Process for producing a printing form

Details Process for producing a printing form Process for producing a printing form

1998-11-03

2001-10-30

Baxter, Janet (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Making printing plates

C430S273100, C430S005000, C430S308000, C430S275100, C430S322000, C430S323000, C430S324000, C430S289100, C430S927000

Reexamination Certificate

active

06309799

ABSTRACT:

BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION The invention relates to a process for producing a printing form.
In the production of screen-printing stencils, it is known to apply a photosensitive resist (lacquer) to a screen serving as the carrier of the printing form or stencil, to expose it and subsequently to develop it, in order to remove the photoresist layer in regions in which later, during printing, ink is intended to be transferred through the screen. The exposure, as conditioned by the pattern, of the photoresist layer is in this case performed, for example, by means of a large film laid onto the printing form blank or point by point with the aid of laser light, the entire photoresist layer being scanned point by point and exposed only at those locations at which the chemical structure of the photoresist is intended to be changed, as conditioned by the pattern, for the later development. Furthermore, it is known to vaporize the photoresist layer away, in accordance with the pattern, using a CO
2
laser.
Both the use of large films and the use of lasers, in particular CO
2
lasers, which have a very high energy demand, makes the production of screen-printing stencils more expensive.
In the production of a flexographic printing form, a radiation-sensitive or photosensitive material, for example a photopolymer or photoelastomer is first applied to a carrier. Then, by means of irradiating or exposing individual regions of the layer, as conditioned by the pattern, the polymer is crosslinked. Ultraviolet radiation is preferably used for the exposure. Subsequently, in a suitable developer, the unexposed and hence noncrosslinked polymer regions are washed out. If appropriate, using a radiation source emitting in the UV range, the photopolymer layer is re-exposed and hence more intensively crosslinked.
In the case of the previously known methods, the regions of the radiation-sensitive layer that were not to be exposed in accordance with the pattern, that is to say in particular of the photoelastomer layer, are covered, either by a full-size film, as in the case of screen-printing stencils, or by means of a layer applied in the shape of the pattern by means of nozzles.
In the case of covering the radiation-sensitive layer with a photographically produced full-size film, a seam in the pattern in the case of cylindrical printing forms is generally impossible to avoid. In any case, such a full-size film usually which must have the size of the printing forme. Therefore, in the case of cylindrical printing formes the size of the cylinder surface, makes the production of printing formes considerably more expensive.
The spraying on process, in accordance with the pattern, of a covering, light-absorbing and/or light-reflecting ink layer with the aid of nozzles onto a radiation-sensitive layer places demands on highly-pure covering inks, which adhere adequately to the radiation-sensitive layer and nevertheless do not block up the nozzles used. In addition to the high requirements on the covering inks, the nozzles used for the spraying on process must also satisfy high requirements, since a large number of droplets of constant size have to be produced per unit time.
Although the problems which occur during the handling of large films may be avoided by spraying covering ink in accordance with the pattern onto the surface of a printing forme, in particular onto a cylinder surface, the spraying on technique requires very precise nozzles, which are complicated to produce and to control.
SUMMARY OF THE INVENTION
Taking this as the starting point, the invention is based on the object of providing a process for producing a printing forme which is simple and cost-effective to carry out and, in the process, delivers printing formes that are true to pattern.
According to the invention, therefore, in the production of printing formes, in particular in the production of cylindrical flexographic printing formes, a radiation-proof or light-proof covering layer is applied to a radiation-sensitive layer, in particular a photopolymer or photoelastomer layer. By means of a subsequent structuring of the covering layer in accordance with a pattern, an irradiation or exposure mask is constructed directly on the radiation-sensitive layer. The exposure and development of the radiation-sensitive layer is then subsequently performed in the usual way.
In the case of the process according to the invention, therefore, the exposure mask according to the pattern is constructed directly on the layer to be exposed, the layer to be exposed firstly being completely covered with the covering layer, in order then to structure the latter. In selecting the covering layer material, it is therefore necessary to take into account only the adhesion properties and the radiation absorption and radiation reflection properties. The absorption and/or reflection capability of the covering layer is in this case to be matched to the radiation sensitivity of the first radiation-sensitive layer. Although it is sufficient if the covering layer completely blocks off only the radiation used for the exposure of the first radiation-sensitive layer, it is advantageous if the covering layer also has a good absorption or reflection capacity even in the adjacent radiation regions.
The process according to the invention may be used not only in the case of the actual production of printing formes, in which the first radiation-sensitive layer is structured in accordance with a pattern to be printed later, but also in the production of screens formed by electrotyping. In this case, the first radiation-sensitive layer, which is arranged on a conductive full-area, in particular a metal surface, is then patterned in accordance with the screen to be produced by electroplating.
The process according to the invention may be simplified further if provision is made for the covering layer to be structured in accordance with the pattern, where the irradiation mask is removed during the development of the first radiation-sensitive layer with mechanical and/or chemical means. With this mechanical and/or chemical means, an additional processing step for the separate removal of the covering layer following the exposure, where such a step complicates the process unnecessarily, is avoided.
In the process according to the invention, provision is expediently made that, in order to structure the covering layer as conditioned by the pattern, parts to be removed of the covering layer are removed directly. In this case, it is possible for the parts to be removed from the covering layer, as conditioned by the pattern. The parts can be removed mechanically, but it is preferred to use radiation.
Any radiation that can be focused precisely to a point can be used as the radiation for removing the parts to be removed of the covering layer. It is thus, for example, conceivable to employ a particulate radiation, in particular electron beams. However, provision is preferably made for electromagnetic radiation, in particular heat radiation or light to be used as the radiation for the removal of the covering layer in accordance with the pattern. In this case, it is possible to use as the radiation source any suitable lamp which emits light or heat in the desired or required spectral range with an adequate intensity. However, it is particularly expedient if laser radiation is used as the radiation for the removal of the covering layer in accordance with the pattern. Laser radiation has the advantage that it is easily to construct and handle very small light spots of high intensity for the removal of the covering layer.
In the case of another refinement of the invention, provision is made for a structuring mask to be applied to the covering layer in accordance with the pattern, preferably to be sprayed on, in particular sprayed on point by point. For the uncovered parts of the covering layer to be removed, the structuring mask (
6
) is formed by means of a wax solution and the uncovered parts of the covering layer are removed chemically, preferably by etching.
The spraying of a

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