Printing – Printing members – Rolling contact
Reexamination Certificate
2000-08-08
2002-06-04
Funk, Stephen R. (Department: 2854)
Printing
Printing members
Rolling contact
C101S382100
Reexamination Certificate
active
06397746
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a camera-ready copy sheet for lithographic printing plates to prevent them from positionally offsetting on plate cylinders in a press.
2. Description of the Related Art
To perform printing with lithographic presses, a lithographic plate is wrapped around a plate cylinder and fixed mechanically.
Conventionally, when lithographic presswork is performed with plates having a non-metallic base on at least the back side as exemplified by a plastic film or paper (coated with a resin on both sides), the softness of the base tends to reduce the accuracy of the positions in which the plate is gripped on to the leading edge of the plate cylinder. If this phenomenon occurs, the accuracy of the plate in the vertical position (i.e., the accuracy in the around-the-cylinder direction) decreases and, in an extreme case, the plate may be fixed slantwise. As a further problem, the friction with the cylinder during printing causes partial distortion in the plate, eventually reducing the accuracy in position relative to the paper to be printed.
Under these circumstances, the use of lithographic plates having a non-metallic base on at least the back side has been limited to the case of short-run work where no problem is caused even if printed pieces have low accuracy in register. On the other hand, exquisite multi-color printing and long-run work on massive presses often fail to achieve the desired color register.
Platemaking and printing operations based on the computer-to-plate (CTP) technology have gained increasing acceptance these days. Compared to the conventional approaches (in which the plate material is subjected to contact exposure with a lith film), the new processes have the advantage of providing good dimensional and positional accuracy for the image (exposure) with respect to the plate material, as well as permitting easy registration in multi-color printing.
Of the two distinctive advantages of the CTP technology, the ease in registration for multi-color printing cannot be realized with lithographic plates based on non-metallic materials such as paper and plastics because they have the problems already described above.
In order to solve the above described problem, it has recently been proposed that a sheeting having an initial elastic modulus of no more than 300 kg/mm
2
should be inserted between the press plate and the plate cylinder [see Unexamined Published Japanese Patent Application (kokai) No. 11-20130)]. The sheet has fine particulate matter such as glass beads adhered and fixed thereto so that it has a center-line average roughness Ra of at least 2.
According to the publication, the sheet is prepared by adhering and fixing fine particulate matter such as glass beads onto the surface of a sheet material at high and uniform density. In other words, a highly concentrated liquid dispersion of fine particles is needed to form asperities on the sheeting.
Since the fine particulate matter of the type used in the patent is generally expensive, adhering and fixing it at high and uniform density inevitably increases the cost of the sheet. In addition, the fine particles cannot be easily dispersed in liquid at high concentration and, what is more, agglomeration often occurs in the highly concentrated dispersion and the resulting coarse particles will deteriorate the quality of printed matter.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a camera-ready copy sheet with which lithographic printing plates can be prevented from being positionally offset on a plate cylinder in a press. In the presence of such camera-ready copy sheet, lithographic plates using bases made of non-metallic materials can be applied to multi-color printing or long-run work. In addition, positional offset of plates can be suppressed by merely providing a small amount of particles of at least two different sizes and hardness values on the surface of the copy sheet and this contributes to both cost reduction and easy production. The larger particles have the added advantage of preventing positional offset from occurring between the lithographic plate and the camera-ready copy sheet before printing starts.
According to a first aspect of the present invention, in a camera-ready copy sheet to be inserted between a plate cylinder and a lithographic printing plate at least the back side of which is made of a non-metallic material, said copy sheet having asperities of a predetermined shape on the front side that are urged against the back side of the lithographic printing plate to depress it, the asperities being formed of projections that consist of at least two groups of particles; a larger particle group having a particle size larger than an intermediate between maximum and minimum particle sizes have an average size at least twice an average size of a smaller particle group having a particle size smaller than said intermediate, and the sum per unit area of maximum cross-sectional areas of planes in the particles of the larger particle group that are parallel to the surface of the sheet ranges from 0.1% to 4% of the unit area. This range is preferably in the range of 0.1 to 3.14%.
In the first aspect of the present invention, the sum per unit area of maximum cross-sectional areas of planes in the particles of the smaller particle group that are parallel to the surface of the sheet preferably ranges from 0.1% to 99.9% of the unit area. This range is preferably in the range of 3 to 80%.
In the following description, the larger particle group having the particle size larger than the intermediate are designated “the larger particles” and the smaller particle group having the particle size smaller than the intermediate are designated “the smaller particles”, and the proportion of unit area that is occupied by the sum per unit area of maximum cross-sectional areas of planes in the particles that are parallel to the surface of the sheet is referred to as “occupied area percentage”.
In the first aspect of the present invention, the larger particles preferably have an average diameter of 5-50 &mgr;m. If the larger particles have an average diameter of 50 &mgr;m, the smaller particles have an average diameter of no more than 25 &mgr;m. If the larger particles have an average diameter of 5 &mgr;m, the smaller particles have an average diameter of no more than 2.5 &mgr;m. In other words, if the larger particles have an average diameter of 5-50 &mgr;m, the smaller particles have an average diameter of no more than 25 &mgr;m, preferably in the range of 0.1-25 &mgr;m, more preferably in the range of 1.0-25 &mgr;m.
In the first aspect of the present invention, the projections that compose the asperities on the camera-ready copy sheet of the invention consist of two or groups of particles, the larger ones being fine inorganic particles such as glass beads or fine particles of comparatively hard polymers such as polystyrene. The smaller particles may be of the same materials as mentioned above; although they may be used in suitably selected amounts, their use is preferably minimized from the viewpoints of cost and production efficiency.
The amount of the smaller particles as relative to the larger particles is preferably in the range of ½ to 1000 times, more preferably 1-200 times, and most preferably 1-25 times, the amount of the larger particles.
If the larger particles are distributed coarsely to occupy 0.1-4% of unit area as set forth above, a comparatively small number of the larger particles suffice to achieve positive prevention of positional offset of lithographic plates on the plate cylinder. By using the smaller particles, the lower limit of the percent area that need be occupied by the larger particles to produce the stated effect is reduced to 0.1 (i.e., the number of the large particles that need be used is reduced) and, at the same, this provides ease in separating adjacent camera-ready copy sheets in a stack. If the larger particles occupy 0.1% of the unit area, the remaining part (99.9%
Kasai Seishi
Nakayama Takao
Ohishi Hiroyuki
Fuji Photo Film Co. , Ltd.
Funk Stephen R.
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