Printing – Planographic – Lithographic plate making – and processes of making or using...
Reexamination Certificate
2001-11-02
2004-03-09
Funk, Stephen R. (Department: 2854)
Printing
Planographic
Lithographic plate making, and processes of making or using...
C101S478000, C510S171000
Reexamination Certificate
active
06701842
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the treatment of an erasable lithographic printing plate. In particular, the present invention relates to a process for the treatment of an erasable lithographic printing plate after imaging or fixing with an agent which provides the imaged printing plate with favourable properties.
2. Description of the Related Art
Digital direct imaging of printing plates has rapidly developed in the last decade into an essentially independent sub-area of printing processes. This technique combines the advantages of digital technology with traditional printing technology. This combination makes it possible to image the printing plate directly from digital integrated word/image processing and to run off small- to medium-run orders in the shortest possible time. A crucial breakthrough was achieved in this connection with an erasable printing plate which remains in the printing machine and can be erased, prepared and re-imaged digitally in the shortest possible time without manual intervention. In order that these operations can proceed without manual intervention in the printing machine, for example installation and removal of the printing plate, substantial automation or control of the individual steps, such as erasure of the printing plates, imaging, fixing, preparation and conditioning, is necessary. This in turn, in contrast to conventional printing technology, requires a different and specific choice of the materials used, for example those of the printing plate, the erasing composition, the imaging material and other necessary auxiliaries.
In the printing process of the generic type, in which a printing-plate cylinder is provided with plastic in a punctiform and imagewise manner, this printing-plate cylinder is then coated with printing ink for an offset process, and the printing ink in the ink-carrying areas is taken up by a rubber roll and transferred to the material to be printed. The process of printing-ink acceptance by the printing-plate cylinder is based on an awkward interaction between hydrophilic areas of the printing-plate cylinder which repel the printing ink (in the present case the non-imaged areas—in the case of a metal printing plate the metal surface) and the ink-carrying areas, in the present case the imaging with polymeric material. In order that this distinction-based mechanism also proceeds sharply and clearly in the edge zones of the image, i.e. in the transition between the metal surface and the imaging layer, clean phase separation between oleophilic printing ink and damping solution must occur in this area. It has been found that, in the process of the generic type, residue particles from the image are present, in particular in this area, these residue particles presumably being ash constituents, separated-out constituents or sprayed constituents of the donor layer of the thermal transfer ribbon used for the imaging. In a conventional process, the following procedure has hitherto been followed. The printing plate was treated with a cleaning agent consisting of two different components. The first component firstly substantially dissolved the oleophilic part, i.e. the printing ink, of the used printing plate. The second component then dissolved the image from the printing plate. Since the image is a substance which is soluble in water at a certain pH, the two components are inevitably incompatible with one another to a certain extent. This means that on use of the second component, slight traces of the oleophilic residues which have not been removed in the first operation cannot be removed, and thus the first component of the erasing composition is used again in order that the printing-plate surface can be fully cleaned of the image, including the printing ink located on the image. This interplay has to be carried out a number of times in stubborn cases. After cleaning, the printing-plate surface is sufficiently hydrophilized for imaging to take place. This is followed by fixing, i.e. warming of the image in order to gain chemical and physical influence on the primary substance making up the image, for example surface treatment of the pixel surface, stronger adhesion to the printing plate, levelling of the pixels, etc. In conventional printing plates, which are handled manually, a rubber layer is generally applied after fixing in order to preserve the printing-plate surface prepared for printing and to protect it, for example against fingerprints. In printing processes which use an erasable lithographic printing plate, this is not really necessary since in the case of print on demand, the next print order is generally carried out and executed immediately after the preceding one. In the case of the process with an erasable lithographic printing plate, conditioning is generally carried out immediately before printing. This conditioning step firstly has the aim of hydrophilization of the non-ink-carrying areas, i.e. restoring the surface quality of these parts, which may have been impaired by the processes of imaging and/or fixing. The second aim is to remove the abovementioned residues located in the edge regions of the pixels and formed during imaging. For this purpose, the acidic component of the abovementioned two-part cleaning medium is used. The acidic component contains phosphoric acid, which, for example, adequately hydrophilizes the metal surface of the printing plate, and it contains a certain very fine abrasive, which is intended to remove the residues in the edge regions. It has now been found that the use of this agent not only removes the residues in the edge regions, but the abrasive action also acts on the polymer material and can thus affect the habit of the pixels which later carry ink.
SUMMARY OF THE INVENTION
The object of the present process is to simplify the sequence of the known process for printing using an erasable lithographic printing plate, in particular during cleaning of the printing plate, namely to achieve the simplest and gentlest possible removal or encapsulation of the residues in the edge regions of the image pixels which does not significantly affect the shape of the pixels, including their surface nature, and adequately hydrophilizes the metal surface. In particular, it is an object of the present invention to provide a simpler cleaning programmed without alternating ink and imaging/erasure sequences. Furthermore, re-deposition of, for example, ink residues which are only soluble in one of the two components in a conventional erasing composition should be avoided. Furthermore, the disadvantage that the current conditioning step has to take place immediately before printing should be overcome in that the printing plate can be employed at any desired point in time, i.e. even after a print stop.
It has now been found that the abovementioned object can be achieved by the use of a heat-curable and water-soluble substance or of a water-soluble substance which has been applied to the printing plate after imaging or after fixing and is washed off with a solution essentially consisting of water immediately before printing.
A further aspect consists in that a one-component cleaning solution is used instead of the two-component cleaning solution. In the case of the two-component cleaning solution, a metal surface which has been essentially hydrophilized on the printing plate by means of phosphate residues is provided before the imaging through the use of the acidic component as the final component. On use of a one-component cleaning solution, an essentially alkaline cleaning solution is used which leaves behind a metal surface provided with oxide or hydroxide groups. This surface appears to have the advantage that strong re-hydrophilization, as in conventional processes due to the re-use of an acidic component containing abrasive elements, is unnecessary.
Surprisingly, it has been found that printing plates having the same print quality as in complex known processes can be achieved through the use of the heat-curable and water-soluble subst
Hartmann Thomas
Schneider Josef
Cohen & Pontani, Lieberman & Pavane
Funk Stephen R.
MAN Roland Druckmaschinen AG
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