Process for reducing halftone dot images

A process for reducing the size of halftone dot images obtained by exposing and developing a metal image-forming material comprising a support, a thin metallic layer composed mainly of aluminum on the support and a photosensitive resin (or photoresist) layer on the thin metallic layer uniformly by first applying a solution capable of swelling the photosensitive resin layer to those parts which are to be subjected to dot size reduction, and then applying a reducer solution to the parts so treated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for reducing the size of halftone dot 
images obtained by imagewise exposing and developing a metal image-forming 
material comprising a support, a thin metallic layer composed mainly of 
aluminum on the support, and a photosensitive resin layer on the thin 
metallic layer. 
2. Description of the Prior Art 
Japanese patent application (OPI) No. 139,720/75 corresponding to U.S. 
patent application, Ser. No. 571,817, filed Apr. 25, 1975, now U.S. Pat. 
No. 4008084, discloses a metal image-forming material comprising a 
support, a thin metallic layer composed mainly of aluminum on the support 
and a photosensitive resin layer on the thin metallic layer. When this 
metal image-forming material is exposed through a image using active 
radiation and then contacted with or immersed in an alkaline solution the 
resin layer is dissolved or swollen at the exposed portion or unexposed 
portion depending on the property of the photosensitive resin layer, and 
the alkaline solution permeates to the metallic layer to dissolve it and 
thus to form the corresponding image. The image so obtained has high 
contrast, and superior resolving power, and is especially suitable for 
preparing a halftone original for printing plates. 
In an ordinary silver halide photographic material for printing (a 
so-called lithographic material), an operation is performed to partly 
reduce the size of the dots forming the halftone images in order to obtain 
better prints. This reducing operation is usually effected by coating a 
reducer solution containing a compound capable of dissolving the image 
(silver) on a part of the image after development. This halftone dot size 
reduction, for example, comprises the conversion of the silver image to a 
silver salt and removal of the silver salt by dissolution. It is 
inevitable as a result for the peripheral edges of the halftone dots to be 
etched away and their area decreased in size, and at the same time, the 
density of the halftone dots as a whole to decrease. 
Since, however, the halftone dot size reduction of the above-described 
metal image-forming material is based on a so-called "side-etching method" 
by which the reducer solution permeates between the support and the 
photosensitive resin and contacts the metal layer at sites where such does 
not contact the photosensitive resin or the support thereby to etch the 
metal layer, this method offers the great advantage that only the sizes of 
the area of the halftone dots is changed without decreasing the optical 
density of the halftone dots as a whole, which advantage cannot be 
obtained by a dot reduction of conventional lithographic materials. 
Because the space between the support and the photosensitive resin layer is 
very narrow in the above-described metal image-forming material, the 
penetration of the reducer solution is non-uniform causing an unevenness 
in the etching of the metal image to occur, and the shape of the halftone 
dots sometimes is deteriorated. This degradation of the shape of the 
halftone dots, that is, the reduced similarity of the shapes of the 
halftone dots before and after the halftone dot size reduction, causes the 
serious defect to occur that prints faithful to the original cannot be 
obtained. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide a process for 
halftone dot size reduction suitable for halftone images formed on the 
above-described metal image forming material. 
Another object of this invention is to provide a process for halftone dot 
size reduction which ensures superior similarity of the shapes of halftone 
dots before and after halftone dot reduction. 
According to this invention, the above objects are achieved by a process 
for reducing the size of halftone dot images obtained by exposing and 
developing a metal image forming material comprising a support, a thin 
metallic layer composed mainly of aluminum on the support and a 
photosensitive resin layer on the thin metallic layer, wherein prior to 
applying a reducer solution, a solution capable of swelling the 
photosensitive resin layer is applied to those parts which are to be 
subjected to halftone dot size reduction. 
DETAILED DESCRIPTION OF THE INVENTION 
The halftone dot reducing method of this invention can be applied to a 
halftone dot image formed on a metal image-forming material comprising a 
support, a thin metallic layer composed mainly of aluminum on the support, 
and a photosensitive resin layer on the thin metallic layer, particularly 
a metallic halftone dot image formed on the metal image-forming material 
described in Japanese patent application (OPI) No. 139,720/75 set forth 
above by the exposing and developing treatments described therein. 
In the above described metal image-forming material any support can be used 
and suitable examples of metals which can be used in the thin metallic 
layer include aluminum and a metal or metals having ionization tendency 
less than that of aluminum such as Fe, Mn, Ga, Cr, In, Co, Ni, Pb, Sn, Sb, 
Bi, Cu, Ag, Pd, Au, etc. the thickness of such a metallic layer can be 
varied as desired and suitable conventional photosensitive resins can be 
employed as the layer of the photosensitive resin. 
The solution capable of swelling the photosensitive resin can be selected 
as desired depending on the type of photosensitive resin used. Generally, 
the solution may be (1) an alkaline aqueous solution having a pH of at 
least about 9 but below about 11, (2) an organic solvent selected from the 
group consisting of alcohols and hydroxyethers, or (3) an alkaline aqueous 
solution containing an organic solvent selected from the group consisting 
of alcohols and hydroxyethers and having a pH of less than about 11. 
Examples of preferred alkaline solutions are aqueous solutions of sodium 
hydroxide, potassium hydroxide, sodium phosphate, sodium carbonate, and/or 
sodium aluminate. These materials are merely exemplary of preferred 
alkaline solutions which can be used. Basically any material can be used 
if such can provide the aqueous solution with a pH of about 9 to 11. As 
described above, examples of organic solvents are alcohols, particualrly 
lower alcohols such as methyl alcohol and ethyl alcohol, and hydroxyethers 
(i.e., hydroxyalkyl alkyl ethers (hereinafter oxyethers for simplicity)) 
such as .beta.-hydroxyethyl methyl ether, .beta.-hydroxyethyl ethyl ether, 
.beta.-hydroxyethyl propyl ether, .beta.-hydroxyethyl butyl ether, etc. 
Alkaline aqueous solutions as described above and containing alcohols 
and/or oxyethers as described above can also be used and are preferred. 
The amount of the alcohol or oxyether in the solution in this case can be 
selected as desired. 
Various kinds of surfactants can be added to the solution capable of 
swelling the photosensitive resin. Examples of suitable surfactants are 
those described in Japanese patent application (OPI) No. 32928/75 
(corresponding to U.S. patent application Ser. No. 491,006, filed July 23, 
1974, now abandoned). A suitable amount of the surfactant which can be 
used is up to about 3% by weight, preferably 0.005 to 1% by weight, based 
on the weight of the alkaline aqueous solution (1) and (3) and up to about 
1% by weight, preferably up to 0.1% by weight, based on the weight of the 
organic solvent (2). 
In the present invention, this swelling solution is applied after exposure 
but prior to halftone dot size reduction, preferably just prior to 
halftone dot reduction, to those parts of the metal image-forming material 
in which the size of the halftone dots is to be reduced. The method of 
application is not critical. When the parts to be reduced cover the entire 
area of the metal image-forming material, the material can be immersed in 
the swelling solution. Where the area of the parts to be reduced is small, 
the swelling solution can be dropped onto the area by squirting, using a 
pipette, or the like. 
Suitable temperatures and times which can be used in the application of the 
swelling solution are about 5.degree. to about 80.degree. C, preferably 
about 15.degree. to about 70.degree. C for about 5 sec. to about 10 min., 
preferably about 20 sec. to about 7 min. in using the alkaline solution 
(1); about 5.degree. to about 50.degree. C, preferably about 15.degree. to 
about 35.degree. C for about 5 sec. to about 5 min., preferably about 10 
sec. to about 3 min. in using the organic solvent (2); and about 5.degree. 
to about 70.degree. C, preferably about 15.degree. to about 60.degree. C 
for about 5 sec. to about 7 min. preferably about 15 sec. to about 5 min. 
in using the alkaline solution containing the organic solvent (3). 
Basically, the reducer solution used in the reducing process of this 
invention may be selected from the various compositions used as an etchant 
solution for the thin metallic layer of the above-described metal 
image-forming material. The solution may be an alkaline or acidic 
soltution. However, since the halftone dot image is composed of a metal 
comprising mainly aluminum, the use of an acidic solution results in the 
formation of a stable oxide on the surface of aluminum, and etching does 
not proceed beyond a certain point. Hence, alkaline solutions are 
preferred. Preferred alkaline solutions are, for example, an aqueous 
solution of sodium hydroxide, and an aqueous solution of potassium 
hydroxide, and aqueous solution of lithium hydroxide, an aqueous solution 
of sodium carbonate, whose pH has been adjusted to at least about 11 e.g., 
up to a pH of about 13.8. Furthermore, the reducer solution used in this 
invention may contain a material which inhibits the formation of bubbles 
at the time of etching, such as sodium bromate or sodium iodate. 
Additional materials which can be present in the reducer solution include 
NaAlO.sub.2, Na.sub.3 PO.sub.4, etc. 
In the present invention, the above-described reducer solution is applied 
to the metal image-forming material to which the above swelling solution 
has been applied. The method of application may be the same as that used 
in applying the swelling solution. Prior to halftone dot size reduction, 
the swelling solution applied to the metal image-forming material may be 
wiped off. 
Since the reducer solution used in this invention is alkaline, it can also 
swell the photosensitive resin layer. But since it etches the halftone dot 
image simultaneously with or before the swelling of the resin layer, 
uniform dot size reduction cannot be expected. According to this 
invention, an alkaline solution or an organic solvent as described above 
effective only for swelling (that is, having only a slight capability of 
etching the halftone dot image) is applied to the metal image-forming 
material, so that subsequent size reduction can be performed uniformly. 
The size reducing process of this invention is especially effective when a 
negative-type photosensitive resin is used as the photosensitive resin 
layer of the metal image-forming material. A negative-type photosensitive 
resin has strong chemical resistance at the exposed portion as compared 
with positive-type photosensitive resins, and is not easily swollen with 
solvents used for ordinary processing solutions, such as water. Thus, the 
application of a swelling agent prior to halftone dot size reduction is 
effective as in the process of this invention. Examples of the 
negative-type photosensitive resins are those described in Japanese patent 
application (OPI) No. 139,720/75 (page 5, left bottom column, line 1 to 
page 6, line 13), and blends of such resins with chlorinated polyolefins, 
e.g., chlorinated polyethylene, chlorinated polypropylene, a mixture of 
chlorinated polyethylene and chlorinated polypropylene, etc. 
The size reducing process of this invention is further effective for a 
halftone dot size reducing operation which is performed after a lapse of 
some time e.g., about 3 hours, from the exposure and developing treatment 
of the above-described metal image-forming material. The size reducing 
process of this invention is useful because in general printing 
techniques, halftone dot size reduction is rarely performed immediately 
after the development. 
According to the size reducing process of this invention, the similarity of 
the shapes of halftone dots after the size reduction to that before the 
size reduction is high, and only the sizes of the halftone dots are 
changed without changing the density of the halftone dots. Hence, prints 
of superior quality can be obtained.

The following examples are given to illustrate the present invention in 
greater detail. Unless otherwise indicated herein, the parts, percentages, 
ratios and the like are by weight. 
EXAMPLE 1 
About 400 mg of an Al-Fe (5:1 atomic ratio) alloy was placed in a tungsten 
boat disposed in a vacuum-deposition apparatus, and a 100 .mu.m-thick 
polyethylene terephthalate film was placed within the vacuum deposition 
chamber at a distance of about 30 cm from the evaporation source. Under a 
vacuum of about 5 .times. 10.sup.-5 torr, an aluminum/iron alloy 
vacuum-deposited film having a thickness of about 600 A was obtained. A 
photosensitive resin composition having the following formulation was 
coated on the vacuum-deposited alloy layer to a dry thickness of about 1.5 
.mu.m by means of a whirler, and dried in a dryer at 100.degree. C for 2 
minutes. 
______________________________________ 
Formulation of the Photosensitive Resin Composition 
Copolymer of Methyl Methacrylate 
and Methacrylic Acid (7:3 molar ratio) 
1 g 
Pentaerythritol Tetraacrylate 
0.85 g 
N-Methyl-2-benzoyl Methylene-.beta.- 
60 mg 
naphthothiazole 
Methyl Ethyl Ketone 12 g 
Methyl Cellosolve Acetate 
12 g 
______________________________________ 
The photosensitive material so obtained was exposed through a halftone 
original for 40 seconds at a distance of 1 m using a Fuji PS Light (a 2KW 
metal halide lamp produced by the Fuji Photo Film Co., Ltd.), and then 
immersed in a developing and etching solution of the following 
formulation. 
______________________________________ 
Formulation of Developing 
and Etching Solution 
NaOH 4 g 
KBrO.sub.3 10 g 
Water 1 liter 
______________________________________ 
After an immersion time of about 18 seconds at a solution temperature of 
26.degree. C, the metal layer at the unexposed portion was removed. The 
photosensitive material so treated was allowed to stand overnight, and 
subjected to halftone size reduction using a reducer solution of the 
following formulation at about 25.degree. C for 1 minute. 
______________________________________ 
Formulation of Reducer Solution 
NaOH 2 g 
KBrO.sub.3 6 g 
Water 100 ml 
______________________________________ 
At this time, the etching of the metal was non-uniform, and the shape of 
the halftone dots was impaired. 
The other parts of the same photosensitive material were immersed in a 
swelling solution of the following formulation at about 25.degree. C for 
about 1 minute. 
______________________________________ 
Formulation of Swelling Solution 
Na.sub.3 PO.sub.4 .multidot. 12 H.sub.2 O 
5 g 
Water 100 ml 
______________________________________ 
The surface of the photosensitive material was wiped off with a filter 
paper, and the material was subjected to halftone dot size reduction using 
the above reducer solution at about 25.degree. C for 1 minute. At this 
time, the metal was uniformly etched, and the shapes of the halftone dots 
before and after reduction were similar to each other. 
EXAMPLE 2 
A halftone dot image prepared in the same way as in Example 1 was allowed 
to stand overnight, and then immersed in a solution of the following 
formulation at about 25.degree. C for about 1 minute, and then subjected 
to halftone dot size reduction using the same reducer solution as used in 
Example 1. 
______________________________________ 
Formulation of Swelling Solution 
Na.sub.2 CO.sub.3 5 g 
Water 100 ml 
______________________________________ 
The metal layer was etched uniformly, and the similarity of the shapes of 
the halftone dots before and after the size reduction was superior. 
EXAMPLE 3 
The same procedure as in Example 2 was repeated except that the 
photosensitive material was immersed in a solution of the following 
formulation at about 25.degree. C for 1 minute prior to halftone dot size 
reduction. 
______________________________________ 
Formulation of Swelling Solution 
NaOH 0.4 g 
Water 100 ml 
______________________________________ 
Satisfactory halftone dot size reduction could be performed. 
EXAMPLE 4 
When the same procedure as in Example 2 was performed except that the 
photosensitive material was immersed in ethanol at about 20.degree. C for 
about 30 seconds prior to halftone dot size reduction, satisfactory 
halftone dot size reduction could be performed. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.