Printing – Printing members – Rolling contact
Reexamination Certificate
2000-11-13
2002-05-14
Funk, Stephen R. (Department: 2854)
Printing
Printing members
Rolling contact
C101S401100, C101S401300, C101S415100, C101S453000
Reexamination Certificate
active
06386104
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an under sheet for a lithographic printing plate to prevent the deviation (i.e., the skid) of the position of a lithographic printing plate on the plate cylinder of a printer (i.e., a printing press).
BACKGROUND OF THE INVENTION
Printing is generally performed in a lithographic printer by winding a lithographic printing plate around a plate cylinder to fix the printing plate mechanically.
As the support of a lithographic printing plate, materials such as a metal, a plastic film and a paper have been conventionally used. A lithographic printing plate using materials other than metals as a support is excellent in handling property but is defective in dimensional stability as compared with a lithographic printing plate using a metal as a support.
When a lithographic printing plate having a support at least the back surface of which is a material other than a metal is used in a lithographic printer, the grasp position to the front end of the plate cylinder is liable to be made worse since the support is soft. In such a case the accuracy in the longitudinal position (the accuracy along the circumferential direction of the plate cylinder) becomes worse, and a lithographic printing plate is shifted diagonally and fixed in certain circumstances. Further, there is a problem that a deformation occurs partially due to the friction with a plate cylinder during printing, for instance, and the positional accuracy to the printing paper is deteriorated.
Accordingly, the use of a lithographic printing plate having a support at least whose back surface is a material other than a metal is limited to printing of a small number of papers where no problem arises even if a register accuracy of a printed matter is low, and if such a lithographic printing plate is used in multicolor precise printing or in printing of a large number of papers with a large-sized printer, there are cases where the deviation of colors is caused.
On the other hand, a plate-making method and a printing method by CTP (computer to plate) which have been prevailed in recent years are advantageous in that the dimension of imaging (exposure) and the positional accuracy are excellent and the register in multicolor printing is easy as compared with conventional plate-making and printing methods (exposure process of a printing plate material is performed by contact exposure using a lith film).
However, when a lithographic printing plate having a support comprising a material other than a metal such as a plastic film or paper is used, the advantage of easiness of the register in multicolor printing of CTP cannot be put to practical use due to the above drawback of the printing plate.
Further, it is suggested in recent years to interpose a sheet having an initial modulus of elasticity of 29×10
8
Pa (300 kg/mm
2
) or less between a printing plate and a plate cylinder (as described in JP-A-11-20130 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). This sheet comprises fine glass beads and the like adhered and fixed so that central line average roughness (Ra) becomes 2 or more.
However, as is described in JP-A-11-20130 that this sheet can be produced by adhering fine glass beads and the like densely and uniformly on the surface of a sheet-like material, a solution having fine particles dispersed therein in high concentration is required to form concavities and convexities on the surface of a sheet.
These kinds of fine particles are generally expensive and a large quantity of fine particles are necessary to adhere them densely and uniformly, which results in the increase of the production cost of a sheet. Moreover, not only it is difficult to disperse fine particles in a solution in high concentration but also dispersion in high concentration is liable to cause the agglomeration of particles in the solution leading to coarse particles, and there arises a problem that the coarse particles deteriorate printing quality.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an under sheet for a lithographic printing plate which can prevent the deviation of the position between the lithographic printing plate and the under sheet before printing and can be produced inexpensively.
The above object of the present invention can be achieved by an under sheet for a lithographic printing plate which has an initial modulus of elasticity of 34×10
8
Pa or more, and has concavities and convexities having central line average roughness (Ra) of less than 2 &mgr;m and Ra/Rz of 0.05 or more on the surface in contact with the back surface of the lithographic printing plate.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, the skid (i.e., the deviation) of a lithographic printing plate on a plate cylinder caused by the pressure during printing on a lithographic printer can be certainly prevented by an under sheet for a lithographic printing plate which has an initial modulus of elasticity of 34×10
8
Pa or more, and has concavities and convexities having central line average (surface) roughness (Ra) of less than 2 &mgr;m and Ra(center line average roughness)/Rz(ten-point mean roughness) of 0. 05 or more on the surface in contact with the back surface of the lithographic printing plate. Ra and Rz can be measured based on JIS B 0601. The term “Initial modulus of elasticity” in the present invention means the modulus of elasticity in the state before the under sheet is used in printing.
An under sheet in the present invention is used by being interposed between a plate cylinder and a lithographic printing plate having a back surface comprising at least a material other than a metal.
The under sheet in the present invention has an initial modulus of elasticity of 34×10
8
Pa or more, preferably from 34×10
8
to 98×10
8
Pa.
As the support for an under sheet, e.g., a metal plate, a resin sheet, and a composite sheet of a metal and a resin are used, preferably a metal sheet such as an aluminum plate, a zinc plate, a titanium plate, and a stainless steel plate, a bimetallic sheet such as a copper-aluminum plate, a copper-stainless steel plate, and a chromium-copper plate, a trimetallic sheet such as a chromium-copper-aluminum plate, a chromium-lead-iron plate, and a chromium-copper-stainless steel plate, a resin sheet such as a PET sheet, a PE sheet, a PP sheet, a polyester sheet, a polyimide sheet, a polyamide sheet, and an acrylate resin sheet, and a metal-resin composite sheet such as an aluminum-PET sheet, an aluminum-PE sheet, an aluminum-polyester sheet, a titanium-PET sheet, and a titanium-PE sheet, and more preferably a metal sheet such as an aluminum plate and a stainless steel plate, a resin sheet such as a PET sheet and a PE sheet, and a metal-resin composite sheet such as an aluminum-PET sheet and an aluminum-polyester sheet can be exemplified.
The thickness of an under sheet is from 50 to 350 &mgr;m, preferably from 75 to 300 &mgr;m, and more preferably from 100 to 250 &mgr;m.
The concavities and convexities are formed on the surface of the under sheet in contact with the back surface for the lithographic printing plate so that the surface of the under sheet has a central line average (surface) roughness (Ra) of less than 2 &mgr;m, preferably from 0.5 to 1.95 &mgr;m, and more preferably from 1 to 1.95 &mgr;m, and Ra/Rz of 0.05 or more, preferably from 0. 05 to 1, and more preferably from 0. 05 to 0.16. The protrusions can be formed by inorganic fine particles, organic fine particles and organic-inorganic composite fine particles.
As the inorganic fine particles, e.g., a metallic powder, a metallic oxide, a metallic nitride, a metallic sulfide, a metallic carbide and composite compounds of these compounds can be exemplified, preferably a metallic oxide and a metallic sulfide, and more preferably a metallic oxide such as glass, SiO
2
, TiO
2
, ZnO, Fe
2
O
3
, ZrO
2
and SnO
2
, and a metallic sulfide such as ZnS and Cus can be exemplified.
As the organic fine particles, e.g
Kasai Seishi
Kato Eiichi
Fuji Photo Film Co. , Ltd.
Funk Stephen R.
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