Incremental printing of symbolic information – Thermal marking apparatus or processes – Multicolor
Reexamination Certificate
1999-03-15
2001-05-01
Le, N. (Department: 2861)
Incremental printing of symbolic information
Thermal marking apparatus or processes
Multicolor
C347S171000, C347S262000
Reexamination Certificate
active
06226020
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method and apparatus for producing prints, especially multicolor prints, especially a proof, by means of thermal transfer.
2. Discussion of the Prior Art
The thermal transfer method has been known for years. In principle, a substrate, which may be the final substrate or an intermediate carrier, is brought into contact with a colored layer which has been applied to a carrier, and this colored layer is transferred to the substrate dot by dot, or in accordance with an image, under the action of heat.
By means of differently colored films, it is also possible for a number of colors to be applied one after another, and thus for a colored print to be produced. If the substrate is an intermediate carrier, the finished multicolored image is then transferred to the target substrate in a further step.
U.S. Pat. No. 5,164,742 describes a method in which the action of heat is achieved by means of laser radiation. This means that very small dots can be achieved. However, the method cited has the disadvantage that each film is laid on the substrate over its entire area. Achieving this without the inclusion of air is complicated. Furthermore, only materials which do not gas out during imaging can be used, since otherwise so-called tenting occurs, that is to say the material is lifted, contact between the adjacent regions of the film and the substrate is lost and it is no longer possible for the film to be imaged properly.
This restricts the usability both of the suitable materials and of the imaging arrangement, since during the (very brief) introduction of the energy in the normal case, not only is the material incipiently melted but also a plasma and gas are produced. In particular, limits are placed on the imaging speed, since the shorter the action time, the earlier the plasma is produced. Furthermore, laying a film on a substrate in an external drum imaging means is not possible unless additional fixing is provided.
German Patent DE 44 30 555 C1 discloses a method from another field of application, namely the production of offset printing forms.
SUMMARY OF THE INVENTION
The object of the present invention is, then, to develop a method and an apparatus for producing a print, especially a proof, to the effect that it can be used for transferring material from the film to the substrate in a simple way without the gases which are produced during laser imaging. Furthermore, this method and the apparatus are to be equally suitable for use with smooth and rough substrates.
Pursuant to this object, and others which will become apparent hereafter, one aspect of the present invention resides in a method of producing a print, a tape-like transfer film is used for imaging a substrate applied to a substrate cylinder. The transfer film has a width which is small in relation to the substrate width so that the gas produced during the laser imaging operation can escape to a sufficient degree because of the substrate and the transfer film being placed opposite one another over only a small area. During the imaging operation the transfer film is guided continuously between the substrate and the laser beam so as to be close to the substrate surface. The transfer film is also moved over the substrate width simultaneously and in synchronism with the movement of the laser imaging unit.
The fact that use is made of a tape-like transfer film with a width which is small in relation to the substrate width and, in an external drum imaging arrangement, this transfer film is guided between the substrate and the laser beam, close to the substrate surface, and is moved with respect to the substrate simultaneously and in synchronism with the movement of the laser beam, means that good contact between the thermal transfer material and the substrate, or a defined distance between these two, can be achieved.
The gas which may be formed can escape to a sufficient extent because the thermal transfer tape and the substrate are placed opposite each other over only a small area.
Since the contact or distance between the thermal transfer film and the substrate can be monitored, it can be set to the widest range of different substrates, from very smooth, such as cast-coated paper, to rough, such as roughened metal.
Furthermore, the carrier of the substrate may be considerably thinner than in the case of the whole-area films of the prior art, since the material can be kept in the form of a narrow tape wound up on reels and, at the time of the imaging operation, only has to be guided, under tension, over a short distance.
The fact that the tape-like transfer film, with a width which is only a fraction of the substrate width, can be guided through between the substrate and the imaging unit, in the immediate vicinity of the substrate surface, by means of the tape transport mechanism, and the fact that the tape transport mechanism works together with a traversing unit which is coupled to the imaging unit, means that the transfer film can be moved over the substrate width uniformly with the movement of the printing head. The laser-based thermal printing head, which is controlled in a known way by means of a control unit in accordance with an image to be transferred, introduces heat into the thermal transfer film at each image dot and hence performing a dot-by-dot transfer of the ink-accepting coating of the transfer tape. The entire substrate surface is covered via the rotation of the substrate cylinder and the traversing action axially parallel to the substrate cylinder.
In this case, a number of colors can be applied by using transfer films of different colors, as is also common in the case of conventional thermal transfer. In this case, the transfer materials must have been provided with absorbers for the laser wavelengths used and must have been applied to the carrier material of the transfer tape.
Thus, the tape width of the transfer film may be selected as a function of the number of imaging channels of a laser imaging unit traversing along the axis of a rotating substrate cylinder, that is to say may be designed to be as wide as the printing head.
Another advantageous refinement of the generic method is designing the transfer tape to have a width which permits a number of tracks to be written alongside one another for different imaging operations, and hence permits the transfer tape to be used for a number of imaging operations.
In this case, it is necessary for the position of the point or points at which the laser beam or beams impringe(s), that is to say the writing track of the laser, on the transfer tape to be shifted in each case, so that the tracks do not overlap. Of course, it is possible either for the laser imaging head to be shifted in relation to the common coupling relating to the traversing means or for the tape transport mechanism, at least the two contact rolls
6
a
,
6
b
(
FIG. 1
) to be shifted in relation to the laser imaging head.
One variant of this is independent mechanical traversing of the tape transport mechanism and the laser imaging unit, the synchronized movement being achieved electronically, that is to say the tape transport mechanism and the laser imaging unit have their own traversing drive, the two being shifted synchronously in an electronically coupled manner. Shifting the transfer tape in relation to the laser imaging head in order to provide unused tracks is then simply possible by changing the distance between the tape transport mechanism and the laser imaging head.
A further refinement is that, for the colors used, use is made of one tape composed of differently colored parts rather than different tapes. The tape may have the differently colored parts alongside one another, for example in the sequence black, cyan, magenta, yellow, or may have the differently colored parts one after another.
In another embodiment, during the imaging operation, the transfer film is guided in the same direction as the relative movement of the substrate cylinder and at an identical surface speed. In the preferred embodiment, th
Feller Bernhard
Fuhrmann Hartmut
Hartmann Thomas
Muller Michael
Probian Dirk
Cohen & Pontani, Lieberman & Pavane
Le N.
MAN Roland Druckmaschinen AG
Pham Hai C.
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