Printing – Planographic – Lithographic printing plates
Reexamination Certificate
2001-03-28
2003-05-27
Funk, Stephen R. (Department: 2854)
Printing
Planographic
Lithographic printing plates
C101S458000
Reexamination Certificate
active
06568325
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to supports for lithographic printing plates, particularly to supports that can be processed into lithographic printing plates having longer press life. More specifically, the invention relates to (1) supports that can be processed into lithographic printing plates having longer press life, higher stain resistance and better surface quality, (2) supports that can be obtained by the process comprising efficient electrochemical graining treatment and which can be processed into lithographic printing plates that have longer press life and which retain this property even after the plate surface is wiped with a plate cleaning solution, and (3) supports that can be processed into lithographic printing plates having longer press life and higher resistance to aggressive ink staining.
BACKGROUND OF THE ART
Photosensitive lithographic printing plates using aluminum alloy plates as supports are extensively used in offset printing. Such lithographic printing plates are prepared by processing presensitized plates. Generally, the presensitized plate is made by roughening the surface of an aluminum alloy plate, anodizing it, applying a photosensitive solution, and drying the applied coat to form a photosensitive layer. The presensitized plate is exposed imagewise, whereupon the exposed areas of the photosensitive layer change in physical properties. The photosensitive layer is then treated with a developer solution so that it is removed from the exposed areas (if the presensitized plate is positive-acting) or from the unexposed areas (if the presensitized plate is negative-acting). The areas from which the photosensitive layer has been removed are hydrophilic nonimage areas and the areas where the photosensitive layer remains intact are ink-receptive image areas. Thus, presensitized plates are processed into lithographic printing plates using the changes in the physical properties of the photosensitive layer that take place upon exposure.
The lithographic printing plate is then mounted on the plate cylinder for printing. In printing, an ink and a fountain solution are supplied to the surface of the plate. The ink adheres only to the image areas of the plate and the image is transferred to the blanket cylinder, from which it is transferred to the substrate such as paper, thereby completing the printing process.
Aluminum alloy plates are conventionally grained by three known techniques, mechanical (e.g. ball graining and brush graining), electrochemical (electrolytic etching with a liquid electrolyte based on hydrochloric acid, nitric acid, etc.; this technique is also hereunder referred to as “electrolytic graining”), and chemical (etching with an acid or alkali solution). Since the plate surfaces prepared by electrolytic graining have homogeneous pits and exhibit better printing performance, it is common today to combine the electrolytic graining method with another method such as mechanical graining or chemical graining.
By electrolytic graining, aluminum alloy plates acquire roughened surfaces that have not only “wavy” or “wrinkled” asperities to an average surface roughness Ra of about 0.30-1.0 &mgr;m but also pits in honeycomb or crater form that are about 0.2-20 &mgr;m in diameter and about 0.05-1 &mgr;m in depth. If the “wavy” or “wrinkled” asperities in the roughened surface obtained by the electrolytic graining method are not adequately uniform, it is preferably combined with the mechanical and/or chemical graining method to increase the uniformity of the asperities.
If the pits formed in the plate surface by graining are not uniform in diameter or depth, several defects occur and this problem is hereunder described with reference to
FIGS. 1 and 2
which show schematically the cross-sectional structure of a conventional presensitized plate indicated by
10
. As shown, the presensitized plate
10
consists of an aluminum alloy support plate
12
having pits P formed in its surface which in turn is coated with a photosensitive layer
14
. First suppose that the pits P do not have uniform depth in the direction of exposure (see FIG.
1
A); if the area where a deeper pit P′ is formed is exposed, halation (nonuniform scattering of light) occurs (see
FIG. 1B
) and not only the exposed areas but also the unexposed areas change in physical properties (see FIG.
1
C). This may produce “fog” in the printed image. If the presensitized plate is exposed over a wide area including pits P and the deeper pit P′ (see FIG.
2
A), the exposure at the bottom of the pit P′ which is far from the light source (see
FIG. 2B
) may turn out insufficient to produce a yet-to-be exposed portion in the “exposed areas” (see FIG.
2
C). The areas from which the photosensitive layer have been removed should inherently become nonimage areas but on account of such yet-to-be exposed portion, the nonimage areas will partly show the characteristics of the image areas. This portion is most likely to become the start point for staining to occur during printing.
Another problem with the nonuniformity of the asperities in the roughened plate surface is decreased adhesion between the photosensitive layer and the support, which in turn leads to a shorter press life of the lithographic printing plate. While direct imaging presensitized plates (for laser platemaking) are drawing increasing attention these days, longer press life is more desired since the adhesiveness of the photosensitive layer to the support is more susceptible than the photosensitive layer of the conventional presensitized plate which requires photographic films during platemaking. Uniformity of the asperities in the plate surface is extremely important to laser platemaking since insufficient exposure is all the more likely to occur.
Therefore, when roughening the surface of an aluminum alloy plate, pits that have appropriate depth and diameter and which are uniform in size must be generated uniformly in the entire surface of the support so that the photosensitive layer adheres strongly to the support while allowing the aluminum alloy plate to hold more water. The deeper the pits, the stronger the adhesion between the photosensitive layer and the support.
As mentioned above, the nonuniformity of the roughened surfaces of supports for lithographic printing plates have considerable effects on press life and other parameters to the printing performance of lithographic printing plates. In order to deal with this problem, many proposals have been made that try to eliminate the nonuniformity by changing the aluminum alloy composition of the plates. Many proposals have also been made concerning the waveform and frequency of the power supply for electrolytic graining.
In offset printing, ink is not directly transferred from the plate onto the substrate such as paper; instead, as shown in
FIG. 3
, ink
4
on the lithographic printing plate
1
wrapped around a plate cylinder
5
is first transferred to an elastic rubber coat (blanket
2
) wrapped around a transfer cylinder
6
and the blanket
2
carrying the layer of ink
4
and the substrate
3
supplied by an impression cylinder
7
are brought into contact under sufficient pressure to perform printing.
If the pits in the nonimage areas are not uniform, the fountain solution is only insufficiently held in the nonimage areas to prevent the ingress of ink which, therefore, adheres to the nonimage areas of the plate surface to stain it. The stain is transferred to the blanket and eventually appears as stain on the print. In order to prevent this problem of stained prints, the pressman who has noted a stain on the blanket usually stops the press, cleans off the ink from the nonimage areas and supplies an increased amount of the fountain solution to prevent further staining of the plate surface. Cleaning is done by wiping the entire plate surface including both image and nonimage areas with a sponge imbibed with a suitable amount of an acidic or alkali liquid plate cleaner. This removes the ink adhering to the nonimage areas of the plate surface.
However
Sawada Hirokazu
Uesugi Akio
Burns Doane , Swecker, Mathis LLP
Funk Stephen R.
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