US3425347A - Resilient printing plate - Google Patents

Description

Feb. 4, 1969 w. D. NARD RESILIENT PRINTING PLATE Filed Feb. 29. 1968 Sheet 2 of2 INvENroR WALLACE D. NARD #MWM/M /QQM A'r'rvs.

United States Patent 3,425,347 RESILIENT PRINTING PLATE Wallace D. Nard, Chicago, Ill., assignor to Electrographic gorfioration, New York, N.Y., a corporation of New or Continuation-impart of application Ser. No. 446,195, Apr. 7, 1965. This application Feb. 29, 1968, Ser. No. 712,331 US. Cl. 101376 6 Claims Int. Cl. B41f 13/10; B41n 1/22 ABSTRACT OF THE DISCLOSURE A printing plate including a rubber layer having an outer printing surface and an inner mounting surface formed by a pattern of spaced uniform projections which relieve tension and concavity of the printing surface when the layer is mounted and adhesively secured to a curved backing member. The projections comprising the mounting surface may be formed from a softer rubber than the printing surface so that they may readily yield and deform when the printing surface is subjected to excessive localized pressures.

This invention relates generally to rubber printing plates and the method by which they may be manufactured.

This is a continuationin-part of application Ser. No. 446,195, filed Apr. 7, 1965 and now abandoned.

Printing plates of the type to which the present invention is applicable are made of rubber with the impression to be printed molded into one surface by an appropriate method. In the past a major limiting factor in attaining high quality printing with rubber plates has been concavity of the printing surface, a defect commonly called cupping. When a rubber plate is positioned on a curved mounting plate, the top printing surface of the plate is drawn into tension causing it to become concave or cupped. The center portions of a printing area are pulled down while the edges remain high. In the printing operation excessive pressure must be applied to level the concave portions, often causing an undesirable halo" around the printed figures. Cupping also occurs, although to a lesser extent, when a rubber plate is vulcanized to a mounting plate. When the rubber cools after the vulcanizing operation, the bottom surface of the plate remains dimensionally stable while the top surface is drawn into tension by shrinkage causing concavity in the printing areas.

A further difliculty in the past with rubber plates is that when excessive pressure exists between the printing plate and the surface being printed, the low viscosity printing ink has a tendency to squeeze out beyond the printing area so that the printed edges are ill-defined. It has been necessary, therefore, that the plates be made and the presses be operated with extreme accuracy so that the pressure applied to each printing area is uniform and within prescribed limits.

Accordingly, it is an object of the present invention to provide an improved rubber printing plate which will produce higher fidelity and sharpness in printing without haloing, unevenness or other imperfections.

Another object of the invention is to provide a rubber plate that may be curved around a mounting plate and vulcanized thereon without causing concavity or excessive tension on the printing surface. It is a related object to provide a rubber plate which has a bottom surface which will bend easily around a cylinder so as to relieve tension on the printing surface.

It is a further object to provide a printing plate that is self-cushioning to prevent high localized pressures during the printing operation caused by inaccuracies inherent in a rubber plate, thereby to achieve higher quality printing than has been attainable by such plates in the past.

Still another object of the invention is to provide a manufacturing procedure for rubber printing plates that is economical and produces an improved and more durable plate.

Other objects and advantages of the invention will become apparent from the following description in conjunction with the attached drawings, wherein:

FIG. 1 is a perspective view of a plate cylinder with a metal-backed rubber printing plate mounted thereon;

FIG. 2 is a fragmentary perspective showing the underside of a rubber plate embodying the present invention;

FIG. 3 is a fragmentary transverse section of a rubber plate embodying the present invention supported on a curved backing plate;

FIG. 4 is a fragmentary enlarged section of a rubber printing plate illustrating the effect of tension on the printing surface when the features of the present invention are not present;

FIG. 5 is a partial section view of a rubber printing plate of the present invention when excessive pressure tends to be applied to the printing surface;

FIG. 6a is a perspective view of a partially cured rubber plate being removed from a mold;

FIG. 6b is a perspective of a rubber plate being cemented to a mounting plate for final vulcanization;

FIG. 6c is a fragmentary section of the cylinder in FIG. 6b showing infra-red rays passing through the wrapping and rubber plate and reflecting back from the mounting plate;

FIG. 6d is a perspective view of a completed rubberbrass back plate.

While the invention is susceptible of various modifications and alternative constructions, a certain illustrative embodiment has been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to specific form disclosed, but, on the contrary, the intention is to cover all modifications, alternative constructions and equivalents following within the spirit and scope of the invention as expressed in the appended claims.

Referring more particularly to the drawings, the invention is shown as applied to a rotary printing cylinder 10 using a metal backed printing plate 11. The printing plate 11 is comprised of a rubber plate 12 cemented and vulcanized to a curved backing or mounting plate 13 preferably made of brass, although a non-rigid backing material such as Tefion could be used in the alternative. The rubber plate 12 has upraised printing lands 14 molded into its top surface. The printing lands 14 may consist of various small raised printing areas having a center portion 14b and an edge or rim 14c. The cylinder 10 cooperates with an impression cylinder 20* with which it is rotated in unison (FIG. 5).

As stated, one of the major difliculties in the use of rubber printing plates has been concavity or cupping of the printing surface. Cupping is caused primarily by the bending of a flat rubber plate into cylindrical shape incident to maintaining it on the backing. The base of the plate is forced into a tighter curve than the printing surface with the result that the base is compressed as indicated by the arrows 12a while the printing surface is peripherally tensioned as indicated at 12b. The center portion 14b is pulled down toward the cylinder as shown in FIG. 4, leaving the unsupported edges high or cupped. Printing with plates with such concave surfaces results in the defects previously mentioned.

In accordance with the present invention I provided relief on the underside of the printing plate in the form of a closely-spaced uniform pattern of discontinuties in the peripheral direction defining projections which are spaced sufficiently from one another to prevent the setting up of compressive forces in the peripheral direction at the mounting surface. More specifically in accordance with the present invention I provide, on the under side of the rubber printing plate 12, a two-dimensional pattern of uniformly spaced projections 16 integral with the plate and being of the same height so as to define a mounting surface for adhesive securing to a backing plate. The ends of the projections are cemented at the interface 18 to the backing plate 13. Preferably, the projections are in the form of flattened cylinders 16.

Considering the relative dimensions, the projections should occupy from to A of the surface area, preferably /2 to A2, and have a height which may range from A to A of the thickness of the printing plate. The maximum spacing of the individual projections should preferably be less than /2 the thickness of the printing plate. Thus, in a practical case, where the plate is /s inch in thickness, I have found it desirable to use cylindrical projections, having a diameter of inch, spaced apart by a distance of inch and having a height of .010 inch. The projections and the under portion of the plate preferably are formed from soft rubber having a durometer rating from 20 to 30, while the top printing portion is to be of harder ru-bber having a durometer rating of 50 to 90.

It is found that a rubber plate made in this way pos sesses a number of useful features and advantages, most notably the almost complete freedom from cupping. This is explained by the fact that compression at the inner surface is substantially eliminated which is accompanied by a corresponding reduction in tension at the outer surface. Thus, where a fiat plate of resilient material is bent, all of the plate material inside of a neutral surface will be compressed while the material on the outside of such surface will be placed in tension. The amount of compressive or tensile stress varies more or less directly with the distance, measured radially, from the neutral plane. By making the inner surface discontinuous in the peripheral direction, the neutral plane, indicated by the dot-dash line in FIG. 3 is moved upwardly. This has two effects. The first is that the radial distance from a given point adjacent the printing surface to the neutral plane is decreased, thus decreasing the tension at such point; and secondly, the curavture at the neutral plane is reduced. In short, relieving or coring out the under side of the plate to define the projections, even where carried to only to A of the average plate thickness, reduces the tension at the printing surface to below that at which objectionable cupping will take place.

A further advantage of the relief provided between the individual projections resides in the increase and control of the cushioning effect for the plate. This is illustrated in FIG. 5, where an impression cylinder 20 is shown pressing a sheet 21 against an upraised printing area '14. While the condition of impression cylinder adjustment shown in FIG. would normally bring about an excessive pressure condition, it will be noted that the projections 16, in the region of contact, deform and yield thereby preventing an excess pressure condition at the printing surface. This cushioning effect is possible since the projections are made of soft rubber with a lower durometer rating than the rubber required for the printing surface.

Since the yielding of the projections 16 in the place of the radially applied force prevents the force from building up, the printing defects which usually accompany use of excessive pressure do not appear. Stated in other words, a rubber printing plate having the features discussed above is much more tolerant of the adjustment of the impression cylinder with respect to the plate cylinder.

In accordance with the present invention, the procedure for making the rubber printing plate is to mold and partially cure a rubber layer to form an outside printing surface and an inside mounting surface having a pattern of spaced projections and then to complete the curing operation after the rubber layer has been afiixed to a curved backing plate. In the present instance, the rubber plate 22 is formed by molding together and partially vulcanizing two rubber layers in a molding press 23. One layer is soft rubber with a durometer hardness rating between 20 and 30, and the other layer is a harder rubber with a durometer rating ranging from 50 to 90. The lower plate 24 of the mold 23 imparts the printing areas into the hard rubber layer while the upper plate 25 molds a pattern of cylindrical projections 27 of the type previously described into the soft layer to form a mounting surface for the plate.

-1" he rubber plate 22 next is taken from the mold and cooled, making it ready for mounting on a backing plate 13. The backing plate 13 is coated with curing cement 27. The cement also is applied to the mounting surface of the rubber plate, although only the end surfaces 18 of the cylindrical projections are coated with the cement since spaces 19 are to remain between the projections. After the rubber plate has been affixed on the backing material, the plate is wrapped in a suitable material 30 such as polyethylene tape for a further curing operation.

The procedure of partially curing a rubber plate with the subsequent completion of the curing process after the plate is mounted on a backing material has been found to substantially reduce cupping of the printing surface normally encountered in bending a fully cured rubber plate. When heat is applied to partially cured rubber in the curved position during the second curing operation, the nerve or memory of the rubber formula is relaxed and sets to the diameter of the curve.

An infra-red heat unit is the preferred means to be used in completing the curing process. Infra-red heat has a decidedly different effect on rubber than conventional steam or electric heat. A significant advantage is that if additional infra-red heat is applied to cured rubber, the surface will not break down or deteriorate as is often the case when steam or electric heat is applied to rubber after an original vulcanization. Steam and electrical heat are directional and to be effective must have direct contact with the rubber, often scorching the rubber surface. Infrared heat can penetrate an insulator such as rubber and be bounced back off of brass or other similar backing material with the refiected wave having nearly the same intensity as the initial wave. The rubber plate can be laid on the reflective surface and will receive infra-red heat on both surfaces almost instantly and with equal intensity.

To complete the curing of the rubber plate, therefore, as shown in FIG. 6c the infra-red rays 31 penetrate the wrapping 30 and the rubber plate 22 to reach the cement and are then reflected by the brass mounting plate. The heat cures the end of the projections at the interface 18 to the backing plate 13 and also completes the curing process of the rubber plate in the curved position without scorching the printing surface or the wrapping. As the infra-red heat is applied to the rubber plate, it expands against the outside wrapping creating a sufiicient overall pressure to secure an even bond between the plate and the backing material. The cupping normally caused by bending a rubber plate to a curved backing material is reduced by the subsequent curing process, as well as by the relief provided on the underside of the plate.

While the term rubber has been used as applied to the material of which the plate is made, it will be understood that this term is used in a generic sense including any material having resilient properties similar to those of rubber, hence the term includes within its scope synthetic rubber and rubber-like plastic materials. Furthermore, although a two dimensional pattern of projections has been shown in the preferred embodiment of the invention, pattern interruptions which run longitudinally and are spaced apart to prevent compressive forces in the peripheral direction at the mounting surface could be used in the alternative. For example, in FIG. 3 the projections 16 could be extended longitudinally along the cylindrical plate.

I claim as my invention:

1. A composite printing plate for use in a printing press, comprising a curved backing member, a layer of rubber having an outer printing surface and an inner surface, said layer of rubber being wrapped about said curved backing member, the inner surface being discontinuous in the peripheral direction to provide uniformlyspaced projections integral with the rubber layer and of the same height to define a mounting surface, and said projections being adhesively secured to the backing mem her and being of a height between one-sixteenth and onequarter of the thickness of the rubber layer.

2. The composite printing plate of claim 1 wherein said projections have a cylindrical shape.

3. The composite printing plate of claim 1 wherein said projections cover one-third to seven-eighths of the surface area of the rubber layer.

4. The composite printing plate of claim 1 wherein the outer printing surface has a higher durorneter hardness rating than the inner surface and said projections being compressible when the printing surface experiences an excessive buildup of pressure.

5. The composite printing plate of claim 4 wherein the outer printing surface has a durorneter hardness rating between 50 and 90 and the inner surface has a durorneter rating ranging from 20 to 30.

6. A composite printing plate for use in a printing press, comprising a curved backing member, a layer of rubber having an outer printing surface and an inner surface, said layer of rubber being wrapped about said curved backing member, the inner surface being discontinuous in the peripheral direction to provide uniformly-spaced projections integral with the rubber layer and of the same height to define a mounting surface, and said projections being adhesively secured to the backing member and covering one-third to seven-eighths of the surface area of the rubber layer.

References Cited UNITED STATES PATENTS 1,119,498 12/1914 Dittman 101401.3 1,559,150 10/1925 Borden 101-4013 X 1,669,416 5/1928 Huebner 101-376 2,741,297 4/1956 Vamuaketis 101395 X 3,216,353 11/1965 Wallenius 101-395 ROBERT E. PULFREY, Primary Examiner.

FRED A. WINANS, Assistant Examiner.

U.S. Cl. X.R. 101-395

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