form the image. Dots of between 30 and 50 ~m will be supported by 20 um non-printing dots and will print reliably.
EX'AMPIaE
A test was run using the Creo ThermoFlex imaging engine sold by 10 Creo Inc of Burnaby, BC, Canada to image a flexographic plate. The plate, once imaged was processed according to u;~ual procedures. The plate contained highlight areas with and without the inclusion of non-printing dots. The photographs shown :in FIG 6-A to 6-C show areas of the plate with a 1o screen @120 lpi. FIG. 6-A ;shows the 1% dots 15 without the support of non-printing dots. Dot 60 has steep shoulders and very little base support. The floor of the flexographic plate shown at 62 is set by a previous back exposure, the level of back exposure chosen for the best overall performance of the plate.
In FIG. 6-B a plurality of 4 pixel non-printing dots have been introduced in the background area of the 1% screen. The floor of the flexographic plate shown at 66 has been effectively raised by the inclusion of these non-printing dots. It should be noted that the shoulders of dot 64 are significantly widened by the inclusion of non-printing dots. It should also be immediately obvious that the inclusion of such dots will reduce the possibility of the small relief features breaking off or folding over. On the other hand, it should also be noted that relief dot 64 has not percept=ively increased in size at the printing relief plane.
In FIG. o-C the non-printing dots have been increased in size to 6 pixels. The corresponding raising of the relief floor is clearly visible. The relief depth can be selectively varied by varying the size and/or number of non-printing dots.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof.
The problem with this type of screening technique when applied to flexographic printing is that the size of dot that may be printed in isolation is actually quite large, typically 40 - 50 pm in diameter.
Even when using this technique, the highlights are difficult to reproduce without having a grainy appearance (which occurs when dots are spaced far apart to represent a very low density).
There remains a need to improve the representation of small dots iv flexographic printing processes.
SUMMARY OF THE INVENTI~N
In a first aspect of the present invention a method for representing a continuous tone image on a flexographic printing plate is presented. A halftone image representation is created by screening 5 the continuous tone image to create a plurality of halftone regions of varying density. Within these regions, one or rnore marginally printable regions that are too small to be reliably printed are identified. One or more non-printing dots are established proximate to but spaced apart from the marginally printable regions. Finally, the flexographic plate is exposed to image forming radiation in accordance with the halftone image representation.
In another aspect of the present invention a method for reproducing halftone image having areas of varying density on a flexographic printing plate is presented. Marginally printable regions that will be too small to be reliably printed are identified within the areas of varying density. One or moz-e non-printing dots are placed, proximate to but spaced apart from the marginally printable regions, the non-printing dots providing a base of support for the marginally printable surfaces.
In another aspect of the invention a method for preparing a flexographic printing plate is presented. The flexographic printing plate is back-exposed to set the relief floor. The halftone image data representing an image to be printed additionally defines a plurality of non-printing dots surrounding marginally printable regions. The flexographic printing plate is patterned in accordance E
with halftone image data provided, the relief floor being selectively raised in the areas containing non-printing dots. The flexographic printing plate is then processed to develop the relief image.
In yet another aspect of the present invention a flexographic printing plate has a non ink-accepting background area and an ink-accepting surface defined in relief to the background area and formed in accordance with an image to be printed. The ink-accepting surface has areas of varying density and within the areas of varying density, marginally printable regions that are too small to be reliably printed. One or more non-printing dots are dish>osed proximate to but spaced apart from the marginally printable regions, the non-printing dots providing a base of support for the marginally printable regions.
For an understanding of the invention, reference will now be made by way of example to a following detailed description in conjunction by accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In drawings which illustrate by way of example only preferred embodiments of the invention:
FIG. 1 is a schematic diagram of a prior art screening method.
FIG. 2 is a schematic diagram of an embodiment of a screening method in accordance with the present invention.
FIG. 3 is a schematic diagram of another embodiment of a screening method in accordance with the present invention.
FIG. 4 is a schematic diagram of a threshold arx:ay based screening method.
FIG. 5 is a schematic diagram of a tile based screening method.
FIG. 6-A - 6-C are a series of photographs of fl_exographic plate samples taken of plates prepared with and withoLa the use of the methods of the present invention.
DESCRIPTI~N
Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and the drawings are to be regarded in an illustrative rather than a restrictive sense.
The invention is described in relation to a screening technique wherein a marginally printable halftone dot that. is desired to print is surrounded by one or more smaller nan-printing halftone dots to provide an extra base of support for the small halftone dot.
An embodiment of the invention. is shown in FIG. 2. Lines 20 demarcate a plurality of halftone cells 22. It will be readily appreciated that each cell has a plurality of possible locations where the imaging device may place a picture element !pixel) to make up a halftone dot. These locations are shown as grid lines 24 in the upper left hand corner of FIG. 1 (these locations are omitted in the rest of FIG. 2 for sake of clarity). The pixel size is commonly a function of the imaging device configuraticn and may be fixed or variable. The halftone cell size is a function of the screening engine and is chosen to suit a particular screening job or a category of screening jobs.
A plurality of small halftone dots 26 representing a low density tone or highlight which it is desired to print in the final product are placed at the centres of a halftone cells 22.. In this case, only one quarter of the halftone cells have printing dots 26. Halftone dots 26 generally comprise a plurality of pixels>. While dot 26 is shown having circular shape, in practice the shape will depend on the shape of the pixels and how many pixels are used to make up the dot and the particular spot function chosen. Dot 2E> could therefore be irregularly shaped or even square. The size of Dot 26 is such that it may not print without the aforementioned problems. In other words, dot 26 is a marginally printable region, which may or may not print properly in a subsequent printing operation. The marginally printable dot size may be chosen by the printer from a knc>wledge of a particular printing process.
Dots 26 each have neighbouring halftone dots 28 which are specifically sized to be non-printing i.e. when the plate is processed dots 28 are small enough that they will be at least partially removed by the processing solvents. Non-printing dots 28 are spaced apart from the dots 26 by an amount sufficient to ensure that dots 28 and 26 do not join. Consequently, dots 28 af'~er processing do not present a relief point that will take up ink and print. Surprisingly, it was found that non-printing dots 28 surrounding printing dots 26, provides an extra base of support for the dot 26 that appears to prevent the dots 26 from being removed by processing. Furthermore, dots 26 were also prevented from breaking off during printing operations. The result is that a printer could now reliably hold smaller dot sizes on press than was previously possible. The inclusion of non-printing dots 28 has the added advantage that since the dots do not print but only contribute to the base of the relief, they do not change the printed density of the particular tone that it is desired to represent.
In another embodiment of the invention, the supporting dots need not necessarily be in adjacent halftone cells. FIG. 3 shows a portion of the screen from FIG. 2 where there is a printing halftone dot 26 in every halftone cell and the non-printing halftone dots 28 form a periphery around dot 26 in the same halftone cell.
There are several practical screening methods for placing non-printing dots. A preferred embodiment, that is also simple, involves using threshold array based AM screening wherein a few centre pixels of every halftone dot on the halftone grid are always turned on.
Threshold array screening is very well known in the art. This is the situation depicted in FIG. 4. A plurality of halftone dot cells are established as indicated by the darkened outlines 50. Each cell has a plurality of locations, in this case 25. The halftone cells are commonly located on what is known as the screen grid shown as dotted lines 52. The screen grid shown has a screen angle of 0°but it should be understood that the screen grid may also be rotated by some angle 5 to form the screen at commonly used screen angles as is well known in the art. In such a case the halftone cells may not be rectangular in shape but will be defined within the confines of the underlying imaging engine resolution grid. In operation, the threshold matrix as shown is tiled across the plane of the image to cover the entire area 10 thereof.
The values at each location determine whether a particular pixel will be on or off, depending on the density of the continuous tone (contone) image at that point. In FIG. 4 the contone image density for the situation and area shown is 3 units (assuming the continuous tone is broken up into 25 levels). Locations in the threshold matrix that have levels less than and equal to 3 will be on. Dot 54 is shown shaded black to indicate that it is on, as it comprises values between 1 and 3. In cells adjacent to the cell contain~_ng dot 54, non-printing dots 56 are formed. These dots are just below the determined printing threshold while dot 54 is on or above t=hat threshold. As the density increases dots 54 may increase in size and dots 56 may also be encompassed by growing other printing dots in these other regions.
Eventually all non-printing dots are obscured by printing dots; the non-printing dots being mainly of use in highlight areas. Once the non-printing dots have disappeared, the screening method returns to conventional AM screening.
The inclusion of non-printing dots has the effect of selectively altering the relief depth in only the areas where such a change is necessary to support marginally printable dots or features.
Advantageously areas where the halftone dot size is large enough to print without problem are unaffected.
In an alternative embodiment, threshold array based FM screening may be adapted to include the non-printing dots., An intermediate tone level is chosen in the FM screen where clusters of dots have formed and the clusters are in close proximity to one another. A non-printing dot is placed at the centre of the clu:~ter. The tone level is chosen having regard to the development of random or pseudo-random dots into clusters of dots according to the particular FM screening technique in use. For example, a 30o tone might be convenient since clusters will generally have formed at this stage. This serves to centre a substantial number of the non-printing dots in the clusters so that as the tone density increases ~~he non-printing dots are mostly overwritten by printing dots, thus rendering them redundant as the density increases.
In another embodiment where it is desired to have more control over the non-printing dots, threshold array screening may be found limiting since it is technically only possible to have the non-printing dot always on. For the situai~ion depicted in FIG. 3 there is no particular problem having all dots 28 on while dot 26 is still small. However, as the tonal density is increased and dot 26 is grown to fill more of the area of halftone cell 22, dot 26 will eventually join with dots 28. When joined to a larger area dot, dots 28 will add to the printing area thus causing undesirable tonal discontinuities.
Preferably, dots 28 should be switched off before reaching this point, or as soon as dots 26 no longer need support. Such control over the dots 28 is not easily achievable using threshold arrays but other known screening methods may be employed.
One particular method is shown in. FIG. 5. A series of tiles 30 -36 contain monotcnically increasing density screens. Tile 38 is at some intermediate density and tile 40 is a solid. There are n tiles in all corresponding to the desired number of tone levels in the image. It should be appreciated that the tiles in FIG. 5 are not physical objects, but rather manifestations of data structures in a computer device. For example, the set of tiles may be a 3 dimensional array of values, each individual tile being represented as a two dimensional array, there being n such individual tiles in the array.
The screening of an image from a contone representation into a halftone representation, involves, for each pixel of the contone image, determining the tone of the pixel and looking up the corresponding tile and applying that tile to the image pixel to achieve the halftone representation.
Tile 30 is the tile that will be used where no-density (Oo tone) is desired. This tile has non-printing dots 28 dispersed over the surface. At this tone level, none of the dots 28 will print on the final article. Tile 32 shows the lowest density step where a plurality of small dots 26 have been placed, each being peripherally surrounded by non-printing dots 28.
In tile 34, the dot 42 is the dot 26 from tile 32 that has now been increased in size. Note though that dot 42 is still surrounded by, but not touching non-printing dots 28. In tile 36 dots 44 are again. increased in size but now non-printing dogs are removed. Dot 44 is large enough not to reed support from the non-printing dots - this is now conventional AM screening. An intermediate tile 38 shows dots 46. If dots 28 were still on at this stage, they would be adding to the size of dots 46 causing undesirable increases or jumps in tone level. The final tile 48 is a solid tile wrerein all dots have grown into each other to completely cover the area of the tile.
I5 While the embodiment in FIG. 5 has been described in relation to AM screening, it is also adaptable to FM screening wherein each tile has a density represented by an FM screening technique and the non-printing dots are placed in close proximity to (but not adjacent to) small clusters of pixels in the FM screen that are desired to be printed. The size, number, and placement of these non-printing dots are allowed to vary for each tile (i.e. may not place any non-printing dots once all of the clusters in the FM screen have reached a sufficient size). This method has an advantage over the threshold array based approach in that the non-printing dots can be placed closer to the clusters (thus providing more support) while avoiding the potential tone jump caused by printing dots joining with non-printing dots. The advantage of using a tiled screen representation is that the non-printing dots can be confined to a few lower density tiles. This allows the highlights to be treated in isolation without affecting the intermediate and high-density tones. With a threshold matrix, it is generally speaking an all or noting proposition.
In yet another embodiment of the invention the .image may be conventionally screened without placing any non-printing dots. The screened image is then post processed to add a supporting infrastructure of non-printing halftone dots, in close proximity to any small halftone dots that meet some pre-determined criteria of needing support of non-printing dots. Likewise, the post processing may also identify thin lines or other delicate structure defining marginally printable regions that may not print properly. A support infrastructure may be placed around the periphery of lines and other small features in the same way as for dots.
Similarly sub-marginally printable dots may be also be removed from the screened image_ By "sub-marginally printable dot" is meant a dot that is smaller than the minimum dot size that is reliably printable with a supporting infrastructure. These sub-marginal dots, even with a supporting infrastructure may be susceptible to the problems previously described. It may be advar.~~ageous to remove these sub-marginal dots from the image after screening. As an example consider a situation where it is determined that dots of diameter smaller than 50 um are marginally printable but that dots smaller than 30 um will still be marginally printable even with the support of non-printing dots. Placing non-printing 20 um diameter dots around the 30 um dot may not be sufficient to guarantee that the dot will print reliably. In this situation, a step may be added to the process 5 whereby all dots smaller than 30 ~m are completely removed form the image. Dots of between 30 and 50 ~m will be supported by 20 um non-printing dots and will print reliably.
EX'AMPIaE
A test was run using the Creo ThermoFlex imaging engine sold by 10 Creo Inc of Burnaby, BC, Canada to image a flexographic plate. The plate, once imaged was processed according to u;~ual procedures. The plate contained highlight areas with and without the inclusion of non-printing dots. The photographs shown :in FIG 6-A to 6-C show areas of the plate with a 1o screen @120 lpi. FIG. 6-A ;shows the 1% dots 15 without the support of non-printing dots. Dot 60 has steep shoulders and very little base support. The floor of the flexographic plate shown at 62 is set by a previous back exposure, the level of back exposure chosen for the best overall performance of the plate.
In FIG. 6-B a plurality of 4 pixel non-printing dots have been introduced in the background area of the 1% screen. The floor of the flexographic plate shown at 66 has been effectively raised by the inclusion of these non-printing dots. It should be noted that the shoulders of dot 64 are significantly widened by the inclusion of non-printing dots. It should also be immediately obvious that the inclusion of such dots will reduce the possibility of the small relief features breaking off or folding over. On the other hand, it should also be noted that relief dot 64 has not percept=ively increased in size at the printing relief plane.
In FIG. o-C the non-printing dots have been increased in size to 6 pixels. The corresponding raising of the relief floor is clearly visible. The relief depth can be selectively varied by varying the size and/or number of non-printing dots.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof.
Claims (15)
1. A method for representing a continuous tone image on a flexographic printing plate, comprising steps of:
creating a halftone image representation by:
i) screening the continuous tone image to create a plurality of halftone regions of varying density;
ii) identifying, within the halftone regions, one or more marginally printable regions that are too small to be reliably printed;
iii) establishing one or more non-printing dots proximate to but spaced apart from the marginally printable regions;
exposing the flexographic plate to image forming radiation in accordance with the halftone image representation.
creating a halftone image representation by:
i) screening the continuous tone image to create a plurality of halftone regions of varying density;
ii) identifying, within the halftone regions, one or more marginally printable regions that are too small to be reliably printed;
iii) establishing one or more non-printing dots proximate to but spaced apart from the marginally printable regions;
exposing the flexographic plate to image forming radiation in accordance with the halftone image representation.
2. The method of claim 1 wherein steps (i) and (ii) are performed in a single operation.
3. The method of claim 1 wherein steps (i), (ii) and (iii) are performed in a single operation.
4. The method of claim 1 wherein steps (i), (ii) and (iii) are performed sequentially in the order as listed.
5. The method of claim 1 wherein steps (ii) and (iii) are performed in a single operation.
6. The method cf claim 1 wherein screening the continuous tone image comprises screening according to a stochastic screening technique.
7. The method of claim 1 wherein screening the continuous tone image comprises screening according to an amplitude modulated screening technique.
8. The method of claim 1 wherein creating a halftone image representation comprises the additional step of removing at least some of the sub-marginally printable regions.
9. The method of claim 1 wherein the non-printing dots each have an exposed surface area no larger than 675 µm2.
10. The method of claim 1 wherein the non-printing dots each have an exposed surface area no larger than 450 µm2.
11. A method for reproducing halftone image having areas of varying density on a flexographic printing plate, the method comprising steps of:
identifying, within the areas of varying density, marginally printable regions that will be too small to be reliably printed;
placing one or more non-printing dots disposed proximate to but spaced apart from the marginally printable regions, the non-printing dots providing a base of support for the marginally printable surfaces.
identifying, within the areas of varying density, marginally printable regions that will be too small to be reliably printed;
placing one or more non-printing dots disposed proximate to but spaced apart from the marginally printable regions, the non-printing dots providing a base of support for the marginally printable surfaces.
12. A method far preparing a flexographic printing plate, the method comprising steps of:
back-exposing the flexographic printing plate to set the relief floor;
receiving halftone image data representing an image to be printed, the halftone image data additionally defining a plurality of non-printing dots surrounding marginally printable regions in the halftone image data;
patterning the flexographic printing plate in accordance with halftone image data provided, the relief floor being selectively raised in the areas containing non-printing dots; and processing the flexographic printing plate to develop the relief image.
back-exposing the flexographic printing plate to set the relief floor;
receiving halftone image data representing an image to be printed, the halftone image data additionally defining a plurality of non-printing dots surrounding marginally printable regions in the halftone image data;
patterning the flexographic printing plate in accordance with halftone image data provided, the relief floor being selectively raised in the areas containing non-printing dots; and processing the flexographic printing plate to develop the relief image.
13. The method of claim 12, wherein the degree to which the relief floor is selectively raised is altered by choosing the size and number of the non-printing dots to be included in the halftone image.
14. A flexographic printing plate, comprising a non ink-accepting background area;
an ink-accepting surface defined in relief to the background area and formed in accordance with an image to be printed, the ink-accepting surface having areas of varying density and within the areas of varying density, marginally printable regions that are too small to be reliably printed;
one or more non-printing dots disposed proximate to but spaced apart from the marginally printable regions, the non-printing dots providing a base of support for the marginally printable regions.
an ink-accepting surface defined in relief to the background area and formed in accordance with an image to be printed, the ink-accepting surface having areas of varying density and within the areas of varying density, marginally printable regions that are too small to be reliably printed;
one or more non-printing dots disposed proximate to but spaced apart from the marginally printable regions, the non-printing dots providing a base of support for the marginally printable regions.
15. The printing plate of claim 14 wherein the marginally printable regions have a plurality of non-printing dots disposed around the periphery thereof.
Priority Applications (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2422132 CA2422132A1 (en) | 2003-03-11 | 2003-03-11 | Screening method for preparing flexographic printing plates |
| US10/736,216 US7126724B2 (en) | 2003-03-11 | 2003-12-15 | Flexographic printing |
| PCT/US2004/007336 WO2004080722A2 (en) | 2003-03-11 | 2004-03-09 | Flexographic printing |
| EP04718872.7A EP1608507B1 (en) | 2003-03-11 | 2004-03-09 | Flexographic printing |
| US11/233,943 US7245402B2 (en) | 2003-03-11 | 2005-09-23 | Flexographic printing |
| US11/611,178 US7486420B2 (en) | 2003-03-11 | 2006-12-15 | Flexographic printing |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2422132 CA2422132A1 (en) | 2003-03-11 | 2003-03-11 | Screening method for preparing flexographic printing plates |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2422132A1 true CA2422132A1 (en) | 2004-09-11 |
Family
ID=32932265
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2422132 Abandoned CA2422132A1 (en) | 2003-03-11 | 2003-03-11 | Screening method for preparing flexographic printing plates |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2422132A1 (en) |
-
2003
- 2003-03-11 CA CA 2422132 patent/CA2422132A1/en not_active Abandoned
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