EP0445273A1 - Procede et moyen de maitrise du surcuit en lithographie imagee par etincelles - Google Patents

Procede et moyen de maitrise du surcuit en lithographie imagee par etincelles

Info

Publication number
EP0445273A1
EP0445273A1 EP19900914611 EP90914611A EP0445273A1 EP 0445273 A1 EP0445273 A1 EP 0445273A1 EP 19900914611 EP19900914611 EP 19900914611 EP 90914611 A EP90914611 A EP 90914611A EP 0445273 A1 EP0445273 A1 EP 0445273A1
Authority
EP
European Patent Office
Prior art keywords
plate
coating
image
ink
spark
Prior art date
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.)
Withdrawn
Application number
EP19900914611
Other languages
German (de)
English (en)
Inventor
Thomas E. Lewis
Michael T. Nowak
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Presstek LLC
Original Assignee
Presstek LLC
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Presstek LLC filed Critical Presstek LLC
Publication of EP0445273A1 publication Critical patent/EP0445273A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
    • B41C1/00—Forme preparation
    • B41C1/10—Forme preparation for lithographic printing; Master sheets for transferring a lithographic image to the forme
    • B41C1/1008—Forme preparation for lithographic printing; Master sheets for transferring a lithographic image to the forme by removal or destruction of lithographic material on the lithographic support, e.g. by laser or spark ablation; by the use of materials rendered soluble or insoluble by heat exposure, e.g. by heat produced from a light to heat transforming system; by on-the-press exposure or on-the-press development, e.g. by the fountain of photolithographic materials
    • B41C1/1033—Forme preparation for lithographic printing; Master sheets for transferring a lithographic image to the forme by removal or destruction of lithographic material on the lithographic support, e.g. by laser or spark ablation; by the use of materials rendered soluble or insoluble by heat exposure, e.g. by heat produced from a light to heat transforming system; by on-the-press exposure or on-the-press development, e.g. by the fountain of photolithographic materials by laser or spark ablation

Definitions

  • This invention relates to offset lithography. It relates more specifically to improved lithography plates and method and apparatus for imaging these plates.
  • Water tends to adhere to the hydrophilic or water-receptive areas of the plate creating a thin film of water there which does not accept ink.
  • the ink does adhere to the hydrophobic areas of the plate and those inked areas, usually corresponding to the printed areas of the original document, are transferred to a relatively soft blanket cylinder and, from there, to the paper or other recording medium brought into contact with the surface of the blanket cylinder by an impression cylinder.
  • the resultant plate now carries a positive or direct image of the original document.
  • a separate printing plate corresponding to each color is required, each of which is usually made photographically as aforesaid.
  • the plates In addition to preparing the appropriate plates for the different colors, the plates must be mounted properly on the print cylinders in the press and the angular positions of the
  • An image has also been applied to a lithographic plate by electro-erosion.
  • the type of plate suitable for imaging in this fashion and disclosed in U.S. Patent 4,596,733 has an oleophilic plastic substrate, e.g. Mylar brand plastic film, having a thin coating of aluminum metal with an overcoating containing conductive graphite which acts as a lubricant and protects the aluminum coating against scratching.
  • a stylus electrode in contact with the graphite containing surface coating is caused to move across the surface of the plate and is pulsed in accordance with incoming picture signals.
  • the resultant current flow between the electrode and the thin metal coating is by design large enough to erode away the thin metal coating and the overlying conductive graphite containing surface coating thereby exposing the underlying ink receptive plastic substrate on the areas of the plate corresponding to the printed portions of the original document.
  • This method of making lithographic plates is disadvantaged in that the
  • thermoplastic image-forming resin or material which has a desired affinity for the printing ink being used to print the copies may be any thermoplastic image-forming resin or material which has a desired affinity for the printing ink being used to print the copies.
  • the image-forming material may be any thermoplastic image-forming resin or material which has a desired affinity for the printing ink being used to print the copies.
  • the image-forming material may be any thermoplastic image-forming resin or material which has a desired affinity for the printing ink being used to print the copies.
  • the image-forming material may be
  • thermoplastic image-forming material that is suitable for jetting and also has the desired affinity (phyilic or phobic) for all of the inks commonly used for making lithographic copies. Also, ink jet printers are generally unable to produce small enough ink dots to allow the production of smooth continuous tones on the printed copies, i.e. the resolution is not high enough.
  • the present invention aims to provide various lithographic plate constructions which can be imaged or written on to form a positive or negative image therein.
  • Another object is to provide such plates which can be used in a wet or dry press with a variety of different printing inks.
  • Another object is to provide low cost lithographic plates which can be imaged electrically.
  • a further object is to provide an improved method for imaging lithographic printing plates.
  • Another object of the invention is to provide a method of imaging lithographic plates which can be practiced while the plate is mounted in a press.
  • Still another object of the invention is to provide a method for writing both positive and negative or background images on lithographic plates. Still another object of the invention is to provide such a method which can be used to apply images to a variety of different kinds of lithographic plates.
  • a further object of the invention is to provide a method of producing on lithographic plates half tone images with variable dot sizes.
  • a further object of the invention is to provide improved apparatus for imaging lithographic plates.
  • Another object of the invention is to provide apparatus of this type which applies the images to the plates efficiently and with a minimum consumption of power.
  • Still another object of the invention is to provide such apparatus which lends itself to control by incoming digital data representing an original document or picture.
  • the invention accordingly comprises an article of manufacture possessing the features and properties exemplified in the constructions described herein and the several steps and the relation of one or more of such steps with respect to the others and the apparatus embodying the features of construction, combination of elements and the arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed description, and the scope of the invention will be indicated in the claims.
  • images are applied to a lithographic printing plate by altering the plate surface characteristics at selected points or areas of the plate using a non-contacting writing head which scans over the surface of the plate and is controlled by incoming picture signals corresponding to the original document or picture being copied.
  • the writing head utilizes a precisely positioned high voltage spark discharge electrode to create on the surface of the plate an intense-heat spark zone as well as a corona zone in a circular region surrounding the spark zone.
  • ancillary data keyed in by the operator such as dot size, screen angle, screen mesh, etc.
  • high voltage pulses having precisely controlled voltage and current profiles are applied to the electrode to produce precisely positioned and defined spark/corona discharges to the plate which etch, erode or otherwise transform selected points or areas of the plate surface to render them either receptive or non-receptive to the printing ink that will be applied to the plate to make the printed copies.
  • Lithographic plates are made ink receptive or oleophilic initially by providing, them with surface areas consisting of unoxidized metals or plastic materials to which oil and rubber based inks adhere readily.
  • plates are made water receptive or hydrophilic initially in one of three ways.
  • One plate embodiment is provided with a plated metal surface, e.g. of chrome, whose topography or character is such that it is wetted by surface tension.
  • a second plate has a surface consisting of a metal oxide, e.g. aluminum oxide, which
  • the third plate construction is provided with a polar plastic surface which is also roughened to render it hydrophilic.
  • a polar plastic surface which is also roughened to render it hydrophilic.
  • certain ones of these plate embodiments are suitable for wet printing, others are better suited for dry printing. Also, different ones of these plate constructions are preferred for direct writing; others are preferred for indirect or background writing.
  • the present apparatus can write images on all of these different lithographic plates having either ink receptive or water receptive surfaces.
  • the plate surface is hydrophilic initially, our apparatus will write a positive or direct image on the plate by rendering oleophilic the points or areas of the plate surface corresponding to the printed portion of the original document.
  • the apparatus will apply a background or negative image to the plate surface by
  • the plate imaging apparatus incorporating our invention is preferably implemented as a scanner or plotter whose writing head consists of one or more spark discharge electrodes.
  • the electrode (or electrodes) is positioned over the working surface of the lithographic plate and moved relative to the plate so as to collectively scan the plate surface.
  • Each electrode is controlled by an incoming stream of picture signals which is an electronic representation of an original document or picture.
  • the signals can originate from any suitable source such as an optical scanner, a disk or tape reader, a computer, etc. These signals are formatted so that the apparatus' spark discharge electrode or electrodes write a positive or negative image onto the surface of the lithographic plate that corresponds to the original document.
  • the spark discharge electrode or electrodes may be incorporated into a flat bed scanner or plotter.
  • the spark discharge writing head is incorporated into a so-called drum scanner or plotter with the lithographic plate being mounted to the cylindrical surface of the drum.
  • our invention can be practiced on a lithographic plate already mounted in a press to apply an image to that plate in situ. In this application, then, the print cylinder itself constitutes the drum component of the scanner or plotter.
  • the plate can be rotated about its axis and the head moved parallel to the rotation axis so that the plate is scanned circumferentially with the image on the plate "growing" in the axial direction.
  • the writing head can move parallel to the drum axis and after each pass of the head, the drum can be
  • each electrode traverses the plate, it is supported on a cushion of air so that it is maintained at a very small fixed distance above the plate surface and cannot scratch that surface.
  • each electrode is pulsed or not pulsed at selected points in the scan
  • the electrode is to write or not write at these locations.
  • a high voltage spark discharge occurs between the electrode tip and the particular point on the plate opposite the tip. The heat from that spark discharge and the accompanying corona field surrounding the spark etches or otherwise transforms the surface of the plate in a
  • controllable fashion to produce an image-forming spot or dot on the plate surface which is precisely defined in terms of shape and depth of penetration into the plate.
  • each electrode is pointed to obtain close control over the definition of the spot on the plate that is affected by the spark discharge from that electrode.
  • the pulse duration, current or voltage controlling the discharge may be varied to produce a variable dot on the plate.
  • the polarity of the voltage applied to the electrode may be made positive or negative depending upon the nature of the plate surface to be affected by the writing, i.e. depending upon whether ions need to be pulled from or repelled to the surface of the plate at each image point in order to transform the surface at that point to distinguish it imagewise from the remainder of the plate surface, e.g. to render it oleophilic in the case of direct writing on a plate whose surface is
  • the apparatus After a complete scan of the plate, then, the apparatus will have applied a complete screened image to the plate in the form of a multiplicity of surface spots or dots which are different in their affinity for ink from the portions of the plate surface not exposed to the spark discharges from the scanning electrode.
  • FIG. 1 is a diagrammatic view of an offset press
  • FIG. 2 is an isometric view on a larger scale showing in greater detail the print cylinder portion of the FIG. 1 press;
  • FIG. 3 is a sectional view taken along line 3-3 of FIG. 2 on a larger scale showing the writing head that applies an image to the surface of the FIG. 2 print cylinder, with the associated electrical components being represented in a block diagram;
  • FIGS. 4A to 4F are enlarged sectional views showing imaged lithographic plates incorporating our invention.
  • FIG. 5A depicts the tendency of non-overlapping image points to leave exposed surface area there between;
  • FIG. 5B depicts the effect of overlapping image points to expose the interstitial surface area
  • FIG. 5C illustrates the manner in which overlapping image pints can produce adverse image effects.
  • FIG. 1 of the drawings shows a more or less conventional offset press shown generally at 10 which can print copies using lithographic plates made in accordance with this invention.
  • Press 10 includes a print cylinder or drum 12 around which is wrapped a lithographic plate 13 whose opposite edge margins are secured to the plate by a conventional clamping mechanism 12a incorporated into cylinder 12.
  • Cylinder 12 or more precisely the plate 13 thereon, contacts the surface of a blanket cylinder 14 which, in turn, rotates in contact with a large diameter impression cylinder 16.
  • the paper sheet P to be printed on is mounted to the surface of cylinder 16 so that it passes through the nip between cylinders 14 and 16 before being discharged to the exit end of the press 10.
  • Ink for inking plate 13 is delivered by an ink train 22, the lowermost roll 22a. of which is in rolling engagement with plate 13 when press 10 is printing.
  • the various cylinders are all geared together so that they are driven in unison by a single drive motor.
  • the illustrated press 10 is capable of wet as well as dry printing. Accordingly, it includes a conventional dampening or water fountain assembly 24 which is movable toward and away from drum 12 in the directions indicated by arrow A in FIG. l between active and inactive positions. Assembly 24 includes a conventional water train shown generally at 26 which conveys water from a tray 26a to a roller 26b which, when the dampening assembly is active, is in rolling engagement with plate 13 and the intermediate roller 22b of ink train 22 as shown in phantom in FIG. 1.
  • the dampening assembly 24 When press 10 is operating in its dry printing mode, the dampening assembly 24 is inactive so that roller 26b is retracted from roller 22b and the plate as shown in solid lines in FIG. 1 and no water is applied to the plate.
  • lithographic plate on cylinder 12 in this case is designed for such dry printing. See for example plate 138 in FIG. 4D. It has a surface which is oleophobic or non-receptive to ink except in those areas that have been written on or imaged to make them oleophilic or receptive to ink.
  • the plate As the cylinder 12 rotates, the plate is contacted by the ink- coated roller 22a of ink train 22. The areas of the plate surface that have been written on and thus made oleophilic pick up ink from roller 22a. Those areas of the plate surface not written on receive no ink.
  • the image written on the plate will have been inked or developed. That image is then transferred to the blanket cylinder 14 and finally, to the paper sheet P which is pressed into contact with the blanket cylinder.
  • the dampening assembly 24 When press 10 is operating in its wet printing mode, the dampening assembly 24 is active so that the water roller 26b contacts ink roller 22b and the surface of the plate 13 as shown in phantom in FIG. 1.
  • Plate 13 which is described in more detail in connection with FIG. 4A, is intended for wet printing. It has a surface which is hydrophilic except in the areas thereof which have been written on to make them
  • the print cylinder 12 is rotatively supported by the press frame 10a and rotated by a standard electric motor 34 or other conventional means.
  • the angular position of cylinder 12 is monitored by conventional means such as a shaft encoder 36 that rotates with the motor armature and associated detector 36a.
  • the angular position of the large diameter impression cylinder 16 may be monitored by a suitable magnetic detector that detects the teeth of the circumferential drive gear on that cylinder which gear meshes with a similar gear on the print cylinder to rotate that cylinder.
  • a writing head assembly shown generally at 42.
  • This assembly comprises a lead screw 42a whose opposite ends are rotatively supported in the press frame 10a, which frame also supports the opposite ends of a guide bar 42b spaced parallel to lead screw 42a.
  • a carriage 44 Mounted for movement along the lead screw and guide bar is a carriage 44. When the lead screw is rotated by a step motor 46, carriage 44 is moved axially with respect to print cylinder 12.
  • the cylinder drive motor 34 and step motor 46 are operated in synchronism by a controller 50 (FIG. 3), which also receives signals from detector 36a, so that as the drum rotates, the carriage 44 moves axially along the drum with the controller "knowing" the instantaneous relative position of the carriage and cylinder at any given moment.
  • controller 50 FIG. 3
  • the control circuitry required to accomplish this is already very well known in the scanner and plotter art.
  • FIG. 3 depicts an illustrative
  • carriage 44 includes a block 52 having a threaded opening 52a for threadedly receiving the lead screw 42a and a second parallel opening 52b for slidably receiving the guide rod 42b.
  • a bore or recess 54 extends in from the underside of block 52 for slidably receiving a discoid writing head 56 made of a suitable rigid electrical insulating
  • An axial passage 57 extends through head 56 for snugly receiving a wire electrode 58 whose diameter has been exaggerated for clarity.
  • Electrode 58 is received and anchored in a socket 62 mounted to the top of head 56 and the lower end 58b of the electrode 58 is preferably pointed as shown in FIG. 3.
  • Electrode 58 is made of an electrically conductive metal, such as thoriated tungsten, capable of withstanding very high temperatures.
  • An insulated conductor 64 connects socket 62 to a terminal 64a at the top of block 52. If the carriage 44 has more than one electrode 58, similar connections are made to those electrodes so that a plurality of points on the plate 13 can be imaged
  • a plurality of small air passages 66 are also formed in head 56. These passages are distributed around electrode 58 and the upper ends of the passages are connected by way of flexible tubes or hoses 68 to a corresponding plurality of vertical passages 72. These passages extend from the inner wall of block bore 54 to an air manifold 74 inside the block which has an inlet passage 76 extending to the top of the block. Passage 76 is connected by a pipe 78 to a source of pressurized air. In the line from the air source is an
  • adjustable valve 82 and a flow restrictor 84. Also, a branch line 78a leading from pipe 78 downstream from restrictor 84 connects to a pressure sensor 90 which produces an output for controlling the setting of valve 82.
  • the writing head 56 and particularly the pulsing of its electrode 58, is controlled by a pulse circuit 96.
  • One suitable circuit comprises a
  • transformer 98 whose secondary winding 98a. is connected at one end by way of a variable resistor 102 to terminal 64a which, as noted previously, is connected electrically to electrode 58. The opposite end of winding 98a is connected to electrical ground.
  • the transformer primary winding 98b is connected to a DC voltage source 104 that supplies a voltage in the order of 1000 volts.
  • the transformer primary circuit includes a large capacitor 106 and a resistor 107 in series. The capacitor is maintained at full voltage by the resistor 107.
  • An electronic switch 108 is connected in shunt with winding 98b and the capacitor. This switch is controlled by switching signals received from controller 50.
  • circuit 96 specifically illustrated is only one of many known circuits that can be used to provide variable high voltage pulses of short duration to electrode 58.
  • capacitor-regenerating resistor may be used to avoid the need for transformer 98. Also, a bias voltage may be applied to the electrode 58 to provide higher voltage output pulses to the electrode without requiring a high voltage rating on the switch.
  • the press 10 When an image is being written on plate 13, the press 10 is operated in a non-print or imaging mode with both the ink and water rollers 22a and 26b being disengaged from cylinder 12.
  • the imaging of plate 13 in press 10 is controlled by controller 50 which, as noted previously, also controls the rotation of cylinder 12 and the scanning of the plate by carriage assembly 42.
  • the signals for imaging plate 13 are applied to controller 50 by a conventional source of picture signals such as a disk reader 114.
  • the controller 50
  • switch 108 synchronizes the image data from disk reader 114 with the control signals that control rotation of cylinder 12 and movement of carriage 44 so that when the electrode 58 is positioned over uniformly spaced image points on the plate 13, switch 108 is either closed or not closed depending upon whether that particular point is to be written on or not written on.
  • switch 108 is closed. The closing of that switch discharges capacitor 106 so that a precisely shaped, i.e. squarewave, high voltage pulse, i.e. 1000 volts, of only about one microsecond duration is applied to transformer 98.
  • the transformer applies a stepped up pulse of about 3000 volts to electrode 58 causing a spark discharge S between the electrode tip 58b and plate 13.
  • That sparks and the accompanying corona field S' surrounding the spark zone etches or transforms the surface of the plate at the point thereon directly opposite the electrode tip 58b to render that point either receptive or non-receptive to ink, depending upon the type of surface on the plate.
  • resistor 102 is adjusted for the different plate embodiments to produce a spark discharge that writes a clearly defined image spot on the plate surface which is in the order of 0.005 to 0.0001 inch in diameter.
  • That resistor 102 may be varied manually or automatically via controller 50 to produce dots of variable size. Dot size may also be varied by varying the voltage and/or duration of the pulses that produce the spark
  • dot size may be varied by repeated pulsing of the electrode at each image point, the number of pulses
  • the electrode has a pointed end 58b as shown and the gap between tip 58b and the plate is made very small, i.e. 0.001 inch, the spark discharge is focused so that image spots as small as 0.0001 inch or even less can be formed while keeping voltage requirements to a minimum.
  • the polarity of the voltage applied to the electrode may be positive or negative although
  • the polarity is selected according to whether ions need to be pulled from or repelled to the plate surface to effect the desired surface transformations on the various plates to be described.
  • the electrode 58 As the electrode 58 is scanned across the plate surface, it can be pulsed at a maximum rate of about 500,000 pulses/sec. However, a more typical rate is 25,000 pulses/sec. Thus, a broad range of dot densities can be achieved, e.g. 2,000 dots/inch to 50 dots/inch.
  • the dots can be printed side-by- side or they may be made to overlap so that substantially 100% of the surface area of the plate can be imaged.
  • an image corresponding to the original document builds up on the plate surface constituted by the points or spots on the plate surface that have been etched or transformed by the spark discharge S, as compared with the areas of the plate surface that have not been so affected by the spark discharge.
  • the press 10 can then be operated in its printing mode by moving the ink roller 22a to its inking position shown in solid lines in FIG. 1, and, in the case of wet printing, by also shifting the water fountain roller 26b to its dotted line position shown in FIG. 1. As the plate rotates, ink will adhere only to the image points written onto the plate that correspond to the printed portion of the original document. That ink image will then be transferred in the usual way via blanket cylinder 14 to the paper sheet P mounted to cylinder 16.
  • Forming the image on the plate 13 while the plate is on the cylinder 12 provides a number of advantages, the most important of which is the significant decrease in the
  • Such a press includes a plurality of sections similar to press 10 described herein, one for each color being printed.
  • the print cylinders in the different press sections after the first are adjusted axially and in phase so that the different color images printed by the lithographic plates in the various press sections will appear in register on the printed copies, it is apparent from the foregoing that, since the images are applied to the plates 13 while they are mounted in the press sections, such print registration can be accomplished electronically in the present case.
  • the controller 50 would adjust the timings of the picture signals controlling the writing of the images at the second and subsequent printing sections to write the image on the lithographic plate 13 in each such station with an axial and/or angular offset that compensates for any misregistration with respect to the image on the first plate 13 in the press.
  • the controller 50 instead of achieving such registration by repositioning the print
  • FIGS. 4A to 4F illustrate various lithographic plate embodiments which are capable of being imaged by the apparatus depicted in FIGS. 1 to 3.
  • the plate 13 mounted to the print cylinder 12 comprises a steel base or substrate layer 13a having a flash coating 13b of copper metal which is, in turn, plated over by a thin layer 13c of chrome metal.
  • plate 13 is a
  • Electrode 58 During a writing operation on plate 13 as described above, voltage pulses are applied to electrode 58 so that spark discharges S occur between the electrode tip 58b and the surface layer 13c of plate 13.
  • Each spark discharge coupled with the accompanying corona field S' surrounding the spark zone, melts the surface of layer 13c at the imaging point I on that surface directly opposite tip 58b. Such melting suffices to modify the surface structure or topography at that point on the surface so that water no longer tends to adhere to that surface area. Accordingly, when plate 13 is imaged in this fashion, a multiplicity of non-water-receptive spots or dots I are formed on the otherwise hydrophilic plate surface, which spots or dots represent the printed portion of the original document being copied.
  • press 10 When press 10 is operated in its wet printing mode, i.e. with dampening assembly 24 in its position shown in phantom in FIG. 1, the water from the dampening roll 26b adheres only to the surface areas of plate 13 that were not subjected to the spark discharges from electrode 58 during the imaging
  • the ink from the ink roll 22a does adhere to those plate surface areas written on, but does not adhere to the surface areas of the plate where the water or wash solution is present.
  • the ink adhering to the plate which forms a direct image of the original document, is transferred via the blanket cylinder 14 to the paper sheet P on cylinder 16. While the polarity of the voltage applied to electrode 58 during the imaging process described above can be positive or negative, we have found that for imaging a plate with a bare chrome surface such as the one in FIG. 4A, a positive polarity is preferred because it enables better control over the formation of the spots or dots on the surface of the plate.
  • FIG. 4B illustrates another plate embodiment which is written on directly and used in a dampening-type press.
  • This plate shown generally at 122 in FIG. 4B, has a substrate 124 made of a metal such as aluminum which has a structured oxide surface layer 126.
  • This surface layer may be produced by any one of a number of known chemical treatments, in some cases assisted by the use of fine abrasives to roughen the plate surface.
  • the controlled oxidation of the plate surface is commonly called anodizing while the surface structure of the plate is referred to as grain or graining.
  • modifiers such as silicates, phosphates, etc. are used to stabilize the hydrophilic character of the plate surface and to promote both adhesion and the stability of the photosensitive layer(s) that are coated on the plates.
  • the aluminum oxide on the surface of the plate is not the crystalline structure associated with corundum or a laser ruby (both are aluminum oxide crystals), and shows considerable interaction with water to form hydrates of the form Al 2 O 3 H 2 O.
  • This interaction with contributions from silicate, phosphate, etc. modifiers is the source of the hydrophilic nature of the plate surface.
  • Formation of hydrates is also a problem when the process proceeds unchecked.
  • Ability to effectively hold a thin film of water required to produce nonimage areas is thus lost which renders the plate useless.
  • Most plates are supplied with photosensitive layers in place that protect the plate surfaces until the time the plates are exposed and developed.
  • the plates are either immediately used or stored for use at a latter time. If the plates are stored, they are coated with a water soluble polymer to protect hydrophilic surfaces. This is the process usually referred to as gumming in the trade. Plates that are supplied without photosensitive layers are usually treated in a similar manner.
  • the images generated on a chrome plate show a similar sensitivity to water contact preceding ink contact. However, after the ink application step, the images on a chrome plate are more stable and the plate can be run without additional steps to preserve the image.
  • the ink remaining on the image points I is quite fragile and must be left to dry or set so that the ink becomes more durable.
  • a standard ink which cures or sets in response to ultraviolet light or heat may be used with plate 122.
  • a standard ultraviolet lamp 126 may be mounted adjacent to print cylinder 12 as depicted in FIGS. 1 and 2 to cure the particular ink.
  • the lamp 126 should extend the full length of cylinder 12 and be supported by frame members 10a close to the surface of cylinder 12 or, more particularly, the lithographic plate thereon.
  • imaging a plate such as plate 122 based on aluminum is optimized if a negative voltage is applied to the imaging electrode 58. This is because positive aluminum ions produced at each image point migrate well in the high intensity current flow of the spark discharge and will move toward the negative electrode.
  • FIG. 4C shows a plate embodiment 130 suitable for direct imaging in a press without dampening.
  • Plate 130 comprises a substrate 132 made of a conductive metal such as aluminum or steel.
  • the substrate carries a thin coating 134 of a highly oleophobic material such as a fluoropolymer or silicone.
  • a highly oleophobic material such as a fluoropolymer or silicone.
  • One suitable coating material is an addition-cured release coating marketed by Dow Corning under its designation SYL-OFF 7044.
  • Plate 130 is written on or imaged by decomposing the surface of coating 134 using spark discharges from electrode 58. The heat from the spark and associated corona decompose the silicone coating into silicon dioxide, carbon dioxide, and water.
  • Hydrocarbon fragments in trace amounts are also possible depending on the chemistry of the silicone polymers used.
  • Silicone resins do not have carbon in their backbones which means various polar structures such as C-OH are not formed.
  • Silanols, which are Si-OH structures are possible structures, but these are reactive which means they react to form other, stable structures.
  • Such decomposition coupled with surface roughening of coating 134 due to the spark discharge renders that surface oleophilic at each image point I directly opposite the tip of electrode 58.
  • that coating is made quite thin, e.g. 0.0003 inch to minimize the voltage required to break down the material to render it ink receptive.
  • FIG. 4D illustrates a lithographic plate 152 suitable for indirect imaging and for wet printing.
  • the plate 152 comprises a substrate 154 made of a suitable conductive metal such as aluminum or copper.
  • Plate 172 comprises a base or substrate 174, a base coat or layer 176 containing pigment or particles 177, a thin conductive metal layer 178, an ink repellent silicone top or surface layer 184, and, if necessary, a primer layer 186 between layers 178 and 184.
  • the material of substrate 174 should have mechanical strength, lack of extension (stretch) and heat resistance.
  • Polyester film meets all these requirements well and is readily available.
  • Dupont's Mylar and ICI's Melinex are two
  • films that can be used for substrate 174 are those based on polyimides (Dupont's Kapton) and polycarbonates (GE's Lexan). A preferred thickness is 0.005 inch, but thinner and thicker versions can be used effectively.
  • this layer is strongly textured.
  • “textured” means that the surface topology has numerous peaks and valleys.
  • the projecting peaks create a surface that can be described as containing numerous tiny electrode tips (point source electrodes) to which the spark from the imaging electrode 58 can jump.
  • This texture is conveniently created by the filler particles 177 included in the base coat, as will be described in detail hereinafter under the section entitled Filler Particles 177.
  • Other requirements of base coat 176 include:
  • the chemistry of the base coat that can be used is wide ranging. Application can be from solvents or from water.
  • 100% solids coatings such as characterize conventional UV and EB curable coating can be used.
  • a number of curing methods chemical reactions that create crosslinking of coating components) can be used to establish the performance properties desired of the coatings. Some of these are:
  • thermoset reactions are those as an aminoplast resin with hydroxyl sites of the primary coating resin. These reactions are greatly
  • an isocynate component reacts with hydroxyl sites on one or more "backbone” resins often referred to as the "polyol” component.
  • Typical polyols include polyethers, polyesters, an acrylics having two or more hydroxyl functional sites.
  • Important modifying resins include hydroxyl functional vinyl resins and cellulose ester resins.
  • the isocyanate component will have two or more isocyanate groups and is either monomeric or oligomeric. The reactions will proceed at ambient temperatures, but can be accelerated using heat and selected catalysts which include tin compounds and tertiary amines.
  • the normal technique is to mix the isocynate functional component (s) with the polyol component (s) just prior to use. The reactions begin, but are slow enough at ambient temperatures to allow a "potlife" during which the coating can be applied.
  • the isocyanate is used in a first approach.
  • solubilizing or dispersing of the resin in water proceeds at ambient temperatures after the water and solubilizing amine (s) have been evaporated upon deposition of the coating.
  • the aziridines are added to the coating at the time of use and have a potlife governed by their rate of hydrolysis in water to produce inert by-products.
  • boron trifluoride complex catalyzed resins can be used, particularly for resins based on cycloaliphatic epoxy functional groups. Another reaction is based on UV exposure generated cationic catalysts for the reaction.
  • Union Carbide's Cyracure system is a
  • Free radicals to initiate the reaction are created by exposure of the coating to an electron beam or by a
  • photoinitiation system incorporated into a coating to be cured by UV exposure.
  • the choice of chemistry to be used will depend on the type of coating equipment to be used and environmental concerns rather than a limitation by required performance properties.
  • a crosslinking reaction is also not an absolute requirement. For example, there are resins soluble in a limited range of solvents not including those typical of offset inks and their cleaners that can be used.
  • the filler particles 177 used to create the important surface structure are chosen based on the following
  • the surface structure of the base coat 176 contributes to the surface structure of the base coat 176. This is dependent on the thickness of the coating to be deposited. This is illustrated for a 5 micron thick (.0002 inch) coat 176 pigmented with particles 177 of spherical geometry that remain well dispersed throughout deposition and curing of the coat. Particles with diameters of 5 microns and less would not be expected to contribute greatly to the surface structure because they could be contained within the thickness of the coating. Larger particles, e.g. 10 microns in diameter, would make significant contributions because they could project 5 microns above the base coat 176 surface, creating high points that are twice the average thickness of that coat.
  • Equidimensional particles such as the spherical particles described above and depicted in FIG. 4F will contribute the same degree regardless of particle orientation within the base coat and are therefore preferred. Particles with one dimension much greater than the others, acicular types being one example, are not usually desirable. These particles will tend to orient themselves with their long dimensions parallel to the surface of the coating, creating low rounded ridges rather than the desirable distinct peaks.
  • Particles that are platelets are also undesirable.
  • coating is a function of the image density to be encountered. For example, if the plate is to be imaged at 400 dots per centimeter or 160,000 dots per square centimeter, it would be desirable to have at least that many peaks (particles) present and
  • Particle sizes, geometries, and densities are readily available data for most filler particle candidates, but there are two important complications. Particle sizes are averages or mean valves that describe the distribution of sizes that are characteristic of a given powder or pigment as supplied. This means that both larger and smaller sizes than the average or mean are present and are significant contributors to particle size considerations. Also, there is always some degree of particle association present when particles are dispersed into a fluid medium, which usually increases during the application and curing of a coating. Resultantly, peaks are produced by groups of particles, as well as by individual particles.
  • Preferred filler particles 177 include the following:
  • Preferred particle sizes for the filler particles to be used is highly dependent on the thickness of the layer 176 to be deposited. For a 5 micron thick layer (preferred
  • the method of coating base layer 176 with the particles 177 dispersed therein onto the substrate 174 may be by any of the currently available commercial coating processes.
  • a preferred application of the base coat is as a layer 5 +/- 2 microns thick. In practice, it is expected that base coats could range from as little as 2 microns to as much as 10 microns in thickness. Layers thicker than 10 microns are possible, and may be required to produce plates of high
  • the base coat 176 may not be required if the substrate 174 has the proper, and in a sense equivalent, properties. More particularly, the use for substrate 174 of films with surface textures (structures) created by mechanical means such as embossing rolls or by the use of filler pigments may have an important advantage in some applications provided they meet two conditions:
  • the films are metalizable with the deposited metal forming layer 178 having adequate adhesion;
  • This layer 178 is important to formation of an image and must be uniformly present if uniform imaging of the plate is to occur.
  • the image carrying (i.e. ink receptive) areas of the plate 172 are created when the spark discharge volatizes a portion of the thin metal layer 178.
  • the size of the feature formed by a spark discharge from electrode tip 58b of a given energy is a function of the amount of metal that is volatized. This is, in turn, a function of the amount of metal present and the energy required to volatize the metal used.
  • An important modifier is the energy available from oxidation of the
  • volatized metal i.e. that can contribute to the volatizing process
  • the metal preferred for layer 178 is aluminum, which can be applied by the process of vacuum metallization (most
  • any metal or metal mixture, including alloys, that can be deposited on base coat 176 can be made to work, a consideration since the sputtering process can then deposit mixtures, alloys, refractories, etc.
  • the thickness of the deposit is a variable that can be expanded outside the indicated range. That is, it is possible to image a plate through a 1000 Angstrom layer of metal, and to image layers less than 100 Angstroms thick. The use of thicker layers reduces the size of the image formed, which is desirable when resolution is to be improved by using smaller size images, points or dots.
  • the primer layer 186 anchors the ink repellent silicone coating 184 to the thin metal layer 178.
  • Effective primers include the following:
  • Silanes and titanates are deposited from dilute solutions, typically 1-3% solids, while polyvinyl alcohols, polyimides, and polyamides-imides are deposited as thin films, typically 3 +/- 1 microns.
  • dilute solutions typically 1-3% solids
  • polyvinyl alcohols, polyimides, and polyamides-imides are deposited as thin films, typically 3 +/- 1 microns. The techniques for the use of these materials is well known in the art.
  • the plates disclosed in the aforementioned U.S. Patent 4,596,733 use a silicone coating as a protective surface layer. This coating is not formulated to release ink, but rather is removable to allow the plates to be used with dampening water applied.
  • the silicone coating here is preferably a mixture of two or more components, one of which will usually be a linear silicone polymer terminated at both ends with functional
  • the second component will be a multifunctional monomeric or polymeric component reactive with the first component. Additional components and types of functional groups present will be discussed for the coating chemistries that follow.
  • Condensation Cure Coatings are usually based on silanon (-Si-OH) terminated polydimethylsiloxane polymers (most commonly linear). The silanol group will condense with a number of multifunctional silanes. Some of the reactions are:
  • Catalysts such as tin salts or titanates can be used to accelerate the reaction.
  • Use of low molecular weight groups such as CH 3 - and CH 3 CH 2 - for R 1 and R 2 also help the reaction rate yielding volatile byproducts easily removed from the coating.
  • the silanes can be difunctional, but trifunctional and tetrafunctional types are preferred.
  • Condensation cure coatings can also be based on a moisture cure approach.
  • the functional groups of the type indicated above and others are subject to hydrolysis by water to liberate a silanol functional silane which can then condense with the silanol groups of the base polymer.
  • a particularly favored approach is to use acetoxy functional silanes, because the byproduct, acetic acid, contributes to an acidic environment favorable for the condensation reaction.
  • a catalyst can be added to promote the condensation when neutral byproducts are produced by hydrolysis of the silane.
  • silanol terminated polydimethylsiloxane polymer is blended with a polydimethylsiloxane second component to produce a coating that can be stored and which is catalyzed just prior to use. Catalyzed, the coating has a potlife of several hours at ambient temperatures, but cures rapidly at elevated temperatures such as 300°F.
  • Silanes, preferably acyloxy functional, with an appropriate second functional group can be added to increase coating adhesion.
  • Addition Cure Coatings are based on the hydrosilation reaction; the addition of Si-H to a double bond catalyzed by a platinum group metal complex.
  • the general reaction is:
  • Coatings are usually formulated as a two part system composed of a vinyl functional base polymer (or polymer blend) to which a catalyst such as a chloroplantinic acid complex has been added along with a reaction modifier(s) when appropriate (cyclic vinyl-methylsiloxanes are typical modifiers), and a second part that is usually a polymethylhydrosiloxane polymer or copolymer.
  • a catalyst such as a chloroplantinic acid complex
  • cyclic vinyl-methylsiloxanes are typical modifiers
  • Typical base polymers are linear
  • Silicone polymers for this approach utilize cycloaliphatic epoxy functional groups.
  • a free radical cure mechanism is used, but requires a high level of inerting to achieve an adequate cure.
  • Silicone polymers for this approach utilize acrylate functional groups, and can be crosslinked effectively by multifunctional acrylate monomers.
  • Preferred base polymers for the surface coatings 184 discussed are based on the coating approach to be used.
  • preferred polymers are medium molecular weight, difunctional polydimethylsiloxanes, or difunctional polydimethyl-siloxane copolymers with
  • dimethylsiloxane composing 80% or more of the total polymer Preferred molecular weights range from 70,000 to 150,000. When a 100% solids coating is to be applied, lower molecular weights are desirable, ranging from 10,000 to 30,000. Higher molecular weight polymers can be added to improve coating properties, but will comprise less than 20% of the total coating.
  • preferred second components to react with silanol or vinyl functional groups are polymethylhydrosiloxane or a polymethylhydrosiloxane copolymer with dimethylsiloxane.
  • selected filler pigments 188 are incorporated into the surface layer 184 to support the imaging process as shown in FIG. 4F.
  • the useful pigment materials are diverse, including:
  • Preferred particle sizes for these materials are small, having average or mean particle sizes considerably less than the thickness of the applied coating (as dried and cured). For example, when an 8 micron thick coating 184 is to be applied, preferred sizes are less than 5 microns and are preferably, 3 microns or less. For thinner coatings, preferred particle sizes are decreased accordingly. Particle 188 geometries are not an important consideration. It is desirable to have all the particles present enclosed by the coating 184 because particle surfaces projecting at the coating surface have the potential to decrease the ink release properties of the
  • Total pigment content should be 20% or less of the dried, cured coating 184 and preferably, less than 10% of the coating.
  • the ink repellent silicone surface coating 184 may be applied by any of the available coating processes.
  • This coating can also be applied as a 100% solids coating (same formula without solvents) via offset gravure and cured using the same conditions.
  • electrode 58 is pulsed, preferably negatively, at each image point I on the surface of the plate.
  • Each such pulse creates a spark discharge between the electrode tip 58b and the plate, and more particularly across the small gap d between tip 58b and the metallic underlayer 178 at the location of a particle 177 in the base coat 176.
  • the repellent outer coat 184 is thinnest. This localizing of the discharge allows close control over the shape of each dot and also over dot placement to maximize image accuracy.
  • the spark discharge etches or erodes away the ink repellent outer layer 184
  • the pulses to electrode 58 should be very short, e.g. 0.5 microseconds to avoid arc "fingering" along layer 178 and consequent melting of that layer around point I.
  • the total thickness of layers 178, 182 and 184, i.e. the depth of well I', should not be so large relative to the width of the image point I that the well I' will not accept conventional offset inks and allow those inks to offset to the blanket cylinder 14 when printing.
  • Plate 172 is used in press 10 with the press being
  • ink roller 22a will adhere to the plate only to the image points I thereby creating an inked image on the plate that is transferred via blanket roller 14 to the paper sheet P carried on cylinder 16.
  • a suitable conductive material for layer 184 should have a volume resistivity of 100 ohm centimeters or less, Dupont's 200 ⁇ C600 Kapton brand film beingone example. This is an experimental film in which the normally nonconductive material has been filled with conductive pigment to create a conductive film.
  • the base coat 176 may also be made conductive by inclusion of a conductive pigment such as one of the preferred base coat pigments
  • the substrate 174 may be a film with a textured surface that forms those peaks.
  • Polycarbonate films with such surfaces are available from General Electric Co.
  • Another possibility is to coat the olephobic surface layer directly onto a metal or conductive plastic substrate having a textured surface so that the substrate forms the conductive peaks.
  • a silicon-coated textured chrome plate has been successfully imaged in accordance with our process. It is also feasible to provide a textured surface on the surface layer so that the spark discharges are localized at the peaks defined by that texturing.
  • All of the lithographic plates described above can be imaged on press 10 or imaged off press by means of the spark discharge imaging apparatus described above.
  • the described plate constructions in toto provide both direct and indirect writing capabilities and they should suit the needs of printers who wish to make copies on both wet and dry offset presses with a variety of conventional inks. In all cases, no subsequent chemical processing is required to develop or fix the images on the plates.
  • the coaction and cooperation of the plates and the imaging apparatus described above thus provide, for the first time, the potential for a fully automated printing facility which can print copies in black and white or in color in long or short runs in a minimum amount of time and with a minimum amount of effort.
  • FIG. 5A illustrates this effect, which is an inherent consequence of the geometry involved.
  • a spark which makes contact with a surface at points 201 will produce surface effects extending radially over a given distance, resulting in circular imaged areas 200. If these areas barely make contact with one another, area 202 will remain unexposed despite its presence within the image area.
  • This difficulty may be overcome by using a more powerful pulse, thereby producing a larger imaged area; or by increasing the number of pulses per unit linear distance as the electrode moves along the plate surface. With either technique, circular imaged areas 200 are made to overlap as shown in FIG. 5B.
  • the increase in the diameter of the imaged areas required to fill area 202 is easily calculated. If the distance between points 201 in the case where circular imaged areas 200 just touch is defined as D, the minimum increased diameter will be D/2.
  • Reference numeral 200a represents the first circular image area produced by the spark, which is burned normally. However, when second circular image area 200b is burned, the area is found extend over additional area 206 even though the spark has been directed to the center of circular area 200b.
  • an image point of a given size may be produced using a relatively low spark energy, because the energy released by the oxidation reaction (triggered by the spark) itself contributes to
  • the volume resistivity of the conductive material must be chosen with care. If the resistance is too great, an
  • resistivity of the conductive layer is partially a function of the plate surface layer or layers.
  • resistivities to be in the range of .5 to 1000 ohm-cm. This range has been found effective with aluminum and copper plate surfaces over a range of image point sizes.

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  • Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Thermal Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Manufacture Or Reproduction Of Printing Formes (AREA)
  • Printing Plates And Materials Therefor (AREA)

Abstract

Procédé de maîtrise de la dégradation indésirable de points d'image se chevauchant dans une plaque lithographique imagée par étincelle. On place une feuille conductrice appropriée présentant une résistivité en volume sélectionné de manière appropriée, en-dessous de la feuille métallique conductrice de la plaque, éliminant ainsi l'énergie d'étincelles excédentaire pendant le processus d'imagerie.
EP19900914611 1989-09-21 1990-09-21 Procede et moyen de maitrise du surcuit en lithographie imagee par etincelles Withdrawn EP0445273A1 (fr)

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US5188032A (en) * 1988-08-19 1993-02-23 Presstek, Inc. Metal-based lithographic plate constructions and methods of making same
US5165345A (en) * 1988-08-19 1992-11-24 Presstek, Inc. Lithographic printing plates containing image-support pigments and methods of printing therewith
IL98453A (en) * 1991-06-11 1996-06-18 Scitex Corp Ltd Method and device for creating a control bar
EP0802067B1 (fr) * 1995-11-08 2002-04-24 Toray Industries, Inc. Plaque planographique originale, sans eau, pour dessin direct

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BE466287A (fr) * 1939-01-27
US3263604A (en) * 1962-01-12 1966-08-02 Timefax Corp Electro-responsive blanks
GB1134742A (en) * 1964-11-23 1968-11-27 Gestetner Ltd Improvements in or relating to the production of planographic offset plates
US4488158A (en) * 1982-01-22 1984-12-11 Exxon Research & Engineering Co. Electrosensitive recording medium
US4911075A (en) * 1988-08-19 1990-03-27 Presstek, Inc. Lithographic plates made by spark discharges

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