US3662050A - Method for making molded printing plates - Google Patents

Abstract

AN IMPROVED DUPLICATE PRINTING PLATE OF THE LETTER PRESS CLASS IS MADE FROM A GELLED SHEET OF FLUID PRINTING PLATE MATERIAL. THE GELLED SHEET IS REQUIRED TO BE PREPARED UNDER SELECTIVELY CONTROLLED HEATING CONDITIONS. THE SELECTIVELY HEATED PRINTING PLATE MATERIAL THUS PRODUCED IS CHARACTERIZED BY BODY AND HANDLING STRENGTH SUCH THAT IT MAY BE MANIPULATED AND CONTAINED IN AN EVACUTATED MOLDING PRESS. IN THE PRESS THE MATERIAL IS SUBJECTED TO HEAT AND PRESSURE WHILE IN CONTACT WOTH A FEMALE MOLD. AN IMPORTANT CONSIDERATION IS CONTROL OF HEATING TO MAINTAIN THE GELLED SHEET IN A STATE OF LIMITED PLASTICITY SO THAT COLD

FLOWING IS POSSIBLE AT THE TIME THE SHEET IS INTRODUCED INTO THE PRESS. IN THIS LIMITED STATE OF PLASTICITY THE MATERIAL THROUGHOUT ITS LAYER THICKNESS MAY, UNDER SUITABLE HEAT AND PRESSURE, BE DISPLACED INTO INTIMATE CONTACT WITH THE MOLD DEPRESSIONS AT ALL POINTS THEREIN. CONTINUING HEAT AND PRESSURE THEN CONVERTS THE DISPLACED PLASTIC OR GELLED MATERIAL INTO A COMPLETELY FUSED AND CURED RUBBER-LIKE MASS TO PRODUCE A PERMANENTLY MOLDED LETTER PRESS PRINTING PLATE WHICH IS FREE FROM STRESSES.

Description

y 9, 1972 J. A. WILLETT 3,662,050

METHOD FOR MAKING MOLDED PRINTING PLATES Filed April 19, 1968 4 Sheets-Sheet 1 May 9, 1972 J. A. WILLETT 3,652,050

METHOD FOR MAKING MOLDED PRINTING PLATES Filed April 19, 1968 4 Sheets-Sheet 2 May 9, 1972 wlLLETT 3,662,050

METHOD FOR MAKING MOLDED PRINTING PLATES Filed April 19, 1968 4 Sheets-Sheet 3 by 7( WM y 9, 1972 J. A. WlLLETT 3,662,050

METHOD FOR MAKING MOLDED PRINTING PLATES Filed April 19, 1968 4 Sheets-Sheet 4 Fig.8.

United States Patent 3,662,050 METHOD FOR MAKING MOLDED PRINTING PLATES John A. Willett, 21 Benvenue St., Wellesley, Mass. 02181 Filed Apr. 19, 1968, Ser. No. 722,731 Int. Cl. B29c 1/02, 25/00; B29d 9/02; B41c 3/06 US. Cl. 264102 6 Claims ABSTRACT OF THE DISCLOSURE An improved duplicate printing plate of the letter press class is made from a gelled sheet of fluid printing plate material. The gelled sheet is required to be prepared under selectively controlled heating conditions. The selectively heated printing plate material thus produced is characterized by body and handling strength such that it may be manipulated and contained in an evacuated molding press. In the press the material is subjected to heat and pressure While in contact with a female mold. An important consideration is control of heating to maintain the gelled sheet in a state of limited plasticity so that cold tflowing is possible at the time the sheet is introduced into the press. In this limited state of plasticity the material throughout its layer thickness may, under suitable heat and pressure, be displaced into intimate contact with the mold depressions at all points therein. Continuing heat and pressure then converts the displaced plastic or gelled material into a completely fused and cured rubber-like mass to produce a permanently molded letter press printing plate which is free from stresses.

This invention is concerned with an improved method of making a molded printing plate having desirable rubber-like characteristics commonly present in conventional duplicate printing plates mad from synthetic or natural rubber.

Duplicate printing plates made from natural or synthetic rubber are molded under heat and pressure to undergo vulcanization. An original of the subject matter to be printed is made by hand setting or machine casting type. The original is then placed between heated platens of a vulcanizing press and preheated. A sheet of molding board impregnated with a thermo-setting resin is then placed on the original and the press is closed forcing the original type faces into the molding board. The charge is held under heat and pressure for about 8 to 12 minutes to complete cure of the molding board. This provides a female mold which is heat and pressure resistant to the. degree required for molding natural or synthetic rubber.

From the mold thus prepared a printing plate is made. A body of uncured natural or synthetic rubber is placed in contact with the mold depressions and heat and pressure are again applied in the vulcanizing press for the same cure period of from about 8 to 12 minutes to form a duplicate printing plate.

There are several disadvantages inherent in the described rubber molding process. A primary concern is the loss of time and labor involved in the 8 to minute curing period required. Also the uncured materials have a tendency to dark cure, that is they cure slowly so as to become useless as molding material after a time. This necessitates keeping the uncured materials under refrigeration and shelf-like may be lengthened to about 90 days at the most. There is thus involved the problem of inventory control and some loss of material. There may also be experienced poor resistance to many solvents and oil.

It has been attempted by those skilled in the art to overcome these disadvantages by utilizing moldable polymeric compositions commonly referred to as plastics" 3,662,050 Patented May 9, 1972 of which the vinyl compounds are particularly illustrative. A plastic such as a conventionally cured vinyl sheet offers several dsirable characteristics over rubber. However, a conventionally cured vinyl sheet does not mold properly and may result in a rejected end product because of non-fill, which is failure of the material to completely fill in the recesses of a female mold and forcing the cured material into the mold depressions tends to set up plate stresses which cause the plate to curl in an objectionable manner.

Non-fill may be explained by reason of the fact thatwhen a conventionally cured vinyl sheet of thermal plastic nature is subject to heating, outer portions of the vinyl sheet tend to become molten before intermediate portions are heated sufficiently to ilow. As a result when pressure is applied to the molten surface, melted material flows too freely at top and bottom sides thereby dissipating material needed to fill depressions in the mold. This condition is aggravated by reason of the fact that the mold acts as an insulator between the vinyl sheet and the lower hot platen and, therefore, the upper side of the sheet may melt faster than the bottom side.

It has also been proposed in the art to use a vinyl type material in a gelled state and laminated with other sheet materials or coatings, or otherwise reinforced. However, gelled materials have not been satisfactorily utilized because the gelled material tends to be lacking in strength and body and cannot be subjected to normal handling technique involved in a vucanizing press operation without breaking, tearing or becoming otherwise dimensionally changed in an objectionable manner.

It is therefore a chief object of the invention to resolve,-

the various problems indicated above in utilizing gelled polymeric compositions in various molded articles as, for example, flexible and rigid plastic printing plates.

Another specific object of the invention is to devise a method whereby an improved duplicate printing plate may be made from a gelled polymeric composition and in the plate thus made desirable characteristics corresponding to those of natural or synthetic rubber plates are substantially realized.

It is also an object to provide a method for making a plastic plate in which can be practically and economically utilized a conventional platemaking vulcanizing press with satisfactory printing quality being obtained.

Another object is to provide a novel process for utilizing a vacuum in a plate-making process.

It is a further object of the invention to provide a method for making duplicate printing plates in a substantially shortened preparation period well below the 8 to 10 minute interval conventionally required and preferably in a range of from 2 to 3 minutes with substantial savings in time and labor costs.

Still another specific object of the invention is to devise a novel method utilizing gelled printing plate material which can be conveniently made and held for substantial periods of time without undesirable deterioration; which is capable of being handled and formed in a molding press without non-fill difiiculties; and which tends to resist excess flowing at temperatures and pressures commonly utilized in conventional vulcanizing presses.

With these objectives in mind, I have devised a method of making duplicate printing plates based on the use of a gelled sheet of printing plate material. In preparing the gelled sheet I may employ various polymeric compositions such as, for example, vinyl compounds and, more particularly, vinyl chloride occurring as a plastisol.

My improved method is essentially based on the concept of forming a gelled sheet and retaining and utilizing the cold molding capabilities of the gelled sheet in order to provide for adequately filling the depressions of a female printing plate mold. An important feature in thus cold molding the gelled sheet is the use of a novel vacuum technique by means of which air pockets may be completely eliminated and all of female mold subject matter may be reproduced.

In one preferred method of making the gelled material I have found that I may also selectively control heating of a formed film of material such as a vinyl chloride plastisol in successive heating steps. A fluid plastisol, having a known formulation and gelling temperature, is first formed on a suitable support and immediately heated to temperatures at which rapid gelling takes place without appreciable fusing. This provides a plastic sheet which retains cold molding capability.

In a second heating step I pre-fuse the gelled sheet to provide at opposite sides thereof thin rubber-like skins of predetermined thickness which reinforce the plastic sheet and envelop the plastic material so that it can be satisfactorily handled and placed in a press. The thickness of the rubber-like skin portions is regulated by controlled heating so that the rubber-like skins and the gelled sheet throughout most of its layer thickness can be cold molded into various shapes and configurations without undue stresses being induced.

In cold molding the gelled sheet a vacuum chamber is created around the gelled sheet and a female mold on which it is supported. A vacuum pump is then operated to completely evacuate air and eliminate air pockets prior to the pressure for cold molding being exerted.

A final heating takes place when the plastic material has been displaced and forced into the depressions of a female mold in a press. Heating at this point is carried out at higher temperatures to cause complete fusing or curing and there is thus produced a rubber-like material which takes a permanent form and which being relatively free from stresses because of its cold molding will lie fiat without any tendency to curl.

Further details as to the nature of the invention and its novel features are hereinafter set forth in greater detail and preferred embodiments of the invention are illustrated in the accompanying drawings, in which:

FIG. 1 is a perspective view illustrating a sheet of gelled material made in accordance with the invention;

FIG. 2 is another perspective view illustrating a duplicate printing plate also made in accordance with the invention from the printing plate material shown in FIG. 1;

FIG. 3 is an exploded view illustrating molding press components and a sheet of printing plate material associated therewith;

FIG. 4 is a cross sectional view further illustrating the molding press components in an open position and with the printing material mounted on a mold member;

FIG. 5 is a view similar to FIG. 4 and further illustrates the molding press components in a closed position indicating a vacuumized step being carried out;

FIG. 6 is another view similar to FIG. 5 illustrating the printing plate material fully molded into the mold member;

FIG. 7 is a detail fragmentary cross sectional view of the mold and the molded material forced into depressions therein;

FIG. 8 is a diagrammatic view illustrating a method of making the printing plate material of the invention;

FIG. 9 is an enlarged cross sectional view of the printing plate material produced by the method illustrated in FIG. 8;

FIG. 10 is a diagrammatic view illustrating a modified method of producing the printing plate material of the invention;

FIG. 11 is a cross sectional view of another modified form of a gelled sheet;

FIG. 12 is a cross sectional view of a modified form of printing plate made from a gelled sheet such as that shown in FIG. 11;

FIG. 13 is a diagrammatic view illustrating printing plate material with a rubber-like skin formed at one side only; and

FIG. 14 is a cross sectional view of a special vacuum plate and seal apparatus employed in FIGS. 5 and 6.

My improved method of making a printing plate consists in providing a gelled material made by forming a layer of a liquid printing plate material such as a vinyl chloride plastisol on a suitable supporting surface. The plastisol is chosen with a known formulation and gelling temperature and immediately upon a film being initiated it is subjected to selectively controlled heating'at a temperature which will induce rapid gelling without appreci able fusing taking place.

There is thus formed a gelled layer which retains a high degree of plasticity throughout its thickness. This layer of plastic material is utilized in accordance with the invention in either one of two difierent ways. It may be processed so as to have shelf-life and to provide reinforced surfaces which facilitate handling and prevent damage or dimensional instability. If the material is not required to have shelf-like and does not require reinforcing surfaces, it may be used without processing.

For many applications in plate-making it is found that better results are obtained and it is therefore preferable to make a reinforced material. In making the reinforced material the layer of gelled plastic is subjected to a controlled degree of heating. This layer of plastic material is subjected to a controlled degree of heating to form at opposite sides of the layer thin fused skin portions of rubber-like consistency. The simultaneous fusing of two sides of the gelled sheet may, for example, be conveniently carried out by passing the sheet between the nip of two heated rolls whose temperature is selectively controlled. This heating step initiates a fusing of the plastic material which is limited in depth to provide skin thicknesses of predetermined value. The two skin portions protectively envelop and contain the plastic gelled material so that the sheet may be handled and manipulated in a printing plate vulcanizing press without damage or dimensional change.

Control of the depth of the fusing to provide the skin portions of a predetermined thickness is accomplished by the use of chilled rolls through which the fused material is caused to immediately pass after leaving the heating rolls.

Thereafter the sheet of pre-fused material is located in a molding press with one of the rubber-like skin portions in contact with the surface of a female mold received in the press. The mold and sheet of material are then sealably enclosed by means of a special vacuum plate apparatus and air is evacuated to remove air pockets at all points between the mold surface depressions and the upper platen of the press. Pressure is then exerted in the press before any appreciable heating takes place. The plastic portion of the sheet material is cold molded and displaced against the rubber-like skin in contact with the female mold so as to force the rubber-like skin portion into intimate contact with the depressions in the mold and hold it there.

Heat and pressure is thereafter continued which converts the relatively thick layer portion of plastic material into a completely fused or cured rubber-like mass while the relatively thin rubber-like skin is held compressed against the mold depressions thereby to form a permanently molded printing surface which is free from stresses and which is an exact reproduction of the subject matter to be printed.

Considering in greater detail one preferred form of printing plate and its method of manufacture, attention is directed to FIG. 1 in which arrow M indicates generally a gelled printing plate material made in accordance with the invention and of the type designed to have shelf-life. As shown in FIG. 1 the material M includes an intermediate layer or core 2 of plastic material and outer protective skin portions 4 and 6 of fused material.

FIG. 8 illustrates diagrammatically apparatus for forming a gelled sheet and pre-fusing the sheet in one preferred form of material which is desired to have shelflife. As noted therein a layer of gelled material M1 is applied by some well-known conventional means on a suitable support such as an endless belt member 30 carried on rolls 32 and 34. 36 indicates a heating device such as a heating oven through which the gelled material is passed to accelerate the gelling operation. Numerals 38 and 40 denote a pair of heating rolls which are capable of being heated to temperatures at and above the temperature at which the gelled material M will fuse. Rolls 42 and 44 denote chilling rolls which are arranged in close proximity to the heating rolls 38 and 40. The chilling rolls are adapted to rapidly cool the fused film and limit the depth of fusing which takes place.

In FIG. 2 there is illustrated a flexible duplicate printing plate P which has been made from the printing material M of FIG. 1, and which is in a completely fused or cured state. FIGS. 3 to 6 inclusive illustrate a method and apparatus for processing the material M to form the printing plate P. FIGS. 11 and 12 illustrate a printing material which is not reinforced to give shelf-life and from which a printing plate has also been made.

The apparatus shown in FIGS. 3 to 6 includes a typical vulcanizing press arrangement with which is combined a special vacuum plate member for evacuating air and other gases from the press when vulcanizing takes place. FIG. 3 illustrates some of the more essential press components occurring in separated relationship and including the upper press platen 10 and a lower press platen 12.

A female mold made as earlier described is denoted by numeral 14 and is formed with depressions 14a. Numeral 16 indicates the vacuum plate of the invention which comprises a rectangularly formed body of metal such as aluminum having at its upper side a sealing member 18 in a groove 18a for enabling the vacuum plate to sealably engage against and solidly contact an underside of platen 10. At its underside the vacuum plate 16 is provided with another relatively larger sealing member 20 which is of a vertical thickness exceeding the thickness of the sheet of printing plate material M.

The dimensional relationship of the sealing member 20 to the printing plate material M is chosen so that the vacuum plate may be arranged to enclose the printing plate material and to engage around the edges of the printing plate material in spaced relationship when the sealing member 20 is sealably engaged against the lower platen 12.

Air evacuating passageways as 22, more clearly shown in FIG. 14, are vertically formed in the vacuum plate 16 and horizontally disposed air evacuating passageways as 24 communicate therewith as shown in FIG. 3 with the result that when a pumping elfect is induced at the vacuum pump fitting 26, the resulting suction will remove air from both the upper and lower sides of the vacuum plate. It will be understood that the fitting 26 is designed to be connected to some conventional form of vacuum pumping apparatus well known in the art and not shown in the drawings.

An important feature of the arrangement described is the provision of an upper vacuum for holding the plate from dishing or deforming when a vacuum is instantaneously exerted in the recessed area at the underside of the plate. If dishing occurs the printing surface may be affected undesirably.

In FIG. 4 the mold 14 is shown mounted on the lower platen 12 with the sheet of printing plate material -M supported at the upper side thereof and the vacuum plate 16 sealably engaged with the upper platen 10 and held in some convenient manner as by latches 10a and 10b.

In FIG. the various components are shown in a further position in which the lower sealing member 20 of the vacuum plate has been moved into sealing engagement with the lower platen 12 and extending around the printing plate material 'M and the mold 14 in spaced relation to the edges thereof all the way around these members. In this position a short dwell period is employed to exert vacuum forces and evacuate small amounts of air or other gas from the enclosed volume and particularly between the female mold 14 and the printing plate material.

In FIG. 6 the press components are shown in a fully closed position illustrating the step of exerting pressure to cold flow the material M and the skin portion 6 into the depressions in the mold and thereafter heating at higher temperatures to fuse or cure all of the plastic material in the sheet M.

As illustrative of a typical method of making a plate using specific materials and operating conditions, the following example is cited.

A fluid composition having the following formulation was prepared Parts Polyvinyl chloride resin r Plasticiser (di-octyl phthalate) 50 Cross linking monomer (SR 350) l0 Pigment 10 This composition was coated on endless belt 22 as shown in FIG. 8 to provide a sheet material M1 which in a fully gelled state was characterized by a fusing temperature of approximately 310 F. Gelling was accelerated by passing the coated material through oven 36 at a temperature of approximately 200 F. for a limited period of timel The sheet of gelled material was then passed between a pair of heating rolls 38 and 40 (FIG. 8) which were heated to maintain them at or above the fusing temperature of the formulation noted. For example, the temperature of the rolls regulated to approximately 350 F. was found to be satisfactory and the contact of the sheet moving between the rolls was limited to one second. Thereafter the material was advanced to pass between a pair of chilling rolls 42 and 44 which limited the fusing action.

These steps produced a fused film thickness of from .003 to .005 inch. By regulating the temperature of the cylinders and the length of time of contact, the depth of fusion may be varied as desired. It has been found that if too great a depth of fusion takes place unsatisfactory results may be experienced in making a printing plate due to the fact that stresses develop in the finished plate and curling results.

FIG. 9 illustrates diagrammatically on a somewhat larger scale the formation of the skin portions 4 and 6' and it is pointed out that with depths of fusion of from .003 to .005 inch, the sheet becomes substantially reinforced and provided with handling strength and body so as to avoid breaking, tearing or otherwise undergoing dimensional change when used to make a printing plate. Moreover, the skin portions 4' and 6' are found to prevent loss of volatile material such as plasticiser and tends to prevent surface reticulation. As a result the plate material M is found to have good shelf-life and satisfactory handling properties.

In forming a printing plate from the material described, a sheet of desired size was placed in a vulcanizing press of standard type in which was received the female mold 14. In accordance with the invention the special vacuum plate 16 was inserted in the mold and attached to the upper platen 10 as shown in FIG. 4, with the sealing member 18 compressed to permit the vacuum plate to engage solidly against the platen surface.

The thickness of the lower sealing member 20 and the combined thickness of the mold 14 and material M were chosen relative to one another so that when the lower platen was moved upwardly a sealing engagement was first affected before any pressure was exerted by the vacuum plate against the upper surface of the printing material and yet a sealed environment was produced.

-As soon as a proper sealing engagement was affected a vacuum pump was operated and air was evacuated through the fitting 26 for a dwell period of approximately 20 seconds. In this period air was pumped from between the upper platen and the upper surface of the vacuum plate, as well as from the space occurring between the bottom side of the vacuum plate and the mold and printing material. During this dwell period of 20 seconds the suction forces operated to solidly lock the vacuum plate against the upper platen. Thereafter pressure was exerted before appreciable heating took place. A pressure was exerted to cold flow the plastic material M and force this material and the skin portion 6 into intimate contact with the depressions in the mold 14. A pressure of approximately twenty-five tons for about one to two seconds, for example, was found to produce the required cold fiow before any appreciable fusing took place.

Pressure and heat at 310 F. were then continued to fuse the skin portions, together with the remainder of gelled material at a temperature of approximately 310 F. This heating under pressure was maintained for a period of from two to three minutes and operated to completely fuse and cure the material in its displaced condition in the mold and there was produced a rubberlike mass in a form of desired printing plate completely free from stress and capable of lying flat on a press.

It will be understood that the formulation noted in the example above has been cited by way of illustration and other compositions and formulations may be employed to achieve a desired durometer in the end product.

In place of the material pre-fused to provide shelf-life, I may also utilize a gelled material M2 as indicated diagrammatically in FIG. 11, which material does not have the pre-fused skin portion. The material may, for some types of printing plate manufacture, be formed into a suitable sheet size and placed on a mold 14b in a vulcanizing press and processed in the manner already decribed.

An important feature in this procedure, as with the earlier method described, is to provide a vacuum and remove all air pockets prior to exerting pressure at opposite sides of the material M2, Also, as before, cold molding takes place 'before appreciable heating occurs so as to fill the mold depressions completely. Heating is continued at a temperature of 310 F. to provide for completely fusing and curing the plastic material to form the printing plate M3 shown in FIG. 12.

In connection with use of vacuum as described to remove all air pockets at either side of the printing mate rial, I may also provide a surface treated material as M4 illustrated in FIG. 13. The surface material is formed with an embossed pattern such as grooves 50 along which passage of air may be conducted to insure complete evacuation.

In forming a reinforced sheet with shelf-life, I may also desire to utilize in place of the fused skin portions described, one or more laminating sheets of material of greater strength than that of the gelled sheet. In FIG. 10, I have illustrated a modified method of making this material. A gelled sheet 52 is formed and heated and then into contact with upper and lower sheets 54 and 56 which may consist of thin fused vinyl chloride sheets which are adapted to become adhesively attached to the gelled layer 52.

It will be understood that the printing plate body can be provided with a fused skin at one side thereof, as generally shown in FIG. 13, and that such a body is located in the molding press with the fused skin in contact with the surface of the mold received within the press.

There are other changes and modifications in the invention, and its method and apparatus may be resorted to within the scope of the accompanying claims.

I claim:

1. In a method of making a flexible printing plate with a printing surface reproduced from a female mold of subject matter to be printed, the steps which include providing a printing plate body consisting of a gelled polymeric compound which is fused at one side to provide a thin outer hardened skin and an underlying portion which occurs in a plastic condition, locating the printing plate body in a molding press with the hardened skin in contact with the surface of the said female mold received between upper and lower components of the molding press, sealably containing outer edges of the mold components and subjecting the printing plate body to vacuum forces to evacuate air from within the mold, exerting pressure in the molding press to cold-flow the said underlying plastic portion of the printing plate body against the hardened skin and to force the hardened skin into intimate contact with the female mold surface throughout all points therein, and then heating to convert the gelled polymeric printing plate body into a cured polymeric composition while the hardened skin is held compressed against the female mold surface, thereby to form a permanently molded printing surface which is a true reproduction of the said subject matter to be printed.

2. A method according to claim 1 in which the thickness of the hardened skin is maintained in a range of from .003 to .005 inch to provide a stress-free printing plate body which is resistant to curling.

3. In a method of making a flexible printing plate with a printing surface reproduced from a female mold of subject matter to be printed, the steps which include providing a printing plate body consisting of a gelled polymeric compound which is fused at one side to form an outer hardened skin portion and an underlying portion occurring in a plastic condition, locating the printing plate body between a vacuum plate and the female mold received in a molding press, sealably containing an upper side of the vacuum plate against an upper platen portion of the press and a lower side of the vacuum plate against a lower platen portion of the press, simultaneously evacuating air from spaces above and below the vacuum plate to form a vacuum, subjecting the printing plate body to pressure exerted through the female mold and the vacuum plate while under vacuum at two sides thereof, and cold-flowing the fused skin and underlying plastic portion into intimate contact with the female mold surfaces at all points therein, and then heating to convert the printing plate body into a cured polymeric compound while the fused skin is held against the female mold surface, thereby to form a permanent molded printing surface of the subject matter to be printed.

4. In a method of making a flexible printing plate with a printing surface reproduced from a female mold of subject matter to be printed, the steps which include coating a liquid polymeric composition on a supporting surface and selectively heating the polymeric material to form a gelled layer, subjecting the gelled layer to a controlled degree of heating to form at opposite outer sides of the layer thin fused skin portions while retaining an intermediate layer portion which remains in an unfused condition, locating the layer of material in a molding press with one of the fused skin portions in contact with the surface of the said female mold received between upper and lower platens of the press, sealably containing outer edges of the platens and subjecting the printing plate body to vacuum forces to evacuate air from within the mold, then exerting pressure in the molding press to coldfiow the said intermediate layer portion against the fused skin portion in contact with the female mold and to force the skin portion into intimate contact with the female molding surface throughout all points therein, and then maintaining pressure and heating to convert the layer portion and the fused skin portions into a cured polymeric composition while the said hardened skin is held compressed against the female mold surface thereby to form a permanent molded printing surface which is a true reproduction of the subject matter to be printed.

5. A method according to claim 4 in which the step of subjecting the sheet of plastic material to controlled heating consists in passing the sheet between the nip of a pair of heated rolls and regulating the thickness of the said rubber-like skin portions by controlling the temperature and contact period of the rolls on the sheet.

6. A method according to claim 4 in which said step of controlled heating to form skin portions consists in passing the gelled material between a pair of heating rolls and controlling the temperature of the rolls and the time interval during which the material is in contact with the rolls to form rubber-like skin portions in a thickness between .003-.005 inch.

References ited UNITED STATES PATENTS 1,377,503 5/1921 Novotny 264320 X 2,452,821 11/1948 Wood, I1. 10l-40l.1 UX 5 2,834,052 5/1958 Hunn 264227 X 2,925,625 2/1960 Souza 264320 3,031,959 5/1962 Libberton 101401.1 X 3,257,944 6/1966 Gray 264259 X 10 ROBERT F. WHITE, Primary Examiner J. H. SILBAUGH, Assistant Examiner US. Cl. X.R.

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