PRE-CORRECTION HEATING STAGE OF LITHOGRAPHIC PRINTING PLATE PROCESSING

Abstract: There is disclosed a lithographic printing plate treatment process and line in which development and rinsing, and preferably also gumming takes place in a processing unit (10), and any required correction is applied to treated plates issuing successively from the processing unit (10), so that any necessary image correction can be carried out. However, downstream of the processing unit (10), there is arranged a heating oven (16), which may be incorporated in the processing unit (10), or arranged separately as a ''stand alone'' oven (21), and which applies a heating action on the developed and rinsed plates treated within the processing unit (10), prior to the correction e.g. on a deletion table. This heating action provides at least some of the advantages of conventional baking ovens, but the heating action is less fierce, so that the plates are still capable of being corrected on the deletion table, whereas the finally issuing corrected plates still maintain improved resistance to subsequent action of mechanical and/or chemical attack normally encountered when a lithographic plate is mounted in a printing process.

Description: PRE-CORRECTION HEATING STAGE OF LITHOGRAPHIC PRINTING PLATE PROCESSING This invention relates generally to the processing of lithographic printing plates, in which exposed plates are developed, rinsed, gummed, dried and then usually stacked for delivery to a printing press. The applicants have developed an automated plate processing equipment line which is known as "Plate Street" and which can take a plate from a print-down frame, and then conveys the plate continuously along the line to receive treatments from successive work stations arranged along the line. A number of drying stages can be incorporated in various pieces of equipment in the line. In the case of a processor unit, this takes the form of a hot air dryer arranged at the downstream end of a single unit in which development, rinsing and gumming takes place successively, and which applies a gentle heating action to the plates after rinsing and gumming, and at temperatures slightly above ambient e.g. up to about 30°C. However, this gentle heating action is only sufficient to dry-off moisture, and does not have any appreciable effect on the constitution of the surface of the treated plate. It is known to use baking ovens in plate processing lines, and usually the oven is arranged in the final stage of a treatment line, and operates so as to bake the treated plate and thereby considerably increase the number of copies which can be derived from the plate when mounted in a printing press. There are other advantages of carrying out baking in the final stage of the treatment line, by increasing the resistance of the plates to materials and conditions to which they will be exposed in service, i.e. increasing the resistance to attack by chemical and / or mechanical action normally encountered in the use of lithographic printing plates, such as alcohol resistance, and abrasion resistance. The final stage of a treatment line incorporati g a separate baking oven includes, downstream of the baking oven, a gumming unit which applies a final storage gum to the surface of the baked plate. However, upstream of the baking oven there is usually arranged a so-called "deletion table" at which minor corrections can be made to the- image on the dried and processed plate e.g. by the use of deletion gel, deletion pens, or correction fluid, prior to passage of the corrected plate to the baking oven. While the use of a baking oven provides benefits which are advantageous to many printers, and especially the larger printers having long production runs, the oven does represent an appreciable capital cost and also increases the length of the treatment line, and these factors can outweigh the advantages to certain users, and especially if space available is somewhat restricted. It would therefore be advantageous to be able to offer a modified treatment line which can omit the baking oven (and thereby reduce the length of the line and also the capital costs of the oven) , and yet achieve at least a proportion of the advantages of the baking stage. This may be achieved by incorporation of a suitable heater at a convenient position along the length of the treatment line, bμt in seeking to meet this desired objective, it should be borne in mind (a) that the heating which takes place in a baking oven is relatively high temperature and (b) the use of these high temperatures on the plates prior to conveyance of the plates to the deletion table would render the plates incapable of correction. Thus, a typical baked plate, after issue from a baking oven, would be incapable of correction of its image, since of course the primary purpose of the baking step is to improve the surface resistance properties of the treated plate so that it can more readily withstand abrasion and chemical attack from substances used in the printing process. The invention is therefore based on the surprising discovery that it is possible to provide a lithographic printing plate treatment in which higher temperature heating can be applied to the treated plates prior to any required correction action (higher temperatures than the gentle heating action presently provided upon issue from the gumming unit) , so as to derive at least a proportion of the benefits of baking in a baking oven, and yet this heating not being of such a fierce character as to adversely affect or to inhibit any required correcting action . According to a first aspect of the invention there is provided a method of processing a lithographic printing plate which comprises subjecting the plate to a treatment of developing and rinsing of the plate, and carrying out any required correction to the image on the treated plate, in which the treated plate is subjected to a heating stage, prior to correction treatment, such heating of the plate being carried out to a temperature level and for a time duration at which improved resistance is provided to subsequent mechanical and / or chemical action normally encountered in use of a lithographic printing plate when mounted in a printing press, while retaining a capability for the treated image on the plate to be corrected. According to a second aspect of the invention there is provided a lithographic printing plate processing line which includes a processing unit in which development and rinsing and preferably also gumming can take place, and a deletion table arranged to receive treated plates issuing successively from said processing unit; in which a heating oven is arranged between the processing unit and the deletion table so as to receive and heat successive plates issuing from the unit to a temperature level at which improved resistance is provided to subsequent mechanical and / or chemical action normally encountered in use of a lithographic printing plate when mounted in a printing press, and yet still retaining a capability of correction on the deletion table. According to a third aspect of the invention there is provided a method of processing a lithographic printing plate which comprises developing and rinsing- and preferably also gumming the plate in a processing unit, and carrying out any required correction to the image on the treated plate: in which a heating oven is arranged downstream of the unit and is operated so as to heat successive plates issuing from the unit to a temperature level at which improved resistance is provided to subsequent mechanical and / or chemical action normally encountered in use of a lithographic printing plate when mounted in a printing press, and yet still retaining a capability for the treated image on the plate to be corrected. Preferably, in the second or third aspects of the invention, the processing unit incorporates a heating oven which is arranged at its downstream end so as to carry out heating of the plates successively prior to correction. However, it is not essential to this invention that the heating carried out prior to image correction should be via an oven incorporated in the processing unit. Thus, there are other means by which the required heating can be carried out in an intermediate phase between treatment in the processing unit and any correction which may be required. Therefore, for certain users, it may be desirable to provide a "stand alone" oven which can be supplied with plates treated by the processing unit and then heated, prior to correction treatment when required. The stand-alone oven conveniently may be arranged in a continuous lithographic plate treatment line. However, the invention does include a method of treatment of lithographic printing plates, in which the stages of developing and rinsing and preferably gumming, and pre- correction heating of the plates may be carried out on an intermittent basis, and not necessarily with the components of the treatment apparatus being arranged in a continuous production line. According to a fourth aspect of the invention there is provided a processing unit operable to develop and rinse and preferably also apply gum to a lithographic printing plate during processing thereof, and to convey the treated plate to a deletion table at which any required correction to the treated image on the plate can be carried out: in which the processing unit incorporates a heating oven arranged at the downstream end of the unit to heat successive plates passing through the unit to a temperature level at which improved resistance is provided to subsequent mechanical and / or chemical action normally encountered in use of a lithographic printing plate when mounted in a printing press, and yet still retaining a capability of correction of the treated image on the deletion table. However, while it is preferable for the gumming to be applied to the plate by the processing unit, this is not essential to the invention and gumming can be omitted from the processing unit, provided that a suitable cleaning and gumming treatment is applied after correction. Thus, the invention provides a beneficial heating action on treated plates prior to any required correction, and this heating action is greater than the substantially ambient temperature moisture removal heating action provided by known units, and therefore at a level sufficient to achieve at least a proportion of the properties achievable by a baking oven, and yet with a less fierce action than a baking oven i.e. lower temperature and / or shorter duration so that the treated image on the heated plates retains a capability of correction. The invention therefore enables at least a proportion of the benefits of a baking oven to be achieved (and especially increased run length obtainable) while keeping the capability of corrections to be carried out e.g. on a deletion table arranged downstream of the processing unit. The preferred temperature range utilised in the carrying out of the invention is 150 to 250°C , which is of course substantially above the known ambient temperature hot air drying, but sufficiently below and less fierce than the heating of the plates achieved in conventional baking ovens so as to retain the subsequent correction capability. The time duration of exposure to this heating action is preferably between ten seconds and one and a half minutes depending upon speed of transit. The invention therefore enables most of the advantages of a baking oven to be obtained, but with a shorter treatment line, and saving on the capital cost of the baking oven. Also, with the use of the known baking ovens, it is usual to apply a pre-baking solution, and this is a specially prepared solution which is relatively expensive, and therefore economies can also be achieved in this respect also. Preferably, the treatment line has a ' continuous conveyor capability, whereby plates are conveyed continuously along the line, and through each working station, and at a substantially constant speed, say 0.5 up to 2.0 meters per minute. Generally speaking, the benefits achieved by the pre- deletion heating in the invention will be less than that which can be achieved by the use of a known baking oven, but nevertheless they will still be appreciable, and of course with the advantage that correction capability is still retained, whereas there is substantially nil correction capability of a treated plate after it has undergone treatment in known baking ovens. For certain applications, it may be desirable to permit re-entry of a corrected plate for further passage through the rinse and gum units of the processor, in which case a re-entry slot may be arranged immediately downstream of the development unit. Embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawing, in which: Figure 1 is a plan view of one embodiment of lithographic printing plate processing unit for use -in carrying out the invention; Figure 2 is a schematic side view of the processing unit shown in Figure 1; and. Figure 3 is a schematic side view of a modification of Figures 1 and 2 to be provided in a further embodiment of lithographic printing plate treatment line and process according to the invention. Referring now to Figures 1 and 2 of the drawings, there will be described an embodiment of lithographic printing plate processing unit for use in carrying out the invention, and which can form part of a treatment line which receives an exposed printing plate from a print-down frame, develops the plate, rinses, preferably gums and then applies special heat treatment to the plate prior to stacking of the treated plate for delivery to a printing press . The processing unit is designated generally by reference 10, and preferably is adapted to be incorporated in a continuous conveyor plate treating line, having an entry or inlet 11, a development stage 12, a rinse stage 13, a gum unit 14. and a gentle hot air dry stage 15 at which moisture is evaporated from the plate surface using air at temperatures slightly above ambient e.g. up to 30°C. Thus far, the embodiment 10 is substan ially in line with existing processing equipment. However, in addition, at the downstream end of the unit 10 there is incorporated a heating oven 16 which is arranged to heat successive plates passing through the unit 10 to a temperature level at which improved resistance is provided to subsequent mechanical and / or chemical action normally encountered in printing processes applied to the plate when mounted in a printing press. However, this temperature level , and the duration of its action on the plate is carefully controlled so that, while improved resistance is obtained, as compared with substantially unheated plates, there is still retained a capability of correction of the treated image e.g. by deletion gel, deletion pen or correction fluid, which may be carried out on a correction work surface e.g. a standard deletion table which is preferably arranged immediately downstream of the unit 10. The heating action applied to the plates by the oven 16 is therefore substantially greater than the substantially ambient temperature heating which takes place in the hot air dry stage 15, but in a carefully controlled and less fierce action than that normally achievable in a conventional baking oven. Typically, a baking oven operates with air at temperatures up to about 300°C, which normally heats the plates to about 250°C during the residence of a plate as it travels through a baking oven, whereas the oven 16 employs hot air at temperatures of more than ambient temperature and preferably in the range 150 to 250°C. This less fierce heating action than achieved in a baking oven still enables a substantial proportion of the advantages of use of a baking oven to be achieved, while still retaining a correction capability. This therefore enables a treatment line to be provided which omits a baking oven, thereby reducing the length of the line and also making a saving by virtue of the capital cost of the baking oven, by the incorporation in the unit 10 of the less fiercely acting oven 16. Therefore, at least for certain users, and especially those for which space is at a premium, a substantial proportion of the benefits achievable by a baking oven can be obtained, and yet in a treatment line which is appreciably shorter. Cooling fans 17 are arranged downstream of the oven 16, and an exit conveyor 18 feeds the treated plates to a deletion table (not shown) arranged downstream of the unit 10 in the treatment line, at which any corrections to the treated image (if required) can be carried out. An extraction unit 19 is arranged above the oven 16 to extract vapour generated during the heating stage applied to the heated plates. While a gumming unit 14 is shown incorporated in the processor, this is not essential and can be omitted, in which case a gumming unit (not shown) will be provided in the line downstream of the deletion table for application of a cleaning gum to the plate. However, while the embodiment of Figures 1 and 2 shows a pre-deletion heating oven 16 incorporated in the processing unit 10, it is not an essential feature of 'the invention for the pre-deletion heating stage to be carried out via an oven incorporated in the processing unit. It is only necessary for an intermediate heating stage to be provided between the processing via stages 12 to 15 in the processing unit, and any necessary deletion work carried out subsequently e.g. on the deletion table. The oven 16 may therefore be detached from incorporation within the processing unit 10, and arranged downstream of the processing unit to have plates treated by the processing unit transferred automatically thereto, or manually if a non-continuous operation is required by the customer. Referring to Figure 3 of the drawings, this shows a modification to the processing unit 10 of Figures 1 and 2, in which a "stand-alone" intermediate pre-deletion unit 20 is provided which includes a stand-alone oven assembly 21. Preferably, there is automatic transfer of treated plates from the processing unit 10 to a mesh belt conveyor 22 feeding oven 21, though certain customers may be content to have a stand alone unit to which treated plates can be transferred manually when required. The unit 20 includes oven 21 which carries out pre-correction heating, in the manner described above, and also has an extraction fan 23 and plate cooling fans 24 which act on the plates prior to issue from the oven 2 via output conveyor stage 25. The treated plates can then be transferred automatically, or intermittently by manual means if desired, for deletion e.g. at a deletion table, at which any necessary correction can be carried out. and this correction will not be adversely affected by the heating action applied in the oven 21. Nevertheless, as described above, the heating action is of such a level as to achieve at least some of the advantages achieved with known baking ovens which operate with air temperatures of about 300°C. While the invention is preferably carried out on a continuous basis, which will suit many customers requiring an automated process, for the smaller printers, who do not wish to incur the capital cost of a full treatment line, or do not have the space to incorporate all of the components for continuous line operation, there may still be advantages in carrying out a lithographic printing plate treatment process, within the scope of this invention, in which the various stages are carried out on an intermittent basis, and possibly with manual transfer of the treated plates from stage to stage, at intervals to suit working conditions . Thus, in its least automated aspect, the method of the invention should be understood as including exposure in a light frame, manual development, manual rinsing, manual gumming, manual transfer to the oven, removal from the oven and then any necessary correction. Finally, the correction chemical fluids may be rinsed-off, and storage gum applied.

Claims: CLAIMS 1. A method of processing a lithographic printing plate which comprises subjecting the plate to a treatment of developing and rinsing of the plate, and carrying out any required correction to the image on the treated plate, in which the treated plate is subjected to a heating stage, prior to correction treatment, such heating of the plate being carried out to a temperature level and for a time duration at which improved resistance is provided to subsequent mechanical and / or chemical action normally encountered in use of a lithographic printing plate when mounted in a printing press, while retaining a capability for the treated image on the plate to be corrected. 2. A lithographic printing plate processing line which includes a processing unit in which development and rinsing and prefera-bly also gumming can take place, and a deletion table arranged to receive treated plates issuing successively from said processing unit; in which a heating oven is arranged between the processing unit and the deletion table so as to receive and heat successive plates issuing from the unit to a temperature level at which improved resistance is provided to subsequent mechanical and / or chemical action normally encountered in use of a lithographic printing plate when mounted in a printing press, and yet still retaining a capability of correction on the deletion table. 3. A method of processing a lithographic printing plate which comprises developing and rinsing and preferably also gumming the plate in a processing unit, and carrying out any required correction to the image on the treated plate: in which a heating oven is arranged downstream of the unit and is operated so as to heat successive plates issuing from the unit to a temperature level at which improved resistance is provided to subsequent mechanical and / or chemical action normally encountered in use of a lithographic printing plate when mounted in a printing press, and yet still retaining a capability for the treated image on the plate to be corrected. 4. A processing line according to Claim 2, in which the heating oven is incorporated in the processing unit at the downstream end thereof, so as to carry out heating of the plates successively prior to transfer to the deletion table. 5. A processing line according to Claim 2. in which the heating oven is separate from and spaced downstream of the processing unit. 6. A method according to Claim 3, in which the heating oven is incorporated in the processing unit, at the downstream end thereof, so as to carry out heating of the plates successively prior to transfer to a correction treatment stage. 7. A method according to Claim 6, in which the stages of developing and rinsing, and pre-correction heating of the plates is carried out on an inter itte ' nt basis. 8. A processing unit operable to develop and rinse and preferably also apply gum to a lithographic printing plate during processing thereof, and to convey the treated plate to a deletion table at which any required correction to the treated image on the plate can be carried out: in which the processing unit incorporates a heating oven arranged at the downstream end of the unit to heat successive plates passing through the unit to a temperature level at which improved resistance is provided to subsequent mechanical and / or chemical action normally encountered in use of a lithographic printing plate when mounted in a printing press, and yet still retaining a capability of correction of the treated image on the deletion table. 9. A method according to Claim 3. in which only developing and rinsing of the plates takes place in the processing unit, and gumming takes place after correction treatment on a deletion table. 10. A method according to any one of Claims 1 , 3, 6. 7 or 9, in which the pre-deletion heating of the plates takes place using hot air at temperatures in the range 150°C to 250°C. 11. A processing line according to any one of Claims 2, 4, 5 or 8, including a continuous conveyor arrangement for conveying the plates continuously along the processing line and through each working station. 12. A processing line according to Claim 11, in which the conveyor arrangement is capable of operating at speeds in the range 0.5 to 2.5 meters per minute. 13. A processing line according to Claim 1, in which a re-entry slot is arranged immediately downstream of the development unit of the processor unit, to permit re-entry of a corrected plate for further passage through the rinse and gum units of the processor unit.

Inventors: PARKINSON JOHN, RITCHIE CAMERON, PARKINSON, JOHN, RITCHIE, CAMERON
Applicants: COOKSON GRAPHICS PLC
Application year: 1989
Main classification: G03F7
IPC codes: G03F7