US2272254A

US2272254A - Flexible printing plate

Info

Publication number: US2272254A
Application number: US214319A
Authority: US
Inventors: Swan Hylton
Original assignee: Bakelite Corp
Current assignee: Bakelite Corp
Priority date: 1938-06-17
Filing date: 1938-06-17
Publication date: 1942-02-10
Anticipated expiration: 1959-02-10
Also published as: GB522648A

Description

FLEXIBLE PRINTING PLATE Filed June 17, 1938 W/lll/jf/llillll/ll 6 f 1 1-4 a I I A. INVENTOR.

- 169/5 0. 5 wan Patented Feb. 10, 1942 UNITED STATES PATENT OFFICE FLEXIBLE PRINTING PLATE Hylton Swan, Upper Montclair, N. J assignor, by

mesne assignments, to Bakelite Corporation, a corporation of New Jersey Application June 17, 1938, Serial No. 214,319

4 Claims.

This invention relates to printing plates and similar articles molded from a plastic material against a matrix. It is particularly directed to printing plates prepared from rubber which at the present time are finding extensive use in the printing industry.

The invention resides in the provision of a fine screen or apertured grid material through the orifices of which the printing face material is extruded, or, itimay be said, sprayed in the form of fine spaghetti-like protrusions into the matrix depressions that correspond to the printing surface areas. The grid or screen can be of any suitable 'foraminous or woven material; for

instance a material found satisfactory and inexpensive in the preparation of rubber plates is acotton fabric of about 80 mesh or less and preferably preshrunk or having the threads forming the meshes locked or secured against substantial lateral movement by impregnation or coating with a fixative, such as a varinsh that in the dried form does not soften or yield under the action of heat. The grid thus remains in the printing face or on the top of the non-printing areas as a reinforcement at the bases of th printing areas and imparts a number of desirable properties to the plate.

A most important and outstanding accomplishment obtained by incorporating the porous grid in the printing face of the plate is that it functions to give sharpness and definition to the printing areas and avoidance of the gasket" effect resulting in poor definition of the printing characters caused by imprisoned air or air pockets" in the type depression of a matrix during the molding of the plates. The accomplishment of these functions can be attributed to the fine subdivision of the rubber with the forcing of the rubber protrusions vertically into the matrix depressions and the escape of air permitted by the surface screen or grid. It further serves to restrict the lateral or side fiow of the rubber, confining its movement to a downward direction, thus insuring sharp, clear reproductions of the matrix characters and below the grid reinforcement is a substantial thickness of resilient rubber giving a desirable cushion effect to the printing plate.

Among other highly useful technical effects accomplished by the present invention is that of controlling shrinkage. This is a well recognized defiect of rubber printing plates wherein the shrinkage varies from about to thousandths of an inch per inch of length depending onthe rubber composition. Resort has heretofore been had to the expedient for checking shrinkage by applying backings to rubber printing plates of variouskinds such as -metal plates, wire screen, paper, fabric, etc,; while effective to some degree, shrinkage is not entirely-corrected by back ings of this nature. But another objection is that backings cause a concave curvature to the printing faces which make it diflicult to secure the plates to printing; rolls without havingthe edges or "comers lift up;.' this is attributable to the greater shrinkage of the rubberprinting faces as compared with that of the backings; It has been found, however, that by the; incorporation of the porous grid in the printing face of the plate the shrinkage-can be reduced to a minimum such as about 1 to 1.5 thousandths of an inch and can even be compensated by takingadvantage of the expansion of the matrix that occurs on heating in the molding operation; also convexity of the printing face is produced by the porous grid in the printing face which is desirable since the plates then fit snugly to the printing rolls without lifting. The reduction or compensation of shrinkage in the printing face is obviously important for registration in multicolor printing.

In practicing, this invention for the manufacture of printing plates from rubber, the grid or screen is first prepared. When a woven fabric is used for this purpose, it is preferably treated with a fixative such as a heat-hardenable phenol resin varnish and then baked. Enough varnish is used to lock or anchor the warp and woof threads where they cross and prevent the lateral movement relatively to each other but not to fill the pores or small apertures. The varnish also serves to shrink the fabric, and the treated fabric is found to give a better shrinkage control than the untreated fabric. Preshrunk fabric, such as sanforized cloth, is also found effective for the purposes of this invention; and where absolute shrinkage control is not a major requirement, untreated fabric can be used.

As fabric material, cotton cloth of 80 mesh, i. e. 80 threads or filaments to the inch, is found sufficiently fine, for the reproductionof fine type matter or line cuts. The mesh of the material used is not critical except that it is generally desirable to use as small a mesh as the plastic material yvill permit for fine reproduction. For the general run of plates coarser fabrics can be used, such as a mesh, and this is preferable from the standpoint of a quicker molding of a rubber plate and decrease in pressure required for molding. Mesh fabric of metal wire or other filaments can be used in place of fibrous textiles,

mediate state characterized by fusibility and sol- H ubility and setting to an infusible insoluble state; water-soluble resins which set up on heating can be applied in the form of water solutions. So-

lutions of other types of heat-hardening resins can be substituted, the purpose in such cases being the impregnation with a resinwhich when hardened by heating does not soften or yield under the action of heat in the operation of molding a printing plate. The viscosity of the resin solution. should be such as not to fill the pores of the fabric while serving to lock the threads together; for instance a fabric of about 80 mesh treated with an alcohol solution of a phenol resin having a kinematic viscosity of about 17.5 as determined by a pipette'gives a grid or screen with open pores that upon baking has a resin content of intheneighborhood of 22 per cent by weight. The kinematic viscosity of an ordinary brushing varnishis about 100.

Other fixatives can-b substituted in place of resin varnishes such as starch,- silicate of soda, glue, etc., depending upon the accuracy of reproduction required and the nature of the plastic material fo'rced or'extruded through the grid material for forming the printing faces.

A composite or laminated structure is preferably made by calenderlng on the grid or screen onto a layer or thin sheet of rubber composition which sets and cures upon heating. A rubber layer of about 1*; inch in thickness is appropriate for most, types of printing plates, but the thickness can be varied as desired. The rubber composition should preferably be one that sets in not less than a minute and up to about 2 /2 minutes and has a curing period of about 10 to minutes. Y The invention is illustrated in the accompanying drawing in which Fig. l is a view in cross-section of fragmentary portions of the open platens of a press and an assembly of a matrix having applied thereto a composite sheet of a fabric grid and rubber;

Fig. 2 is a similar view showing. the platens closed and the rubber forced through the grid into the depressions in the matrix surface Fig. 3 is a similar view with the platens again separated and a backing of rubber applied to the sheet and bearers placed to limit the closing of the platens:

4 is a "view similar the platens closed:

Fig. 5 is an enlarged elevation of a grid material (the thread spacing being exaggerated) Fig. 6 is a view similar to Fig. 5 showing the fixative for locking the threads;

Fig. 7 is a fragmentary cross section of a molded printing plate; and

Fig. 8 is a fragmentary cross-section of a molded printing plate with an added fabric backing.

For making a printing plate the composite sheet consisting of the grid 5 and rubber material 6 is placed upon a matrix 3 having depressions 4 with the fabric grid in contact with the matrix. The assembly is then placed between to Fig. 3 but showing the platens I and 2 of a press and subjected to heat. and pressure; a temperature of about 135 C. (sixty pounds steam pressure) and a pressure in the neighborhood of about 1000 pounds per square inch are normally used with a rubber composition setting in about 1 minute and with an 80 mesh fabric grid, but these conditions vary with the character of the rubber composition and the fabric mesh. During this molding step which continues for a minute or longer, depending on the set time of the rubber, the rubber is forced through the apertures of the grid formed by the cross. threads 1 and 8 and bonded by the fixature 9 into the depressions of the warm matrix as shown in Fig. 2. In passing through the fine grid'openings the rubber is separated into fine protrusions or into a spray directed perpendicularly into'the recesses of the matrix; the grid therefore acts as a guide for the rubber as well as to finely subdivide it. Since the rubber is thus charged into every matrix depression however small in the form of fine streams, the matrix depressions are'readily filled out and the displaced air canescape. Upon filling the depressions and the remaining space defined by the fabric grid and the matrix, the rubber again unites or coalesces to form the printing areas H and the floorareas I0.

In the preferred practice'the foregoing molding step is carried out as a preliminary step with a two-to three minute cure and without any bearers in the press to regulatefthe thickness. The full pressure is. thus transmitted to the rubber in forcing it into the matrix as shown in Fig. 2. In order to produce plates of a given thickness, ranging say from .125 to .187 inch, an

additional backing of uncured rubber i2, as shown in Fig. 3, is added and the assembly again submitted to heat and pressure in the press but with bearers I3 inserted to govern the thickness as shown in Fig. 4. The heating in the press is continued until-the rubber is curved which generally requires about 10 minutes. By this division into two steps, the printing surfac receives the full pressure during the set time for the rubber, and a plate of uniform thickness is obtained by means of the added filling material on the back during the second molding and curing operation. Types of rubber are preferably used that will give a durometer hardness of from 50m 75 in curing periods of from ten to fifteen minutes;

The invention can be applied to other plastic materials besides rubber which lend themselves to subdivision by forcing through a grid; it is not their chemical composition that is involved so much as the physical characteristics of plasticity, possible subdivision by extrusion through a grid and then coalescing and setting up by the action of heat or otherwise. Other extrudable plastic materials which can be substituted include synthetic rubbers as isopren and other vinyl polymers. It has also been found that resinous molding materials, such as theintimate mixtures pref erably in sheeted form'of wood flour or other finely divided fibrous or granular fillers and heathardening resins of the phenol-aldehyde or ureaaldehyde type can be grids of the character herein described into matrices to yield excellent printing surfaces; the relative inflexibility of printing faces so molded from heat-hardening resin compositions dictate their use principally for the plates. and flatness can be ensured by applyin backings H as shown in Fig. 8 to the plastic maextruded through porous preparation of fiat I terial of fabrics similar to the materials used for the grids or dissimilar materials. Backings can also be applied to flexible rubber and other plastic materials, though for attachment of plates to curved printing rolls the ba-ckings are not required.

The practice of the invention as described in the foregoing applies particularly to the molding of plates from plastics-for the reproduction of ruled forms, fine type matter or line cuts, or any form of plate in which there are no large solid printing areas. will have the fabric of rubber or other thin plastic surfacing material can be made part of the composite sheet applied to the matrix. On the other hand, the imparting of a fabric pattern to solid printing areas may in some cases be found desirable and the designs can be varied in accordance with the pattern of the fabric or grid used.

The invention while intended for printing plates is not restricted to such articles. It is susceptible of use in the preparation of matrices and other articles molded by forcing or extruding a plastic against an original or a matrix through a grid in accordance with the disclosure herein.

What is claimed is:

1. Method of molding printing plates and the like in a matrix from a curing rubber composition which comprises forming a grid from a mesh fabric by varnishing to set the meshes of the fabric, applying to the grid a layer of the curing rubber composition, placing the assembled grid and rubber layer on the matrix with th grid contiguous to the matrix, applying heat and pressure to force the rubber through the grid and into the matrix depressions for a period corresponding to the set time of the rubber, filling out the plate structure with an amount of the rubber composition requisite for a desired thickness, and applying heat and pressure with bearers to govern the thickness to cure the composition,

- 2. Method of molding printing plates and the like in a matrix from a curing rubber composition which comprises applying to a mesh fabric grid a layer of the rubber composition, placing the assembly of grid and rubber layer on the matrix with the grid contiguous to the matrix, and applying heat and pressure to force the rubber through the grid and into the matrix depressions.

3. Method of molding an article having discontinuous surface areas from a plastic in a matrix with depressions conforming to the desired surface areas of the article which comprises placing a grid with apertures contiguous to the matrix and forcing a plastic through the apertures of the grid into conformity with.the depressions in the molding surface of the matrix.

4. Method of molding an article having discontinuous surface areas from a plastic in a matrix with depressions conforming to the desired surface areas of the article which comprises assembling a grid with a layer of plastic material, placing the assembly with the grid apertures contiguous to the matrix, and forcing the plastic through the apertures of the grid into conformity with the depressions in the molding surface of the matrix.

HYLTON SWAN,

CERTIFICATE OF CORRECTION. Patent No. 2,272,25h, February 10, 19!;2.

HYLTON SWAN.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, first column, line 51, for "deflect read --defect--; page 2, second column,

line 11.2, for "curved" read -cured-; and that the said Letter Patent shouldbe read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signedand. sealed this 11mm day or April, A. 1). 191m.

I Henry Van Arsdale, (Seal) Acting Commissioner of Patents.