PLANOGRAPHIC PRINTING

Abstract: A formulation comprising a heat sensitive cross-linkable ink and carbon black is coated over substantially the entirety of an ink-repellent/ink-releasing surface of a printing cylinder of a printing press by heating the formulation and/or the outer surface of the printing cylinder so that the formulation adheres to the cylinder to provide a continuous coating thereon. The coated printing cylinder is then allowed to cool to ambient temperature whereafter it can be imaged using a digitally controlled laser. The imaged printing cylinder may be developed during press start up by application of waterless ink over the cylinder surface in order to remove the non-imaged areas comprising non-cross-linked ink. After completion of printing, the printing cylinder can be cleaned by using an appropriate solvent to solubilise the cross-linked ink in the imaged areas and subsequently re-used.

Description: PLANOGRAPHIC PRINTING This invention relates to planographic printing and particularly, although not exclusively, relates to waterless lithographic printing. Lithographic processes involve establishing image (printing) and non-image (non-printing) areas on a substrate, substantially on a common plane. When such processes are used in printing industries, non- image areas and image areas are arranged to have different affinities for printing ink. For example, non- image areas may be generally hydrophillic or oleophobic and image areas may be oleophilic. In "wet" lithographic printing, a dampening or fount (water-based) liquid is applied initially to a printing form prior to application of ink so that it adheres to the non-image areas and repels oil based inks therefrom. In "dry" or "waterless" printing, ink is repelled from non-image areas due to their release property. Most lithographic printing at the present time is of the "wet" type and, therefore, requires application of a fount -solution during the printing process. However, lithographic printing forms for such processes may suffer from a number of disadvantages. Some of these are: a) Providing the appropriate ink-water balance during press operation is difficult and requires great experience. If the correct ink-water balance is not achieved, "scumming" is caused wherein the printed ink image extends into the non-image areas thereby ruining the printed image . b) Adjustment of the ink-water balance at start-up or re-start is particularly difficult and cannot be stabilised until a large number of sheets have been printed, thus incurring waste. c) The ink tends to become emulsified which leads to poor adherence of the ink to ink-accepting areas of the printing form which causes problems in colour reproduction and dot reproduction. d) The printing process has to be provided with a dampening system, thus increasing its size and complexity. In addition, dampening solutions contain undesirable volatile organic compounds. e) The printing form care chemistry and fount solutions require careful control and selection. In addition, printing form cleaners contain significant levels of solvent which is not desirable. Whilst waterless printing forms have been known for many years and potentially have advantages over forms of the "wet" type, there are few commercial waterless forms and those that are sold tend to be more expensive than those of the "wet" type. One problem associated with waterless printing is that the printing forms become hot during printing and it is found that this tends to increase the adhesion of ink to non-image areas of the form and lead to scumming, with consequent reduction in the quality of prints typically produced. This problem is combatted on commercial presses by cooling the printing form and/or the ink during printing to maintain the temperatures within a narrow temperature band. Currently, there is interest in imaging and/or developing printing forms directly on a printing cylinder of a printing press itself, thereby removing the need to image and/or develop a plate in a separate apparatus prior to attaching it to a press. For example, our co-pending patent application number GB 96 12233.8 describes a printing form that may be prepared directly on the press, by coating a hydrophillic surface of a support with a radiation sensitive ink; imaging the ink coating by digital laser means; and acting on the form with aqueous dampening rollers to remove unexposed areas of the ink coating to reveal the hydrophillic surface of the support and to leave an ink image formed from the ink which is oleophilic after exposure. It is an object of the present invention to address problems associated with known planographic printing forms and methods for their preparation. According to a first aspect of the present invention, there is provided a method of preparing a waterless printing form precursor using a support having a surface which is relatively non-accepting of an ink-accepting formulation at a first viscosity and relatively accepting of the ink-accepting formulation at a second viscosity, the method comprising contacting the ink-accepting formulation at said second viscosity with said surface in order to coat said surface with said formulation. Viscosity of a fluid is generally related to the temperature of the fluid. Accordingly, the invention extends to a method of preparing a waterless printing form precursor using a support having a surface which is relatively non-accepting to an ink-accepting formulation at a first temperature and relatively accepting to the ink-accepting formulation at a second temperature, the method comprising contacting said surface and said ink- accepting formulation at said second temperature in order to coat said surface with said formulation. In the method, the temperature of said ink-accepting formulation and/or said surface may be increased so that during contact of the ink-accepting formulation and said surface said ink-accepting formulation has said second viscosity and/or said second temperature so that said surface may be coated. Said support may comprise a conventional support used to prepare a waterless lithographic printing plate. However, it preferably comprises a support, for example a cylinder, of a printing press. In this case, the printing form is suitably prepared on press. Said surface may be an integral part of a printing cylinder or may be separable from a printing cylinder. For example, it may be part of a covering means, for example a sleeve, which is suitably releasably securable to a printing cylinder. Thus, the method may include the step of arranging a support in position prior to said contacting step and/or associating a said covering means with a said support prior to said contacting step. Properties of said ink-accepting formulation are suitably changeable on exposure to imaging radiation. The viscosity of said formulation may be changeable on exposure to radiation. For ' example, said formulation may be cross-linkable and/or polymerisable on exposure. Exposure of said ink-accepting formulation to imaging radiation is preferably arranged not to reduce the tendency of the formulation to adhere to said surface. Preferably, exposure is arranged to increase the tendency of the formulation to adhere to said surface. Imaging radiation as described may comprise visible, ultra-violet or infra-red radiation. Radiation may be supplied by any known means, for example, imaging radiation in the form of heat may be supplied using a heated body, for example a heated stylus. Alternatively, said radiation may be supplied using a laser for example which suitably emits in the near-IR region between 700 and 1500nm. Said ink-accepting formulation preferably comprises a first component which is suitably ink-accepting and is preferably changeable on exposure to radiation as described. Said first component preferably comprises a material which is cross-linkable and/or polymerisable on exposure to radiation. Said first component is preferably heat-sensitive. It preferably comprises an ink, especially waterless ink. Said printing form precursor preferably includes a component arranged to absorb radiation and suitably to cooperate with said first component for changing its properties. For example, said ink-accepting formulation may include a second component which is suitably arranged to absorb radiation, especially infra-red radiation. Radiation from an Ar-ion source may be absorbed by some materials and converted to heat. Alternatively, a second component as described herein may be incorporated in a material in contact with said ink-accepting formulation. Said second component may comprise a black body absorber such as carbon black or graphite. It may be a commercially available pigment such as Heliogen Green as supplied by BASF, Nigrosine base NGl as supplied by NH Laboratories Inc. or Milori Blue as supplied by Aldrich. Said second component may be an organic pigment or dye such as phthalocyanine pigment; or it may be a dye or pigment of the squarylium, merocyanine, cyanine, indolizine, pyrylium or metal dithioline classes. Examples of such compounds are:- and Dye B and Dye C, KF654 B PINA as supplied by Riedel de Haen UK, Middlesex, England, believed to have the structure: Cf Said formulation may include at least lwt%, preferably at least 2wt% of said second component. Preferably said formulation includes less than 25wt%, more preferably less than 15wt% of said second component. Said formulation may include one or more second components of the type described wherein the wt% referred to refer to the total of all such second components. Said ink-accepting formulation preferably comprises said first component as a major portion thereof. Said relatively non-ink accepting surface of said support may comprise any surface conventionally used as part of a printing surface in waterless lithographic printing. Said surface suitably comprises a release material which may include a siloxane component. Said second temperature is preferably greater than said first temperature, suitably by at least 5°C, preferably at least 10°C, more preferably at least 15°C, especially at least 20°C. Said first temperature may be in the range 20°C to 40°C, preferably in the range 25°C to 35°C. The thickness of the ink-accepting formulation coated on said surface may be adjusted in dependence upon the desired run length of a printing form preparable from said precursor. Thus, the method may include the step of predetermining the desired run length of the form and adjusting the thickness of the coating accordingly. For example, for a lower run length a coating thickness of 0.1 to 0.5 μm may be suitable; for a higher run length a thickness of 3 μm may be suitable. One advantage of using thinner coatings for short run lengths is that such coatings require a shorter imaging time than thicker coatings . In another aspect of the present invention, there is provided a waterless printing form precursor comprising a support having a relatively non-ink-accepting surface coated with an ink-accepting formulation. Said surface is suitably relatively non-ink accepting when dry - that is, suitably in the absence of water. In another aspect of the present invention, there is provided a method of preparing a waterless printing form having image and non-image areas, the method comprising: imagewise exposing a printing form precursor which comprises a support having a surface which is relatively non-accepting of an ink-accepting formulation at a first viscosity and relatively accepting of the ink-accepting formulation at a second viscosity and an ink-accepting formulation coated over said surface; and selectively removing areas under conditions wherein the viscosity of the ink-accepting formulation is different to said second viscosity. In another aspect of the present invention, there is provided a method of preparing a waterless printing form having image and non-image areas, the method comprising: imagewise exposing a printing form precursor which comprises a support having a surface which is relatively non-accepting of an ink-accepting formulation at a first viscosity and relatively accepting of the ink-accepting formulation at a second viscosity and an ink-accepting formulation coated over said surface; and selectively removing areas, for example unexposed areas, at a temperature, wherein the ink-accepting formulation has a viscosity which is higher than said second viscosity. The invention also provides a method of preparing a waterless printing form having image and non- image areas, the method comprising: imagewise exposing a printing form precursor which comprises a support having a surface which is relatively non-ink accepting to ink at a first temperature and relatively ink-accepting to ink at a second temperature and an ink-accepting formulation coated over said surface; and removing unexposed areas at a third temperature which is less than said second temperature. An or said ink-accepting formulation may be used in the removal of said unexposed areas. For example, said unexposed areas may be contacted with ink-accepting formulation. Said ink-accepting formulation of said coating preferably comprises a waterless lithographic ink which may be substantially the same as the ink-accepting formulation used in said removal. As an alternative, a separate developer wash may be used in the removal of said unexposed areas . Said first temperature and said third temperature may be substantially the same. Imagewise exposure of said precursor may comprise heat-mode imaging. This may be achieved using a heated body for example a heat stylus. Preferably, however, it is carried out using a laser which is suitably computer controlled. Preferably, the laser emits at above 600 nm. The invention extends to a method of preparing a printing form as described above for re-use, the method comprising removing image-areas of said form, for example by dissolution and/or by adjustment of temperature. According to another aspect of the present invention, there is provided a printing press comprising: a support having a surface which is relatively non- ink-accepting; contact means for contacting said surface with an ink-accepting formulation in order to coat said surface; and means for adjusting the viscosity of said ink- accepting formulation. Said means for adjusting viscosity may include a temperature adjustment means for adjusting the temperature of said ink-accepting formulation at the time of contact between it and the surface. Said temperature adjustment means is preferably for adjusting the temperature of contact between a first temperature and a second temperature. Means may be provided for maintaining the temperature of the surface within a predetermined temperature range. Said press preferably includes means for adjusting the amount of formulation applied to said surface to adjust the thickness of the coating. A reservoir means is preferably provided for delivery of ink-accepting formulation to said contact means. Said press preferably includes imaging means for imaging the coating on said surface. Said imaging means preferably comprises a digital imaging means, for example a digital imaging head which is suitably an image setter comprising a laser which can scan in an imagewise manner across the surface in response to image signals stored in computer. Said press preferably includes means for delivering printing ink so that it adheres to ink-accepting areas for use in printing a copy. Said press preferably includes means for delivering a development means for removing unexposed areas of an imaged layer. In one embodiment, the development means may comprise said ink-accepting formulation; in another embodiment it may comprise printing ink used in printing a copy. In a further alternative, said developer means may be a separate developer wash. Said reservoir means and said means for delivering printing ink may be one and the same if said ink-accepting formulation and the printing ink are the same. In addition, where said development means comprises said ink- accepting formulation, said means for delivering the development means and said reservoir means may be one and the same. Said press may include renewing means for renewing said surface of said support. Such means may comprise means for removing material from said support and/or may comprise a means for providing a covering means, for example a sleeve. For example a storage means for material, for example a roll of material which is arranged to provide the surface on the support may be associated with the press. When the surface is to be renewed, material may be dispensed from said storage means; and, suitably, removed material may be directed to another or the same storage means. Thus, with such a system, all operations can be carried out in situ on the press, with the exception of the need for occasional renewal of the storage means with new material. The invention extends to a method of waterless printing using a printing form as described herein. Preferably, waterless ink is used in said printing which is preferably substantially the same as said ink- accepting formulation described herein and/or is substantially the same as the waterless ink used for removing unexposed areas . Any feature of any aspect of any invention or embodiment described herein may be combined with any feature of any other aspect of any invention or embodiment described herein. Specific embodiments of the invention will now be described, by way of example. The following products are referred to hereinafter: PS444 - a vinyl -terminated polydimethylsiloxane obtained from Petrarch Systems Inc of Bristol U.S.A. PS123 - a (30-35%) methylhydro (65-70%) dimethylsiloxane copolymer from Fluorochem Limited of Derbyshire, England. PC072 - a platinum/vinylsiloxane catalytic complex solution in xylene obtained from Petrarch Systems Inc. of Bristol U.S.A. UV Cyan ink PB377 - Sahara (Trade Mark) low viscosity UV curing waterless cyan ink from Classic Colours Ltd of Berkshire, England. KF654B - a polymethine dye obtained from Riedel-de Haen AG of Germany. Example 1 - General Description Commercially available heat-sensitive cross-linkable inks for use in waterless lithographic printing are found to adhere more strongly to an ink-repellant or ink- releasing surface of a printing form as the temperature of the form and/or the ink increases. This factor is detrimental in relation to conventional waterless lithographic printing forms (since it leads to scumming) and, accordingly, steps are normally taken to prevent rises in the temperature of the form and/or ink that may cause the effect. In the present specific embodiment, however, the increased adhesion of ink as temperature is increased is used to advantage. In the embodiment, a formulation comprising a heat-sensitive cross-linkable ink and carbon black is coated over substantially the entirety of a generally ink-repellant/ink-releasing surface of a printing cylinder of a printing press by heating the formulation and/or the outer surface of the printing cylinder so that the formulation adheres to the cylinder to provide a continuous coating thereon. By using heat as described, the ink repellency/ink-releasing property of the printing cylinder surface can be overcome sufficiently to allow adhesion in the manner described. The coated printing cylinder is then allowed to cool to ambient temperature whereafter it can be imaged using a digitally controlled laser. In imaged areas, laser energy is captured by the carbon black and heat is generated which causes cross-linking of the ink to provide an imaged area made up of the cross-linked ink adhered to the cylinder surface. (There is an increased adhesive force between the cross-linked ink and the cylinder surface compared to that between non-cross-linked ink and the surface) . The imaged printing cylinder may be developed during press start-up by application of waterless ink over the cylinder surface in order to remove the non- imaged areas comprising non-cross-linked ink. As a result, only imaged areas remain and these are ink-accepting since a major component of them is made up of cross-linked ink. Non-image areas are defined by the ink-repellant/ink- releasing surface of the printing cylinder. Thus, the image on the printing cylinder can be printed. During printing, the printing cylinder is maintained within a temperature band which is sufficiently low to prevent scumming of non-image areas. Thus, it is suitably cooled. After completion of printing, the printing cylinder can be cleaned by using an appropriate solvent to solubilise the cross-linked ink in the imaged areas, and subsequently re-used. Example 2 - Preparation of substrate having release layer A formulation was prepared by barrel rolling PS444 (1.25g) , PS123 ( 0.25g) , 0.2g of 0.15% catalyst PC072 in xylene and methyl ethyl ketone (9.5g) for 1 hour. The formulation was then coated onto a standard electrograined and anodized aluminum substrate using a Meyer bar, followed by drying at 130°C for 3 minutes to give a dry film weight of 3grtϊ 2 . Example 3 - Assessment of effect of temperature on release properties of substrate A water bath was adapted to pump water through a steel heat exchanger which was attached to a piece of substrate prepared in Example 2. A thermocouple was placed in close contact with the substrate. 0.6g of UV cyan ink PB377 was evenly rolled onto a piece of glass approximately 25x20cm in size using a 15cm long 2.5cm diameter rubber roller. The temperature of the piece of substrate was varied by adjusting the temperature of the water bath. PB377 ink was applied to the release layer of the substrate by rolling at three different speeds: lOcrns "1 (slow); 30cms " ' (medium) and SOcms "1 (fast) . After each application, the substrate was observed to assess whether it accepted ink (indicated by a "/" in Table 1 below) ; whether it did not accept ink (indicated by a "x"); and whether slight ink acceptance was observed (indicated by "?") . TABLE 1 *The ink was damaged about 45°C In general, the assessment shows that ink can be applied to the release substrate by slow rolling at a temperature of about 25°C or above. Example 4 - Imaging In an initial test it was found that UV cyan ink PB377 did not absorb strongly at 830nm and, accordingly, it was necessary to sensitize the ink by incorporation of a suitable dye. In this regard KF654B (5wt%) was worked into the ink using a palette knife and shown to have been incorporated by UV-Vis analysis. The dye containing ink was then dabbed onto a white plastics plate to give a film weight of 5gm "2 and, thereafter, a rectangle was imaged using a laser at 830nm, lJcm '2 . The plate was washed in a UV blanket wash and wiped with cotton wool. The non-imaged area was easily removed leaving an imaged rectangle. The scratch resistance of the image was very good, although the film weight of the image was low. In order to increase the film weight of imaged areas a further free radical initiator (dicumyl peroxide) which is sensitive at 830nm was incorporated into the ink. The depth of the imaged area obtained in this case was found to be increased. Thus, it has been shown that waterless ink can be applied at an elevated temperature to a support having a release layer and removed from the support by reducing the temperature; and that the ink can be imaged using a 830nm laser. The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings) , and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment (s) . The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings) , or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims: CLAIMS 1. A method of preparing a waterless printing form precursor using a support having a surface which is relatively non-accepting of an ink-accepting formulation at a first viscosity and relatively accepting of the ink- accepting formulation at a second viscosity, the method comprising contacting the ink-accepting formulation at said second viscosity with said surface in order to coat said surface with said formulation. 2. A method of preparing a waterless printing form precursor using a support having a surface which is relatively non-accepting to an ink-accepting formulation at a first temperature and relatively accepting to the ink- accepting formulation at a second temperature, the method comprising contacting said surface and said ink-accepting formulation at said second temperature in order to coat said surface with said formulation. 3. A method according to claim 1 or claim 2, wherein, in the method, the temperature of said ink-accepting formulation and/or said surface is increased so that during contact of the ink-accepting formulation and said surface said ink-accepting formulation has said second viscosity and/or said second temperature so that said surface can be coated. 4. A method according to any preceding claim, wherein said support is a cylinder of a printing press. 5. A method according to any preceding claim, wherein said surface is an integral part of a printing cylinder or is separable from a printing cylinder of a printing press. 6. A method according to any preceding claim, wherein said surface is part of a covering means which is releasably securable to a printing cylinder. 7. A method according to any preceding claim, wherein the viscosity of the ink-accepting formulation is changeable on exposure to radiation. 8. A method according to claim 7, wherein said ink- accepting formulation is cross-linkable and/or polymerisable on exposure . 9. A method according to any preceding claim, wherein said ink-accepting formulation comprises a first component which is ink-accepting and is changeable on exposure to radiation. 10. A method according to claim 9, wherein said first component comprises a material which is cross-linkable and/or polymerisable on exposure to radiation. 11. A method according to claim 9 or claim 10, wherein said first component is heat sensitive. 12. A method according to any of claims 9 to 11, wherein said first component comprises an ink. 13. A method according to any of claims 9 to 12, wherein said printing form precursor includes a component arranged to absorb radiation and to co-operate with said first component for changing its properties. 14. A method according to any preceding claim, wherein said ink-accepting formulation includes a second component which is arranged to absorb radiation. 15. A method according to claim 13 or claim 14, wherein said component arranged to absorb radiation comprises a black body absorber. 16. A method according to any preceding claim, wherein said surface comprises a release material to render it relatively non-ink-accepting of an ink-accepting material at said first viscosity. 17. A method according to any preceding claim when dependent upon claim 2, wherein said second temperature is greater than said first temperature by at least 5°C . 18. A waterless printing form precursor comprising a support having a relatively non-ink-accepting surface coated with an ink-accepting formulation. 19. A method of preparing a waterless printing form having image and non-image areas, the method comprising: imagewise exposing a printing form precursor which comprises a support having a surface which is relatively non-accepting of an ink-accepting formulation at a first viscosity and relatively accepting of the ink-accepting formulation at a second viscosity and an ink-accepting formulation coated over said surface; and selectively removing areas under conditions wherein the viscosity of the ink-accepting formulation is different to said second viscosity. 20. A method of preparing a waterless printing form having image and non-image areas, the method comprising: imagewise exposing a printing form precursor which comprises a support having a surface which is relatively non-accepting of an ink-accepting formulation at a first viscosity and relatively accepting of the ink-accepting formulation at a second viscosity and an ink-accepting formulation coated over said surface; and selectively removing areas at a temperature wherein the ink-accepting formulation has a viscosity which is higher than said second viscosity. 21. A method of preparing a waterless printing form having image and non-image areas, the method comprising: imagewise exposing a printing form precursor which comprises a support having a surface which is relatively non-ink accepting to ink at a first temperature and relatively ink-accepting to ink at a second temperature and an ink-accepting formulation coated over said surface; and removing unexposed areas at a third temperature which is less than said second temperature. 22. A method according to any of claims 19 to 21, wherein an or said ink accepting formulation is used in the removal of said areas. 23. A method according to any of claims 19 to 22, wherein imagewise exposure of said precursor comprises heat -mode imaging. 24. A method of preparing a printing form according to any of claims 19 to 23, the method comprising removing areas of said form by dissolution. 25. A printing press comprising: a support having a surface which is relatively non- ink- accepting; contact means for contacting said surface with an ink- accepting formulation in order to coat said surface; and means for adjusting the viscosity of said ink-accepting formulation. 26. A press according to claim 25, wherein said means for adjusting viscosity includes a temperature adjustment means for adjusting the temperature of said ink-accepting formulation at the time of contact between it and the surface . 27. A press according to claim 25 or claim 26, said press including imaging means for imaging the coating on said surface. 28. A method of waterless printing using a printing form prepared as described in any of claims 21 to 27.

Inventors: MCCULLOUGH CHRISTOPHER DAVID, RAY KEVIN BARRY, MCCULLOUGH, CHRISTOPHER, DAVID, RAY, KEVIN, BARRY
Applicants: HORSELL GRAPHIC IND LTD, MCCULLOUGH CHRISTOPHER DAVID, RAY KEVIN BARRY, HORSELL GRAPHIC INDUSTRIES LIMITED, MCCULLOUGH, CHRISTOPHER, DAVID, RAY, KEVIN, BARRY
Application year: 1998
Main classification: B41C1
IPC codes: B41C1, G03F7