EP1077129B1 - Process and apparatus for reversibly imaging a printing forme - Google Patents

Description

The invention relates to a method for reversibly imaging a printing form by image-controlled heating of a surface by means of thermal transfer according to the preamble of claim 1.

State of the art

Printing processes are known in which a printing form is preferably on a Printing form cylinder with a thermal transfer film Thermal transfer material is provided. The transfer of the transfer material is preferably carried out laser-induced, but also others Energy sources such as heating elements, lines, matrices or the like are conceivable would. The printing form is then used in particular for an offset process Ink colors, the ink of the ink-guiding areas is optionally over a rubber roller on the substrate to be printed transfer.

For a quick change of print motifs, especially for short runs aspired to the process within the printing press on the one hand computer-controlled and secondly without changing moving parts.

For example, DE 38 09 915 A1 shows such a printing process. through an image information transmission unit within the printing press Image information in the form of color-accepting surface elements on the transfer lipophobic surface of the printing form or the printing form cylinder. The thermal transfer foil is with a thermo or electrothermal sensitive Provide coating that oleophilic, i.e. has ink-accepting properties. The image information transfer unit includes a printhead which is one Row of heating elements, an electrode, an energy beam or any other heat generating unit, in particular a laser printhead. to The print head is transferred via the corresponding image information Image signals controlled so that he is at each pixel heat and pressure on the Thermal transfer film initiates and thus a selective transfer of Coating the film on the surface of the printing form cylinder causes. The Printing form cylinder rotates, the print head is traversed accordingly, see above that from the thermal transfer film, for example, spirally on the printing form the printing form cylinder can be illustrated.

Process for imaging a printing form by means of thermal transfer, one of which Transfer film is carried out between the printing form and a printhead and the Imaging of the surface of the printing form by selective, i.e. as image data Control and transfer of the coating of the transfer film is carried out, are e.g. known from US-A-5 395 729 or WO-A-98/08142.

For repeated imagewise coating of such a printing form Assemblies within the printing press, consisting of a feed device for a thermal transfer film to the printing form cylinder, one with the Rotational movement of the printing form cylinder coordinatable laser print head, and from an electronically controlled pixel transmission unit for Activation of the laser printhead and an element that the imagewise Coating can lift off the printing form, arranged. The Indian The subject presented in EP 0 698 488 B1 fulfills this need. Like the 196 24 441 C1 shows, the element for lifting off the imagewise coating, or the thermal transfer material from the surface of the printing form Be a pressure washer.

The band-shaped used in the subject of EP 0 698 488 B1 Thermal transfer foil is characterized by a comparatively thin coating with thermal transfer material. The imaging layer thickness thus falls on the Printing form cylinder thin, so that this pictorial coating again can be easily lifted off, i.e. the printing form cylinder corresponding to the Reduction of the layer thickness of the thermal transfer material easier, or can be deleted more quickly.

However, it is also known in the professional world that the durability of a thermotransfer-imaged printing form, so the consistent print quality over the total number of copies of a given to be printed

Print product, directly from the layer thickness of the thermal transfer material depends. If this layer thickness is increased in favor of the durability, however, there are problems with lifting the imagewise coating the printing form when deleting what color veil, or ghosting, so so-called toning on the printing form or in the printed image.

However, it should not only be ensured that the image is coated Print form a sufficient service life for the longest possible circulation offers consistent print quality, but also that the image transferred thermal transfer material on the printing form after the printing process can be removed quickly and easily in an environmentally friendly manner a new imaging and printing process can begin immediately.

For this reason, one has already tried, on the one hand, the a thin layer thermal transfer film to use, but then infrared curing the imagewise coating perform the printing form, so that additional curing of this Imaging layer reached and the print run resistance is increased. At this thermal aftertreatment (fixation) is the polymer of the Imaging layer heated by the introduction of heat above the glass temperature.

For this purpose, an infrared radiation is transmitted from the point transmitted Thermal transfer material carried out on the layer formed on the printing form, whereby the adhesion to the surface of the printing form compared to unirradiated areas on the printing form is increased. Such IR radiation also includes laser-induced heat input. It has However, it has been shown that this infrared curing (since carried out over a large area) leads to a strong heating of individual areas of the printing form, in particular a subject-dependent, uneven heating of the imaging layer the printing form, i.e. spot areas heat up more than graded areas Halftone image areas. In extreme cases, this means that the property of a improved circulation resistance is distributed unevenly on the printing form and due to the subject-dependent thermal expansion e.g. a loss of Register accuracy occurs or that none for the printing form as a whole improved circulation resistance was achieved.

Object of the invention

It is the object of the invention to use a printing form Reversible images of thermal transfer material so that a high circulation with consistent print quality is ensured without the layer thickness of the Adjust thermal transfer material according to the desired edition need and to simplify the pictorial deletion of the printing form.

solution

According to the invention, it is therefore proposed to start one in a first step well-known imaging of the printing form by selective control, so imagewise controlled heating of the thermal transfer material on the Transfer film, and transfer of the pixels on the surface of the printing form to carry out, then the transfer film between the printing form and the Image information transmission unit to remove and in a second step an image data-oriented control, i.e. imagewise controlled heating of the Make surface of the printing form, so that in the first step pixel-like transferred image information on the printing form pixel for pixel a second time is heated. After printing, the image information thus fixed, i.e. the polymer parts are removed from the printing form.

Because already without the interposition of the transfer film transmitted image information a second time, preferably from one Laser print head is driven, the imaging energy is increased and the Pixels harden better, so that the durability of the Thermal transfer material layer is clearly improved without the layer thickness must be enlarged.

As is known, the preferably ribbon-shaped thermal transfer film consists of a Substrate layer, i.e. the carrier film or the carrier tape, from one if possible transparent, heat-resistant plastic and applied one Donor layer, i.e. the heat sensitive transferable layer. By Exposure preferably to a laser beam from the back of the Thermal transfer ribbons, i.e. from the uncoated side, is in the Donor layer induces heat, which is used to soften and eventually detach of the thermal transfer material that is transferred to the printing form there due to the high thermal capacity of the printing plate material, e.g. metal, immediately cools and adheres. The thermal transfer film, in particular the layer of Thermal transfer material, preferably has a layer thickness of about 0.5 to 3 µm.

Fig. 1 shows schematically a known device for performing an illustration of a printing form by thermal transfer (Fig. 1a: side view, Fig. 1 b is a perspective view). On a printing form cylinder 1 is one Printing form 2 as a printing plate or as a sleeve-shaped printing form (es but it would also be conceivable to image the surface of the cylinder itself). On Belt transport mechanism 3 carries a belt-shaped thermal transfer film 4 the width b close to or in contact with the surface of the printing form 2. A known and therefore not shown image information transmission unit comprises a printhead 5 which has at least one laser source includes one or more beams focused on the transfer belt 4. The Laser source 5, preferably an IR laser and the tape transport mechanism 3 are together on a traversing unit 6 in the preferred arrangement arranged, by means of this they over the width B of the printing form, which during any control with the cylinder 1 rotates, can be moved.

The duration of laser imaging of a printing form is typically 1 to 2 Minutes.

Now, for the second step, for the second step, so to speak, with the first identical illustration of the surface of the printing form, but without Interposition of the transfer film, the same duration as for the first Step spent so that the imaging time is almost doubled.

There are two ways of doing this. Once the image data oriented Control of the surface of the printing form (second step) identical to that in the the first step is image-controlled heating of the transfer film become. Fig. 2 shows schematically a graph of the image data oriented Control of the laser source, with a) the image data stream during imaging (first Step) and b) the image data-oriented control on the surface of the Show printing form (fix, second step). On the other hand, the Image data-oriented control of the surface of the printing form (fixing, second Step) in the reverse order to that which was controlled by the image in the first step Heating of the transfer film can be made. That the print head 5 (Fig. 1) traverses once on the printing form 2 along the width B of the Printing form from left to right. After removing the transfer film 4 the Print head 5 once from right to left back to the starting position and controls the surface of the printing form in reverse order to the way there. This will optimize the path reached because the "idling" of the print head 5 in the initial position and so a distance B is avoided.

It is preferably provided that both for the first step and the same laser source is used for the second step. The preferred combined training of the laser source as an imaging unit and as However, the fixing device does not preclude the use of different laser sources out.

The polymer layer applied to the printing form is by means of the laser source 5 warmed briefly above the Tg temperature (glass transition temperature) of the polymer. The Heating takes place locally and for a limited time. This can cause damage of the "image-free" space are almost excluded. In contrast, can in the case of surface heating / radiation, as has been carried out up to now lead to a disturbance in the ink-water balance. The spatial Limitation can be achieved by changing the intensity distribution to reach. Diffractive hybrid elements are ideally suited for this.

The deletion process that follows the completed printing process Removing the image information from the surface of the printing form can be done in particularly advantageously also by means of a laser source be carried out in an image data-oriented manner by adjusting the power of the Laser beam is made so that the image information pixel by pixel from the Surface of the printing form can easily be burned away.

A laser source is already described in DE 195 03 951 C2 several intensity levels can be switched, i.e. a corresponding Power adjustment, i.e. the radiance per unit area, i.e. the radiance can be set.

This is also for the structural unit of the printhead with laser source for imaging and fixing the possibility to include the deletion process, so a laser source in the print head first for imaging, second for fixing the Picture elements on the printing form and third to delete the picture information use.

Of course, training the device for one is also Realize thermal transfer such that the print head 5 is a first laser source for imaging the printing form, a second laser source with special adapted intensity distribution for the image data oriented heating of the Surface of the printing form 2 and another laser source with accordingly adjusted radiance to remove image information from the surface the printing form as a structural unit.

The method according to the invention is said to be feasible in a printing press be, however, the invention is in no way on the imagewise coating of a Printing form limited within a printing press, but is suitable in principle also for the production of a printing form outside of a printing press. The print image carrier, i.e. the printing form a seamless Printing form cylinder, a cylinder sleeve, or also a conventional, uncoated printing plate, which is clamped on a printing cylinder.

Claims (10)

1. A process for reversibly imaging a forme by means of thermal transfer, wherein a transfer foil is passed through between the forme and an image-information transfer unit comprising a print head, in a first step the image information is undertaken in the form of an imaging of the surface of the forme by selective, i.e. image-data-oriented, control and transfer of the coating of the transfer foil, and thereupon the transfer foil between the forme and the image-information transfer unit is removed, characterised in that in a second step an image-data-oriented control of the surface of the forme is undertaken, so that the image points on the forme which were transferred in the first step are heated once again and can be removed after the printing operation.
2. A process according to Claim 1, characterised in that the image-data-oriented control of the surface of the forme (second step) is undertaken in a manner identical with the heating of the transfer foil which is controlled in image-specific manner in the first step.
3. A process according to Claim 1, characterised in that the image-data-oriented control of the surface of the forme (second step) is undertaken in reverse sequence in relation to the heating of the transfer foil which is controlled in image-specific manner in the first step.
4. A process according to Claims 1 to 3, characterised in that the same period of time is expended for the second step as for the first step.
5. A process according to Claim 1, characterised in that a laser source is used by way of print head for the image-data-oriented heating both of the transfer foil (first step) and of the forme (second step).
6. A process according to Claim 1, characterised in that the erasing operation for the purpose of removing the image information from the surface of the forme by means of a laser source is likewise carried out in image-data-oriented manner by a power adjustment of the laser beam being undertaken in such a way that the image information can be burnt away from the surface of the forme.
7. A process according to Claim 1, characterised in that a common laser source is used for both the first step and the second step.
8. A process according to Claim 7, characterised in that the common laser source is also used for the erasing operation for the purpose of removing the image information from the surface of the forme.
9. A device for implementing the A process according to Claim 1, which comprises assemblies consisting of a unit for feeding a thermal-transfer foil onto the forme, an image-information transfer unit comprising a print head, and an element for removing the image information from the surface of the forme, characterised in that the print head exhibits at least one laser source, by means of which the imaging of the forme (first step), the image-data-oriented heating of the surface of the forme (second step, fixing) and the removal of the image information from the surface of the forme (erasing) are capable of being carried out.
10. A device according to Claim 9, characterised in that the print head comprises, as a modular unit, a first laser source for the imaging of the forme, a second laser source with specially adapted intensity distribution for the image-data-oriented heating of the surface of the forme, and an additional laser source with appropriately adapted radiance for the purpose of removing the image information from the surface of the forme.

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