Record receiver having plural interactive leaves or a colorless – Having plural interactive leaves
Patent
1996-04-02
1997-12-23
Hess, Bruce H.
Record receiver having plural interactive leaves or a colorless
Having plural interactive leaves
428195, 428323, 428331, 428336, 4284884, 428500, 428704, 428913, 428914, B41M 5035, B41M 538
Patent
active
057007560
DESCRIPTION:
BRIEF SUMMARY
The invention relates to dyesheets for forming printed images on receiver sheets by thermal transfer of dyes, using such heating means as thermal heads controlled by electronic image signals; and in particular to heat resistant backcoats therefor.
Thermal transfer printing is a process for generating printed images by transferring thermally transferable dyes from a dyesheet to a receiver. The dyesheet comprises a base sheet coated on one side with a dyecoat containing one or more thermally transferable dyes, and printing is effected while the dyecoat is held against the surface of the receiver, by heating selected areas of the dyesheet so as to transfer the dyes from those selected areas to corresponding areas of the receiver. This generates an image according to the areas selected. By repeating the transfer process with dyesheets of the three primary colours, full colour images can be obtained. Further panels, e.g. black, may also be provided.
Thermal transfer printing using a thermal head with a plurality of tiny heaters to heat the selected areas, has been gaining widespread attention in recent years, mainly because of its ease of operation in which the areas to be heated can be selected by electronic control of the heaters (e.g. according to a video or computer-generated signal), and because of the clear, high resolution images which can be obtained in this manner.
The base sheet of a thermal transfer dyesheet is usually a thin thermoplastic film, generally orientated polyester film on account of its superior surface smoothness and good handling characteristics. The thermoplastic materials used in such films, however, may lead to a number of problems. For example, for high resolution printing at high speed, it is necessary to provide the thermal stimulus from the heaters in pulses of very short duration to enable all the rows to be printed sequentially within an acceptably short time, but this in turn requires higher temperatures in the printer head in order to provide sufficient thermal energy to transfer sufficient dye in the time allowed. Such temperatures may be well in excess of the melting or softening temperatures of the thermoplastic base sheet, typically rising to 300.degree.-400.degree. C. during pulses of a few milliseconds. One adverse effect of such high temperatures can be localised adhesion between the dyesheet and the printer head, with a result that the dyesheet is unable to be moved smoothly through the printer, and in severe cases the base sheet can lose its integrity, with tearing of the dyesheet resulting.
These problems are usually addressed by providing the dyesheet with one or more protective backcoats of various heat-resistant, highly crosslinked, polymers. By "backcoats" in this context we mean coatings applied either directly or indirectly on the base sheet surface remote from that to which the dyecoat is applied. Thus it is to the backcoat side to which heat is applied by the thermal head during printing. In addition to providing a heat resistant layer to combat sticking, backcoats may also be formulated to improve slip and handling properties.
Poor slip and handling properties can lead to printing defects such as ribbing, and smiles. "Ribbing" is the appearance of lines transverse to the movement through the printer, which normally extend the full width of the print. They are formed by longitudinal variation in the optical density of the print, and occur when there are variations in the amount by which the dyesheet and receiver feed through the printer after each row of pixels has been printed. "Smiles" are short, usually curved, transverse lines caused by creasing of the dyesheet as it passes though the printer.
These problems have previously been attacked by adding heat resistant particles to stand proud of the binder surface, together with one or more lubricants and/or surfactants, but inappropriate slip/handling additives can also lead to the printed image having low colour density, streaks and/or indentations in the direction of travel of the receiver sheet through the
Hess Bruce H.
Imperial Chemical Industries plc
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