Incremental printing of symbolic information – Thermal marking apparatus or processes
Reexamination Certificate
1999-03-15
2001-04-24
Le, N. (Department: 2861)
Incremental printing of symbolic information
Thermal marking apparatus or processes
Reexamination Certificate
active
06222567
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the production of prints, especially multicolor prints or offset printing forms, by means of thermal transfer, using a narrow tape as a transfer film. More particularly, the invention relates to a method and apparatus for optimizing the imaging quality of a thermal transfer print of this type.
2. Discussion of the Prior Art
The thermal transfer method has been known in principle for a long time. A substrate, which may be the final substrate or an intermediate carrier, is brought into contact with a colored layer which is applied to a carrier and transfers this colored layer to the substrate, dot by dot and in accordance with an image, by means of the action of heat.
By means of different-colored films, a number of colors can also be applied one after another, and a colored print can thus be produced. If the substrate is an intermediate carrier, the finished multicolor image is then transferred to the target substrate in a further step. Furthermore, a printing form can also be coated in accordance with an image by means of a suitable polymer. If, for example, the base of the printing form is hydrophilic and hence does not accept ink, the image-carrying parts are transferred to this printing form by thermal transfer as a positive and are then hydrophobic, that is to say they accept ink.
Coating a substrate in this way, in particular by means of a laser, is disclosed by German reference DE 44 30 555 C1. This document describes a method and an apparatus by means of which a printing form can be produced simply and in a manner which can be integrated into the printing machine, in particular on a seamless printing-form cylinder with a smooth surface, without the gases which arc produced during the laser imaging operation noticeably interfering with the transfer of material from the thermal transfer film, that is to say the imaging quality.
Here, a tape-like transfer film with a tape width which is only a fraction of the substrate width is guided through between the substrate and the imaging unit, in the immediate vicinity of the substrate surface, by means of the tape transport mechanism. The tape transport mechanism, together with the imaging unit and electronically or mechanically coupled, is fitted to a traversing unit, so that the transfer film can be moved over the substrate width uniformly with the movement of the imaging unit. Together with the laser-induced thermal imaging unit, which is controlled in a known way by means of a control unit in accordance with an image to be transferred and, for each image point, introduces heat into the thermal transfer film, and thus performs a dot-by-dot transfer of the ink-accepting coating of the transfer tape, it is thus possible for the complete substrate, in particular the complete seamless printing-form cylinder, to be imaged all round.
It has transpired that transferring the thermal transfer material in the laser-induced thermal transfer process gives particularly good imaging results as a result of the transfer film rolling synchronously on the cylindrical substrate surface. This can be attributed in particular to the distance between the transfer tape and substrate then being a minimum.
As a result of this small distance, a low relative speed close to zero between the transfer film and the substrate surface leads to adhesion of the transfer tape to the substrate. This is desirable, but necessitates truly exact synchronization, since any slight relative speed which may occur leads to “smearing” of the thermal transfer material on the substrate surface. The laser-induced thermal transfer process leads to the transfer film adhering temporarily to the substrate. If, in the event of a positive relative speed, the force causing the tape to advance is greater than the adhesion, then the bond will be broken and the transfer material will be transferred only to a partial extent and with smearing. As a result of the inherent elasticity of the transfer tape, adhesion alternates with sliding; the so-called “stick—slip” effect occurs: it is therefore essential for the speed of the transfer film and substrate surface to be exactly the same.
SUMMARY OF THE INVENTION
According to the invention, this object, of achieving a defined and minimum distance between the transfer tape and substrate, is achieved by using the transfer film to exert a contact force which acts on the substrate so as to produce a static friction force. The magnitude of the static friction force is used to control exact synchronism between the tape-like transfer film and the substrate cylinder.
The nub of the method is that a contact force is produced in a suitable way. This force produces a static friction force, which is used to control exact synchronism.
Furthermore, the contact force leads to the distance between the transfer film and substrate being minimized, in particular to the gases which occur as a result of the thermal transfer and the air which is dragged in between the transfer tape and substrate as a result of dynamic and boundary-layer effects being compressed or led away.
This control process is preferably carried out actively, but can also be carried out passively.
Active control is based on the effect that when there is exact synchronism, that is to say when there is no relative speed between the passage speed of the transfer film and the surface speed of the substrate, the tensile force that has to be applied by the rewinding drive in order to wind the thermal transfer tape around once the latter has been accelerated is minimal. This minimal tensile force will be referred to below as the synchronous winding force. The synchronous winding force is essentially determined by the frictional force which has to be overcome in order to deflect the tape being guided, the force required to tear the transfer tape off the substrate surface during a thermal transfer (the thermal transfer leads to the transfer film “sticking” at the point of laser influence), the force components, which are brought about by contact pressure measures, in the direction of movement of the tape when it is running synchronously, for example as a result of the tape being blown on obliquely, and the opposing force needed by the unwinding drive in order to apply tape tension.
As an alternative embodiment, control may also be carried out passively, by a defined speed, which differs only very little from the circumferential speed of the substrate, being predefined and the differential speed being compensated for via the plastic expansion of the transfer tape. However, this necessitates the static friction force being greater than the force needed for plastic expansion of the transfer tape.
For all types of control, the contact force for producing the static friction force is produced on the one hand by the tape tension in conjunction with the thermal transfer tape wrapping around the substrate cylinder, and on the other hand by further force components which press the tape against the substrate cylinder. These force components are preferably produced by air being blown on. Developments make use of electrostatic forces by applying charge to the rear of the tape, or a vacuum which is produced by extracting the air in the entry gap, that is to say at the location where the transfer tape and substrate surface run together.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.
REFERENCES:
patent: 5045865 (1991-09-01), Crystal et al.
patent: 5129321 (1992-07-01), Fadner
patent: 38 37 978 (1990-05-01), None
patent: 44 30 555 (1996-04-01), None
patent: 63-125534 (1988-08-01), None
Feller Bernhard
Hartmann Thomas
Muller Michael
Probian Dirk
Schuster Alfons
Cohen & Pontani, Lieberman & Pavane
Feggins K.
Le N.
MAN Roland Druckmaschinen AG
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