Printing – Planographic – Lithographic plate making – and processes of making or using...
Reexamination Certificate
2000-12-14
2002-11-26
Funk, Stephen R. (Department: 2854)
Printing
Planographic
Lithographic plate making, and processes of making or using...
C101S425000, C101S478000
Reexamination Certificate
active
06484638
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a cleaning method for recycling the lithographic substrate of a printing master.
BACKGROUND OF THE INVENTION
In conventional lithographic printing, ink and an aqueous fountain solution are supplied to the surface of a printing master which contains ink-accepting (oleophilic) and water-accepting (hydrophilic) areas. The inked image pattern is then transferred from the surface of the master to a blanket cylinder having a compressible surface. From the blanket cylinder the image is impressed onto paper. The master is typically a printing plate which carries an image on a dimensionally stable substrate such as an aluminium sheet. The imaged aluminium plate is secured to the plate cylinder of a printing press by a mechanical lock-up mechanism which defines positional registration between the plate and the surface of the cylinder. After the end of the press-run, the mechanical lock-up system is released so that the printing plate carrying the printed image can be removed and discarded and another printing plate can be positioned and locked into place. A new print job can then be started.
Printing masters are generally obtained by the so-called computer-to-film method wherein 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. These steps are usually performed in dedicated exposure and processing equipment and the printing plates are then transported to the printing press and attached to the printing cylinder by press operators using a lock-up mechanism built into the cylinder itself. Although the attachment of the printing cylinder is generally a manual operation, robotic means have been developed for positioning and securing the printing plates.
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 data are transferred directly to a plate precursor by means of a so-called plate-setter. On-press imaging is a direct-to-plate method (also called direct-to-press), wherein the image is exposed on the plate while said plate is mounted on the plate cylinder of a printing press. The major advantage of the latter method compared to off-press plate-making is the improved registration between printing stations of a multi-colour printing press.
Two types of such on-press imaging methods are known. According to a first type, a printing plate precursor is mounted on a printing press, image-wise exposed, optionally developed, and then used as a printing master and finally removed from the press and disposed of, thus requiring a new plate material for each image. An example of this technology is the well-known Heidelberg Model GTO-DI, manufactured by Heidelberg Druckmaschinen AG (Germany) which is described in detail in U.S. Pat. No. 5,339,737. A drawback of this method is the need to use a new plate for each press-run, thus increasing the cost of the printing process.
In a second type of on-press imaging systems, the same lithographic substrate is used in a plurality of press-runs (hereinafter called printing cycles). In each printing cycle, a heat-sensitive or photosensitive layer is coated on the lithographic substrate to make a printing plate precursor and after image-wise exposure and optional development a printing master is obtained. After the press-run, the ink-accepting areas of the printing master are removed from the lithographic substrate in a cleaning step so that the substrate is recycled and can be used in a next cycle of coating, exposing and printing without the need to mount a new plate on the cylinder. Examples of such on-press coating and on-press imaging systems are described in e.g. U.S. Pat. Nos. 5,188,033; 5,713,287; EP-A 786 337 and EP-A 802 457. The latter patent application describes an apparatus comprising a printing member, means for applying a uniform coating, means for scan-wise exposing said uniform coating in accordance with an image pattern and means for developing said uniform coating to leave an image on said printing member, the image consisting of ink-accepting areas on an ink-repellent background or ink-repellent areas on an ink-accepting background. According to a preferred embodiment, the coating comprises hydrophobic thermoplastic polymer particles in a hydrophilic binder.
In the known on-press coating methods, the cleaning of the lithographic substrate often fails because no suitable compromise can be found between the chemical reactivity of the cleaning liquid versus the ink-accepting areas which have to be removed on the one hand and the required inertness of said cleaning liquid versus the fragile lithographic surface on the other hand. A typical lithographic surface is mechanically as well as chemically quite vulnerable. A lithographic surface consists generally of a micro-pore structure in order to differentiate the spreading properties of the ink and the fountain. Anodised aluminium plates comprise a lithographic surface containing one or more metal oxides on which absorption phenomena can take place. These metal oxides are very susceptible to chemical conversion into forms which are no longer lithographically active.
The above mentioned micro-porosity of a lithographic surface is also highly susceptible to mechanical damage. The presence of solid particles in cleaning liquids, which is often required for efficient mechanical cleaning of the lithographic surface, results inevitably in a disturbance of the micro-structure of said surface. Because ink and the coated imaging layer penetrate in the micro-pore structure, it is necessary to carry out a vigorous cleaning so as to avoid phantom images in the subsequent printing cycle, which are due to an insufficient removal of the previous image.
In addition, the known cleaning liquids typically contain solvents which are harmful to hoses, pumps and sealings and/or require a very thorough rinsing with water because these liquids are not compatible with the coating step in the next printing cycle.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a lithographic printing method comprising a cleaning step whereby the ink-accepting areas of a printing master can be removed effectively so that the substrate can be reused in a next print cycle. More particularly, a cleaning step is required which is characterised by a low risk of deteriorating the lithographic surface of the substrate. It is also an object of the present invention to provide a printing method wherein a cleaning liquid is used which does not affect the hardware of the printing press or the cleaning apparatus, in particular a liquid which is inert towards rubber, and which does not require a long rinsing step after the cleaning.
The above objects are obtained by the method of claim 1. The cleaning liquid defined in claim
1
effectively removes the ink-accepting areas of the printing master defined in claim 1. No ghost images are observed after several (>10) print cycles of coating, exposure, printing and cleaning. Rubber hoses and seals are not affected by the cleaning liquid and low amounts of water suffice in the optional rinsing step.
Further objects of the present invention will become clear from the description hereinafter.
Preferred embodiments of the method of the present invention are defined in the dependent claims.
DETAILED DESCRIPTION OF THE INVENTION
The cleaning liquid used in the method of the present invention contains an aliphatic or aromatic compound having a structure comprising at least 6 carbon atoms and preferably at least one double bond. Suitable examples of such compounds are: toluene, xylene, propylbenzene, methylhexane, 3-methyl-6-isopropyl-1,4-cyclohexadiene, 3-(1-methylpropylidene)-cyclohexene, 6-methyl-1-(1-methylethyl)-1,3-cyclohexadiene, 4-methyl-5-(1-methylethenyl)-cyclohexene,
Geerts Peter
Verschueren Eric
Agfa-Gevaert
Funk Stephen R.
Leydig , Voit & Mayer, Ltd.
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