Printing – Stenciling – Stencils
Reexamination Certificate
1999-08-02
2001-03-20
Funk, Stephen R. (Department: 2854)
Printing
Stenciling
Stencils
C118S325000
Reexamination Certificate
active
06202553
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a process for the production of a screen printing form comprising a screen web, for example of plastic filaments, by coating with an emulsion which in particular is photosensitive. The invention also concerns a screen printing form comprising a coated screen web.
Centuries after it was first used in China the screen printing process has been known in Europe approximately since the 19th Century, a fine-mesh textile cloth or wire mesh material is stretched out in a screen printing frame and covered in the image-free regions by a stencil, so as to be impermeable to ink. Besides manual cut stencils—for example for labelling or writing—nowadays preferably photographically produced direct or indirect stencils are the usual practice; the choice of the kind of stencil adopted—in the case of direct stencils those with emulsion, with direct film and emulsion or with direct film and water—is left to the discretion of the screen printer.
A plurality of steps are usually required to produce a screen printing form. Firstly a screen printing cloth is stretched out over a clamping frame of light metal or alloy, wood or the like, and is glued to the frame in its stretched position. The cloth is then coated with photosensitive emulsion, for example using a coating channel manually or by machine using an automatic coating apparatus. As the coating thereon cannot be produced exactly as far as the inward side of the frame, the remaining surface area must be subsequently sealed off using screen filler. The coated surface is now exposed by means of a copy original (film) corresponding to the print image. The regions of the print image which are not exposed are washed out. The operation of drying the stencil is followed by the retouching operation and the operation of covering over the edges with screen filler.
It has been found inter alia difficult to produce an exact flat layer surface on the screen cloths which are usually produced employing a so-called taffeta weave. Due to the low proportion of solids—usually below 50%—in the emulsion the layer sinks down into the mesh openings of the cloth in the subsequent drying procedure, whereby the surface may exhibit a certain degree of roughness. For certain areas of use, that surface quality may be sufficient, but for higher levels of quality requirement, at least one further coating operation is required.
If a plurality of small frames are to be covered with the cloth—for example for producing print images of small surface area—, the procedure involves the use of a so-called mother frame, and the screen cloth is stretched out thereon; it is held to the mother frame for example by clamping clips which are described in the present applicants' EP 0 650 832 A1. The smaller screen printing frames are put on to a support and the screen web which is held under tension is fitted thereover.
A process for the production of photochemically coated material for screen printing cylinders is disclosed in German laid-open application DE 3441593A. In that procedure a chromium steel wire mesh of suitable mesh size is coated with nickel electrolytically, chemically or by vapour deposition, to such an extent that the points of intersection of the cloth are fixed in such a way that no movement is possible; the stabilized material is then coated with at least one layer comprising a photosensitive emulsion by applying that emulsion to a flat surface and embedding the cloth into the rear side of the layer of emulsion and joining same by applying a further layer of emulsion, so that the result is a flat copying layer on the surface of the screen printing stencil, which is at the printing side.
U.S. Pat. No. 3,834,348A describes a clamping frame—which is variable in width—with a cloth portion fixed therein using clamping elements. Displaceable over the cloth portion is a channel for a viscous fluid or an emulsion, the latter flowing out of adjacent slots in the wall of the channel on to the cloth portion in order to form a coating thereon. The coating is then dried or hardened on the cloth portion.
JP 5 216 239A discloses a container with a clamping frame in which a cloth is coated with a photosensitive film. The fluid is introduced into the container until a desired layer thickness has been produced on the cloth. The fluid is then partially exposed; the unexposed regions are removed after termination of the treatment so as to result in an image.
SUMMARY OF THE INVENTION
The foregoing object is attained by the teaching of the independent claims; the appendant claims set forth advantageous configurations.
In accordance with the invention a screen web is partially provided within a predetermined outline contour with coating areas comprising the emulsion, wherein the emulsion is applied preferably in web form or also in point form to the screen web. It is advantageous to use at least one nozzle for the application procedure, which nozzle is guided parallel to the longitudinal direction of the screen web over the screen cloth and produces laterally mutually adjoining coating strips which together give the coating area.
The coating areas can be produced in different ways. On the one hand—in the case of low levels of requirement, rough surface—they can be applied directly to the screen web and then dried. It is also possible for those coating areas which are applied directly to the screen web to be lined with a protective foil and only then moved to the drying station; that is done in particular when the situation involves high levels of requirement and a good roughness value on the Z-axis. It is particularly advantageous however for the coating areas to be produced on a strong stable carrier material and for the latter to be fed with the wet emulsion regions to the screen web; those emulsion regions are then transferred on to the screen web and subsequently dried. Then, the carrier material is drawn off or however provisionally kept as a protective foil on the screen web.
It is in accordance with the invention for a center line of the coating area produced—defined by its preferably four contour lines—to include an angle of inclination with a longitudinal edge of the screen web; the contour lines are turned about the center of the coating area through that angle, which is preferably less than 45°.
In accordance with the invention, application of the coating areas at an angle of inclination relative to the screen web permits the coated cloth portions to be stretched out in an angled position on a clamping frame. Cloths which are stretched out in an angled condition permit cleaner and tidier printing of lines which have to be printed parallel to the screen printing frame. The crucial consideration is that in that situation there is no parallelism between the yarn path of the screen cloth and the line to be printed.
A simplification in manufacture is achieved in particular when the coating area is composed of individual coating webs which are each produced by a respective nozzle or the like tool and which are adjacent in parallel relationship and the longitudinal edges of which blend into each other so that no contour strips which pass through the coating area remain.
In accordance with a further feature of the invention each narrow edge of the strip-like coating web is associated with a respective one of the contour lines of the coating area which is angled relative to the axis of the mother frame.
It is in accordance with the invention for the center line of a clamping frame associated with the coating area to be positioned on the center line of the contour of the coating area, which clamping frame is arranged at a spacing relative to the contour lines and is then glued to the screen web in the taut condition thereof outside the coating area. After the screen web regions which remain between the frame and the coating area have been covered over by an impermeable layer, a coated printing stencil, in particular for smaller surface areas, is produced in a simple fashion, wherein the number of contour lines of the polygonal coating
Bachman & LaPointe P.C.
Funk Stephen R.
Sefar AG
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