AN APPARATUS AND A METHOD FOR PROCESSING PHOTOGRAPHIC MATERIAL

Abstract: An apparatus for the processing of photographic material, and in particular for the development of photographically exposed plates intended for printing plates in a printing operation, comprises an upwardly open tank (5) containing a bath of liquid, conveyor means for conveying the plates (2) along a path (4) in which they are descended obliquely down into the bath and withdrawn again, and at least one movable brush (9) for brushing the top face of the plate. According to the invention the brush is enclosed in a downwardly open cavity (12) which has its lower edge immersed into the bath and which is evacuated in order that liquid is sucked thereinto. The invention also comprises a method for the processing of the photographic material.

Description: AN APPARATUS AND A METHOD FOR PROCESSING PHOTOGRAPHIC MATERIAL Cross Reference to Related Application This application is based on patent application No. 374/97 filed in Denmark on April 1, 1997, the contents of which are incorporated hereinto by reference. Background of the Invention 1. Field of the Invention The present invention relates to an apparatus and a method for processing photographic material, and in particular to the development of photographically exposed plates in order to prepare them for use as printing plates in a printing operation. Photographic materials and photographic exposure, respectively, as used herein and in the following refer to all kinds of photographic materials, related materials, and processes, respectively, irrespective of whether they are based on the use of visible light or of another form of radiation as long as they serve to reproduce an image by similar general principles. Sheets herein comprise all flat objects that may be conveyed as a web along a path, e.g. foils, laminates, plates whether in short formats, long formats or continuous webs. The sheets may be flexible or inflexible. The printing operations may be offset printing, flexographic printing, or other. Treatment by liquid may comprise reacting an emulsion on the face with chemicals in order to fix an image, to dissolve or to swell parts of the emulsion in order to clean the surface, or in order to provide the surface with a finishing, etc. as known in the art of photographic image processing. 2. Description of the Prior Art EP-A 0 410 500 discloses a litographic printing plate adapted for offset printing in which a pattern is formed in the printing plate by a photographic process. This pattern is formed by coating the plate with a light sensitive emulsion, exposing said emulsion and developing the image of the exposed emulsion. The developing process comprises different baths through which the plate passes successively and which have different purposes, such as fixation of the exposed image, washing off of chemicals, and surface treatment of the finished plate. EP-A 0 623 854 discloses an apparatus for processing a litographic offset printing plate in such a process, which apparatus comprises different baths through which the plate is conveyed, controlled by suitable guide rollers. The guide rollers define a conveyor path guiding the plate down into the respective baths and up again, the plate being flexed in order to follow this course. EP-A 0 415 392 discloses a processing apparatus for processing a printing plate, provided with a bath and guide rollers which control the plate so that it is descended obliquely into the solution in the bath, flexed to follow a curved passage below the solution level and pulled upwardly out of the bath. The apparatus comprises a rotating brush roller, accommodated submerged in the solution, and a floating cover for shielding a portion of the surface of the solution, including the area above the brush. The purpose of the floating cover is to counteract the deterioration of the processing solution by the air. EP-A 0 050 818 discloses a processing apparatus for printing plates provided with a tank and with conveyor means for guiding the plate down into the bath along a curved passage and up again, which apparatus comprises a rotating brush partially submerged in the bath and adapted for brushing the upper surface of the plate. This prior art apparatus is provided with a protective shield above the rotating brush, the rear edge of said protective shield being disposed completely below the solution level. The purpose of this is to counteract foaming in the processing solution and to counteract deterioration of the processing solution by the air. EP 0 260 615 discloses a processing apparatus for printing plates, comprising conveyor means for conveying the printing plate along a straight path descending into a bath to exit the bath at a lower portion thereof through the nip between a pair of rollers adapted for holding back the liquid. A rotating, partially submerged brush roller brushes the upper face of the printing plate. US patent 4 515 456 discloses a processing apparatus adapted for conveying the plates along a straight path through a bath, wherein sealing of entry and exit ports are provided by supple lip seals forced against the face of the plate. Brushing generally enhances the processing process, but generally has an adverse effect on the useful life of many developer baths. Many processing chemicals are highly reactive and may exchange different gasses with the air, e.g. absorb C0 2 or oxygen which react with chemicals in the baths so that these no longer have the desired effect and therefore must be reactivated or replaced. Movable brushes stirring the surface of the solution strongly accelerate this deterioration process. For reasons of minimizing handling of the delicate emulsion it is generally favoured to design the processing apparatuses for conveying the plates with the emulsion-covered side facing up. An apparatus provided with brushes for brushing the sheet uppper face, which brushes are completely submerged below the solution surface, is limited in its scope of design, which restricts the use and functionality of the apparatus. If the apparatus is to be designed with conveyor means for conveying the plates downwards and deep into the bath and subsequently upwards to exit the bath, and if the bath is not very long, the plate must be flexed by a relatively acute radius of curvature. This strains the plates and causes resistance to the motion. Furthermore, as a result of the small radius of curvature, the distance between the plate and the brush and hence the force of engagement by the brush may vary depending on several factors, among which the size, resilience and thickness of the plate. Due to the delicate nature of the emulsion on the surface to be brushed, the force of engagement by the brush is, however, a very critical parameter, and an apparatus of the above type is therefore not particularly suitable for processing plates of different grades and thicknesses. Summary of the Invention The invention in one aspect provides an apparatus with the features defined in claim 1. In this apparatus liquid is sucked into a cavity, which cavity is arranged to extend above and at least partially around the brush means. The level of liquid inside the cavity is raised, thereby partially or completely submerging the brush means with the result of reducing foaming and deterioration effects of the liquid caused by the operation of the brush means. Application of the invention to an apparatus with a conveying path curving down through a bath has the advantage that the conveying path may be designed for a quite shallow submersion of the plate, corresponding to a very large radius of curvature, avoiding the disadvantages referred to above. Application of the invention to an apparatus designed for a straight passage of the sheet through a bath has the advantage that the apparatus may be designed for a lower pressure differential across any seal means at entry and/or exit ports. According to a preferred embodiment the lower edge of the vessel dips into the bath liquid in order that the evacuation will suck liquid into the cavity from the stock in the tank. This will cause the level of liquid prevailing in the tank to drop, unless the stock of liquid is made up by other means. However, it is no problem to provide a bath with a surface area sufficient to ensure that the drop in the level of liquid in the tank becomes quite insignificant. According to a preferred embodiment the apparatus comprises a pair of movable brushes, one of the brushes being arranged above and the other below the conveying path. This allows one and the same apparatus to process plates exposed on one face or plates exposed on both faces . According to a preferred embodiment the evacuation means comprises a powered pump, associated with a discharge conduit communicated to the tank. This simplifies control and ensures an advantageous exchange of liquid in the cavity with liquid in the tank. According to a preferred embodiment the pump is communicated to the cavity by way of a suction conduit provided with an aeration valve. This valve may be operated expediently in connection with maintenance works, thus allowing air to enter the cavity so that the level of liquid there may drop in a controlled manner as may be preferred before removal or opening of the vessel for maintenance works . The conveyor means may comprise at least one guide roller which is at least partially submerged in the the solution bath. The guide roller may be arranged within the cavity or outside the cavity. The guide roller controls the course followed by the sheet, i.e. the flexing of the sheet, relieving the brushes or the brush roller of this function, by which a more accurate control of the point of engagement of the brushes can be obtained. In another aspect, the invention provides a method with the features defined in claim 7. This method offers advantages corresponding to those mentioned above with reference to the apparatus according to the invention. Further expedient embodiments of the invention will appear from the sub-claims. Brief Description of the Drawing The invention will appear from the following description of a preferred embodiment which is given with reference to the drawing which in Fig. 1 shows a schematic diagram of an apparatus adapted for processing photographic material . The drawing is schematical and not necessarily to scale, and it is simplified so as to merely schematically represent significant elements, whereas other elements which may be suggested by those skilled in the art have been omitted for the sake of clarity. Description of the Preferred Embodiments The figure shows a processing apparatus designated as a whole with the reference numeral 1. The apparatus essentially comprises a tank 5 with an upper edge 6, which tank is adapted for holding a stock of processing liquid 7 up to the level of the surface 17. The tank is provided with a circulation conduit 10 fitted with a circulation pump 11 for circulating the liquid. The tank is provided with two deflector plates 8, a pair of guide rollers 3 and two pairs of brush rollers 9 for brushing the top face and the bottom face of the plate. The brush rollers are provided with drive means (not shown) for rotation. A pair of inlet guide rollers 18 and a pair of outlet guide rollers 19 are arranged outside respective edges of the tank. These components define a conveyor path 4 for a photographic plate 2. The guide rollers are powered by controlled drive means (not shown) in order that plates may be conveyed along the conveyor path in a controlled fashion. Suitable drive means may be suggested by those skilled in the art. The conveyor path is so adapted that in the area adjacent the guide rollers 3 it curves upwardly by a radius of curvature of 225 mm, which is considered to represent a mild curving, imparting but a gentle strain onto the plates. However, in the area adjacent the brush rollers the conveyor path curves upwardly by an even longer radius of curvature of 1200 mm. With this longer radius of curvature the effect of any variations in the plate resilience and thickness will have but minimal effect on the locus defined by the passage of the plate and will therefore cause but minimal variations in brush engagement pressure. This makes the apparatus well suited for treating a wide variety of plates. The figure shows, in the area by the brush rollers 9, a closed vessel 12 which has a downward opening, the opening being delimited by the lower edge 13. This vessel is positioned in such a manner that the lower edge extends down below the expected normal level of the surface 17 of the processing liquid. The vessel defines a downwards open cavity 19 generally extending above and around an upper portion of the brush rollers. The conveyor path is designed for conveying the plates below the cavity lower edges with a suitable clearance. An upper part of the cavity 19 communicates with a suction conduit 14 connected to an inlet of an evacuation pump 16. The discharge of the evacuation pump merges into a discharge conduit 20, carrying also liquid from the circulation conduit 10 for returning it to the tank. As schematically suggested in the figure the evacuation pump may comprise an ejector pump powered by the flow driven by the circulation pump 11. Other pumps and in particular self-priming liquid pumps may also be used, the only important requirement being that they are capable of transporting both air and liquid at suitable pressure and capacity. As will be understood from the Figure, the evacuation pump 16 evacuates the cavity 19 at the upper portion of the vessel 12 to below atmospheric pressure. Since the edge 13 dips into the liquid in the tank, the low pressure will suck liquid into the cavity, thereby raising the level of liquid therein. Preferably the pressure differential is suficient to fill the cavity completely so that the rotating brushes will operate fully submerged in liquid. In a preferred embodiment adapted for treating plates of widths extending to 1350 mm, it is estimated that a cavity having a cubic content of 5.0 liters will be sufficient for containing the upper portions of two pairs of rotating brushes of a diameter of 55 mm. Given a bath length of 432 mm and a bath width of 1470 mm, the surface area may be computed as 6350 cm 2 . The vessel covers but a small proportion of this bath area. On these conditions the drawing of 5.0 liters of liquid into the cavity will cause the level of liquid in the ambient portion of the bath to drop by about 8 mm. Consequently the vessel should be arranged with the lower edge of the vessel dipping into the liquid to a level at least 8 mm below the level of the liquid which prevails prior to evacuation of the cavity in order that the cavity may be filled without having to make up the bath liquid. The Figure further shows the aeration valve 15 arranged on the suction conduit 14. This valve may selectively be manipulated to allow air to be introduced into the conduit for equalization of the pressures in order that the liquid in the cavity drains to the level of the bath. This is expedient because it provides a facility for emptying the cavity in a controlled manner as may be desired for maintenance work purposes, etc. The embodiment of the Figure shows the guide rollers 3 placed outside the cavity. In other embodiments the guide rollers may be arranged inside the cavity. In the embodiment of the Figure brush rollers are provided at the top face as well as the bottom face of the plate. This provides the apparatus with the capacity of processing plates with photographic emulsion on one side just as effectively as plates with photograhic emulsion on both sides. Other embodiments may employ oscillating brushes as an alternative or as a supplement to rotating brushes . Although specific embodiments have been described above it is emphasized that the invention may be exercised in several ways and that the explanation given above exclusively serves to clarify the invention and not to limit the scope of protection conferred, which is exclusively defined by the appended claims.

Claims: C L A I M S 1. An apparatus for the processing of a sheet of photographic material, and in particular for the development of photographically exposed plates in order to prepare them for use as printing plates in a printing operation, which apparatus comprises: - a tank adapted for containing a body of liquid, - conveyor means adapted for conveying the sheet along a conveying path which path comprises in succession an entry section in which the sheet enters the body of liquid, a treatment section in which the sheet is treated by being exposed to liquid of the body of liquid, and an exit section in which the sheet leaves the body of liquid, - brush means adapted for brushing a face of the sheet at a stage located within the treatment section, - a vessel defining a downwards open and substantially air tight cavity which extends above and at least partially around the brush means, and - evacuation means adapted for being operated to evacuate the cavity defined by the vessel so as to create a subatmospheric pressure within the cavity. 2. The apparatus according to claim 1, c h a r a c t e r i z e d by the tank being upwardly open and adapted for holding a bath of liquid, by the entry section comprising a leg for descending the sheet into the bath, by the treatment section comprising an upwardly curved leg and by the exit section comprising a leg for withdrawing the sheet from the bath. 3. The apparatus according to claim 1, c h a r a c t e - r i z e d by the vessel comprising a lower edge and by the vessel being arranged with the lower edge located below the expected surface level of bath liquid, in order that the operation of the evacuation means will have the effect of sucking liquid from the tank into the cavity defined by the vessel and raising the level of liquid within the cavity. 4. The apparatus according to claim 1, c h a r a c t e r i z e d by the brush means being adapted for brushing the upper face of the sheet, and by the cavity being adapted for holding a body of liquid which fully submerges the brushing means . 5. The apparatus according to claim 1, c h a r a c t e - r i z e d by the evacuation means comprising a powered pump associated with a discharge conduit communicated to the tank. 6. The apparatus according to claim 5, c h a r a c t e - r i z e d by the pump being associated with a suction conduit communicated with the cavity, and by the suction conduit being provided with an aeration valve, which may be operated to selectively permit the equalisation of the pressure within the suction conduit with the ambient pressure. 7. The apparatus according to claim 1, c h a r a c t e r i z e d by the brush means comprising a rotating brush roller or an oscillating brush. 8. The apparatus according to claim 1, c h a r a c t e r i z e d by the brush means comprising a pair of movable brushes, a first movable brush arranged above the conveying path, and a second movable brush arranged below the conveying path. 9. A method for the processing of a sheet of photographic material, and in particular for the development of photographically exposed plates in order to prepare them for use as printing plates in a printing operation, which method comprises: - conveying the sheet along a conveying path which path comprises in succession an entry section in which the sheet enters a body of liquid, a treatment section in which the sheet is treated by being exposed to liquid of the body of liquid, and an exit section in which the sheet leaves the body of liquid, - operating brush means to brush a face of the sheet while exposed to the liquid, - enclosing the brush means within a downwards open and substantially air tight cavity which extends above and at least partially around the brush means, and - evacuating the cavity so as to create a subatmospheric pressure within the cavity. 10. The method according to claim 9, c h r a c t e r i z e d by comprising the steps of - providing a tank containing a bath of the liquid, - entering the sheet into the body oi liquid by descending it into the liquid, - treating the sheet with the liquid while conveying it along an upwards curved path, - exiting the sheet from the bath of liquid by conveying the sheet upwards to withdraw it. 11. The method according to claim 10, c h a r a c t e r i z e d by comprising the step of providing a substantially air tight communication between the cavity and the tank in order that evacuating the cavity will have the effect of sucking liquid from the tank into the cavity and raising the level of liquid within the cavity. 12. The method according to claim 11, c h a r a c t e - r i z e d by comprising the steps of raising the level of liquid to fully submerge the brush means and by brushing the upper face of the sheet. 13. The method according to claim 9, c h a r a c t e - r i z e d by comprising the step of returning to the body of liquid any fluid withdrawn during the step of evacuating the cavity. 14. The method according to claim 9, c h a r a c t e - r i z e d by the brushing being effected by means of a rotating brush roller or by means of an oscillating brush.

Inventors: BOCKMANN PETERSEN HENRIK, HOUGAARD FINN, JENSEN TOMMY, BOCKMANN PETERSEN, HENRIK, HOUGAARD, FINN, JENSEN, TOMMY
Applicants: BOCKMANN PETERSEN HENRIK, GLUNZ & JENSEN, HOUGAARD FINN, JENSEN TOMMY, BOCKMANN PETERSEN, HENRIK, GLUNZ & JENSEN A/S, HOUGAARD, FINN, JENSEN, TOMMY
Application year: 1998
Main classification: G03D3
IPC codes: G03D3, G03F7