CA1038692A - Method of making a printing plate from a porous substrate - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A thermoplastic plate, for example one of polypropylene or nylon, fabricated so it has an open-cell structure, has a radiation transparent cover sheet applied to one face thereof. The cover sheet has an energy absorbing coating (e.g. of carbon and nitrocellulose) in intimate contact with the plate. A modulated laser beam is then trans-mitted through said cover sheet to selectively transfer some of the energy absorbing material to the plate according to the configuration required to define the areas of relief desired in the plate. The cover sheet is then removed except for the portion of the energy ab-sorbing coating transferred to the plate. The entire surface of the plate is then exposed to infra-red rays. The portions of said surface to which energy absorbing material was transferred are elevated in temperature, by the absorbed infra-red energy, to the point that the structure beneath the transferred material collapses, thus causing those portions to sink to a plane below the plane of the other portions of the plate. Additional infra-red heating steps may be performed to produce further collapse of said portions of the surface, as herein-after more fully explained.

Description

This i8 a division of application Serial No. 222,238 filed March 17~ 1975.
This invention relates to a method and apparatus for making a printing plate from a porous substrate.
It has previously been proposed to produce a printing plate by selectively collapfiing the open cell structure of a thermoplastic plate to provide relief (depression oE non-printing areas), and thereby to define the non-depreased portions necessary for performing a printing operation.
It is an ob~ect of this invention to carry out the foregoing basic method in a more effective manner and at lower cost.
It iB a further obiect of the invention to achieve a more complete collapse of the cell structure in the areaa ~here relief i8 desired, and to better define the planar difference between the raised and relief portions of the plate.
I will first summarize the basic invention and then I will explain certain inventive improvement~ that may be applied to the basic invention.
A low-energy ab~orbing thermoplastic printing plate, having an open-cell structure, has energy absorbing material selectlvely applied to thofie areas of its surface where relief (depression of non-printing areas) is desired. The plate 18 then exposed to infra-red energy to collapse the cells in said areas and provide relief in the plate.
Alternatively, the plate may have high energy absorbing characteristicfi if the portions thereof to which said material is applied are thereby given low energy absorbing characteristic~.
Accordingly there is provided in accordance with the present invention a method of making an open-cell thermoplastic plate into a printing plate comprising bringing a coating material into contact with a surface of the plate, causing said material to adhere to the plate only in selected areas, whereby to form coated and uncoated areas of the plate, to .~
-1- ~

~L038692 thereby create a printing configuration and change the absorptivity of said selected areas to radiant energy, and applying radiant energy to said plate to cause the coated and uncoated areas of the surface to be heated differentially, relieving one of ~aid areas to create a printing configuration on said surface.
There is also provided in accordance with the present invention an apparatus for making a printing plate from an open-cell type of thermoplastic plate comprising mean6 for selectively applying a coating to the surface of said plate to form thereby a replica of the printing configuration desired on the surface of the plate, means for heating said plate to a temperature just below the temperature at which the thermoplastic radically changes viscosity, and meana for e~posing the heated plate to infra-red radiation to collapse the structure under said coating without collapsing the structure not covered by the coating.
~aving thus de8cribed the basic concept of the invention, I
will now describe several inventive improvements which may be applied to the basic concept:
The thermoplastic printing plate, at the fitart of the process, may be polypropylene, nylon, or other similar material.
~ithin the scope of the ba~ic invention described above, the said "material" may be selectively applied to the plate in any suitable way. Two such ways, each of which is an improvement upon the basic concept, will now be described. First, a cover sheet applied to the plate may have "material" in the form of a coating which, when exposed to radiant energy directed through the cover sheet, is transferred to the plate. Secondly, the cover sheet may include the "material" and will transfer it to the plate when the cover sheet is impressed with a mechanical force (such as when one draws on a sheet of carbon paper or causes the type bar of a typewriter to strike the ribbon to effect a ~03B69Z
transfer of an image~, Preferably the transfer of the "material" to the plate should be an "impact" type of transfer, A typical and suit-able impact transfer method will now be described.
According to a further improvement, the radiation transparent cover she~t on the plate 18 polyethylene terephthalate (sold under the trade mark Mylar by ~.I. du Pont de Nemours & Co.) and this cover sheet has a coating of an energy absorbing material such as carbon and nitrocellulose, in contact with one surface of the plate. The coating is maintained in intimate contact with the plate in any suitable way, such as by applying a vacuum to the opposlte surface of the porous open celled plate. A beam, of suitable radiation and power~ such as from a laser, then traverses those areas of the plate where relief i8 desired and transfers a portion of the coating to the surface of the plate.
The Mylar layer is then removed, leaving a pattern of coating material that has been transferred to the surface of the plate. The plate can then be glven an infra-red eYposure, to selectlvely collapse ant seal the areas where rellef is desired, thereby providng shallow rellef in the order of 0.0003 to 0.01 i~ches.
Another improvement upon the invention includes a second treatment of the plste with infra-red energy to achieve a more complete collapse of the cells in the areas of relief. If a cooling fluid, such as air, is passed through the plate during thl~ second treatment, it will selectively cool the printing areas, assisting in the prevention of cell collapse of those areas. Since the cells have at least partially collapsed and become sealed in the areas where relief is desired, the cooling fluid will not keep these cells cool, and they will be heated to a degree necessary to achieve the desired relief.
Another improvement is that either or both of the treatments of the plate with infra-red energy is preferably carried out after the plate has been raised to a temperature just below that at which the plate material undergoes a radical change of viscosity. Following such a preheating of the plate, the infra-red energy falling on the deposited ~038692 energy-absorbing coating on the~surface of the plate adds energy to those areas of said surface on which some of the coating has been deposited.
This energy causes the structure under said areas to collapse, thus resulting in a relief of the surface in those areas.
Either or both of the infra-red treatments may be improved by using feed-back. This may be accomplished by having an infra-red source that scans the surface of the plate and supplies energy to it. The reflectivlty of the surface is continuously determined as the scanning proceeds, and the infra-red source intensity is increased in those areas where the reflectivity is low (areas where deep relief in the surface i8 desired) and the source intensity is reduced in those areas where the reflectivity is high (areas where shallow relief is desired).
A further improvement results by filtering out, from the infra-red rays applied to the plate, those wavelengths to which the plate (without treatment with energy absorbing material) has maximum absorptlon.
This still further enhanceR the thermal contrast between the areas of the plate wlth the energy absorbing coating and the areas without the coating.
The present tivisional application is directed to the method of increasing the relief existing in an interconnected open-celled thermo-plastic plate, which rellef was achleved by at least partlal sealing of the open-cell struct~re of the plate, comprising pAssing a coollng fluid through the remaining interconnected open-cell structure, and heatin8 the plate to effect further collapse of the cells.
The invention is illustrated by way of example with reference to the accompanying drawings wherein:
Figure 1 iQ a side view of Step I of the process;
Figure 2 is a perspective view of Step II of the process;
Figure 3 is a side view of Step III of the process;
Figure 4 i8 a side view of one optional form of Step IV of the process;
Figure 4A is a first alternate form for carrying out Step rv of the process;

Figure 4B i9 a second alternate form for carrying out Step rv of the process;
Figure 5 is a side view of apparatus for carrying out Step V

of the process. (This step is optional, but its inclusion is an improvement~;
Figure 5A is a greatly enlarged view of a portion of plate 10 of Figure 5;
Figure 6A illustrates Step VI and shows how the resulting plate may be inked for letterpress or letterset printing;
Figure 6B illustraees a modified form of Step VI and shows how the plate may be inked for screen printing; and Figure 7 illustrates modified apparatus for carrying out Steps IV and/or V.
The plate 10, after the processing hereinafter described, becomes the printing plate. At the start, this is a plate fabricated of polypropylene, nylon, or other thermoplaatic material. Preferably the plate 10 should exhibit a sharp transition between its solid and its semi-solid Ytates as its temperature rises. This characteristic is exhibited by polypropylene between 150 and 180C. If the msterial has the desired sharp transition, and is preheated to a temperature ~ust below that at ~hich the plate becomes semi-solid, a further sur-face temperature rise of several degrees Centigrade, resulting from exposure to infra-red rays, will ca~se structural collapse in the plate and cauae the portions of the plate exposed to the infra-red energy to sink below the surface of the plate by 0.0003 inch or more.
In other ~ords, the plate material should have a high "melt index".
The melt index is sufficiently "high" for the purpose of this invention if it is greater than 3.
It is also preferable that plate 10 have an interconnected open-cell structure, to permit transpiration cooling. This can be easily achieved by preparing the plate in accordance wlth the instructions specified in lines 57 et seq., of column 3, of my ~.S.
Patent No. 3,779,779 entitled "Radiation Etchable Plate", issued December 18, 1973.
In the first step of the method, a radiation transparent cover sheet 11 of polyethylene terephthalate ~sold under the trade 10;~9Z
name Nylsr~, haYing an energy absorbing coating 12, $Uch a~ a mixture of carbon and nitrocellulose, on its underside, is placed in intimate contac~ with the upper side of plate 10. Thu6 the carbon and nitro-cellulo~e coating is in direct contact with the upper surface of plate 10..
The afore~aid intimate contact may be maintained in any suit-able way, such as by applying a vacuum to the underside of open-celled plate 10, or by applying electro~tatic charge(s) to one or both of plate 10 and/or cover sheet 11.
In ~tep IT, the plate 10, with its cover sheet 11, is next exposet to a very fine laaer beam of infra-red energy, ~hlch is scanned across the plste and modulated as necessary to tranfifer the information to be printed to plate 10. This is done ln accordance with Figure 2 of my prior ~.S. Patent No. 3,739,088, granted June 12, 1973, and entitled "Printing Plate Productlon Method and Apparatu ". Figure 2 of that patent i8 reproduced here (as Plgure 2~ except that in the present drawlng the cover sheet 11, bearing energy absorbing coating 12 thereon, is superimposed on plate 10.
In the apparatus illustrated in Figure 2, the pa6te-up lS and

2~ plate 10 are supported in curved conditlon concentrically relative to the axis of an elongated rotatlng double scanning assembly 18. The lasers 16 and 17 are carried at Qpposlte ends of assembly 18 for their beams to be deflected by rotating angular mirrors 19 and 20 through focusing lenses 21 and 22 to impinge respectively on the paste-up 15 and the plate 10.
AB indicated by thearrows 24a and 24, the mirror and len~
is rotated by a drive mechanism 23 and is simultaneously moved axially by suitable translational drive means such as a linear induction motor 80 that the beams from lasers 16 and 17 scan along a spiral path. The entire scanning assembly is suitably mounted on an air bearing member.
The beam from the laser 16 as focussed on the paste-up 15 by the lens 21 is reflected back to a detector 25 which converts the reflected light of the bea~ into electric signals ~hose inten~ities are proportional to the intensity of the reflected light received. The detector 25 is suitably a photomultiplier, or photodiode, and is connected to actuate a modulator 34. The modulator 34 is connected to modulate the intensity of the beam from the laser 17 in a binary man-ner corresponding to the signals received from the detector 25 for reproducing a template on the plate 10 corresponding to the material represented on the paste-up 15 as described above.
The laser 16 is suitably a neon heli~m laser which has an operating wavelength of 0.6328 microns, and the lens 21 is selected to focus the beam from laser 16 into a spot of about 0.001 inch dia-meter on the paste-up 15.
~hen the beam from laser 17 passes through lens 22 and im-pinges on transparent cover sheet 11, a portion of the energy absorbing carbon and nitrocellulose coating 12 is transferred to the plate 10, where it forms a pattern, normally as a negative of the material to be printed, afi will appear.
My prior U.S. Patent 3,816,659, for l'Scanning Apparatus", issued June 11, 1974, contains suggestions that may be helpful in constructing the apparatu shown in Figure 2 of the present application.
The polypropylene plate 10 is formulated to exhibit minimum infra-red absorption. However, where the laser beam has transferred carbon and nitrocellulose to the plate, the absorption of infra-red energy will be much greater. Thus, in response to the infra-red heat-ing steps described below, the energy absorbing portions of the plate will be heated more than the untreated portions of the plate.
The vacuum previously described in connection with Step I
may be continued during Steps II and III.
Step III consists merely of peeling cover sheet 11 from plate 10, as shown in Figure 3. This leaves that portion 12a of coating 12 ch ~as transferred to plate 10 intact on that plate.
Instead of employing a polypropylene plate 10 with minimum ~1038692 infra-red absorption, and a coating of carbon and nitrocellulose to increase the absorption, the reverse may be done. That is, one may fabricate a polypropylene plate 10 ~ith maximum absorption and a coat-ing 12 that will reduce the absorption of the plate 10 in the areas to which the coating is transferred. In event such a reversal is employed, the ~riting step should also be reversed so that transfer of the coat-ing occurs in the areas which will receive ink and print the desired text, insteat of in the areas of relief (non-printing areas~.
Furthermore, instead of using a carbon and nitrocellulose coating 12 and a laser beam, various other energy absorbing coatings and methods of transferring the same may be employed. Transfer to the plate 10 may be accomplished in any suitable way, including any suit-able mechanical method. For example, the pressure transfer of a carbon coating from carbon paper, or of heat-absorbing ink from a typewriter ribbon, may be used. Furthermore, suitable thin metallic foils may be used as energy reflecting material, and methods of transferring quch metallic foils to other objects may be used to transfer such thin metallic foils to plate 10. Other suitable coatings and transfer tech-niques are described in U.S. Patent No. 3,745,586, issued July 10, 1973 to Robert S. Braudy for "Laser Writi~g", U.S. Patent No. 3,787,210, issued January 22, 1974 to Donald Lee Roberts for "Laser Recording Technique Using Combustible Blow-Off", and Woodward, IBM Technical Disclosure Bulletin Vol. 9, No. 11, April 1967, page 1592. Preferably the transfer of the coating to the plate should be by an impact method, several of whlch methods have been referred to above.
Step IV comprises directing infra-red or other suitable radi-ant energy onto the imaged surface of plate 10. The time of application, and the intensity of this energy, are carefully selected so that the areas of the surface of plate 10 to which carbon and nitrocellulose 12a have been selectively transferred change viscosity. Consequently, the open-cell structure under such areas collapses, causing the surface in such areas to sink below the surface of the printing areas, which remain 103869~
solid since the tempe~ature to which they are heated is lower. To facilitate this, the plate ~ay be pre-heated in an oven or by trans-piration methods to a temperature just below the thermoplastic transition temperature, 90 that the lnfra-red heating step may then be of short duration. This limits the conduction process in the plate, and is there-fore a desirable result since heat conduction in the plate, when part of the plate has reached a semi-solid atate, reduces the resolution of the resulting printing plate.
I will next describe three ways that the infra-red heating step, just referred to, may be carried out:
1. As shown ln Figure 4, the upper side of plate 10 may be exposed to an infra-red source 30 which heats the entire upper surface of plate 10 simultaneously.
2. As shown in Figure 4A, the plate 10 may be held in oven 31 until it achieve~ a temperature ~ust below the transition temperature. It is then moved to the right under the elongated heater such as sold under the trade mark Calrod (or other electric heater in the form of a long rod).
The heater 32 may have a suitable reflector 32R to concentrate its heating power along a very limited but ~traight segment of plate 10. As a given segment of plate 10 passes under heater rod 32, that portion of the segment having the carbon and nitrocellulose coating transferred thereto is heated more, by the absorption of energy. This collapses the structure of the plate under the coated areas of that segment.
If plate 10 has the necessary ~harp transition from a solid to a semi-solid state, and the other desired characteristics explained above, and if the heater 32 emits suitable energy toward the plate 10, a cell collapse, sufficient to cause the surface of plate 10 to sink about 0.0003 to 0.01 inches in the areas to which carbon has been transferred, will occur as a result of an exposure to the infra-red rays for about one second. The preferred speed of plate 10 past the infra-red heater 32 will give the plate an exposure for about one second.

3. As shown in Figure 4B, the preferred way of heating the _ g _ ~038692 plate is by a controlled beam of infra-red energy, such a9 the beam of a tungsten halogen lamp (such as sold by General Electric under the trade mark Quartzline, Type DYS, rated at 600 watts and 120 volts), that scans the surface of plate 10, Energy reflected by the surface of plate 10 operates detector 71 to provide the input to control apparatus 72, which controls radiant source 70 to lncrease the beam intensity incident upon those areas where the plate surface has a large heat absorptivity due to the transferred coating 12 and to decrease the intensity where the plate surface is uncoated and has a low heat absorptivity. Apparatus for determining the surface reflectivity and for controlling the beam is shown in Cra~g U. S. Patent 2,342,025, issued July 8, 1958, entitled "Photo-graphic Method", and in Folse U. S. Patent 3,036,497, issued May 29, 1962, entitled "Photographic Dodging Apparatus".
Step V of the process, shown in Figure 5, is an improvement, and will now be described. After Steps I through III have been completed, the plate 10 i8 passed under Calrod heater 32. The infra-red energy from rod 32 passes through filter 33, which may be made of the same material as the plate 10. The filter 33 ls therefore particularly absorbent to the ratiant energy which has optlmum heating effect on those portions of plate 10 which have had no part of the carbon and nitrocellulose coating 12 transferred thereto. This enhances the differential heatlng effect between the coated portions o~ the surface of plate 10 (the portions to whlch some of said coating 12 has been transferred) and the uncoated portions of said surface, resulting in a more complete collapse of the cell structure under the coated portions. This step will not, however, create any collapse of the cell structure of those areas to which no part of the coating 12 was transferred.
As shown in Figure 7, the filter plate 33 is rotated by motor 34 past the outlet of cold air 35. Hence, any heat from the radiant energy source 32 (directed through filter plate 33 at plate 10) which has been absorbed by filter plate 33 is dissipated without significantly elevating the temperature of the filter plate 33.

If a vacuum i~ applied to the underside of plate 10 during Step V, air will be induced to flow through the open cells in the surface of plate 1~, that is, through the cells in the areas to which no part of coating 12 was transferred. Since the other surface cells have at least partially collapsed, the air flow through them will be wholly or partially impaired. The transpiration cooling therefore enhance~ the local temperature differences. It does not interfere with collap~e of cells in the areas to which some of the coating 12 was trans-ferred, and may, in fact, enhance the cell collapse as a result of the presaure gradient created. On the other hand, air does flow through those portions of the upper surface of plate 10 ~here there haa been no collapse of the cell structure, thus keeping those portions cool and free from collapse.
Instead of applying a vacuum to the lower side of the plate, to generate the above-mentioned air flow, any ~uitable ~ir pressure differential may be applied acrofi~ the plate.
Figure 5A is a greatly enlarged sectional vlew of Figure 5.
It ia noted that the upper surface of the plate 10 has printing portions 50 and areas of relief Sl. The cells 52 in the printing portions SO
have not collapsed and are interconnected wlth the open cells 53 in the body of the plate. The cells 54, ~ust beneath each area of relief, have, however, collapsed and are at least partly sealed against trans-mission of air therethrough.
If the process is carried out as aforesaid, a printing plate fiuitable for letterpress or letterfiet ~ork is produced and may be inked by a roller 80, as shown in Step VI, Figure 6A.
For screen pr:inting (Figure 6B), the ink may be forced through the plate from the side which does not contact the paper to the printing side. The ink will travel through the non-collapsed portion of the cell structure to the raised printing portions on the plate and will thus wet those portions with ink. Ink will not, however, pass through those relieved portions of the plate where the structure has been sealed.

If the starting plate 10 of Figure 1 is composed of urethane rubber (e.g " a product of B.F. Goodrich Co. designated 58105 and sold under the trade mark Estane) the end product (after Steps I to V) will be su~table for flexographic printing and may be inked as shown in Figure 6A.

Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of increasing the relief existing in an inter-connected open-celled thermoplastic plate, which relief was achieved by at least partial sealing of the open-cell structure of the plate, comprising passing a cooling fluid through the remaining interconnected open-cell structure, and heating the plate to effect further collapse of the cells.

2. The method of claim 1 in which the heating step comprises exposing the surface of the plate in which there is relief to infra-red radiant energy to heat that surface of the plate.

3. The method of claim 2 in which the fluid flowing through the interconnected open-cell structure is cooler than the plate and keeps the portion of the plate wherein there is no relief cooler than the portion of the plate where relief is desired.

4. The method of claim 3 in which the fluid flow through the plate increases the pressure gradient between the portion of the plate where relief is desired and the portion of the plate where cell collapse is not desired.

5. The method of claim 4 in which the surface of the plate has been so treated that the portion of the surface of the plate where relief is desired has greater absorption of infra-red rays than the portion wherein relief is not desired.

6. The method of increasing the relief existing in an interconnected open-celled thermoplastic plate, which relief was achieved by at least partial collapse of the open-cell structure of at least one limited area of the plate, comprising exposing one side of the plate to a cooling fluid and providing a pressure differential to attempt to move the fluid through the inter-connected open-cell structure, while heating the plate to effect further collapse of the cells, said cooling fluid being obstructed from moving through the cells beneath said limited area due to said collapse of those cells, whereby said last-named heating step will further collapse the cells beneath said limited area without causing such collapse beneath other areas.

7. The method of claim 6 in which the heating step comprises exposing the surface of the plate in which there is relief to infra-red radiant energy to heat that surface of the plate.

8. The method of claim 7 in which the fluid flowing through the interconnected open-cell structure is cooler than the plate and keeps that portion of the plate that is outside said limited area cooler than the portion of the plate beneath said limited area.

9. The method of claim 8 in which the fluid flow through the plate increases the temperature differential between the portion of the plate where relief is desired and the portion of the plate where cell collapse is not desired.

10. The method of claim 8 in which the surface of the plate has been so treated that the portion of the surface of the plate outside said limited area has greater absorption of infra-red rays than the portion within said limited area.

11. In a method of making a printing plate in which an open-celled thermoplastic plate has been selectively coated on a first side with a heat absorbing coating and then heated to provide sealed, partially collapsed areas and unsealed, non-collapsed areas, the improvement comprising:
(a) exposing one side of said plate to a cooling fluid and providing a pressure differential to attempt to move the fluid through the plate so that the sealed, collapsed areas provide impedance to said movement of said cooling fluid and the unsealed, non-collapsed areas allow the cooling fluid to pass therethrough, and (b) applying infra-red heating to said one side while said cooling fluid and pressure differential are also applied to said plate, to effect further collapse of the partially collapsed cells without causing further collapse of the non-collapsed cells.

12. The method of providing relief in an open-celled thermoplastic printing plate comprising:
a first heating step for at least partially collapsing the cells in a first set of selected areas of the plate, a subsequent, second heating step comprising heating the plate to effect cell collapse while passing a cooling fluid through the open-celled structure of limited, selected areas of the plate to prevent cell collapse in those areas.