Corrugated board printing plate

Abstract: The invention relates to a printing plate for corrugated board which comprises a rubber or plastic foam sheet having a smooth skin layer on its surface, the thickness of the skin layer being 0.3 to 2.0 mm and the surface hardness of the printing plate is SRIS (C) 35 to 55. Preferably, the surface roughness of the skin layer is not more than JIS R max = 100 µm. Preferably, the printing plate further includes a reinforcing sheet comprising a base cloth and a rubber or resin cladding layer disposed on either side thereof and the interface between the foam sheet and one surface of the base cloth of the reinforcing sheet is separably bonded with comparatively weak adhesion.

Description: Background of the Invention The present invention relates to a printing plate for corrugated board. The hitherto-known printing plates for corrugated board include a vulcanized assembly comprising a face layer and a reverse layer, each made of natural or synthetic rubber with a JIS rubber hardness of 30 to 60, and a textile or other reinforcing sheet interposed between said layers and a vulcanized assembly comprising a reverse rubber layer, a reinforcing sheet, an intermediate rubber layer and a thin face rubber layer built up in the order mentioned, said reverse rubber layer and intermediate rubber layer having a JIS rubber hardness of 30 to 60 and said face rubber layer having a JIS rubber hardness lower by 10 to 20 than the hardness of said intermediate rubber layer (Japanese Utility Model Publication No. 52064/1981). The conventional printing plates for corrugated board are invariably solid rubber plates but there is a lower limit to hardness and it is impossible to lower the hardness to less than the JIS-A rubber hardness of 20. If the hardness is reduced to less than 20, the printing plate will be too soft to preclude an uneven print. On the other hand, if a material plate having a hardness of 30 or more is sculptured to prepare a relief printing plate and a corrugated board is printed using it, the printing pressure will be so high that an excess of printing ink deposits on the leading side of the projections of the plate to yield the so-called marginal zone. Another trouble is that the interliner is crushed by the printing pressure to cause a decrease in board strength. Particularly when the corrugated board is scored for folding and printed across the scores, the printing ink is not readily deposited at the scores to cause a local omission of printing. The attempt to overcome this drawback by increasing the printing pressure results in collapse of the inter-liner. JP-A-56-52064 discloses a printing plate comprising a reverse rubber layer, a reinforcing layer, an intermediate rubber layer and a thin rubber face layer. The reverse rubber layer and the intermediate rubber layer have a JIS rubber hardness of 35 to 60 and the thin rubber face layer has a JIS rubber hardness lower by 10 to 20 than the hardness of the intermediate rubber layer. Thus, the thin rubber face layer has a JIS rubber hardness of 15 to 50. FR-A-2 064 032 discloses a printing plate having a printing layer consisting of a material of low hardness, such as natural or synthetic caoutchouc, a synthetic plastics material or elastomeric material. The printing layer is formed by superposition of several layers of different hardness and elasticity and is bonded to a further layer consisting of a soft, non-extendable material. GB-A-659 924 discloses a multi-layer resilient printing block having a hard printing layer, an underlying softer cushion layer being composed of rubber mixtures and a covering layer which is resistant to shrinking and crumpling. The hardness of the printing layer between 55 and 100° Shore and the hardness of the cushion layer is about 15 to 50° Shore less than the hardness of the printing layer. US-A-2 741 297 discloses a laminated printing plate which is constituted essentially of a relatively thick resilient backing sheet of soft rubber in the unvulcanized state, having adhered to one surface a relatively thinner layer of a hard but flexible blend of a thermosetting resin with a synthetic rubber. It is an object of this invention to provide an improved printing plate free of the above-mentioned disadvantages, wherein a rubber or resin foam sheet is used for reducing the required printing pressure to prevent a decrease in board strength, eliminating the occurrence of said marginal zone and the uneven impression at folding scores to assure a uniform and sharp print and facilitating removal of unwanted areas in sculpturing. Summary of the Invention The above-mentioned and other objects are accomplished by providing a printing plate for printing on corrugated board comprising: a rubber foam or resin foam sheet (1) having a skin layer (1a) with a smooth surface formed on a foam layer (1b), said skin layer (1a) having a thickness in the range of 0.3 to 2.0 mm and a surface roughness JIS B 0601 Rmax not greater than 100 µm, and a surface hardness in the range of SRIS 0101 (c) 35 to 55, wherein SRIS 0101 is identical to JIS K 6301 hardness test, a reinforcing sheet (2) for the foam sheet comprising a base cloth (2a) and a rubber or resin cladding layer (2b, 2c) on either side thereof, the reinforcing sheet (2) being releasably bonded with adhesive to the opposite side of said foam sheet (1). The printing plate of this invention is manufactured by backing a sculptured rubber or plastic foam surface layer with a reinforcing sheet consisting of a base cloth and a rubber or resin cladding layer disposed on either side thereof, with the reinforcing sheet being releasably bonded with adhesive to the foam sheet or releasably bonded to the base cloth. The peeling strength of said releasable interface is 0.5 to 3.0 kg/25 mm and the peeling strength of the remaining unreleasable interface is preferably not less than 4 kg/25 mm. Only if the above bond strength is obtainable, said releasable interface may be constituted by a vulcanized bond, an adhesive bond, a two-sided adhesive tape or any other appropriate bonding means. Brief Description of the Drawings Fig. 1 is a sectional view showing a printing plate for corrugated board according to this invention; Fig. 2 is a sectional view of the printing plate illustrated in Fig. 1 which is provided with a reinforcing sheet; Fig. 3 is a sectional view showing the printing plate illustrated in Fig. 2 which has been sculptured; Fig. 4 is a view similar to Fig. 3, showing a sculptured printing plate as another embodiment of the invention; Fig. 5 is a view similar to Fig. 2, showing a printing plate as another embodiment of the invention; and Fig. 6 is a sectional view showing the printing plate of Fig. 5 which has been sculptured. Detailed Description of the Preferred Embodiments Referring to Fig. 1, a printing plate made of a rubber or resin foam sheet 1 comprises a foam layer 1b and a smooth skin layer 1a disposed on top of said foam layer 1b, said skin layer 1a having a thickness of 0.3 to 2.0 mm and an SRIS 0101 (C) hardness of 35 to 55. The SRIS 0101 (c) hardness test is identical to the JIS K 6301 hardness test. If the surface hardness of the printing plate is less than 35, the plate will be too soft to preclude an uneven print, while an SRIS 0101 (C) hardness in excess of 55 will provide an excess printing pressure as does the conventional solid rubber plate. This, coupled with the provision of a smooth skin layer 1a, assures that the printing ink is deposited evenly on the board surface, producing an even print. However, if the thickness of the skin layer 1a is less than 0.3 mm, the durability will be adversely affected, while if the thickness exceeds 2.0 mm, the dimensional stability of the plate is sacrificed to cause curling and produce an excess printing pressure as does the conventional solid rubber plate. The surface roughness of the skin layer 1a is preferably not more than JIS B 0601 R max = 100 µm. If it exceeds 100 µm, an uneven print will tend to be produced. According to the embodiment shown in Fig. 1, a printing plate was prepared from natural rubber. This printing plate had a skin layer 1a with a thickness of 0.5 mm, a surface hardness of SRIS 0101 (C) 47, a skin layer surface roughness of JIS 10-point mean roughness of Rz = 11 µm and a JIS maximum height of R max = 15 µm. On the other hand, a control solid rubber printing plate was fabricated using the same kind of natural rubber with a JIS-A hardness of 40. Each of these printing plates was sculptured and mounted on the plate drum of a corrugated board printing press and a corrugated board [paper quality RK 220 g/m² x SCP 160 g/m² x MC 200 g/m², B-flute, sized (box inner size) 480 x 373 151 mm, R-form (Type A)] was printed at a printing pressure (clearance between the surface of the printing plate and the drum) of 2.5 mm and a drum speed of 60 RPM and the changes in thickness of the corrugated board and the compressive strengths of corrugated board boxes were compared. The results are shown in Table 1. This invention Control Thickness of corrugated board (mm) Before printing 2.844 2.844 After printing 2.825 2.736 Loss 0.019 0.108 Compressive strength of corrugated board box (kgf) Before printing 164.2 164.2 After printing 161.4 155.0 Loss 2.8 9.2 It is apparent from the above table that with the printing plate of this invention, the loss of thickness was as low as about 18% and the loss of compressive strength as low as about 30%, both compared with the control plate. Substantially no difference was found in the uniformity of impression between the printing plate of this invention and the control printing plate. However, an uneven print developed frequently with a printing plate of a foam sheet having a thin (0.3 mm) skin layer with a JIS 10-point mean roughness of Rz = 37 and a JIS maximum height of R max = 55. Referring to the printing plate illustrated in Fig. 2, this printing plate comprises a surface rubber layer 1 which is a natural rubber foam sheet having a thickness of 6 mm and has a surface skin layer 1a which is 0.5 mm thick. It has a surface JIS 10-point mean roughness of Rz = 11 and a JIS maximum height of R max = 15 µm. The printing plate further comprises a reinforcing sheet 2 which consists of a base cloth 2a and a 0.85 mm thick rubber cladding layer 2b, 2c vulcanized to either side of the base cloth, with the upper rubber cladding layer 2b being bonded to the underside of said surface rubber layer 1 with the aid of a two-sided acrylic adhesive tape in such a manner that it can be peeled off. The peeling strength was 1.2 kg/25 mm. The surface rubber layer 1 of this printing plate was sculptured as shown in Fig. 3. Thus, the unwanted portions of narrow width were formed into shallow recesses 3 by cutting off the surface of the surface rubber layer (rubber foam sheet), while the unwanted portions of broad width were formed into deep recesses 4 by cutting through the surface rubber layer 1 from its surface to the surface of the upper rubber cladding layer 2b of the reinforcing sheet 2 and peeling off the surface rubber layer from the interface. In addition, in order to prevent peeling in the course of printing, the edges of wanted portions were reinforced with a chloroprene rubber adhesive. A control printing plate was fabricated in the same manner except that a solid natural rubber sheet with a JIS-A hardness of 40 was used in lieu of the surface rubber layer 1 and was similarly sculptured. Each of the above printing plates was mounted on the plate drum of a corrugated board printing press and a corrugated board [paper quality RK 220 g/m² x SCP 160 g/m² x MC 200 g/m², B-flute, size (inner size of box) 480 x 373 x 151 mm, R-foam (Type A)] was printed at a printing pressure (clearance between the surface of the printing plate and the drum) of 2.5 mm and a drum speed of 60 RPM. After 17 hours of cumulative operation on an intermittent basis, no peeling was found of the surface rubber layer 1 in the releaf areas. Comparison of changes in the thickness of corrugated board and the compressive strength (JIS Z 0212) of corrugated board boxes showed the following results. This invention Control Thickness of corrugated board (mm) Before printing 2.844 2.844 After printing 2.820 2.736 Loss 0.024 0.108 Compressive strength of corrugated board box (kgf) Before printing 164.2 164.2 After printing 161.7 155.0 Loss 2.5 9.2 It is apparent from the above table that with the printing plate of this invention, the loss of thickness was as low as about 22% and the loss of compressive strength as low as about 27%, both compared with the control plate. Substantially no uneven impression occurred with either printing plate. In the printing plate illustrated in Fig. 4, a base cloth 2a of a reinforcing sheet 2 and an upper rubber cladding layer 2b are separably bonded to each other with a natural rubber adhesive having a comparatively week adhesive power, while the upper rubber cladding layer 2b and the underside of the surface rubber layer 1 are inseparably bonded to each other with a chloroprene rubber adhesive having a comparatively high adhesive power. Then, after incisions are made down to the top of the base cloth 2a, the surface rubber layer 1 together with the upper rubber cladding layer 2b of reinforcing sheet 2 is peeled off. In this case, the rubber foam sheet of surface rubber layer 1 can be more readily peeled off without the risk of breakage of the rubber foam sheet, with the result that the peeled surface is neat and the adhesive power of the adhesive can be increased. In the printing plate illustrated in Fig. 5, a peeling rubber layer 2d is interposed between the upper rubber cladding layer 2b of reinforcing sheet 2 and the base cloth 2a, and the upper rubber cladding layer 2b of reinforcing sheet 2 and the rubber surface layer 1 are initimately bonded to each other while the upper rubber cladding layer 2b of reinforcing sheet 2 and the peeling rubber layer 2d are bonded comparatively weakly to each other. Then, as shown in Fig. 6, incisions are made down to the interface between the upper rubber cladding layer 2b of reinforcing sheet 2 and the peeling rubber layer 2d. Thereafter, the surface rubber layer 1 together with the upper rubber cladding layer 2b of reinforcing sheet 2 is peeled off.

Claims: A printing plate for printing on corrugated board comprising: a rubber foam or resin foam sheet (1) having a skin layer (1a) with a smooth surface formed on a foam layer (1b), said skin layer (1a) having a thickness in the range of 0.3 to 2.0 mm, a surface roughness JIS B 0601 Rmax not greater than 100 µm, and a surface hardness, in the range of SRIS 0101 (c) 35 to 55, a reinforced sheet (2) for the foam sheet comprising a base cloth (2a) and a rubber or resin cladding layer (2b,2c) on either side thereof, the reinforcing sheet (2) being releasably bonded with adhesive to the opposite side of said foam sheet (1). A printing plate as claimed in claim 1, wherein the reinforcing sheet (2) is separably bonded to the foam sheet (1) with a two-sided adhesive tape. A printing plate as claimed in any one of the preceding claims, wherein the foam sheet (1) is inseparably bonded to the cladding layer (2b) of the reinforcing sheet (2), while the cladding layer (2b) is separably bonded to said base cloth. A printing plate as claimed in any one of the preceding claims, wherein said foam sheet (1) is made of natural rubber or polyvinyl chloride. The printing plate as claimed in any one of the preceding claims, wherein the base cloth (2a) is separably bonded to the rubber cladding layer (2b) with a natural rubber adhesive, while the foam sheet (1) is inseparably bonded to the rubber cladding layer (2b) with a chloroprene rubber adhesive. The printing plate as claimed in any one of the preceding claims, wherein the cladding layer (2b) facing the foam layer includes a peeling rubber layer (2d) separably bonded to the cladding layer (2b) and inseparably bonded to the base cloth (2a). A printing plate as claimed in any one of the preceding claims additionally comprising relatively shallow recesses (3) sculptured into the printing plate from the side of the skin layer (1a), the relatively shallow recesses (3) having a depth at least as deep as the thickness of the skin layer (1a) of the foam sheet (1) but not as deep as the thickness of the foam layer (1b), and relatively deep recesses (4) having a depth at least as deep as the thickness of the foam layer (1b) and the skin layer (1a) combined and extending no deeper than the base cloth (2a). The printing plate as claimed in claim 7, wherein the relatively deep recesses (4) are formed by cutting the printing plate through the skin layer (1a) and the foam layer (1b) to a depth at least as deep as the cladding layer (2b) and peeling away at least the foam sheet (1b) in portions delineated by the cutting of the printing plate. The printing plate as claimed in claim 8, wherein the relatively deep recesses (4) are formed by cutting the printing plate past the cladding layer (2b) to the base cloth (2a) and peeling away the foam sheet (1b) and cladding layer (2b) in portions delineated by the cutting of the printing plate. The printing plate as claimed in claim 7, wherein the relatively deep recesses (4) are formed by cutting through the skin layer (1a) and foam layer (1b) only to the peeling rubber layer (2d) and then peeling away those portions of the cladding layer (2b) and foam layer (1b) delineated by the cutting of the printing plate.

Inventors: INOUE SHUJI, INOUE,SHUJI
Applicants: KUREHA ELASTOMER CO LTD, KUREHA ELASTOMER CO., LTD.
Application year: 1988
Main classification: B41N1
IPC codes: B41N1