Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2000-10-24
2002-06-04
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S097000, C430S124300, C101S467000, C427S550000, C427S556000, C427S559000, C427S208200, C427S270000, C427S130000, C427S201000
Reexamination Certificate
active
06399280
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a heat-mode method for preparing lithographic printing plates.
More specifically the invention is related to a method for making a lithographic printing plate by a negative working non-ablative process.
BACKGROUND OF THE INVENTION
Rotary printing presses use a so-called master such as a printing plate which is mounted on a cylinder of the printing press. The master carries an image which is defined by the ink accepting areas of the printing surface and a print is obtained by applying ink to said surface and then transferring the ink from the master onto a substrate, which is typically a paper substrate. In conventional lithographic printing, ink as well as an aqueous fountain solution are fed to the printing surface of the master, which is referred to herein as lithographic surface and consists of oleophilic (or hydrophobic, i.e. ink accepting, water repelling) areas as well as hydrophilic (or oleophobic, i.e. water accepting, ink repelling) areas.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, colour separation, screening, trapping, layout and imposition are accomplished digitally and each colour selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
In recent years the so-called computer-to-plate method has gained a lot of interest. This method, also called direct-to-plate method, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate-setter. In the field of such computer-to-plate methods the following improvements are being studied presently:
(i) On-press imaging. A special type of a computer-to-plate process, involves the exposure of a plate precursor while being mounted on a plate cylinder of a printing press by means of an image-setter that is integrated in the press. This method may be called ‘computer-to-press’ and printing presses with an integrated image-setter are sometimes called digital presses. A review of digital presses is given in the Proceedings of the Imaging Science & Technology's 1997 International Conference on Digital Printing Technologies (Non-Impact Printing 13). Computer-to-press methods have been described in e.g. EP-A 770 495, EP-A 770 496, WO 94001280, EP-A 580 394 and EP-A 774 364. The best known imaging methods are based on ablation. A problem associated with ablative plates is the generation of debris which is difficult to remove and may disturb the printing process or may contaminate the exposure optics of the integrated image-setter. Other methods require processing with chemicals which may damage the electronics and other devices of the press.
(ii) On-press coating. Whereas a plate precursor normally consists of a sheet-like support and one or more functional coatings, computer-to-press methods have been described wherein a composition, which is capable to form a lithographic surface upon image-wise exposure_.and optional processing, is provided directly on the surface of a plate cylinder of the press. EP-A-101 266 describes the coating of a hydrophobic layer directly on the hydrophilic surface of a plate cylinder. After removal of the non-printing areas by ablation, a master is obtained. However, ablation should be avoided in computer-to-press methods, as discussed above. U.S. Pat. No. 5,713,287 describes a computer-to-press method wherein a so-called switchable polymer such as tetrahydro-pyranyl methylmethacrylate is applied directly on the surface of a plate cylinder. The switchable polymer is converted from a first water-sensitive property to an opposite water-sensitive property by image-wise exposure. The latter method requires a curing step and the polymers are quite expensive because they are thermally unstable and therefore difficult to synthesise. EP-A-802 457 describes a hybrid method wherein a functional coating is provided on a plate support that is mounted on a cylinder of a printing press. This method also needs processing. A major problem associated with known on-press coating methods is the need for a wet-coating device which needs to be integrated in the press.
(iii) Elimination of chemical processing. The development of functional coatings which require no chemical processing or may be processed with plain water is another major trend in plate making. WO-90002044, WO-91008108 and EP-A-580 394 disclose such plates, which are, however, all ablative plates. In addition, these methods require typically multi-layer materials, which makes them less suitable for on-press coating. A non-ablative plate which can be processed with plain water is described in e.g. EP-A-770 497 and EP-A-773 112. Such plates also allow on-press processing, either by wiping the exposed plate with water while being mounted on the press or by the fountain solution during the first runs of the printing job.
(iv) Thermal imaging. Most of the computer-to-press methods referred to above use so-called thermal materials, i.e. plate precursors or on-press coatable compositions which comprise a compound that converts absorbed light into heat. The heat which is generated on image-wise exposure triggers a (physico-)chemical process, such as ablation, polymerisation, insolubilisation by cross-linking of a polymer, decomposition, or particle coagulation of a thermoplastic polymer latex. This heat-mode process then results in a lithographic surface consisting of ink accepting and ink repelling areas.
EP-A-786 337 discloses a process for imaging a printing plate, wherein the printing plate is charged over the whole surface and wherein the whole surface is covered with fluid toner particles, which are charged oppositely. Thereon is the layer, formed by the particles imagewise fixed or imagewise ablated by infrared exposure on the surface of the printing plate. Thereafter the parts which are not fixed are removed and optionally the non-ablated areas are fixed by heating over the whole surface of the plate. This process requires a cumbersome development.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a method for making lithographic printing plates having excellent printing properties, which is suitable for on-press coating and on-press thermal imaging and which does not require chemical processing.
It is still a further object of the invention to provide a heat sensitive imaging material for making lithographic printing plates which can be used in computer to plate application.
Further objects of the present invention will become clear from the description hereinafter.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method for making a lithographic printing plate comprising the steps of
applying a first magnetic field to a dry, light absorbing powder, which comprises a magnetic material and a hydrophobic thermoplastic binder, thereby coating said powder on a surface of a metal support;
image-wise exposing to light the powder in contact with the surface of the metal support, thereby increasing the adhesion of the powder to the surface of the metal support without substantially ablating the powder; and
removing the non-exposed magnetic powder from the surface of the metal support under action of a second magnetic field with a polarity substantially opposite to the first magnetic field.
DETAILED DESCRIPTION OF THE INVENTION
The metal support is preferably pure aluminum or an aluminum alloy, the aluminum content of which is at least 95%. The thickness of the support usually ranges from about 0.13 to about 0.50 mm.
The preparation of aluminum or aluminum alloy foils for lithographic offset printing comprises the following steps: graining, anodizing, and optionally sealing of the foil.
Graining and anodization of the foil are necessary to obtain a lithographic printin
De Meutter Stefaan
Meisters Augustin
Vermeersch Joan
Agfa-Gevaert
Baxter Janet
Breiner & Breiner L.L.C.
Lee Sin J.
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