Method of producing support for planographic printing plate

Disclosed are (i) a method of producing a support for planographic printing plate, which comprises melting an aluminum ingot having an aluminum content of not less than 99.7 wt % to prepare a cast ingot, scalping the surface of the cast ingot, soaking the scalped cast ingot, cold rolling the soaked ingot to a thickness of 0.1 to 0.5 mm, correction of the resulting sheet to prepare an aluminum support, and then graining the aluminum support and (ii) a method of producing a support for planographic printing plate, which comprises melting an aluminum ingot having an aluminum content of not less than 99.7 wt % to prepare a cast ingot in a melt holding furnace, directly subjecting the cast ingot to continuous casting to prepare a thin sheet having a thickness of 2 to 30 mm, cold rolling the thin sheet, correction of the resulting sheet to prepare an aluminum support, and then graining the aluminum support.

FIELD OF THE INVENTION 
The present invention relates to a method of producing a support for 
planographic printing plate and more particularly relates to a method of 
producing an aluminum support which is superior in an electrolytically 
graining property. 
BACKGROUND OF THE INVENTION 
As an aluminum support for printing plate, particularly for offset printing 
plate there is used an aluminum plate (including aluminum alloy plate). 
In general, an aluminum plate to be used as a support for offset printing 
plate needs to have a proper adhesion to a photographic light-sensitive 
material and a proper water retention. 
The surface of the aluminum plate should be uniformly and finely grained to 
meet the aforesaid requirements. This graining process largely affects a 
printing performance and a durability of the printing plate upon the 
printing process following manufacture of the plate. Thus, it is important 
for the manufacture of the plate whether such graining is satisfactory or 
not. 
In general, an alternating current electrolytic graining method is used as 
the method of graining an aluminum support for a printing plate. There are 
a variety of suitable alternating currents, for example, a normal 
alternating waveform such as a sinewaveform, a special alternating 
waveform such as a squarewaveform, and the like. When the aluminum support 
is grained by alternating current supplied between the aluminum plate and 
an opposite electrode such as a graphite electrode, this graining is 
usually conducted only one time, as the result of which, the depth of pits 
formed by the graining is small over the whole surface thereof. Also, the 
durability of the grained printing plate during printing will deteriorate. 
Therefore, in order to obtain a uniformly and closely grained aluminum 
plate satisfying the requirement of a printing plate with deep pits as 
compared with their diameters, a variety of methods have been proposed as 
follows. 
One method is a graining method to use a current of particular waveform for 
an electrolytic power source (JP-A-53-67507). (The term "JP-A" as used 
herein means an "unexamined published Japanese patent application".) 
Another method is to control a ratio between an electricity quantity of a 
positive period and that of a negative period at the time of alternating 
electrolytic graining (JP-A-54-65607). Still another method is to control 
the waveform supplied from an electrolytic power source (JP-A-55-25381). 
Finally, another method is directed to a combination of current density 
(JP-A-56-29699). 
Further, known is a graining method using a combination of an AC 
electrolytic etching method with a mechanical graining method 
(JP-A-55-142695). 
As the method of producing an aluminum support, on the other hand, known is 
a method in which an aluminum ingot is melted and held, and then cast into 
a slab (having a thickness in a range from 400 to 600 mm, a width in a 
range from 1,000 to 2,000 mm, and a length in a range from 2,000 to 6,000 
mm). Then, the cast slab thus obtained is subjected to a scalping step in 
which the slab surface is scalped by 3 to 10 mm with a scalping machine so 
as to remove an impurity structure portion on the surface. Next, the slab 
is subjected to a soaking treatment step in which the slab is kept in a 
soaking furnace at a temperature in a range from 480.degree. to 
540.degree. C. for a time in a range from 6 to 12 hours, thereby to remove 
any stress inside the slab and make the structure of the slab uniform. 
Then, the thus treated slab is hot rolled at a temperature in a range from 
480.degree. to 540.degree. C. to a thickness in a range from 5 to 40 mm. 
Thereafter, the hot rolled slab is cold rolled at room temperature into a 
plate of a predetermined thickness. Then, in order to make the structure 
uniform and improve the flatness of the plate, the thus cold rolled plate 
is annealed thereby to make the rolled structure, etc. uniform, and the 
plate is then subjected to correction by cold rolling to a predetermined 
thickness. Such an aluminum plate obtained in the manner described above 
has been used as a support for a planographic printing plate. 
However, electrolytic graining is apt to be influenced by an aluminum 
support to be treated. If an aluminum support is prepared through melting 
and holding, casting, scalping and soaking, even though passing through 
repetition of heating and cooling followed by scalping of a surface layer, 
scattering of the metal alloy components is generated in the surface 
layer, causing a drop in the yield of a planographic printing plate. 
In this connection, the present inventors have previously proposed a method 
of producing a support for planographic printing plate, which comprises 
continuously performing casting and hot-rolling from molten aluminum to 
form a hot-rolled coil of a thin plate, transforming the hot-rolled coil 
into an aluminum support through cold-rolling, heat-treatment and 
correction, and finally, graining the aluminum support (U.S. Pat. No. 
5,078,805 which corresponds to JP-A-3-79798). 
However, even the preparation methods which have been previously proposed 
by the present inventors give the non-uniformity of the yield of 
electrolytic graining and the graining property due to the components of 
aluminum support. 
Further, in order to prepare an aluminum alloy having the foregoing 
composition, a method is normally employed which comprises melting an 
ingot having an aluminum content of not less than 99.7%, and then adding 
an aluminum mother alloy containing predetermined amounts of Fe, Si and Cu 
to the molten aluminum. This aluminum mother alloy is expensive as 
compared with an aluminum ingot, raising the cost of aluminum alloy. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method of producing a 
support for planographic printing plate which is superior in graining 
property and which reduces the non-uniformity in quality of materials for 
aluminum support, thereby improving the yield of electrolytic graining as 
well as enabling the production of a low cost planographic printing plate. 
The present inventors have made extensive studies on the relationship 
between aluminum support and electrolytic graining. As a result, the 
present inventors worked out the present invention. 
In particular, the foregoing object of the present invention is 
accomplished with: 
(i) a method of producing a support for planographic printing plate, which 
comprises melting an aluminum ingot having an aluminum content of not less 
than 99.7 wt % to prepare a cast ingot, scalping the surface of the cast 
ingot, soaking the scalped cast ingot, cold rolling the soaked ingot to a 
thickness of 0.1 to 0.5 mm, without followed by annealing, correction of 
the resulting sheet to prepare an aluminum support, and then graining the 
aluminum support; and 
(ii) a method of producing a support for planographic printing plate, which 
comprises melting an aluminum ingot having an aluminum content of not less 
than 99.7 wt % to prepare a cast ingot in a melt holding furnace, directly 
subjecting the cast ingot to continuous casting to prepare a thin sheet 
having a thickness of 2 to 30 mm, cold rolling the thin sheet, without 
followed by annealing, correction of the resulting sheet to prepare an 
aluminum support, and then graining the aluminum support.

DETAILED DESCRIPTION OF THE INVENTION 
In the present invention, as the method for preparing an aluminum cast 
ingot from molten aluminum in, e.g., a fixed casting mold, a casting 
technique such as DC method has been put into practical use. 
Further, as a continuous casting method employing a driven casting mold 
there can be used a method employing a cooling belt such as Hapelett 
method or a method employing a cooling roller such as Hunter method and 3C 
method. Moreover, JP-A-60-238001, JP-A-60-240360, etc. disclose a method 
for preparing a coil of thin sheet. 
According to conventional methods, when a support for printing plate is 
prepared only from an aluminum ingot having an aluminum content of not 
less than 99.7 wt %, it is disadvantageous in that the shape of grain is 
collapsed during electrolytic graining. The present invention provides a 
method of producing a support for planographic printing plate having a 
good adaptability to electrolytic graining by correction without heat 
treatment after cold rolling. 
Referring to FIGS. 1(A), 1(B), 2, 3 and 4, an embodiment of the method of 
producing an aluminum support according to the present invention will be 
further described. As shown in FIG. 1(A), the reference number 1 is a 
casting mold in which an ingot is formed into cast ingot 2. Alternatively, 
as shown in FIG. 1(B), molten aluminum may be supplied into cast ingot 
receiving tray 4 from molten aluminum supplying nozzle 5 through 
water-cooled casting mold 3 to prepare cast ingot 6. Further, as shown in 
FIG. 2, an aluminum ingot may be melted in melt holding furnace 7, and 
then formed into a sheet having a thickness of 2 to 30 mm by means of 
twin-roller continuous casting machine 8. In a case of using a cast ingot, 
it is scalped to a certain extent, soaked, cold rolled to a thickness of 
0.1 to 0.5 mm as shown in FIG. 3, and then corrected as shown in FIG. 4 to 
prepare an aluminum support. In this process, soaking is effected before 
cold rolling. In the case where an aluminum ingot is melted in melt 
holding furnace 7 and formed into a sheet having a thickness of about 4 to 
30 mm by twin-roller continuous casting machine 8, the sheet is then cold 
rolled by cold rolling machine 10 as shown in FIG. 3, and then, without 
followed by annealing, correction by correction machine 11 as shown in 
FIG. 4 to prepare a support. 
The feature of the present invention is that no annealing treatment is 
effected after cold rolling. 
In the present invention, the soaking treatment is conducted at a 
temperature of 280.degree. to 650.degree. C., preferably 400.degree. to 
630.degree. C., more preferably 500.degree. to 600.degree. C. for a period 
of 2 to 15 hours, preferably 4 to 12 hours, more preferably 6 to 11 hours. 
In the present invention, while a variety of known continuous casting 
methods is applicable, preferred are a twin-roller continuous casting 
method and a twin-belt continuous casting method. In a case of using the 
twin-roller continuous casting method, it is preferred that a cast ingot 
is cast to a thin sheet having a thickness of 2 to 10 mm. In a case of 
using the twin-belt continuous casting method, it is preferred that a cast 
ingot is cast to a sheet having a thickness of 10 to 30 mm, subsequently 
the sheet is hot rolled to a thickness of 2 to 10 mm (before cold 
rolling). 
As the method for graining the support for planographic printing plate 
according to the present invention, there is used mechanical graining, 
chemical graining, electrochemical graining or combination thereof. 
Examples of mechanical graining methods include ball graining, wire 
graining, brush graining, and liquid honing. As electrochemical graining 
method, there is normally used AC electrolytic etching method. As electric 
current, there is used a normal alternating current such as sinewaveform 
or a special alternating current such as squarewaveform, and the like. As 
a pretreatment for the electrochemical graining, etching may be conducted 
with caustic soda. 
If electrochemical graining is conducted, it is preferably with an 
alternating current in an aqueous solution mainly composed of hydrochloric 
acid or nitric acid. The electrochemical graining will be further 
described hereinafter. 
First, the aluminum is etched with an alkali. Preferred examples of 
alkaline agents include caustic soda, caustic potash, sodium metasilicate, 
sodium carbonate, sodium aluminate, and sodium gluconate. The 
concentration of the alkaline agent, the temperature of the alkaline agent 
and the etching time are preferably selected from 0.01 to 20%, 20.degree. 
to 90.degree. C. and 5 sec. to 5 min., respectively. The preferred etching 
rate is in the range of 0.1 to 5 g/m.sup.2. 
In particular, if the support contains a large amount of impurities, the 
etching rate is preferably in the range of 0.01 to 1 g/m.sup.2 
(JP-A-1-237197). Since alkaline-insoluble substances (smut) are left on 
the surface of the aluminum plate thus alkali-etched, the aluminum plate 
may be subsequently desmutted as necessary. 
The pretreatment is effected as mentioned above. In the present invention, 
the aluminum plate is subsequently subjected to AC electrolytic etching in 
an electrolyte mainly composed of hydrochloric acid or nitric acid. The 
frequency of the AC electrolytic current is in the range of 0.1 to 100 Hz, 
preferably 0.1 to 1.0 Hz or 10 to 60 Hz. 
The concentration of the etching solution is in the range of 3 to 150 g/l, 
preferably 5 to 50 g/l. The solubility of aluminum in the etching bath is 
preferably in the range of not more than 50 g/l, more preferably 2 to 20 
g/l. The etching bath may contain additives as necessary. However, in mass 
production, it is difficult to control the concentration of such an 
etching bath. 
The electric current density in the etching bath is preferably in the range 
of 5 to 100 A/dm.sup.2, more preferably 10 to 80 A/dm.sup.2. The waveform 
of electric current can be properly selected depending on the required 
quality and the components of aluminum support used but may be preferably 
a special alternating waveform as described in JP-B-56-19280 and 
JP-B-55-19191. (The term "JP-B" as used herein means an "examined Japanese 
patent publication"). The waveform of electric current and the liquid 
conditions are properly selected depending on required electricity as well 
as required quality and components of aluminum support used. 
The aluminum plate which has been subjected to electrolytic graining is 
then subjected to dipping in an alkaline solution as a part of desmutting 
treatment to dissolve smutts away. As such an alkaline agent, there may be 
used caustic soda or the like. The desmutting treatment is preferably 
effected at a pH value of not lower than 10 and a temperature of 
25.degree. to 60.degree. C. for a dipping time as extremely short as 1 to 
10 seconds. 
The aluminum plate thus etched is then dipped in a solution mainly composed 
of sulfuric acid. It is preferred that the sulfuric acid solution is in 
the concentration range of 50 to 400 g/l, which is much lower than the 
conventional value, and the temperature range of 25.degree. to 65.degree. 
C. If the concentration of sulfuric acid is more than 400 g/l or the 
temperature of sulfuric acid is more than 65.degree. C., the processing 
bath is more liable to corrosion, and in an aluminum alloy comprising not 
less than 0.3% of manganese, the grains formed by the electrochemical 
graining is collapsed. Further, if the aluminum plate is etched by more 
than 0.2 g/m.sup.2, the printing durability reduces. Thus, the etching 
rate is preferably controlled to not more than 0.2 g/m.sup.2. 
The aluminum plate preferably forms an anodized film thereon in an amount 
of 0.1 to 10 g/m.sup.2, more preferably 0.3 to 5 g/m.sup.2. 
The anodizing conditions vary with the electrolyte used and thus are not 
specifically determined. In general, it is appropriate that the 
electrolyte concentration is in the range of 1 to 80% by weight, the 
electrolyte temperature is in the range of 5.degree. to 70.degree. C., the 
electric current density is in the range of 0.5 to 60 A/dm.sup.2, the 
voltage is in the range of 1 to 100 V, and the electrolysis time is in the 
range of 1 second to 5 minutes. 
The grained aluminum plate having an anodized film thus obtained is stable 
and excellent in hydrophilicity itself and thus can directly form a 
photosensitive coat thereon. If necessary, the aluminum plate may be 
further subjected to surface treatment. 
For example, a silicate layer formed by the foregoing metasilicate of 
alkaline metal or an undercoating layer formed by a hydrophilic polymeric 
compound may be formed on the aluminum plate. The coating amount of the 
undercoating layer is preferably in the range of 5 to 150 mg/m.sup.2. 
A photosensitive coat is then formed on the aluminum plate thus treated. 
The photosensitive printing plate is imagewise exposed to light, and then 
developed to make a printing plate, which is then mounted in a printing 
machine for printing. 
The present invention will be further described in the following 
non-limiting examples. Unless otherwise indicated, all parts, percents, 
ratios and the like are by weight. 
EXAMPLE 1 
A commercially available ingot having an aluminum content of not less than 
99.7% (including 0.085% of Fe, 0.034% of Si and almost 0 (zero) % of Cu as 
impurities) was melted, and then formed into a cast ingot in a carbon 
casting mold at a casting temperature of 750.degree. C. as shown in FIG. 
1(A). The cast ingot was scalped by about 10 mm, subjected to soaking at a 
temperature of 550.degree. C. for 10 hours, and then finished to a 
thickness of 0.24 mm only by cold rolling to prepare a sample of Example 1 
of the present invention. 
COMATIVE EXAMPLES 1 AND 2 
In order to prepare a JIS1050 material that can be widely used as a support 
for planographic printing plate, various mother alloys were added to a 
commercially available ingot to make a composition consisting of 0.35% of 
Fe, 0.07% of Si, 0.01% of Cu, 0.03% of Ti, and a balance of Al and 
unavoidable impurities. The ingot was then formed into a cast ingot in the 
same manner as in Example 1. The cast ingot was scalped by an ordinary 
method, subjected to soaking, subjected to cold rolling and intermediate 
annealing (using an apparatus as shown in FIG. 5) once or more times, and 
then cold rolled again so that it was finished to a thickness of 0.24 mm 
to prepare a sample of Comparative Example 1. 
As another comparative example, a cast ingot was prepared from an ingot 
having an aluminum content of 99.7%. The cast ingot was then finished to a 
thickness of 0.24 mm in the same manner as in Comparative Example 1 to 
prepare a sample of Comparative Example 2. 
The aluminum plates thus prepared were used as supports for planographic 
printing plate. These supports were etched with a 15% aqueous solution of 
caustic soda at a temperature of 50.degree. C. at an etching rate of 5 
g/m.sup.2, washed with water, desmutted with a 150 g/l sulfuric acid at a 
temperature of 50.degree. C. for 10 seconds, and then washed with water. 
These supports were then subjected to electrochemical graining with an 
alternating current as described in JP-B-55-19191 in a 16 g/l nitric acid. 
The electrolysis conditions were 14 V for anode voltage V.sub.A, 12 V for 
cathode voltage V.sub.c, and 350 coulomb/dm.sup.2 for anodic electricity. 
Without coating a photosensitive layer, the substrates 1 to 3 thus prepared 
were then evaluated for uniformity in appearance and grain shape 
(evaluated by observing a view of grained surface enlarged by a scanning 
electron microscope). At the same time, the cost of the raw materials of 
these substrates were compared. The results are set forth in Table 1. 
TABLE 1 
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Cost 
Uniformity ratio 
Rolling in Grain of raw 
Component method appearance 
shape materials 
______________________________________ 
Ex. 1: 
Al, 99.7% Cold Good Good 100 
rolling 
C.Ex. JIS1050 Cold Streak Good 106 
1: rolling unevenness 
+ Inter- 
mediate 
annealing 
C.Ex. Al, 99.7% Cold Streak Melted, 
100 
2: rolling unevenness 
poor 
+ Inter- 
mediate 
annealing 
______________________________________ 
As mentioned above, the example of the present invention exhibits a good 
appearance and grain shape and an excellent adaptability to graining. 
Further, the example of the present invention has a great effect of 
reducing the cost of raw materials. In accordance with the present 
invention, a planographic printing plate can be prepared only from a 
commercially available ingot having an aluminum content of not less than 
99.7%, thereby enabling a drastic cost reduction. 
Moreover, the present invention can employ a simplified rolling method, 
enabling a production cost reduction. 
While casting is effected with a carbon casting mold in Example 1, the 
present invention is not limited thereto. Twin-roller continuous casting 
method as shown in FIG. 2 and twin-belt continuous casting method can be 
used to accomplish the same effects as above. 
EXAMPLE 2 
Referring to FIG. 2 which illustrates the concept of a casting process, 
another embodiment of the process for producing an aluminum support to be 
used in the present invention will be described below. 
An aluminum ingot having an aluminum content of not less than 99.7% 
(including 0.085% of Fe, 0.034% of Si, and almost 0 (zero) % of Cu as 
impurities) was melted in melt holding furnace 7, and then continuously 
casted into a sheet having a thickness of 7 mm by twin-roller continuous 
casting machine 8. The sheet was wound on coiler 9, and then subsequently 
subjected to treatment by cold rolling machine 10 and correction machine 
11 as shown in FIGS. 3 and 4, respectively, to prepare an aluminum support 
as a sample of Example 2 of the present invention. 
COMATIVE EXAMPLE 3 
An aluminum ingot having an aluminum content of not less than 99.7% was 
melted and held with a mother alloy of Fe, Si, Cu and Ti being added 
thereto so that a composition comprising 0.35% of Fe, 0.07% of Si, 0.01% 
of Cu and 0.03% of Ti was made. The cast ingot thus prepared was then 
casted in the same manner as in Example 2 to prepare an aluminum support 
as a sample of Comparative Example 3. 
These samples were then subjected to graining in the same manner as in 
Example 1 and Comparative Examples 1 and 2, anodized by an ordinary 
method, and then coated with a photosensitive layer to prepare 
photosensitive planographic printing plates. These photosensitive 
planographic printing plates were exposed to light, developed, and then 
gummed to prepare planographic printing plates. These planographic 
printing plates were then used for printing in an ordinary manner. The 
results of the printing properties as well as the results of uniformity in 
appearance after graining and the comparison of the cost of raw materials 
are set forth in Table 2. 
TABLE 2 
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Results Uniformity 
Cost ratio 
Com- Rolling of in of-raw 
ponent method printing appearance 
materials 
______________________________________ 
Ex. 2: 
Al, Cold Good Good 100 
99.7% rolling 
C.Ex. JIS1050 Cold Acceptable 
Poor 106 
3: rolling 
______________________________________ 
As mentioned above, the sample of the present invention can provide 
improved results of printing, a drastically improved appearance and a 
reduction of the cost of raw materials. 
As mentioned above, the planographic printing plate prepared according to 
the method of producing a support for planographic printing plate of the 
present invention exhibits an improved adaptability to electrolytic 
graining as compared with conventional planographic printing plates, 
thereby enabling a drastic reduction of the cost of raw materials. 
Further, the present invention eliminates the necessity of blending of raw 
materials with a mother alloy, eliminating the drop of yield due to 
blending and hence enhancing the yield. 
Moreover, the simplification of cold rolling process gives a great effect 
of reducing the production cost, providing a great contribution to the 
quality improvement and cost reduction of support for planographic 
printing plate. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one of ordinary skill 
in the art that various changes and modifications can be made therein 
without departing from the spirit and scope thereof.