METHOD AND APPARATUS FOR WASHING-OUT PRINTING PLATES

Abstract: The invention provides a method and apparatus for washing out a pre-exposed printing plate, according to which the plate is subjected to a continuous and steady flow of fresh, clean wash-out fluid which is allowed to flow in a continuous and steady manner to waste, after its contact with the plate. Thus, the same wash-out fluid is not reused unless and until it has been cleaned and/or filtered ready for reuse, and the further plate is continuously treated using clean wash-out fluid. The supply of fluid in a continuous and steady flow, under control and on demand, for as long as required, rather than in circulation from a reservoir of pre-heated fluid, leads to a reduction in water consumption and electrical energy savings, with the benefit of improvement in the quality of the washed out plate.

Description: METHOD AND APPARATUS FOR WASHING-OUT PRINTING PLATES Field of he Invention This invention relates to a method and apparatus for washing-out a pre-exposed printing plate such as a photopolymer relief printing or moulding plate or a presensitised aluminium plate. The invention also relates to a method and apparatus for processing such a plate, incorporating the washing-out method and apparatus respectively. Background to the Invention Photopolymer relief printing and moulding plates, and presensitised aluminium plates, are now extensively used in the printing industry. The production of a printing plate is generally carried out in the following manner: imagewise irradiation (main exposure) , followed by removal of unhardened areas (washout) , drying of the washed plate and post irradiation (post exposure) . The removal of unhardened areas (washout) is generally carried out by brushing the plate in a heated fluid such as water and/or a chemical solvent such as alcohol, butyl alcohol, etc. Printing plates fall into two categories - those that are water washable and those that need to be washed using some other chemical solvent. Recently, water washable type photopolymer plates have gained considerable market presence, largely replacing the organic solvent washable type. The most important reason for the popularity of water washable plates is their environmentally friendly nature and working conditions, especially compared to those of organic solvent washable plates. Methods for washing out such plates, either of the water or of the organic solvent washable type, are known. These are generally carried out in either automatic in¬ line or stacker type processors, and usually involve the recirculation of a fixed volume of washout fluid, through a washout chamber, a number of times before the fluid is discarded. For instance, an existing processor might typically operate with a fixed volume of washout fluid, contained and heated in a holding tank within the processor. This fluid is pumped, in the case of an in-line automatic processor, to the washout chamber of the processor, where it is brought into contact with a washout brush and a plate to be washed. Having been in contact with the printing plate, the fluid then returns - usually via gravity - to the holding tank ready to be re-pumped back to the washout chamber in a closed loop. In the case of a stacker type processor, the plate being washed, along with the washout brush, is immersed directly into the holding tank. There are several problems associated with the use of such washing-out methods, as follows:- 1. As subsequent plates are processed, the quality of the washout fluid in the holding tank deteriorates. Its viscosity increases and its washout efficiency is reduced, because the level of waste material within the fluid and holding tank increases with each plate processed. 2. The efficiency of the methods, with regard to energy consumption and practical operation of the processors used, is low due to the fact that a) there is a considerable delay in initial start-up of the processor, in order for the fixed volume of washout fluid to be heated to its optimum operating temperature; and b) a high proportion of energy is wasted in the continuous re-heating of fluid in the holding tank in order to maintain the operating temperature (i.e. to combat natural heat loss) whilst the processor is in 'stand-by' mode. Stand-by mode is typically utilised for as much as 75% of a working day or shift when processing a plurality of plates in a normal daily production cycle. 3. At the end of a processing cycle or working day, the user is presented with a high volume of waste washout fluid which contains a high concentration of photopolymer waste. Should this volume of fluid require filtration (as is normally the case) and/or subsequent processing for re-use of the fluid, the user must have available a filtration system that can physically handle the large fluid volume and high waste concentration involved. Such filtration systems are generally expensive, physically large, inconvenient and require large amounts of additional energy and operator attention. 4. Current washout systems usually have an upturned washout brush (bristles uppermost) which tends to suffer from *brush clogging' . This happens because there is insufficient washout fluid circulation within the area of the brush and also because the printing plate being processed is above the brush, so that gravity causes the washed out particles/residue of plate material to fall into and collect within the bristles of the brush and reduce its effectiveness. In order to overcome this common problem, it is usually necessary to continue washing the plate for extended periods to achieve the required amount of relief. However, this can often lead to unacceptable swelling of the plate if it absorbs too much washout fluid into its exposure hardened areas, because of its extended contact time with the fluid. It is an aim of the present invention to provide a method and apparatus for washing out a printing plate, which overcome or at least mitigate the above described problems. An effective and economical washing out procedure is sought which will also respect the need for energy conservation and environmental issues. Statement of the Invention According to the present invention there is provided a method for washing-out a pre-exposed printing plate, comprising the steps of subjecting the plate to a continuous flow of fresh, clean washout fluid and allowing the washout fluid to flow continuously to waste after its contact with the plate, such that the same washout fluid is not re-used in the method unless and until it has been cleaned and/or filtered ready for re¬ use. Thus, in contrast to previously used washing-out methods, the washout fluid is not recirculated unless and until it has been cleaned and can be used in a fresh, clean form. The plate is thus continuously treated using clean washout fluid. The fluid is also supplied to the plate in a continuous flow, rather than as a fixed volume which is used and re-used in a closed-loop system until it has become unacceptably dirty and needs to be discarded. The plate which can be washed out using the method of the invention may be a photopolymer relief printing or moulding plate, a presensitised aluminium printing plate, or any other type of plate which needs to be washed out post-exposure to remove unexposed areas. The plate may be either water washable, in which case the washout fluid is water or a suitable aqueous solution, or organic solvent washable, in which case the washout fluid comprises one or more appropriate organic solvents. Preferably, the plate is water washable. The flow of washout fluid to which the plate is subjected is preferably at a steady, relatively slow rate. The fluid is preferably directed to flow onto the plate through a spray bar of conventional construction, such as to ensure an even distribution of the fluid over the plate. The spray bar may form part of a washout brush used to brush the plate during the washing-out process, the fluid thus being sprayed onto the plate through the brush used to clean it. The washout brush preferably has downwardly-directed bristles during use, such that the brush contacts the printing plate from above. Such a brush suffers very little brush clogging compared to conventionally-used brushes which have upwardly-directed bristles during use. The washout fluid allowed to flow to waste may subsequently be discarded. Alternatively, it may be filtered and/or otherwise cleaned ready for re-use. Such filtered fluid can then be re-directed into the flow of fresh, clean fluid over the printing plate, for example by means of a closed-loop system in which cleaned water is recycled to maintain economy and efficiency. The washout fluid to which the plate is subjected is preferably heated to a desired operating temperature before it is allowed to flow over the plate. This is conveniently achieved by means of an instantaneous fluid heater positioned in the flow of the fluid, upstream of the printing plate. The temperature of the fluid flow to which the plate is subjected can thus be controlled to be at the correct operating temperature prior to the washout procedure. The method of the invention may form part of an overall method for processing a printing plate, which may include other processing steps such as main exposure, post and detack exposure and drying. The present invention additionally provides apparatus for washing-out a pre-exposed printing plate, comprising a source of fresh, clean washout fluid; means for supplying fluid from the source continuously to a plate positioned in the apparatus so as to subject the plate to a continuous flow of the clean fluid; and means for continuously removing the fluid for filtration and re- circulation, or for filtration and thereafter to waste, after its contact with a plate positioned in the apparatus. The apparatus preferably additionally comprises one or more of the following:- 1. a plate holder (typically of conventional construction) for holding one or more plates in position in the apparatus; 2. a washout brush for brushing a plate positioned in the apparatus, the bristles of the brush preferably being downwardly directed during use, such that the brush contacts the plate from above; 3. an instantaneous fluid heater, upstream of the plate, for heating washout fluid from the source prior to its supply to the plate; and 4. a washout chamber in which the plate may be positioned. The plate holder may be either a fixed position, a linear-tracking, a multi-directional or any other suitable type of plate holder which is available. The washout brush may be capable of an omni-directional, bi¬ directional, multi-directional, orbital or linear brushing action, or may be fixed, depending on the type of plate holder in association with which it is to be used. The means for supplying washout fluid to the plate preferably comprises a spray bar, which conveniently forms part of a washout brush. The means for removing the fluid for filtration preferably comprises a catchment tray (conveniently positioned below a plate holder) for collecting the fluid and directing it towards an outlet, either to filter and thence to waste or to be filtered and/or cleaned. The apparatus may additionally comprise filtration means for filtering and/or cleaning removed washout fluid. The apparatus may then also comprise means for recirculating the washout fluid, after it has been filtered by the filtration means, back to the washout fluid source ready for re-use. This forms a closed-loop system for the washout fluid. The invention is most appropriately designed with the aid of the catchment tray to allow for a steady flow of removed washout fluid to be introduced into the filtration system. This additional filtration system for filtering and cleaning the washout fluid having the option to return the cleaned water back to the washout brush head or to introduce the clean water to main drain. The apparatus may be adapted for either automatically or manually controlled operation. The apparatus of the invention may form part of an overall apparatus for processing a printing plate. The processing apparatus may be of either the "in-line" or the "stacker" type, both well known and commonly available types of plate processor. The present invention will now be described, by way of example only, with reference to the accompanying illustrative drawings, of which:- - Figure 1 shows schematically apparatus for washing out printing plates, in accordance with the invention; - Figures 2-5 show alternative types of brush and plate holder for use in the apparatus shown in Figure 1; - Figure 6 shows schematically an automatic in-line printing plate processor incorporating the apparatus shown in Figure 1; and - Figure 7 shows schematically a stacker type printing plate processor incorporating the apparatus shown in Figure 1. Detailed Description of the Drawings Apparatus in accordance with the invention is shown schematically in Figure 1. It comprises: - A down-turned brush pad 1 (bristles lowermost) , with integral spray bars 2 for even distribution of washout fluid. This brush pad may have a fixed position or an omni-directional or bi-directional orbital action when used in conjunction with the moving plate holder 3 which may be flat or of the drum type. - An instantaneous heating device 4 situated within a main washout fluid supply circuit, complete with its own electronic temperature monitoring and control apparatus. - A plate holding device 3, which may be flat or cylindrical and can be in a fixed position or have multi-directional motion. The purpose of the plate holder 3 is to physically hold the printing plate 5 so that the surface to be washed out is held in, or brought into, contact with the washout brush 1. - A catchment tray 6 which is located under and in close proximity to the plate holding device 3. Its function is to collect waste washout fluid (as it falls through gravity) and channel it towards a point of exit (7) or to an internal or externally mounted filtration unit and/or fluid recycling system. Electronic/electromechanical sequential control (not shown) which has the function of timing the operation of the instantaneous fluid heating device 4 along with fluid flow to the brush head 1. This control system is useful in achieving a system of 'plate wetting' which is the automatic application of fluid to the plate surface, prior to contact with the washout bristles (sometimes necessary to protect the print surface of photopolymer relief printing or moulding plates) . In use, fresh, clean washout fluid is fed continuously from a source (not shown) to the spray bars 2 via heater 4. It is then, after its contact with plate 5, allowed to flow continuously into catchment tray 6, from which it flows either to waste, to filtration ready for re-use or to filtration and then to drain. There is no holding tank for the fluid, and it is not recirculated in a closed loop directly from tray 6 back to heater 4 and thence to spray bars 2. Closed-loop recirculation is introduced only after filtration and/or cleaning of the waste washout fluid. Figures 2-5 show how different types of brush pad may be used with different types of plate holder in the apparatus shown in Figure 1. In Figure 2, a fixed brush assembly 10 is shown for use with an omni- or multi¬ directional moving plate holder 11. In Figure 3, fixed brush assembly 12 is used with an omni- or multi- directional moving cylindrical plate holder 13. Figure 4 shows the use of an omni- or multi-directional, or an orbital, moving brush 14 with a fixed plate holder 15. Figure 5 shows the same brush 14, this time for use with a linear tracking or omni- or multi-directional moving plate holder 16 (i.e. both brush and plate holder moving) . Figures 6 and 7 show how the apparatus of Figure 1 may be incorporated into printing plate processors, either of the "in-line" type (Figure 6) or of the "stacker" type ( Figure 7 ) . In the processor shown in Figure 6, washout fluid collecting in the catchment tray 6 of the washout apparatus passes to the filtration unit 20, where it is filtered and/or cleaned ready to be re-introduced into the washout brush head, thus achieving an economical and highly efficient closed-loop system. Alternatively the washout fluid, after filtration and/or cleaning, may be introduced to main drain. Washed out plates pass through the processor to air knife 21, post and detack exposure section 22 and drying section 23, all conventional components in a printing plate processor. In the multi-cabinet or 'stacker' type processor shown in Figure 7, plates are manually introduced to the different treatments forming part of a plate making procedure. The production of the plates is carried out in main exposure (25) , washing out (top of the processor) , post and detack exposure (26) and drying (27) sections of the processor. The washout fluid from the catchment tray (6) is again passed to a filtration unit (24) for filtering and either re-circulation or passage to main drain. In both processors, the filtration unit could either be a component of the processor or a free-standing unit connected to the processor. The advantages of the method and apparatus of the invention, and particularly of the apparatus shown in Figure 1, are as follows: a. Efficient washout of non-irradiated areas of the printing plate is achieved by a downward facing brush which does not suffer from 'brush clogging', coupled with a continuous supply of clean washout fluid evenly distributed over the area of the washout brush via the integral spray bars. b. Washout time is thus reduced by up to 50% when compared to present washout systems. c. The risk of subsequent plate swell is also greatly reduced, if not totally irradicated. d. Simplicity of construction is evident when compared to conventional washout systems, as the requirement for heater pumps and fluid level control devices (in the holding tank) no longer exists. e. An additional benefit is the lower level of day to day and routine engineering maintenance required. f. The apparatus is more suitable for inclusion within modern printing plate processing equipment used for photopolymer and presensitised aluminium plates, due to the reduction in physical space required to accommodate its component parts. This is again achieved by the removal of any requirement for recirculation pumps, fluid holding tanks and associated control equipment. g. A high degree of energy conservation is achieved, since energy is only utilised when the invention is actually in operation calling for heated fluid and the setting up of the sequential actions of further processing of printing plates. h. The production of environmentally friendly water washable photopolymer relief printing and moulding plates is further enhanced since the system of the invention releases a controlled low volume flow of waste washout fluid, having a relatively low concentration of waste photopolymer. This allows the use of an automatic, convenient to use, compact and energy efficient filtration system. i. A substantial reduction in the level of water consumption is achieved - up to 50% less than in conventional in-line systems - as the invention uses only as much water as it requires to achieve optimum washout which in turn is determined via the control board setting. j . Pre-heated washout fluid is introduced into the washout chamber for only as long as it is necessary to wash out a plate. As there is no fluid holding tank, energy requirements are greatly reduced. k. The apparatus of the invention is particularly efficient in that the filtration system filters and/or cleans only the pre-calculated amount of fluid which is needed to achieve the washout of a printing plate.

Claims: Claims 1. A method for washing-out a pre-exposed printing plate, comprising the steps of subjecting the plate to a continuous flow of fresh, clean washout fluid and allowing the washout fluid to flow continuously to waste after its contact with the plate, such that the same washout fluid is not re-used in the method unless and until it has been cleaned and/or filtered ready for re-use. 2. A method according to Claim 1, wherein the printing plate is water washable, and the washout fluid is water or a suitable aqueous solution. 3. A method according to Claim 1 or Claim 2, wherein the flow of washout fluid to which the plate is subjected is at a steady, relatively slow rate. 4. A method according to any one of the preceding claims, wherein the fluid is directed to flow onto the plate through a spray bar, so as to ensure an even distribution of the fluid over the plate. 5. A method according to Claim 4, wherein the spray bar forms part of a washout brush used to brush the plate during the washing-out process, the fluid thus being sprayed onto the plate through the brush used to clean it. 6. A method according to Claim 5, wherein the washout brush has downwardly-directed bristles during use, such that the brush contacts the printing plate from above. 7. A method according to any one of the preceding claims, wherein the washout fluid allowed to flow to waste is subsequently filtered and/or otherwise cleaned ready to be re-directed into the flow of fresh, clean fluid over the printing plate. 8. A method according to any one of the preceding claims, wherein the washout fluid to which the plate is subjected is heated to a desired operating temperature before it is allowed to flow over the plate, by means of an instantaneous fluid heater positioned in the flow of the fluid, upstream of the printing plate. 9. A method for processing a printing plate, comprising the step of washing-out the exposed printing plate using a method according to any one of the preceding claims. 10. Apparatus for washing-out a pre-exposed printing plate, comprising a source of fresh, clean washout fluid; means for supplying fluid from the source continuously to a plate positioned in the apparatus so as to subject the plate to a continuous flow of the clean fluid; and means for continuously removing the fluid for filtration and re- circulation, or for filtration and thereafter to waste, after its contact with a plate positioned in the apparatus. 11. Apparatus according to Claim 10, additionally comprising a plate holder for holding one or more plates in position in the apparatus. 12. Apparatus according to Claim 10 or Claim 11, additionally comprising a washout brush for brushing a plate positioned in the apparatus. 13. Apparatus according to Claim 12, wherein the bristles of the washout brush are downwardly directed during use, such that the brush contacts the plate from above. 14. Apparatus according to any one of Claims 10-13, additionally comprising an instantaneous fluid heater, upstream of the plate, for heating washout fluid from the source prior to its supply to the plate. 15. Apparatus according to any one of Claims 10-14, wherein the means for supplying washout fluid to the plate comprises a spray bar which forms part of a washout brush. 16. Apparatus according to any one of Claims 10-15, additionally comprising filtration means for filtering and/or cleaning removed washout fluid. 17. Apparatus according to Claim 16, additionally comprising means for recirculating the washout fluid, after it has been filtered by the filtration means, back to the washout fluid source ready for re-use. 18. Apparatus for processing a printing plate, comprising apparatus according to any one of Claims 10-17, for washing out the exposed plate.

Inventors: DANON RICHARD, DANON, RICHARD
Applicants: DANTEX GRAPHICS LTD
Application year: 1992
Main classification: G03F7
IPC codes: G03F7