Method and apparatus for making a lithographic offset plate by the silver salt diffusion transfer process

A method and apparatus for making an offset printing plate according to the silver salt diffusion transfer process comprises the step of washing the imaging element by applying a curtain of an aqueous medium onto the surface of the element at its silver halide emulsion layer side, across the width of the element, substantially transverse to its processing direction, and in countercurrent to the processing direction of the element.

FIELD OF THE INVENTION 
The present invention relates to an apparatus and a method for the 
processing of photographic sheet materials and particularly for processing 
such materials according to the silver salt diffusion transfer process for 
producing a lithographic offset printing plate. 
BACKGROUND OF THE INVENTION 
The principles of the silver complex diffusion transfer reversal process, 
hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 
2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" 
by Andre Rott and Edith Weyde--The Focal Press--London and New York, 
(1972). 
In the DTR-process non-developed silver halide of an informationwise 
exposed photographic silver halide emulsion layer material is transformed 
with a so-called silver halide solvent into soluble silver complex 
compounds which are allowed to diffuse into an image receiving element and 
are reduced therein with a developing agent, generally in the presence of 
physical development nuclei, to form a silver image having reversed image 
density values with respect to the black silver image obtained in the 
exposed areas of the photographic material. 
A DTR-image bearing material can be used as a planographic printing plate 
wherein the DTR-silver image areas form the water-repellant ink-receptive 
areas on a water-receptive ink-repellant background. 
The DTR-image can be formed in the image receiving layer of a sheet or web 
material which is a separate element with respect to the photographic 
silver halide emulsion material (a so-called two-sheet DTR element) or in 
the image receiving layer of a so-called single-support-element, also 
called mono-sheet element, which contains at least one photographic silver 
halide emulsion layer integral with an image receiving layer in 
waterpermeable relationship therewith. It is the latter mono-sheet version 
which is preferred for the preparation of offset printing plates by the 
DTR method. 
Two types of the mono-sheet DTR offset imaging elements exist. According to 
a first type disclosed in e.g. U.S. Pat. No. 4,722,535 and GB- 1,241,661 a 
support is provided in the order given with a silver halide emulsion layer 
and a layer containing physical development nuclei serving as the 
image-receiving layer. After informationwise exposure and development, the 
imaged element is used as a printing plate without the removal of the 
emulsion layer. 
According to a second type of mono-sheet DTR offset imaging element a 
hydrophilic support, mostly anodized aluminium, is provided in the order 
given with a layer of physical development nuclei and a silver halide 
emulsion layer. After informationwise exposure and development, the imaged 
element is washed with water to remove the emulsion layer so that a 
support carrying a silver image is left wich is used as a printing plate. 
Such type of lithographic printing plate is disclosed e.g. in U.S. Pat. 
No. 3,511,656. 
EP-A-410500 discloses an imaging element containing a support provided with 
an image receiving layer and a silver halide emulsion. In said document is 
disclosed a diffusion transfer reversal process for obtaining a 
lithographic printing plate in which said imaging element is imagewise 
exposed and subsequently developed using a developing liquid or activating 
liquid in the presence of a silver halide complexing agent. The imaging 
elemnt is then guided through a diffusion transfer zone so that the silver 
halide complexes formed during the development step are allowed to diffuse 
into the image receiving layer where they are converted to silver. When 
the imaging element leaves the diffusion transfer zone a silver image is 
formed in the image receiving layer. Hereafter, the imaging element passes 
a washing and a rinsing station to remove the silver halide emulsion layer 
so that the silver image is exposed. Finally the imaging element, now 
carrying a silver image on its surface, is treated with a finishing liquid 
that contains a so called hydrophobizing agent for improving the 
hydrophobicity of the silver image. 
However, said washing requires several precautions, as e.g. a gently or 
softly washing of the developed monosheet layer assemblage alone is not 
sufficient to remove the now superfluous emulsion layers from the imaging 
element, but a certain mechanical force is required. 
Therefor, sometimes brushes, sponges, scrapers or squeegee blades, wiping 
or scrubbing rollers, etc are applied--either individually or separately, 
or in combination together-, but these elements can damage the image, 
especially in case of high resolution images. 
According to the state of the art relating to the present invention, 
several attempts have been made of using a spray in a washing station of a 
processing apparatus. 
U.S. Pat. No. 3,589,261 applies in a washing station at the entrance a 
first spray which gives a hard scrubbing action to the surface of the 
element and at its exit a second spray which gives a light rinsing action 
to the element. 
In U.S. Pat. No. 4,142,194 a rinsing section comprises water impact sprays 
and barrier means (e.g. a wiper) extending longitudinally of the upper 
drive roller. 
The apparatus "Silverlith", e.g. processor SLT, marketed by Du Pont, 
comprises a washing section and a rinsing section. In the washing section 
warm water is dripping out of a tube, without any substantial pressure, on 
an upper drive roller, which is a scrub roller, as well as on the element. 
In the rinsing section, water is sprayed on the element which is passed 
below a squeegee set to remove the water of the element. 
The use of common sprays or jets tends to damage the image on the imaging 
element, or tends to cause clogging of pumps, valves and filters by 
fragments of the layers removed from the imaging element after its 
development, both phenomena being detrimental to image quality. 
OBJECT OF THE INVENTION 
It is an object of the present disclosure to provide an alternative and 
improved method for washing lithographic offset printing plates according 
to the DTR-process. 
It is a further object of the present invention to provide an alternative 
method for obtaining lithographic offset printing plates according to the 
DTR-process with improved printing properties. 
It is a further object of the present invention to provide an alternative 
apparatus for obtaining lithographic offset printing plates according to 
the DTR-process with improved printing properties. 
Further objects and advantages will become apparant from the description 
given hereinbelow. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a method for making 
an offset printing plate according to the silver salt diffusion transfer 
process comprising the steps of: 
informationwise exposing an imaging element comprising in the order given 
(i) a hydrophilic support, (ii) an image receiving layer containing 
physical development nuclei and (iii) a silver halide emulsion layer; 
developing said informationwise exposed element using an aqueous alkaline 
processing solution in the presence of at least one developing agent and 
at least one silver halide solvent to obtain a silver image on said 
hydrophilic support; 
removing said silver halide emulsion layer and any optional layer on top of 
said silver image to uncover said silver image by washing with an aqueous 
medium, wherein said washing is carried out by applying a curtain of said 
aqeous medium onto the surface of the element at its silver halide 
emulsion layer side, across the width of the element substantially 
transverse to its processing direction, and in countercurrent to said 
processing direction. 
According to the present invention, there is also provided an apparatus, 
comprising a washing station 4 which comprises a washing chamber, a sump 
or tank 28 for collecting wash-solution, entrance and exit rollers 42 to 
45 for transporting or processing an imaged element along a transporting 
or processing path 12 through the station, and at least one spraying means 
for applying a curtain of a wash-solution to said element, located between 
the entrance and the exit transport rollers and being oriented towards the 
processing path 12 of the element 9 at an inclined angle .beta. to said 
processing path 12 in a direction 26 countercurrent to the processing 
direction 11 of the element.

DETAILED DESCRIPTION OF THE INVENTION 
Reference is firstly made to FIG. 1 in which there is illustrated a 
longitudinal cross section through an apparatus 10 for processing exposed 
printing plates 19 of the type described in EP-A-410500. The apparatus 10 
is mounted within a generally rectangular housing (not shown) for 
supporting the various parts of the apparatus. An informationwise exposed 
imaged element 9 is progressing in processing direction 11 along a 
processing path 12 by being sequentially fed through an entrance station 
1, a developing station 2, a diffusion transfer station 3, a washing 
station 4 containing a wash-solution 15, a rinsing station 5 and a 
finishing station 6. After having passed a drying station 7, the printing 
plate 19 may then leave the apparatus 10 through the output station 8 and 
a lithographic printing plate 19 is obtained. 
According to an embodiment of uncovering the imaged element, the developed 
monosheet layer assemblage is washed with an aqueous medium 15 such as 
water or water with an additive, sometimes also indicated as wash-solution 
15 so that the now superfluous emulsion layers are removed from the 
element. Therefor, in washing station 4, the element is throughputted by 
an entrance pair 13 of rollers and subsequently washed with a solution 15 
which is sprayed onto the surface of the element 9 from a washing nozzle. 
Washing station 4 also comprises an exit pair 14 of squeegee rollers for 
squeezing excessive wash-solution 15 from the surface of the element. In 
the absence of an element, the sprays do not touch neither the entance nor 
the exit roller pair, but directly flow into a tank. 
Now, first a description will be given,in reference to FIGS. 1 and 2, of 
the method(s) according to the present invention. In a first preferred 
embodiment of the present invention, a method for making an offset 
printing plate 19 according to the silver salt diffusion transfer process 
comprises the steps of: 
informationwise exposing (not described in detail) an imaging element 
comprising in the order given (i) a hydrophilic support, (ii) an image 
receiving layer containing physical development nuclei and (iii) a silver 
halide emulsion layer; 
developing (in station 2 and 3) said informationwise exposed element 9 
using an aqueous alkaline processing solution in the presence of at least 
one developing agent and at least one silver halide solvent to obtain a 
silver image on said hydrophilic support; 
removing said silver halide emulsion layer and any optional layer on top of 
said silver image (in station 4) to uncover said silver image by washing 
with an aqueous medium 15, said washing being carried out by applying a 
curtain 21 of an aqueous medium 15 onto the surface of the element at its 
silver halide emulsion layer side, across the width of the element, 
substantially transverse to its processing direction 11, and in 
countercurrent 26 to said direction 11. 
In a second preferred embodiment of the present invention, said applying of 
a curtain 21 of an aqueous medium 15 is carried out by spraying while 
transporting said element through the washing station along a processing 
path 12, thereby spraying either continuously or discontinuously. 
Advantageously, said spraying is carried out under an inclined angle .beta. 
(referral 29) to the processing path 12 (to be explained later on in the 
further descriptions; cfr. the later FIG. 3). 
It is also of great advantage that the temperature of the aqueous medium 15 
is maintained between 30.degree. C. and 80.degree. (also to be described 
later on). 
Before starting with the extensive description of the apparatus in full 
details, it is noticed that e.g. the guidance control of the imaged 
element in the developing bath is disclosed in our patent application 
EP-A-93201305.5; that the squeegee rollers for squeezing excessive aqueous 
medium 15 from the surface of the element are disclosed in our patent 
application EP-A-93202154; that two solutions to the problem of rollers 
adhering together when the processing apparatus is idle are disclosed in 
our patent applications EP-A-92203312 and 93201957. 
Now reference is made to FIG. 3, which is a longitudinal cross section 
through a washing station 4 and to FIG. 4, which is a transversal section 
through a washing station of an apparatus according to the present 
invention. As it can be seen on these figures, the developed imaging 
element 9 is passing along processing direction 11 under a spray curtain 
21 of aqueous medium 15. The nozzles 22 on the spray heads 23 are 
preferably designed to ensure that droplets of solution impinge on all 
points on the surface of the element 9 to ensure an even coverage of the 
surface and the pressure of the spray curtain 21 is adjusted to ensure 
that spraying droplets impinge with sufficient impact to wash the element. 
As shown in FIG. 3 and in FIG. 4, washing station 4 essentially comprises a 
washing or spraying chamber 20 and two times an antechamber 24 and 25, one 
at the incoming side of the spraying chamber and one at the outgoing side 
of the spraying chamber. More in detail, washing station 4 further 
advantageously comprises vertical walls 41 with suitable openings 27 for 
progressing the imaged element 9 into processing direction 11, a sump 28 
partly filled with an aqueous medium 15, rubbered upper rollers 42, 43 and 
rubbered bottom rollers 44, 45 (mounted within the antechamber 24, 25) for 
gripping and advancing the element 9, one middle roller 46 or several 
middle rollers 47 which are possibly segmented middle rollers 47, a cover 
plate 48 and sealing means 49 to prevent solution leakage and air 
entrainement, as well as associated pump and temperature control means 
(not shown). Said middle roller 46 (or rollers 47) is (or are) arranged 
for engaging only the rear side of the element 9. 
As a consequence of the preferably very intensive atomisation of the 
droplets, the air within the washing station 4 is highly saturated. By 
further consequence, the possible danger of leakages of aqueous medium 15 
liquid to the preceding (diffusion) station 3 or to the following 
(rinsing) station 5, is also very high. In order to prevent possible 
contamination problems, special precautions are to be taken taken. It is 
noticed that e.g. sealing means for maintaining a liquid seal 40 between 
at least one roller of at least one roller pair and an adjacent wall of 
the washing station is disclosed in our patent application EP-A-93201957. 
According to a preferred embodiment of the present invention, an apparatus 
for carrying out the method according to any of the preceding claims, 
comprises a washing station 4 which comprises a washing chamber 20, a sump 
or tank 28 for collecting aqueous medium, entrance and exit rollers 42 to 
45 for transporting an imaged element along a processsing path 12 through 
the station, and at least one spraying means for applying a curtain of an 
aqueous medium 15 to said element, located between the entrance and the 
exit transport rollers and being oriented towards the silver halide 
emulsion layer side of said element at an inclined angle .beta. (referral 
29) to the processing path 12 in (a direction) countercurrent (referral 
26) to the transporting direction 12 of the element. 
In another preferred embodiment of the present invention, the washing 
station further comprises at least one middle roller 46 arranged for 
engaging the rear side only of the imaged element. 
In another preferred embodiment of the present invention, said processsing 
path 12 is substantially horizontal and said spraying means is located 
above the processing path 12 of the imaged element. 
In a further preferred embodiment of the present invention, the washing 
station 4 comprises a sump or tank 28 provided with an overflow (not 
shown) for maintaining the aqueous medium 15 at a level below that of the 
rollers in the station, and with heating means (not shown) for heating the 
aqueous medium 15. Further, a circulation pump (not shown) draws water 
from said sump 28 and pumps it to the spray heads 23. An additional pipe 
(not shown) is fitted to the tank at its lowest point to permit drainage. 
In a still further preferred embodiment of the present invention, the 
washing station 4 additionally comprises a piping connection (not shown) 
to the water mains supply for optional replenishing fresh wash-water and 
means (not shown) for dosing an amount of fresh water into tank 28 during 
replenishment. Upon a signal from e.g. a level detector (not shown) dosing 
means is controlled to add a certain amount of fresh wash-water into tank. 
The apparatus according to the invention comprises in principle no filters 
in the washing circuit, so that the reliability of operation becomes high. 
This is different from prior art apparatus operating with an unheated 
aqueous medium whereby material removed from the imaged element is present 
in the liquid circuit in the form of skins or flakes which can raise large 
problems with filtering and/or with spraying. 
In a further preferred embodiment of the present invention, said spraying 
means are spray nozzles 22. Spray nozzles 22 are usually classified as to 
the pattern they produce, such as hollow cone, solid cone, flat spray and 
square spray. Here, reference can be made to FIG. 5 which gives a survey 
of some producable spray patterns; to FIGS. 6a and 6b which give a 
principal layout of two different spray nozzles, each with their 
corresponding spray pattern; and to FIG. 7, which illustrates the term 
"theoretical coverage" of a spray nozzle. 
Whereas the shape of the spray pattern is controlled by the geometry of the 
nozzle, the output of the spray nozzles 22 depends on the diameter of the 
nozzle and the hydraulic pressure at the outlet of the nozzle, further 
indicated as "nozzle pressure". 
A wide variety of nozzles are available and by selection of nozzle size and 
by controlling the nozzle pressure an almost infinite variety and 
combination of outputs can be obtained. 
The size and shape of the spray nozzles and the nozzle pressure are 
parameters which can be adjusted to ensure an even coverage of the surface 
of the imaged element 9 and enough force or impact of the spray droplets. 
The flow rate (or capacity) from the spray nozzles can be accurately 
controlled by controlling the pressure at the nozzles. This is achieved by 
using sensitive pressure regulating valves or other devices (not shown for 
reason of simplicity). 
In another preferred embodiment of the present invention, the nozzles have 
a spray angle .alpha. (defined in FIG. 7 and further indicated in FIGS. 4 
and 8a and 8b) of a spray nozzle situated between 100.degree. and 
150.degree. and wherein the spray curtain 21 has an inclined angle .beta. 
(also indicated with referral 29 and figurated in FIGS. 2 and 3) situated 
between 45.degree. and 80.degree.. 
In another preferred embodiment of the present invention, the aqueous 
medium 15 is sprayed with a common pressure at the nozzle ranging from 0.5 
bar to 2.0 bar and with a global capacity (or total flow) ranging from 20 
liters pro minute to 40 l/min. 
In one preferred embodiment of the present invention, the spraying means 
comprises at least one spray nozzle. 
Preferrably, the spray nozzle is a nozzle flat spray type (see FIG. 8) 
Floodjet K12 or K10 or K15, made by Spraying Systems Co., North Avenue at 
Schmale Road, Wheaton, Ill.--U.S.A. 
In another preferred embodiment of the present invention, the spraying 
means comprises a plurality of spray nozzles 22 spaced from one another. 
Herein, the spray nozzles are spaced in at least one single row across the 
width of the imaged element transverse to the processing direction of the 
element. This arrangement is particularly suited for obtaining an abundant 
supply of aqueous medium. 
According to a further embodiment, the nozzles are arranged in a row across 
the width of the imaged element with an offset angle .gamma. between 
1.degree. and 10.degree. to a transverse to the processing direction of 
the element, thereby producing a plurality of partial spray curtains. FIG. 
9 gives a perspective view of one embodiment according to the present 
invention with "offsetted" spray nozzles, whereas FIG. 10 gives a front 
view and an upper view of an embodiment according to FIG. 9. 
Some of the advantages resulting from using an offset angle .gamma. are 
that the spray of aqueous medium impinges not only in countercurrent, but 
also comprises a lateral component, so that the uncovered layer flows away 
more easily, that no accumulation of wash-water on the imaged element 
takes place, that no impingement force is reduced by any accumulation of 
wash-water on the element, and that any interference with adjacent sprays 
can be avoided. 
In another embodiment of the present invention the nozzles are spaced in 
some "intervalled-but parallel" rows across the width of the imaged 
element with an offset distance .delta. between 1 and 15 mm transverse to 
the processing direction of the element, thereby producing a plurality of 
partial spray curtains. FIG. 11 gives a front view and an upper view of 
another embodiment according to the present invention with 
"intervalled-parallel" rows of spray nozzles. 
Therabove, it is important that the rate of application of the aqueous 
medium 15 be controlled within a predetermined range of flow rates being 
related to the rate of passage of the imaged element through the apparatus 
10. It is also important that the temperature and the nozzle pressure of 
the aqueous medium 15 are maintained accurately at their appropriate 
levels. 
Another preferred embodiment of the present invention, further comprises 
recirculation means (not shown) for collecting and recirculating the 
aqueous medium 15 to the spraying means. 
Another preferred embodiment of the present invention, further comprises 
replenishing means (not shown) for replenishing the sump 28 of the washing 
station 4 with a predetermined amount of fresh wash-liquid. 
Still another preferred embodiment of the present invention, further 
comprises a reservoir (not shown) for holding an antibacterium and dosing 
means for dosing a predetermined amount of said antibacterium into the 
sump 28 of the washing station 4. Preferably, said antibacterium comprises 
a liquid with a pH between 10 and 13.5. 
Still another preferred embodiment of the present invention, comprises a 
developing station 2, a diffusion transfer station 3, a rinsing station 5 
and a finishing station 6. 
According to the present invention, prefeably nozzles with relatively large 
openings so that there is no risk for the nozzles to become clogged by 
particles in the aqueous medium 15 that is received from the tank and 
returned to the spray nozzles. 
Thus included in the present invention is a suitable automatically operated 
apparatus for processing lithographic offset printing plates according to 
a silver complex diffusion transfer reversal process, wherein a washing 
station as described hereabove is integrated. 
Using a spray enables successful development of higher resolution images 
and also offer cost saving advantages because the omission of brushes and 
wiping rollers not only reduces the initial manufacturing cost but also 
the maintenance cost. 
According to a preferred embodiment of the present invention, a centrigugal 
pump is used. However, other types of hydraulic pumps might be applied, 
e.g. piston, membrane, gear or screw type pumps. 
In selecting the type of pump cost, efficiency, and maintenance generally 
are the governing factors in making a final selection. 
The present invention clearly can be applied in the case of processing 
materials which are suitable to produce a lithographic offset printing 
plate by the silver salt diffusion transfer process, but can also be 
applied in the case of processing other photographic sheet materials. 
Of course, many modifications are possible in the arrangement of this 
invention.