JP2001113852A - Block copy sheet for lithographic printing plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a block copy sheet for a lithographic printing plate which prevents a positional slippage of the plate on a plate cylinder of a printing machine and facilitates correction of distortion of the plate or position thereof between the printing plate and the block copy sheet and which is easy to separate from another in a stacked state. SOLUTION: A block copy sheet for a lithographic printing plate is interposed between a plate cylinder and the lithographic printing plate of which the rear, at least, is constituted of a material other than metal. Indentations in a required shape which are brought into pressure contact with the rear of the plate so as to indent the rear are provided in the surface of the sheet and projections constituting these indentations are formed of a plurality of grain groups. In the block copy sheet thus constituted, the mean grain size of the grain group of which the grain sizes are larger than the intermediate value between the maximum and minimum grain sizes is twice as large as or larger than that of the grain group of which the grain sizes are smaller than the above intermediate value, and the total amount of the projections formed of the grains, having the mean grain size of the grain group of which the grain sizes are larger than the intermediate value, per the unit area of the maximum cross-sectional area on the side parallel to the surface of the sheet, is made 0.1% or more in the unit area.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithographic printing plate undersheet for preventing a lithographic printing plate from being displaced on a plate cylinder of a printing press.

[0002]

2. Description of the Related Art Generally, in a lithographic printing press,
The lithographic printing plate is wound around the plate cylinder and mechanically fixed, and printing is performed.

Hitherto, as a support for a lithographic printing plate, a support made of a material such as metal, plastic film, and paper has been used. A lithographic printing plate using a material other than a metal as a support is excellent in handleability, but lacks dimensional stability, as compared with a lithographic printing plate using a metal as a support.

[0004]

In a lithographic printing press, when a lithographic printing plate having at least a back surface made of a material other than metal as a support is used, the support is soft and the front end of the plate cylinder is bitten. The position tends to be poor, in which case the vertical position accuracy (the accuracy along the circumferential direction of the plate cylinder) is deteriorated, and in some cases, the plate may be fixed at an angle. Further, for example, there is a problem in that distortion is partially generated due to friction with the plate cylinder during printing, and the positional accuracy with respect to paper is reduced.

Therefore, a lithographic printing plate having at least a back surface made of a material other than metal as a support is used only for printing a small number of sheets which do not cause a problem even if the registration accuracy of the printed matter is low. When printing a large number of sheets using a large printing machine, color shift may occur.

On the other hand, CTP (Computer
The plate making and printing method using plate to plate is compared with the conventional conventional plate making and printing method (the exposure process of the plate material is based on contact exposure using a lith film). And the advantage is that registration in multicolor printing is easy.

However, in the case of a lithographic printing plate using a material other than metal such as a plastic film or paper as a support as described above, the lithographic printing plate has the above-mentioned problems,
The advantage of easy registration in multicolor printing of P is not utilized.

Recently, it has been proposed that a sheet material having an initial elastic modulus of 300 kg / mm ² or less be interposed between the printing plate and the printing cylinder in order to solve the above-mentioned problems (see, for example, Japanese Patent Application Laid-Open No. H11-157556). JP-A-11-20130). This sheet
Fine glass beads and the like are bonded and fixed so that the center line average roughness Ra is 2 or more.

[0009] However, as described in the publication, this sheet material can be formed by densely and uniformly fixing fine glass beads or the like to the surface of the sheet material, and the sheet material has irregularities. To do so, a liquid in which fine particles are dispersed at a high concentration is required.

Since fine particles of this kind are generally expensive, a large amount of fine particles are required to fix them densely and uniformly as described above, which leads to an increase in sheet cost and a high concentration of fine particles. Not only is it difficult to disperse in the liquid, but also in the case of high-concentration dispersion, aggregation tends to occur in the liquid to form coarse particles, which causes a problem that the coarse particles deteriorate the print quality.

An object of the present invention is to reliably prevent the lithographic printing plate from being displaced on a plate cylinder of a printing press, whereby, for example, a lithographic printing plate having a support other than metal as a multicolor printing plate can be used. Or, a lithographic printing plate that can be applied to printing of a large number of sheets, can suppress plate misregistration, is excellent in cost reduction and manufacturing suitability, and can easily separate an underlay sheet from a laminated state. An object of the present invention is to provide an under-print sheet.

[0012]

The inventor of the present invention has conducted various studies to solve the above-mentioned problems, and as a result, it has been found that printing is performed only by attaching two or more types of particles, large and small, to the underlay sheet surface under specific conditions. The present invention has been found that, during lithographic printing by a press, the lithographic printing plate can be reliably prevented from being displaced on the plate cylinder due to, for example, pressure applied during printing, and facilitates separation of the underlay sheet in a laminated state. Was completed.

That is, the present invention provides a plate cylinder and at least a back surface which is interposed between lithographic printing plates made of a material other than metal, and pressed against the back surface of the lithographic printing plate to depress the back surface of the lithographic printing plate. Is provided on the surface,
In the lithographic printing plate underlay sheet in which the projections constituting the irregularities are composed of a plurality of particle groups, the average particle diameter of a particle group having a particle diameter larger than the intermediate value between the maximum particle diameter and the minimum particle diameter is larger than the intermediate value. The unit of the maximum cross-sectional area of the surface parallel to the sheet surface of the projection made of particles having an average particle diameter of the particle group having a particle diameter larger than the intermediate value, which is at least twice the average particle diameter of the particle group having a small diameter. A lithographic printing plate underlay sheet, wherein the total per area is 0.1 or more per unit area.

In the following description, particles having a particle size larger than the intermediate value are referred to as large particles, and particles having a particle size smaller than the intermediate value are referred to as small particles. Further, the ratio of the sum of the maximum cross-sectional area of the surface parallel to the sheet surface of each particle per unit area to the unit area is called an area occupancy, and is calculated as follows. A photograph of the surface of the sample is taken from directly above with an optical microscope, and the number n of particles (large particles or small particles) existing in S in a certain area is read. Average diameter R of this particle
The area occupancy is obtained from Area occupancy = ^{[n × (πR 2/4)} / S ] × 100
(%)

In the present invention, the large particles have an average diameter of 5 to 5.
It is preferably 50 μm. If it exceeds 50 μm, printing unevenness is likely to occur, which is not preferable. In the present invention, when the average diameter of the large particles is 50 μm, the average diameter of the small particles is 25 μm or less, and the average diameter of the large particles is 5 μm.
When m, the average diameter of the small particles is 2.5 μm. That is, when the average diameter of the large particles is 5 to 50 μm, the average diameter of the small particles is 25 μm or less, the preferable average diameter of the small particles is in the range of 0.1 to 25 μm, and more preferably 1.0 to 25 μm. Range.

In the present invention, the projections constituting the unevenness of the underlay sheet are composed of a plurality of particle groups, and the large particles are, for example, inorganic fine particles such as glass beads or polymer fine particles having relatively high hardness such as polystyrene. Can be As the small particles, particles of the same material as described above can be used, and the amount of the particles can be appropriately selected, but is preferably small in view of cost and production suitability.

In the present invention, as described above, if the area occupation ratio of the large particles is 0.1% or more, the displacement on the plate cylinder can be surely prevented. In the present invention, by using the small particles together with the large particles, it is possible to lower the lower limit of the area occupancy of the large particles required to exhibit the above-described effect to 0.1% (that is, to reduce the number of the large particles). In addition, the underlay sheet can be easily separated from the laminated state.

Although the distribution of the large particles is not particularly limited, as described above, when the average diameter of the large particles exceeds 50 μm, printing unevenness is likely to occur. It is preferable that the particles are provided on the underlaying sheet surface so that the particles do not agglomerate during the formation of the projections. In this case, the upper limit of the area occupancy of the large particles is not particularly limited, but is preferably 90% or less. Also, by setting the area occupation ratio of the large particles to 4% or less, the overlapping of the particles can be effectively prevented, and the occurrence of printing unevenness can be suppressed.

The content of the small particles is not particularly limited. For example, when the area occupation ratio of the large particles is 0.1%, the small particles may occupy the remaining region. In this case, the area occupancy of the small particles is 99.9%. In addition, small particles are 0.1
If the area occupation ratio is not less than%, the above-described effect can be obtained. Particularly preferably, it is 3% to 80%.

The addition amount of the small particles to the large particles is preferably in the range of 1/2 to 1000 times, more preferably in the range of 1 to 200 times, and particularly preferably in the range of 1 to 25 times. .

In the underlaying sheet for lithographic printing plate according to the present invention, the small particles intervene, so that the large particles forming the large projections constituting the irregularities of the required shape provided on the surface are reduced as described above. However, it is enough to prevent misalignment, and the large projections formed by the large particles are pressed into contact with the back surface of the lithographic printing plate, so that the back surface of the lithographic printing plate is dented and bitten. Therefore, during printing by the printing press, it is possible to reliably prevent the lithographic printing plate from being displaced on the plate cylinder due to, for example, pressure applied during printing. By specifying the press contact portion with the back surface of the printing plate as in the underlay sheet of the present invention, the printing unevenness of the printed matter can be reduced to a level having no practical problem. Further, the effect that the stacked sheets are easily separated by the interposition of the small particles can be obtained.

In the present invention, the reason why the back surface of the lithographic printing plate is depressed is that the lithographic printing plate and the underlay sheet are wound around the plate cylinder while the underlay sheet is sandwiched between the lithographic printing plate and the plate cylinder. In the step of sandwiching the underlay sheet between the lithographic printing plate and the plate cylinder, the back surface of the lithographic printing plate may be recessed only when the printing pressure is applied after sandwiching.

The type of the lithographic printing plate used in the present invention is not particularly limited.
Plates, those having a silver diffusible photosensitive layer, those using electrophotographic plate making, and the like.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION As a method of forming irregularities on the surface of a support of an underlay sheet, a particle group of glass or the like harder than a support of a lithographic printing plate (particles having a particle size of an intermediate value between the maximum particle size and the minimum particle size) is used. Particles having an average particle diameter of a group of particles having a large diameter is at least twice the average particle diameter of a group of particles having a particle diameter smaller than the above-mentioned intermediate value) are fixed to the surface of a support under the plate to form irregularities. Method.

Specific examples of the method of forming the irregularities by fixing the particles on the surface of the support of the underlay sheet include a method of coating and drying a liquid in which the particles are dispersed in a binder, and a method of applying a mechanical pressure after forming the binder film. And a method of electrodepositing particles after forming the binder film.

In order to form unevenness with high uniformity particularly by suppressing aggregation, a method of dispersing and coating in the form of an aqueous dispersion in the presence of a latex containing a trace amount of a surfactant, or a method of using a large amount of binder is generally used. A method of dispersing coating in the coexistence is preferable. It is also effective to add a surfactant alone.

Examples of latexes or binders that can be used include polyamide, polyolefin, ethyl acrylate-ethyl methacrylate copolymer, acrylonitrile-methyl methacrylate copolymer, amylose acetate,
Styrene-butadiene copolymer, polycarbonate, polyvinyl formate, poly-p-chlorostyrene, polyvinyl acetate, polydimethylsiloxane, polystyrene, polyethyl acrylate, polyacrylonitrile, polyacenaphthylene, 1,4-polyisoprene, Poly-p-isopropylstyrene, polyethylene terephthalate, polyethylene naphthalate, polyethylene, polyvinyl chloride, polyoxymethylene, polypropylene oxide, polybutyl methacrylate, polyethyl methacrylate, polyethyl methacrylate-2-ethylbutyl, polymethacrylic acid-n
-Butyl, polymethyl methacrylate, poly-n-lauryl methacrylate, poly-α-methylstyrene, poly-p
-Methylstyrene, poly-o-methoxystyrene, poly-p-methoxystyrene, polytetrahydrofuran, polyvinyl alcohol, poly-N-vinylcarbazole,
Examples thereof include poly-1-vinylnaphthalene, poly-2-vinylnaphthalene, polyvinylbiphenyl, poly-2-vinylpyridine, polyphenylene oxide, polybutadiene, polybutene, polybutene oxide, polypropylene, and copolymers thereof. Of these, polyethylene terephthalate (PETP) films are most preferred.

As the surfactant, soap (metal salt of higher fatty acid), higher alcohol sulfate,
α-olefin sulfates, anionic surfactants such as alkylbenzene sulfonates, higher alcohol ethylene oxide adducts, alkylphenol ethylene oxide adducts, nonionic surfactants such as fatty acid diethanolamide, pluronic nonionic detergents, amino acids Examples include amphoteric surfactants such as amphoteric detergents, pedine-type amphoteric detergents, and Ultravon-type amphoteric detergents.

Further, a thin layer (overcoat layer) may be provided on the underlay sheet on which the projections obtained as described above are formed, for the purpose of further preventing falling off. The overcoat layer can be provided by a method such as spray coating or bar coating.

The support of the underlaying sheet may be any as long as it has excellent fit with the plate cylinder. That is, for example,
Examples include plastics such as polyethylene terephthalate, polypropylene, and polyethylene, metals such as aluminum and SUS, and paper, synthetic paper, and cloth. The thickness of these supports used is preferably in the range of 30 to 500 μm.

As a method of fixing the underlay sheet to the plate cylinder, an adhesive layer is provided on the back surface of the support of the underlay sheet, and an adhesive or an adhesive such as a spray paste or a double-sided tape is used for the adhesive layer. Method. Also, there is a method in which the underlay sheet is not provided with an adhesive layer, and the leading end and the rear end of the underlay sheet are locked by claws provided on the plate cylinder. Alternatively, a method in which they are combined can be used.

The operation of the present embodiment will be described. At the time of printing by a lithographic printing press, each lithographic printing plate is mounted on a plate cylinder of each printing unit via an underlay sheet.
At this time, each underplate sheet is pressed into contact with the back surface of the lithographic printing plate, so that the projections constituting the irregularities on the surface of the underplate support are cut into the back surface of the lithographic printing plate to be recessed. Thus, each under-sheet adjusts the printing pressure of the blanket cylinder and the impression cylinder, respectively, and also prevents the lithographic printing plate from being displaced on the plate cylinder due to the pressurization.

Next, a method of preventing a lithographic printing plate from shifting will be described. An underlay sheet provided with irregularities of a required shape on the surface of the support is interposed between the lithographic printing plate and the plate cylinder of each printing unit. At this time, the unevenness on the surface of each underlay sheet is pressed against the back surface of each lithographic printing plate, and the unevenness on the surface of each underlay sheet is cut into the back surface of each lithographic printing plate. Thereby, the back surface of each lithographic printing plate is recessed in accordance with the unevenness of each underlay sheet.

[0034]

EXAMPLE As a comparative example, an underlay sheet of this embodiment and an underlay sheet having irregularities on its surface outside the scope of the present invention were actually printed by a lithographic printing press, and lithographic printing was performed on a plate cylinder. The displacement of the plate was measured. Specific conditions and results are shown below.

Example 1 A 100 μm thick polyethylene terephthalate (PE)
A solution in which the following formulation was dispersed by a homogenizer was applied to the surface of the support composed of T) by roller coating. An underlay sheet of the present embodiment having a convex portion made of glass particles on the surface was prepared. Glass particles (GB made by Toshiba Glass
731) was converted to 1.5 to 60 μm with a powder centrifuge.
And the average diameter is 3μm, 5μm, 20μ
A mixture was prepared by mixing the same amount of large particles of m, 40 μm, 50 μm, and 60 μm and small particles having an average diameter of の of the large particles. In addition, a mixture of small particles 10 times the amount of large particles was prepared. Acrylic resin was used as the binder. By changing the mixing ratio in a system consisting only of large particles, a formulation was prepared in which the area occupancy was within a predetermined range, and small particles were added thereto.

Glass beads: large particles (diameter 3 to 60 μm) X (0.1 to 0.3) g average diameter 3 μm 0.1 g average diameter 5 μm 0.1 g average diameter 20 μm 0.2 g average diameter 40 μm 0.2 g average Diameter 50μm 0.3g average diameter 60μm 0.3g

Glass beads: small particles (1.5 to 30 μm in diameter) mixed in the same amount as X (Example) Average diameter 1.5 μm 0.1 g Average diameter 2.5 μm 0.1 g Average diameter 10 μm 0.2 g Average diameter 20 μm 0.2g Average diameter 25μm 0.3g Average diameter 30μm 0.3g

Mixing 10 times the amount of X (Example) Average diameter 1.5 μm 1 g Average diameter 2.5 μm 1 g Average diameter 10 μm 2 g Average diameter 20 μm 2 g Average diameter 25 μm 3 g Average diameter 30 μm 3 g Acrylic resin (40% toluene solution) 20 g 80 g of toluene

As a lithographic printing plate, a silver-diffusible photosensitive material having a thickness of 100 μm made of polyethylene terephthalate (PET) as a support, Super Master Plus (130 μm in total thickness) manufactured by Agfagewald Co., Ltd. was made by a dedicated plate maker SPM415. What was done was used. The lithographic printing plate can be obtained by a direct drawing printing plate in which an image receiving layer is provided on a support other than a metal or an electrophotographic plate making using an electrophotographic method.

Next, the obtained underlay sheet and planographic printing plate were each cut to a width of 560 mm and a length of 400 mm, and the planographic printing plate was so cut that the uneven surface of the underlay sheet was in contact with the back surface of the planographic printing plate. The printing plate and the underlay sheet were stacked. The stacked lithographic printing plate and underlay sheet were mounted on a plate cylinder of a single-sided printing press of Oliver 52 manufactured by Sakurai Co., Ltd.
Printed.

Before printing, the surface of the lithographic printing plate was squeezed with a sponge impregnated with the treatment liquid G671c. As the fountain solution on the printing press, the treatment solution G671c was mixed with water at 1:
The diluted one was used, and as the ink, New Champion F gloss 85 manufactured by Dainippon Ink and Chemicals, Inc. was used.

After printing, the position of the ruled line printed on the printed matter immediately after the start of printing is compared with the position of the ruled line printed on the printed matter after printing 2,000 sheets. The misalignment of the lithographic printing plate above was measured.

COMPARATIVE EXAMPLE 1 As an underlaying sheet, the surface of a support made of polyethylene terephthalate (PET) having the same thickness of 100 μm as in the above example was coated on a surface of 5 μm, 20 μm, 40 μm and 50 μm.
Using each of the particles alone, an underlay sheet was prepared. Using the obtained underlaying sheets, 2,000 sheets were printed in the same lithographic printing plate and printing machine as in the above example, in the same manner as in the above example. Tables 1 and 2 show the evaluation results of the examples. The symbols in each column indicate, from the left, misregistration, printing unevenness, and ease of position correction.

The evaluation criteria in each table are as follows. "Evaluation Criteria" Misregistration less than 30 μm: ◎ 30 μm or more and less than 50 μm: ○ 50 μm or more and less than 100 μm: Δ 100 μm or more: × Printing unevenness None: ◎ Slightly: ○ Many: △ Extremely easy: × Easy: ◎ Easy: ○ Slightly difficult: △ Difficult: ×

In addition to the misregistration, print unevenness on the printed matter was evaluated. The printing unevenness is interpreted as follows. If there are coarse particles on the surface of the underlay sheet, the coarse particles deform the support of the soft printing plate such as PET and push up the surface of the printing plate in that part in a state where the printing plate is overlaid and printed. It is interpreted that spot-shaped printing stains are generated, and as a result, printing unevenness is confirmed on printed matter.

[0046]

[Table 1]

As is clear from the results shown in Table 1, when the small particles are added in the same amount as the large particles in addition to the large particles having a particle diameter in the range of 5 to 50 μm, the area occupancy is 0.1 to 4 μm. .
In the range of 0, the misregistration was less than 50 μm, there was no printing unevenness, the position of the plate was easily corrected, and good results were obtained. When the area occupancy was in the range of 0.1 to 3.14, the misregistration was less than 30 μm, there was no printing unevenness, and the position of the plate was extremely easy to correct, and particularly good results were obtained. From this, it can be seen that when the same amount of the small particles as the large particles is added, good printing can be performed even if the number of the large particles is small. In addition, the underlay sheets in the stacked state were easily separated, and double feeding to the printing apparatus could be prevented.

[0048]

[Table 2]

As is clear from the results shown in Table 2, when the small particles were added in an amount 10 times the large particles in addition to the large particles having a particle diameter in the range of 5 to 50 μm, the area occupancy was 0.1 to 0.1 μm.
In the range of 4.0, the misregistration was less than 50 μm, there was no printing unevenness, the position of the plate was easily corrected, and good results were obtained. When the area occupancy is in the range of 0.1 to 3.14,
The misregistration was less than 30 μm, there was no printing unevenness, and the position of the plate was extremely easy to correct, and particularly good results were obtained. From this, it can be seen that if the small particles are added in an amount 10 times that of the large particles, good printing can be performed even if the number of the large particles is small. In addition, the underlay sheets in the stacked state were easily separated, and double feeding to the printing apparatus could be prevented. Further, from the results of Tables 1 and 2, it is estimated that the same effects as described above can be obtained if the added amount of the small particles is in the same amount to 10 times the amount of the large particles.

On the other hand, the particle diameters of the comparative examples 5 μm, 20 μm,
In the underlay sheet prepared by using particles of 0 μm and 50 μm alone, each item of misregistration, printing unevenness, and ease of position correction did not satisfy the level of ◎.

Particularly, an underlay sheet using particles having a particle size of 5 μm alone has a large misregistration, and an underlay sheet using particles having a particle size of 50 μm alone has many printing irregularities. Above was difficult.

Next, the ease of separation of the underlay sheet from the layered state was evaluated by changing the area occupancy of the small particles. Table 3 shows the results. As for the symbol in each column, ◎ is extremely good,
が is good and X is bad.

[0053]

[Table 3]

As shown in Table 3, when the average diameter of the small particles is in the range of 1.5 to 25 μm, the area occupancy is in the range of 0.1 to 9
In the range of 9.9%, the underlay sheet was easily separated from the laminated state. In particular, when the area occupancy was in the range of 30 to 80%, separation was extremely easy. In contrast, when the average diameter of the small particles was 30 μm and the area occupancy was 0.05%, it was difficult to separate the underlay sheet from the laminated state. Accordingly, small particles having an average diameter of 25 μm or less are occupied by an area occupancy of 0.1 to 9%.
When it is mixed in the range of 9.9%, preferably in the range of 30% to 80%, it can be seen that the underlay sheet is easily separated from the laminated state, and double feed can be prevented.

From the above results, the underlay sheet of the present invention, in which the area occupancy of large particles is in the range of 0.1 to 4%, indicates that misregistration and uneven printing (non-image area) Dirt)
And the ease of correcting the position of the plate and the ease of separation from the laminated state can be satisfied, which is preferable.

Example 2 Thickness 10 with improved coatability and adhesion by corona treatment
A mixture of particles and a binder was applied to the surface of a support made of polyethylene phthalate (PET) having a thickness of 0 μm in the same manner as in Example 1 to prepare an underlay sheet. Except that the classified glass particles used in Example 1 were used, and the mixing ratio of large and small particles and the binder were changed to acrylic latex, the formulation was exactly the same as in Example 1, and the binder ratio was also the same. The mixed solution was dispersed using a homogenizer (manufactured by Nippon Seiki Seisakusho) at 5000 rpm for 1 minute, and then dispersed.
Ultrasound was continued until just before application with a doctor blade to prevent reaggregation of the particles. Table 4 below shows the relationship between the area occupancy of the large particles and the average diameter of the large particles for the system in which the same amount of the small particles as the large particles was added.

[0057]

[Table 4]

As is apparent from Table 4, if the small particles are added to the large particles in the range of 5 to 50 μm, and if the area ratio occupied by the large particles is 0.1% or more, the underlay sheet is used. It can be seen that the performance as can be fully exhibited. In addition,
Observation of the underlay sheet obtained in Example 4 with a microscope revealed that most of the particles were present alone without agglomeration.

[0059]

According to the present invention, the plate cylinder and at least the back surface are interposed between lithographic printing plates made of a material other than metal,
The surface of the lithographic printing plate is provided with unevenness having a required shape to dent the back surface of the lithographic printing plate by being pressed against the back surface of the lithographic printing plate. The average particle diameter of a particle group having a particle diameter larger than the intermediate value between the maximum particle diameter and the minimum particle diameter is at least twice the average particle diameter of the particle group having a particle diameter smaller than the intermediate value, and When the total of the maximum cross-sectional area per unit area of the plane parallel to the sheet surface per unit area of the projections composed of the particles having the average particle diameter of the large particle group is 0.1 or more in the unit area, the misregistration occurs. In this case, an underlay sheet having the above-mentioned characteristics, reduced costs, eliminated printing unevenness, and excellent manufacturing suitability can be obtained.

According to the present invention, the twist and the position between the lithographic printing plate and the underlay sheet before printing can be easily corrected by the presence of the large particles, and the large particles can be corrected by the presence of the small particles. Can be set to a small amount, thereby not only reducing costs and eliminating printing unevenness, but also has an excellent effect of making it easier to separate the underlay sheets in a laminated state.

Claims (2)

[Claims] An unevenness having a required shape which is interposed between a plate cylinder and a lithographic printing plate having at least a back surface made of a material other than metal and pressed against the back surface of the lithographic printing plate to dent the back surface of the lithographic printing plate. Is provided on the surface, the projections constituting the unevenness in the lithographic printing plate under sheet consisting of a plurality of particle groups, the average particle diameter of the particle group having a particle diameter larger than the intermediate value between the maximum particle diameter and the minimum particle diameter is A surface parallel to the sheet surface of a projection made of particles having an average particle diameter of at least twice the average particle diameter of the particle group having a particle diameter smaller than the intermediate value and having a particle diameter larger than the intermediate value. Wherein the total of the maximum cross-sectional area per unit area is 0.1% or more of the unit area. 2. The sum of the maximum cross-sectional area per unit area of the surface parallel to the sheet surface of particles having an average particle diameter of a particle group having a particle diameter smaller than the intermediate value is 0.1 to 99 in the unit area. .9
%. The lithographic printing plate underlay sheet according to claim 1, which is in the range of%.