Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
1999-06-29
2001-10-16
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S944000, C430S945000, C430S306000, C430S325000, C101S466000, C101S465000, C101S467000
Reexamination Certificate
active
06303271
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a digital printing method and especially to a method for preparing an imaged lithographic plate on-or-off press using a digitally controlled laser output.
BACKGROUND OF THE INVENTION
Currently the commonest method of preparing a lithographic plate is to image a photosensitive lithographic plate using an image mask, such as a photographic negative, and to prepare the plate therefrom using an aqueous developing solution. This procedure is time consuming and requires facilities and equipment to support the necessary chemistry.
Thus recently, various methods have been proposed for preparing lithographic plates on the press which is to be used to produce prints from the plate. These methods prepare the image using a digitally controlled laser image head. As described in E.P.A. 38039, such methods include inkjet methods digitally controlled, spark-discharge methods and the production of electromagnetic-radiation pulses that create chemical changes of the place blank. Also etching methods have been described as well as blank plates which are ablated by the laser to form an ink-receptive image.
SUMMARY OF THE INVENTION
We have discovered a novel method of preparing a printing form using a digitally controlled laser output from an imaging head which may be employed on-or-off a press.
According to the present invention there is provided a method of preparing a printing form which comprises coating on a lithographic support having a hydrophilic surface a layer of a radiation sensitive ink, imaging the ink coating by digital laser means, then acting on the plate with aqueous dampening rollers to remove the unexposed areas of the ink coating to reveal the hydrophilic surface of the support and to leave an ink image formed from the ink, which is oleophilic after exposure.
DETAILED DESCRIPTION OF THE INVENTION
The support is a material suitable for use on lithographic presses and may be metal, plastic or paper. Typical metals are aluminum, chromium or steel. Typical plastics are polyethylene terephthalate or polycarbonate.
The surface of the lithographic support is suitably treated to render it hydrophilic and adherent for the ink. Thus it may be anodized aluminum, chromium or it may be of a plastic material which is either hydrophilic or which has been treated to render it hydrophilic, for example polyethylene terephthalate coated with hydrophilic layers as described in our PCT Application GB96 02883 and WO94/18005 (Agfa).
Most preferably the support is metal and this is in the form of a sleeve or cylinder which firs on to a printing press. Most preferably the method of the present invention is carried out in situ in a printing press. Thus the printing press comprises an ink train which when the metal sleeve is mounted on the press can be lowered to coat on the sleeve an ink coating of a required thickness, together with a digital laser imaging head, means to disengage the metal sleeve from the printing press and to rotate it at a speed suitable for imaging, and water dampening rollers.
A preferred method for the use of flexible lithographic supports is to have a roll of the hydrophilic support within the press which when new material is required dispenses the new substrate and recoils the used substrate automatically. Such a system is utilized commercially in the Heidelberg Quickrmaster DI press and on-press imaging system. In such a system all operations are carried out in-situ on the press with the exception of occasional renewal of the roll of hydrophilic support material.
Means are present in the ink-train to coat any required thickness of ink on the metal sleeve. For example for a lower run length an ink thickness of 0.1 to 0.5 microns is suitable. But for a higher run length a thickness of 3 microns is suitable.
The digital laser imagine head is in essence an image setter attached to the printing press and comprises a laser which scans in an imagewise manner radiation across the plate in response to image signals stored in a computer.
The laser may emit in the U.V waveband as white light or preferably in the infra-red region of the spectrum.
Preferably the radiation sensitive ink comprises a radiation absorbing material which allows the ink to be sensitive to the wavelength of the radiation emitted by the image scanning means.
Conveniently the scanning means is a laser beam having a wavelength of above 600 nm. Usefully the radiation sensitive ink comprises an infra-red absorbing compound. Suitable infra-red absorbing compounds include pigments such as phthalocyanine pigments or dyes of the following classes. squarylium, cyanine, merocyanine, indolizine, pyryhinium or metal dithiolene dye.
Preferably the infra-red absorbing compound is one whose absorption spectrum is significant at the wavelength output of the laser which is to be used in the method of the present invention. For example gallium arsenide diode lasers emit at 830 nm and Nd YAG lasers emit at 1064 nm.
Carbon black is also a useful radiation absorbing compound and in the context of this invention it can also be used as the colorant for the black radiation sensitive ink.
Preferably the radiation sensitive ink comprises a radiation sensitive resin which hardens or cross-links when irradiated. Suitable radiation sensitive resins are certain acryiate resins, for example polyether acrylare, epoxy acrylate, and alkyl acrylate. Suitable solvents for example styrene or methyl acrylate may also be present as well as a photopolynerization initiator such as benzophenone or p-dialkyl-aminobenzoic acid.
Preferably the dampening rollers are covered with a lithographic fountain solution.
Thus in the preferred method of the present invention a metal sleeve or cylinder which has a hydrophilic surface and which forms part of the printing surface of a printing press is coated with a predetermined thickness of a radiation sensitive ink the metal sleeve is disengaged from the roller drive of the printing press and is caused to rotate at a speed suitable for imaging, the digital laser-head attached to the printing press images the ink layer on the metal sleeve, after imaging the metal sleeve is re-engaged to the roller drive of the printing press and the rollers of the press rotate and act as water-dampening rollers, thus removing the unexposed areas of the ink on the surface of the sleeve and to reveal the hydrophilic surface of the sleeve in the unexposed areas of the sleeve, the rollers of the press are then inked up and the printing press prints on to paper fed to it. After the print run has finished a plate washer can be employed to remove all the ink from the sleeve which can then be re-used.
Preferably the metal sleeve can be removed from the press to clean it thoroughly and also to renew it periodically.
Preferably details of the required film thickness to be coated on the sleeve are fed directly into the laser imaging head which is programmed to adjust incident power and scanning speed to provide the optimum cure and imaging resolution.
Conveniently the same radiation sensitive ink is used to form the initial coating on the metal sleeve and in the actual print run. Thus ensures that the ink used in the print run will have a high affinity for the image areas.
Some advantages of the proposed method of the present invention are that only the film thickness necessary to do the job need be employed which in turn means recording time is minimized. This means for this system that make ready time As directly proportional to run length which is exactly what is required for a Direct-to-Press system i.e. make ready time reduces as run length reduces in cases where imaging power is constant. The digital inking controls can be arranged to communicate with the digital head allowing feedback loops to ensure maximum added value in terms of make-ready. The idea of a removable sleeve is beneficial in case the surface becomes scratched and a spare can be used. It may also be possible to have them conditioned on a maintenance basis for optimum hydrophilicity.
EXAMPLES
Testing Sensitivity of Coatings
The
Bayes Stuart
Bennett Peter A. R.
Riley David S.
Smith Carole-Anne
Baxter Janet
Kodak Polychrome Graphics LLC
Lee Sin J.
Ratner & Prestia
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