PROCESS AND APPARATUS FOR PRODUCING PRINTING PLATES

Abstract: A process for decoating organic photo-conductor plates have a first soaking step in which an aqueous developer solution is applied to the plate to soften but not remove the coating and a second, washing step in which water is applied to the plate to remove the softened coating, usually by brushing the coating. The plate is preferably subsequently rinsed. The invention includes an apparatus for carrying out the process.

Description: PROCESS AND APPARATUS FOR PRODUCING PRINTING PLATES The present invention relates to a new process for producing printing plates and apparatus for carrying out the process. Specifically the process is for decoating organic photo-conductor (OPC) plates to which a toner has been image-wise applied and cured. Litho plates comprise a metal substrate, usually formed of aluminium although sometimes formed of zinc or other metals. A plate manufacturer provides the substrate with a coating, usually a resin, areas of which car. be selectively removed by the plate maker to provide a printing plate having areas of different degrees of hydrophilicity/phobicity. Inks used to print from the plates are oleophilic (hydrophobic) so are deposited only on the hydrophobic areas of the plates. The resin that is coated onto the plate has conventionally been a phctc-sensitive resin, which on exposure to light becomes more or less easily removable by the development process. In the case of negative plates exposure cures the reεir. by initiating polymerisation of prepolymer. The unexposed and unpoly eriεed resin can then be selectively removed by application of, for instance, an acid or alkaline solution or an organic solvent, most commonly an aquecuε alkaline solution containing surfactants and solvents. In the case of positiv plates exposure to light cleaves bonds to render the coating more soluble in alkali. Thus, exposed reεin can be removed by applicaticn of a developer, usually an alkaline developer. Conventionally the εa e proceεsorε have been used for developing both positive and negative preεensitiεed plates. In the development of photo-sensitive plates there is preferably a rinse or wash step following development by the alkaline or other developer. Rinsing is generally carried out by application of water. For some photo-sensitive resins application of water terminates the development process making it i poεεible to remove unexposed resin. It is therefore essential that all of the uncured areas are removed before the plate passeε to the v.ashing step. In order to promote removal, development baths for positive and negative plates have been provided with brushes or other mechanical means for abrading the resin on the plate surface. In the developer bath developer solution is generally applied to the plateε by spraying or other means and is conveniently also applied direct to any brushes to maximise removal of the reεin. In the waεh stage water iε generally applied to the plate surfaceε by spraying or other means without application of abrasive force. The washing is intended tc remove residues of the developer solution rather than of resin. Although there have been processes which do not apply abrasive force by brusheε in the developer εtep, replacing this by the use of high preεεure liquid jetε or the use of organic solvents or omitting agitation altogether, these have always been intended to remove resin in that developer stage. Such processes have not been found to provide entirely satisfactory development. Another type of litho plate compriεeε an electrostatic sensitive coating of an "organic photo conductor" or OPC reεin. Tc produce a printing plate, the coating iε electrostatically charged in discrete areas of the plate. This may be achieved in one of two ways. The coating can be charged over the entire plate surface, and then the charge is selectively cancelled in areas, generally the non-image areas. Generally the charge iε cancelled by application of electromagnetic radiation, uεually light. Alternatively the coating iε pretreated with electromagnetic radiation in discrete areas which prevents electrostatic charge being refined and is then charged. Light or other radiation is directed at the plate by a camera, cr using laser beam from a laser printer. In the next εtep a toner iε applied, which can be in liquid or εolid particulate form and generally compriεes a low-melting reεin and a pigment, for instance a carbon pigment. The toner iε deposited selectively onto the charged areas of the plate. The plate is then heated to evaporate any εolvent and fuεe the toner. Application and curing of the toner is sometimes termed developing since it produces a black image on the plate, although those steps are not strictly speaking analagous tc the development steps of a conventional preεenεitised plate production process, ie in which the resin is photo-sensitive. The plate iε then subjected to a decoating operation, in which the OPC resin iε removed from the toner-free areas of the plate. The decoating operation has generally been carried out using the same apparatus aε has been used for developing photo-sensitive plates. The process has thus consisted of application of developer solution, generally an alkaline solution, to the plate with brushing, optionally followed by a washing stage. Plates produced by development in the conventional apparatus were not entirely satisfactory and produced low quality plates with inadequate resolution. One solution to the problem that had been proposed was the proviεion of extra brushes in the developer bath but this can reduce the quality yet more. According tc the present invention there is provided a process for decoating an electrostatic sensitive reεin coated plate that has toner image-wise coated over the resin, in which the plate is decoated in the areas unprotected by toner by contact with a developer solution and then a washing liquid, and which iε characterised by co priεing a) a firεt, soaking step in which an aqueous developer solution iε applied to the plate tc soften but not remove the coating, and b) a second, washing step in which an aqueous liquid iε applied to the plate to remove the εoftened coating. Preferably substantially no resin is removed from the plate by the developer solution; the developer merely softens the coating and enables it to be removed by the aqueous wash liquids . The procesε according to the invention for producing a litho printing plate product from a plate having a coating of an electro εtatic sensitive resin comprises the following steps: 1. electrostatically charging the coating in diεcrete areaε of the plate, 2. applying a toner, which iε εelectively attracted to the charged areaε, to the plate, 3. curing the toner, 4. subjecting the plate to a decoating operation to remove the electrostatic sensitive coating material from the areaε not protected by toner, and iε characterised in that the decoating operation comprises (a) a first, soaking step in which a developer solution is applied to the plate to soften but not remove the coating, and (b) a second, washing step in which water is applied to the plate to remove the softened coating. The present invention also provides printing plates produced by the new process. The new procesε has been found to produce plates having a far higher quality than the conventional process of brushing in developer solution. The process can be used tc make OPC plates having much improved quality than has yet been possible. The process thus benefitε from the advantages afforded by the OPC process in general as compared to the use of photo-sensitive plates, including savings made by dispensing with the requirement for photographic film and the associated development stages. Vie found that development of OPC plates is not terminated by the application of water in the washing εtep. The new proceεs is highly advantageous since it εignificantly increases the lifetime of the developer bath since it does not become contaminated and neutralised by reεir. that haε been removed from the plate. Resin iε removed into the waεh bath which generally does not comprise any large amounts of active chemicals and so is inexpensive and easier to 'dispoεe of. Furthermore we have found that the plateε produced by the proceεε have improved definition or reduced "sharpening" compared to OPC plates made by the usual proceεεeε. We believe this improvement, iε due to a reduction in removal of toner and OPC reεin thereby left unprotected in the developer bath. The developer iε any conventional OPC or positive developer solution and iε generally an aqueous alkaline solution, uεually compriεing silicate, optionally containing surfactant. Suitable developers comprise for instance alkali metal silicate, uεually sodium silicate, for instance sodiu metaεilicate in concentrations of about 2 to 10% by weight usually 5 to 6% by weight, suitable surfactantε, which aid penetration of the coating, comprise cationic εurfactanctε eg. carboxy-methyl fatty acid amido-ethyl amine . The solution is applied in amounts sufficient to wet the entire εurface, for instance by flooding with solution or by dipping the plate into a developer bath. The developer solution is generally recirculated in the process, by collecting excess solution and recirculating it to the application means. In the washing step, the reεin is generally removed from the plate by abrasion, although sometimes it may be possible to use high pressure liquid jets. In the most convenient embodiment of the procesε the coating iε removed by brushing the plate εurface. The washing iε uεually carried out by water, usually mains water, and iε preferably free of additives. Sometimes it may be necessary to recirculate the wash water, in which case it may be neceεsary to incorporate buffers or other components tc neutralise developer that contaminates the water. Preferably the wash water is not recirculated. In the process it iε preferable that soaking is carried cut in the developer for at least 10 secondε, preferably at least 15 seconds and more preferably at least 20 or 30 secondε. Uεually it iε net necessary for the period to be more than 5 minutes and up to 60 seconds is often sufficient. In general it iε preferred for the period to be kept to a minimum to prevent attack by the developer of resin in the image areas, for instance under the toner in an OPC plate. Thus the rate of progress of a plate through the proceεε iε suitable tc ensure that the time between the first application of developer solution, for instance by spraying, flooding or dipping or other means, and before the application of water in the washing step, is at least those minimum periods mentioned. The procesε may comprise a further rinεing step after the washing step, if desired, for instance in which the plate iε rinsed with pure water. The proceεε may also compriεe a subsequent step in which a gum is applied to the plate, to protect the plate during storage or for ether purposes including protection of the plate during a subsequent baking stage in the process. Such gum solutions are conventional for application to lithσ plates and compriεe, for inεtance, dextrin gums in concentrations of 2 to 15% by weight, preferably 6 to 10% by weight. The plates may also be subjected to a subsequent drying stage, for instance in which they are heated and/or air dried. The plates may sometimes be baked, to increase the poεεible print run, for instance at a temperature of about 220°C. According to the invention there iε also provided apparatus suitable for carrying out the decoating prcceεε, comprising a developer bath, means for applying developer solution from the developer bath to a plate passing through the apparatus, means for applying wash water tc the plate and means for removal of resin from the plate whilst it is in contact with the wash water. Usually the apparatus arranged for passage of the plate substantially horizontally with its coated εurface uppermcε . In the new apparatus the means for removal of the reεin generally comprises means for applying abrasion tc the εurface, preferably roller brushes. Suitable brushes may have very soft bristleε or may comprise a sleeve of a velour material for instance of the type uεed aε wetting rollers o printing presses. For most applications the brushes are suitable for applying abrasion to one side of the plate only although in some circumstances a double sided plate .may be produced in which case brushes can be provided on both sides of the plate. Uεually the brushes are driven for instance in the εa e direction aε the plate moves. The means for application of developer may comprise spray means, flooding means or means for dipping the plate into developer εolution. Preferably the apparatuε compriεeε means for recirculating exceεε developer to the application eanε, for instance compriεing a bath positioned for collecting excess solution and pump eanε for pumping liquid from the bath to the application meanε. The meanε for applying wash water may compriεe εpray meanε for applying water to the plate surface for instance immediately on entry of the plate to the waεhing εection. Preferably there are means also for applying water to the brushes to optimise removal of reεin. Conveniently there are further means for applying water after the abrasion has been applied, to act as a final rinse before the plate iε transported out of the washing section. The apparatus generally compriseε dam meanε for preventing solutions from flowing backwards or forwards in the apparatuε to contaminate other areaε. The apparatuε alεo compriεeε means for collecting wash water and removed resin. The apparatus may be an open bath apparatuε which iε open to the air or the developer bath at leaεt may be closed. This increases the life of an alkaline developer εolution by minimising uptake by the solution of carbon dioxide from the air which would neutralise the alkaline developers. The apparatuε may also compriεe means for applying a solution of preserving or other gum to the plate after the washing step. The apparatus may also comprise means for carrying out other intermediate stepε, for instance intermediate rinsing steps. The means for application of gum solution/or and immediate rinsing means may comprise similar application means as for the developer solution and washing solution, e.g. spray means. The apparatus may also comprise a drying means for a final drying step. This may comprise meanε for directing a flow of warm air at the plate εurface. Although the apparatuε may be manually operated, it is preferably provided with automated transport means, for instance comprising motorised tranεport rollerε aε well aε sensorε. Theεe tranεport means allow the apparatus to be set for optimum operating conditionε, for instance to maintain the plate for the optimum periods in the various stageε of the apparatuε and for application of optimal amounts of developer, water and gum, aε well aε for driving the various rollers and brushes when and how necessary. The apparatuε may be part of an automated plate line that may compriεe also modules for carrying out the other t εtageε of the plate making proceεs, including, for an OPC proceεε, charging meanε, meanε for discharging selected areas usually comprising an optical camera or a laser and meanε for application and curing of toner. For a process for forming plates from a positive photoεenεitive plate the line includes means for exposing image areas tc light. Also the line may comprise means for carrying out subsequent εtageε, εuch aε baking which can further cure the reεin and increase the life of the plate, as well aε means for applying and/or removing various protective coatings, for instance fusing gums used to protect the plate during baking. The accompanying drawing showε a εchematic diagram of a preferred embodiment of the apparatus. A processor 1 comprises a feed table 2 on which can be supported a plate 3 prepared for procesεing with itε coated εide uppermost. The plate is fed via rubber coated transport rollers 4 to a position above a developer bath 5. Whilst it is above that bath developer solution iε applied to wet the upper coated εurface of the plate via a spray bar schematically represented aε 6. Excess solution is collected in the bath and iε recirculated by recirculating pump 8 to the spray bar. Developer εolution is prevented from flowing backwards by rubber dams 9 and to some extent by the rollers 4 acting as squeegees. The plate is then tranεported by transport rollers 10 to a poεition above a washing bath 11. Rollers 10 prevent flow forwards of developer solution into the washing bath 11 by acting as squeegeeε . Whilεt in position above the wash bath water iε applied tc the upper, coated surface of the plate by spray bar schematically illustrated as 12. Excess water is collected in the bath 11 and is prevented from flowing backwards into the developer solution by rubber dams 14. Rubber da s 9 or 14 could be omitted by providing the plate path at a slope to the horizontal so that gravity would prevent flow of liquid either backwards or forwards. Two brushes 15 on the upper side of the plate are rotated whilst in contact with the upper εurface of the plate which bears against pressure bars 16 underneath the plate, thereby removing resin from the parts of the plate which are soluble in developer. The brushes are preferably rotated so that they are moving in the same direction aε, but faster than, the plate. They may be capable of reversing fcr a part of the procesε, for inεtance aε the back end of the plate paεεeε them. To improve removal of resin further water is applied to the plates via the first brush 15 from spray bar 13. Additionally further water iε applied as a rinse from spray bar 17 located further upstream from the brusheε. Excess wash water iε collected in the bath 11 and iε sent to waste. Spray barε 12, 13 and 17 are εupplied with fresh water from the ainε. From the waεh εection the plate iε transported by transport rollerε 18 to the gumming section. Rollers 18 prevent the washing εolution from flowing forward in the procesεor and contaminating the gumming εection. In the gumming εection a gum solution is contained in a bath 19 and iε applied via a εpray bar 20 which is εupplied with gumming solution from the tank 19 by recirculating pump 21. From the gumming section the plate is transported by transport rollerε 22 to a drier assembly generally illustrated as 23 which consists of a heater for air and a fan to blow warm air onto the plate surface. An ejector assembly 24 transports the plate onto a delivery table 25 from where the plate can be removed for storage or for printing. The following example illuεtrateε the invention: An aluminium plate that had been provided with an OPC coating cόmpriεing an oxadiazole derivative of vinyl acetate copolymer and an anthraquinone pigment at a coating weight of about 5g/m 2 was electrostatically charged over its entire εurface. Areaε of the coating were selectively discharged by application of light from an optical camera. The plate was εubεequently contacted with a liquid toner which waε selectively attracted to the partε of the coating remaining electrostatically charged. The plate was then heated to a temperature of 180°C to drive off solvent and to fuεe the toner, and waε then cooled to ambient temperature. The plate was then processed through the apparatuε deεcribed above. In the developer bath waε a developer solution comprising 5 to 6% sodium metaεilicate and 0.2% carboxymethyl fatty acid amidoethyla ine. This waε applied to wet the plate at room temperature. The rate of progress of the plate through the apparatus was such that the time from first application of the developer solution to tranεport out of the developer area by the tranεport rollerε waε 40-45 εecondε. The plate then passed into the washing section, where pure mains water at ambient temperature waε applied to the plate via spray bars and from the brushes. The plates were brushed using 2 rollers with velour sleeves rotating in the same direction as the movement of the plate. The plates were then passed by transport rollerε into the gumming εection where an aqueous solution of about 8% of dextrin gum was applied by spray bar. The plate was then dried by warm air and ejected. This plate waε compared with a comparative plate produced by the same procesε but which was decoated by the conventional method, that is by application of developer whilst bruεhing with the same type of brushes as used in the invention but in the developer bath, followed by rinsing with pure water applied from spray bars only, followed by the same gumming and drying steps. The plate produced by the proceεε of the invention and the comparative plate were εubjected to teεts to compare the quality of printing by meaεuring the εcreen range for each plate. The screen range iε the range of halftone dot εizes that can be reproduced on the plate at a given screen ruling (dots per inch) expressed in percentage aggregate area of the dots per unit area. A conventional processor gave a screen range of 20% tc 80% at 100 lines per inch screen ruling on the plate. Under the same conditions the new processor gave a screen range of 10% to 80%, showing less undercutting of the highlight dots. The capacity of developer in the conventional processor is 7 square metres per litre. The developer is exhausted by the build up of products of the development reaction and coating solids. In the new processor capacity is increased to 12 square metres per lite.

Claims: CLAIMS 1. A process for decoating an electrostatic εenεitive resin coated plate that has toner image-wise coated over the resin, in which the plate is decoated in the areaε unprotected by toner by contact with a developer solution and then a washing liquid, and which is characterised by comprising a) a first, soaking step in which an aqueous developer solution is applied to the plate to soften but not remove the coating, and b) a second, washing step in which water is applied to the plate to remove the softened coating. 2. A process according to the invention for producing a litho printing plate product from a plate having a coating of an electro static senεitive material compriεing the following εtepε: 1. electroεtatically charging the coating in diεcrete areaε of the plate, 2. applying a toner, which iε selectively attracted to the charged areaε, to the plate, 3. curing the toner, 4. εubjecting the plate to a decoating operation to remove the electroεtatic εenεitive coating material from the "' areas not protected by toner, and is characterised in that the decoating operation compriεeε (a) a first, soaking step in which a developer εolution iε applied to the plate to soften but not remove the coating, and (b) a second, washing step in which water is applied to the plate to remove the softened coating. 3. A proceεε according to any preceding claim in which the developer εolution iε an alkaline solution, preferably comprising alkali metal silicate, more preferably sodium metraεilicate. ' 4. A proceεε according to any preceding claim in which the exceεε developer solution that haε been applied to the plate iε recirculated in the proceεε. 5. A process according to any preceding claim in which the softened resin is removed by abrasion, preferably by brushing the plate coating. 6. A process according tc any preceding claim in which substantially no coating iε removed from the plate during the soaking step. 7. A proceεε according to any preceding claim in which the εoaking step iε carried out for a period of at leaεt 20 secondε preferably at leaεt 30 seconds. 8. A proceεs according to any preceding claim in which the developer solution is applied to the plate by spraying. . A plate produced by a proceεs according to any preceding claim. 10. Apparatuε comprising a plate processor and an electrostatic sensitive plate, in which the plate processor comprises a developer bath, means for applying developer solution from the developer bath to the plate passing through the apparatuε, means for applying wash water to the plate and mieans for removal of reεin from the plate whilεt it iε in contact with the waεh water. 11. Apparatuε according to claim 10 in which the meanε for removal of reεin cc priseε abrasion meanε, uεually brushes, provided above the waεh bath. 12. A plate line for producing litho plates from electrostatic sensitive plates comprising means for electroεtatically charging discrete areas of a plate coated with an organic photo-conducting polymer, meanε for applying a toner that iε selectively attached to the charged areas of the plate, means for curing the toner and means fcr decoating the plate characteriεed in that the decoating meanε comprises a processor aε defined in claim 10 or 11.

Inventors: BROWN STEPHEN DENIS, GRANT EUAN MACDONALD, PATEL JAYANTI, BROWN, STEPHEN, DENIS, GRANT, EUAN, MACDONALD, PATEL, JAYANTI
Applicants: POLYCHROME CORP, POLYCHROME CORPORATION
Application year: 1989
Main classification: G03G13
IPC codes: B41N1, G03F7, G03G13