US3113514A - Flexible printing plate - Google Patents

Description

De@ 1o, 1963 FIG.

INTING PLATE FLEXIBLE PR Fi1ed April 14, 1960 AIR FIG. I

FIG. 2

United States Patent Office 3,113,5l4 Patented Dec. 10, 1963 3,113,514 FLEXBL'E PRINTING PLATE Edward F. Hogan, Carmel Valley, Calif., assignor to Chemical Products Corporation, East Providence, RJ., a corporation of Rhode Island Filed Apr. 14, 1960, Ser. No. 22,308 7 Claims. (Cl. 101-379) This invention relates to flexible printing plates and to an improved method of manufacture which provides plates of uniform characteristics on a commercially practical basis.

While printing plates made of rubber or synthetic plastic compositions have been employed in the past, they have suffered from certain deficiencies and shortcomings which have limited their use. The rubber-composition plates, although flexible in nature, have required refrigeration of the rubber composition during extended storage in order to avoid a pre-cure of the material which would prevent its proper molding and vulcanization to form the desired plate. In addition, the highly elastic nature of rubber compositions has tended to result in distortion of the printing elements at the face of the printing plate under the pressure applied in the operation; with rubber plates it has therefore been difficult to obtain clear, sharp, undistorted impressions, even on good calendered paper stock but particularly on irregular surfaces such as corrugated box-board and the like. Generally, rubber plates are not suited for reproducing half-tone screens finer than 85 lines per inch.

Although a large portion of the plates used to print books are fully rigid plates from synthetic plastic material, such plates are practically impossible to curve and use on rotary presses and have some drawbacks even for flat bed press work. Because of their rigidity, plastic plates are, for instance, not suited for providing satisfactory impressions on irregular and uneven surfaces.

Plastisols have long been used to form all sorts of molded products, but printing plates from plastisols have not been successfully produced commercially. A common defect is the presence of voids, holes, and pockets formed by gases present at the printing surface during the hardening of the plastisol.

This invention is based in part on a novel way of making plastisol printing plates which eliminates voids or bubbles at the printing surface, and provides an improved flexible plastisol printing plate. This invention moreover provides a two (or more) layer printing plate which consists of a facing layer of a relatively hard plastic composition which forms the printing face of the characters, and a backing of a softer composition, which carries the facing layer and renders the complete printing plate ilexible and compliant throughout.

Briefly, the process is carried out by brushing or spraying a light coat of a relatively hard liquid plastisol compound having a low viscosity onto the molded side of a conventional printing plate matrix, covering the bottom of the cavities in the matrix to form a facing layer for each character. To assure faithful molding of the facing layer an air jet is then directed against the liquid and the surface of the matrix at an angle from one side to the other, to agitate the liquid material. The liquid plastisol is thereby caused to wet the matrix surface completely, and to fill or partially llll the character cavities. It has been found that the air stream passing over the plastisol removes the entrapped air from within the plastisol and assures complete wetting of the character surfaces, and a clean, unimpaired printing surface is thereby formed on the facing layer. The excess plastisol is wiped from the matrix surface and this first thin layer of plastisol forming the facing of the characters is gelled by heating the coated matrix. A second liquid layer or backing of a relatively soft plastisol composition sufficient to fill the cavities completely and alone, if desired, to form a backing layer of plastisol above the matrix is then cast on the matrix. With the first layer present at the matrix surface forming the facing of the characters, the second layer readily wets the exposed back surface of the facing layer so thoroughly that the bubbles do not create a problem. The entire mass is now fused and allowed to cool, causing the mass to harden and the two layers to bond together, with the printing characters impressed thereupon. The second layer may be cast to a suf'n`cient thickness to form the backing layer, or it may only fill the character cavities in which case an additional backing layer must be provided. This may be cast in the same manner or it may be a fabric or like material to which the already cast layers are caused to adhere.

Two-layer plastic plates made by prior art processes have a printing layer and a backing layer. The printing layer is generally a harder plastic material, forming both the printing characters and the spaces between and within the characters. These plates are not well suited for printing on irregular surfaces, for when a character deforms to conform to the surface to be printed the bridging effect of the hard plastic which joins the characters causes the adjacent characters also to deflect. Therefore, in printing on irregular surfaces, much distortion results. The plates of the present invention comprise two layers, but differ from other two-layer plates in that although the hard printing layer forms the surface of the raised characters, it does not form a bridge between the characters. The characters are resiliently supported and separated from each other by the second backing layer of soft elastic material, which forms all or part of the inter-character spaces. This backing of soft, resilient material imparts to the chraacters a sufficient resilience to allow them individually to accommodate an irregular surface without affecting adjacent or nearby characters. This feature permits even printing on corrugated box-board or other irregular surfaces. The hard surface of the character causes substantially all distortion or deflection to be taken up in the soft backing layer. The printing surface is therefore not deformed as it would be if it were also soft,

and subject to mushrooming, and may be used with halftone screens of up to lines per inch or more.

There are other resulting advantages to having the characters alone distinct from the backing layer. The characters may be different, contrasting color from the backing layer, thus facilitating proof-reading. Also the characters are formed from a harder, more durable plastisol mixture, thus making the plate more wear-resistant and yet still allowing the desired resiliency imparted by the soft backing layer.

The preferred embodiment of the invention is described in detail below with reference to the drawings in which:

FIG. l is a cross section elevation through a printing plate matrix showing the casting of a facing which forms the printing surface of the individual characters;

FIG. 2 shows the matrix of FIG. l with the resilient backing layer cast against the printing surface facing;

FIG. 3 is an exaggerated view in cross section showing the manner in which a flexible plate of this invention accommodates an uneven printing surface; and

FIG. 4 is a cross section elevation showing a modified embodiment of that of FIG. 1.

In making a printing plate according to the preferred embodiment of this invention, a conventional printing plate matrix 10 is placed within a closely tting pan 12 and a quantity of a liquid plastisol composition is applied in an amount at least sufllcient to cover the bottoms of the individual characters 14. This first quantity of plastisol forms the facing 15 of the characters, and should be a formulation which is relatively hard when cured.

The hardness of the facing composition may vary within Wide limits, depending on the use to which it is to be put and on the desired characteristics of the iinal print. For book work Vand similar fine printing on a smooth iiat surface, a facing having a hardness of 60 more on the Shore durometer D scale is preferred. For rougher work, however, as paste board printing a facing hardness of S on the Shore durometer A scale is satisfactory, while on uneven surfaces as on corrugated boxboard, which require compressions and compliances of the character facing, a softer composition, of 65 Shore durometer A hardness will be required.

After the plastisol for forming the facing of the characters has been spread over the matrix to iill the characters, an air jet 16 is played at an angle over the surf. face of the plastisol illed characters to cause mild agitation of the liquid plastisol. A puddling action results which helps to eliminate entrapped gas bubbles within the characters, with the result that complete wetting of the matrix within the characters is assured. The air jet should be of velocity sufficient to cause a noticeable agitation of the plastisol composition, and by playing it across the surface of the characters the combined eifects of the agitation and the cross flowing stream of air serve very well to assure perfect fidelity of the casting. The excess plastisol is then wiped from the matrix surface, so as to leave plastisol only within the characters, and the matrix and plastisol are then heated, e.g. to about 180 C. for about 5 minutes to cause the plastisol to gel, to a semi-solid state.

It is important in forming the facing portion of the plate that only thte individual characters possess the rigidity of the relatively hard composition and that the material between the characters be relatively soft. The amount of relatively hard plastisol composition should accordnigly not be suiicient to till the characters and spread over the matrix surface. Any excess of this composition should be removed prior to the formation of the backing layer.

The flexible printing plate is completed by casting a second portion of plastisol on the matrix to form a backing 13 which carries the printing characters previously cast. For this, a plastisol which is soft relative to the facing is employed. By way of example, if the facing is designed for tine work, having a hardness of e.g. 60 Shore durometer i), a backing of 85 Shore durometer A would give satisfactory compliant support. At the other extreme however, where a plate is designed for rough work on corrugated boxboard, the backing layer may be as soft as Shore durometer A.

When the backing portion 18 of plastisol is cast, special precautions to assure contact of the matrix surfaces and with the backing surfaces of the character facings are generally unnecessary. The plastisol readily wets the character facings and the exposed portions of the matrix surface are generally smooth and do not tend to trap air. The amount of backing portion of plastisol should be sufiicient to form a backing layer suiiiciently thick to be handled and to be applied to printing press bearers. The cast assembly contained within the tray 12 is iinally cured, effecting also a cure of the previously gelled character facings, by heating the casting in an oven to about 350 C. for about 20 minutes. Upon cooling the cured plate is pressed to a press plate 20 and then removed, trimmed, and iinished preparatory to actual printing.

If desired, additional backing layers, e.g. of cloth or screen, may also be applied to the plate for additional strength, but suitable plates can be made entirely of plastisol.

A modied embodiment of this invention is shown in FIG. 4. The characters are more or less individually carried on a supporting layer 24, which may be a rubber impregnated fabric or other relatively uncompressible non-compliant sheet. The facings 15 of the characters of a hard composition are formed as described above. The soft composition which forms the backing 18 serves to carry the facings but does not provide a layer or supporting sheet for the characters; the supporting layer 24 is rather applied to the cast characters after the backing 1S has been poured, and is caused to adhere to the characters in the cured product. The supporting layer 24, may for instance be layed over the matrix after the backing iS is poured, and pressed flat against the backing, before the final curing, and the entire assembly then cured to form the plate.

Plastisois useful in this invention consist essentially of a thermoplastic material, typically polyvinyl chloride, dispersed in a plasticizer, forming at room temperature a liquid. Heating causes the plastic and plasticizer to combine into a solid rubbery composition having essentially the same dimensions as the cast liquid. Suitable plastisols, designated according to the hardness of the cured product, are available commercially, and their nature and compositions are well known in the plastics molding trade.

The improved effect of forming a plate with relatively hard character facings carried by a relatively soft backing material is shown somewhat schematically in FIG. 3. A portion of the plate carried by the press plate 20 having three adjacent characters is shown in printing contact with an uneven portion of corrugated box-board 22. The center character is in contact with a relatively thick part of the box-board and is shown depressed upwardly. The facing i5, although displaced, is not distorted at its surface since essentially all distortion is taken up in the relatively soft backing composition 18. Moreover, since only the backing composition connects adjacent characters, there is no displacement of either adjacent character.

Plates embodying this invention are accordingly superior to flexible plates heretofore known in providing flexibility suitable for such uses as printing on uneven surfaces while avoiding distortion of printing surface. It is for instance possible to reproduce half-tones from a screen of lines per inch with the plates of this invention.

Although this invention has been described with specific reference to the use of plastisols, and also features improvements in the casting of plastisols, it is contemplated that the composite plate structure which includes relatively hard character facings and relatively soft backing material could be provided by other plastic molding techniques familiar to those skilled in the art. Modifications to the embodiment described which will occur to those skilled in the art and familiar with this disclosure may be made without departing from the scope of this invention.

Having thus disclosed my invention and described in detail the preferred embodiment thereof, I claim and desire to secure by Letters Patent:

1. The method of making plastic printing plates comprising providing a matrix having cavities corresponding to the characters to be produced, partially lling and covering the bottoms of said cavities with a iirst quantity heat-hardenable liquid plastic composition forming a relatively hard plastic and heating said composition to harden it and form the print facings of the corresponding characters, applying a second quantity of heat-hardenable liquid plastic composition forming a relatively soft plastic over said facings to iill the cavity, applying an uncompressible supporting layer over the lled cavities, and heating said second quantity of heat-hardenable composition to harden it and to bond the back of the cavities to the supporting layer to form a plastic printing plate.

2. A process for forming flexible printing plates which comprises: providing a matrix having cavities adapted to produce the printing characters on the face of said plate; covering the bottom of said cavities with a first layer of liquid, heat-hardenable material; directing an air stream across the surface of said matrix thereby eliminating entrapped air within said first layer and causing said heathardenable material to spread on the inner surface of said cavities; applying heat to said matrix and said rst layer of liquid to cause it to harden; adding a sufficient amount of a second layer of liquid heat-hardenable material to lill the cavities of said matrix and to form a continuous exible layer above said matrix; and applying heat to harden said second layer and cause it to bond to said rst layer.

3. The method of making plastic printing plates cornprising providing a matrix having cavities corresponding to the characters to be formed, covering the bottoms of said cavities with a heat-hardenable liquid plastic material without covering the area between the cavities and heating said composition to harden it and form individual separate print facings of the corresponding characters, applying a second quantity of heat-hardenable liquid plastic composition over said facings and heating said second quantity of heat-hardenable composition to harden it and thereby provide a support for said facings, said second quantity of heat-hardenable plastic composition forming a softer plastic than said facings whereby said facings are free to deflect without substantial distortion.

4. The method defined by claim 3 wherein the second quantity of heat-hardenable plastic composition is provided in a quantity to cover the entire matrix and upon hardening forms a backing layer carrying said facings.

5. A printing plate comprising a backing and characters protruding from said backing, said characters having facings lying in a common plane and composed of a relatively hard plastic material extending toward said backing less than the entire depth of the characters, said facings being supported by a relatively soft elastic material which forms the balance of said characters and separates said facings from said backing.

6. A printing plate as dened in claim 5 wherein the relatively soft elastic material and the relatively hard plastic material is a. heat-hardened plastisol composition.

7. The method of making plastic printing plates cornprising: providing a matrix having cavities corresponding to the characters to be formed; covering the bottoms of said cavities with a heat hardenable vinyl plastisol composition without covering the area between the cavities; wiping excess plastisol composition from the surface of the matrix; heating said composition to form individual separate print facings of the corresponding characters; applying a second quantity of a heat hardenable vinyl plastisol composition over said facings; and heating said second quantity of heat-hardenable plastisol composition to harden it and thereby provide a support for said facings, said second quantity of heat hardenable plastisol composition forming a softer vinyl plastic than said facings whereby said facings are free to deect Without substantial distortion.

References Cited in the file of this patent UNITED STATES PATENTS 336,481 Hall Feb. 16, 1886 374,048 Evans Nov. 29, 1887 518,826 Sachers Apr. 24, 1894 2,028,712 Swan et al. Jan. 21, 1936 2,101,838 Cochran Dec. 14, 1937 2,741,297 Vamvaketis Apr. 10, 1956 2,777,824 Leeds Jan. 15, 1957 2,812,549 Wall Nov. 12, 1957 FOREIGN PATENTS 388,482 Great Britain Mar. 2, 1933

Claims (1)

1. 5. A PRINTING PLATE COMPRISING A BACKING AND CHARACTERS PROTRUDING FROM SAID BACKING, SAID CHARACTERS HAVING FACINGS LYING IN A COMMON PLANE AND COMPOSED OF A RELATIVELY HARD PLASTIC MATERIAL EXTENDING TOWARD SAID BACKING LESS THAN THE ENTIRE DEPTH OF THE CHARACTERS, SAID FACINGS BEING SUPPORTED BY A RELATIVELY SOFT ELASTIC MATERIAL WHICH FORMS THE BALANCE OF SAID CHARACTERS AND SEPARATES SAID FACINGS FROM SAID BACKING.

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