Patent Application
20090095183
The present invention relates to lithographic printing, and particularly to the manufacture of imageable plates that are stacked with interleaving to protect the imageable coating.
Modern lithographic printing plates, especially for the newspaper and commercial printing industries, are typically manufactured by coating a substantially continuous sheet of metal substrate material with a liquid resin, curing the resin, cutting the coated and cured sheet into rectangles having the desired end product dimensions, optionally punching to create alignment holes, and then stacking and bundling the plates for shipment to the imaging and printing facility. To protect the coating and facilitate removal of individual plates from the stack, an interleaf is provided between successive plates on the stack. Commonly, such interleaving consists of loose polymer coated papers having substantially the same rectangular dimension as the plates.
The plates must be removed from the stack and individually imaged, such as by an infrared, violet, or ultraviolet laser, depending on the radiation sensitivity characteristics of the coating. Each imaged plate is then developed in a bath of alkaline developer solution or solvent, depending on the coating. Regardless of the radiation sensitivity or type of developer bath, the interleaf must be removed before the plate is imaged or subjected to the developer.
The present inventor has recognized that such conventional manufacture and use of the plates includes at least two wasteful steps. The first is the insertion of interleaves between the plates during stacking, and the second is the removal of the interleaves before imaging or developing.
It is an object of the invention to eliminate the loose interleaf in the manufacture and use of lithographic printing plates.
This object is achieved by providing an interleaf that is permanently integrated to the bottom surface of the plate substrate.
In general, a thin, flexible, water and solvent insoluble film is adhered to and covers the bottom surface of the elongated sheet before the sheet is cut into plates. The interleaf has a lesser thickness than the thickness of the substrate.
A preferred method comprises the steps of selecting a wound coil of aluminum sheet, unwinding the coil and advancing the sheet through a coating station at which a liquid coating of radiation imageable material is applied to the top surface of the sheet, curing the material to form a cured coating adhered to the top surface of the sheet, laminating a polymeric film to the bottom surface of the sheet, and advancing the laminated sheet to a cutting device where individual plates are cut from the sheet.
Preferably, after lamination, the sheet is rewound into a coil, whereby in the windings of the recoil, the polymeric film is interposed between the coating on the top surface of a given winding and the bottom surface of the next winding, and the coil is unwound and the unwound sheet advanced to a cutting device and/or slitter.
The film is preferably a polyolefin material laminated to the substrate by heat.
The plate embodiment preferably comprises a metal substrate having top and bottom surfaces, a radiation imageable coating covering and adhered to the top surface of the substrate, and a polymeric film adhered to the bottom surface of the substrate, wherein the interleaf has a lesser thickness than the thickness of the substrate.
The polymeric film is preferably a polyolefin laminated to the bottom surface of the substrate, at a thickness in the range of about 0.5-2.0 mils.
The interleaving technique disclosed herein, provides several significant advantages.
The interleaf film can be applied automatically and continuously without human intervention to the sheet of material during manufacture, without limiting the rate of throughput.
When the interleaf film is applied before cutting of the sheet, the film provides lubrication for the knife blades. This is especially important when plates are stacked for trimming to a non-standard size, via guillotine blades. This self-lubrication avoids the need for either a lubrication additive or the slowing down of the cutting rate to assure that excessive friction heat does not build up and fuse the plates against the knife edge.
In manufacturing lines where the coated sheet is recoiled and relocated to a cutting station, the integrated interleaf applied after curing of the coating protects the coating on the sheet during such recoiling and subsequent uncoiling at the cutting station.
The integrated interleaf permits the rapid stacking of cut plates without the intervening step of inserting a loose interleaf, and without the risk that an interleaf may have been inadvertently omitted or inserted incorrectly so as to render a portion of the underlying plate unprotected. The protection provided by the integrated interleaf remains active during stacking, handling and shipping of the bundle, and during unbundling and handling of the plates before feeding of the individual plates to the imager.
The plates can be directly fed to the imager, thereby avoiding the need for the commonly used interleaf removal device. The integrated interleaf is non-removable, i.e., it does not come loose, dissolve, or otherwise disintegrate during the imaging or in the developer solution and associated rollers, washers, and dryers.
The interleaf remains intact when the imaged plates are restacked or otherwise gathered for attachment to the printing press cylinder. The thin, flexible interleaf film can bend with the curvature of the plate when attached to the cylinder. For example, a typical newspaper plate is about 12.5×22 inches and several such plates are attached to a printing press cylinder having a diameter of about 18 inches.
Although the film is flexible, it is preferably of sufficient density and thickness (0.5-2.0 mils) that the thickness of the metal substrate (8-12 mils) can be reduced a similar amount, thereby preserving the overall developed plate thickness required in the printing press (about 8-12 mils), while saving on the high cost of substrate material.
A representative embodiment will be described below with reference to the accompanying drawing, in which:
In
According to the present disclosure, an interleaf adhesion station 24 is situated between the curing at 20 and the recoiling at 38. Preferably, the sheet carrying the cured coating passes between heated roll 26 and an opposed roll 28, whereby a thin, flexible film 32 from film supply 30 is laminated to the entire bottom surface of the sheet in a continuous operation. This process may include a device 34 for guiding and/or feeding the continuous sheet of film from source 30, to the roll 26.
The resulting sheet at 36 consists essentially of a metal substrate, a cured radiation sensitive coating adhered to the top side of the substrate, and a thin, flexible film adhered to the bottom side of the substrate. The sheet may optionally receive a top coat of, for example, PVOH, as a barrier to the penetration of oxygen during shipment and storage. The sheet at 36 may be immediately cut into individual plates, but typically the sheet is recoiled at 38 and moved to a cutting line as represented in
In
The sheet 36 and each cut plate have a cross section as shown in
It should be appreciated that in the broadest sense, the integrated interleaf can be adhered to the substrate at any point in the manufacturing line of
Nevertheless, regardless of the step during manufacture at which the interleaf is applied, an individual plate having the thin integrated leaf, is believed novel and non-obvious. Moreover, a stack of such plates exhibits significant advantages in handling and imaging.
A resin or polymeric film, especially a polyolefin such as polyethylene and polypropylene, has the ideal characteristics of ready availability, low cost, known lamination equipment and procedures, high lamination adherence, insolubility in developing fluids, inks and fountain solutions, and dimensional stability at ambient temperature even when subjected to the pressures associated with a printing press.
It should also be appreciated that as used herein, a “film” of interleaf material adhered to the substrate does not require that the material was in the form of a film before adhesion to the substrate. In this context, “film” means a thin layer or layers, even if applied by coating, rolling, or processes other than heat lamination.
