Screen printing stencil

Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates

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522 27, 522 28, 522 33, 522 35, 522 44, 522 46, 522 63, 522149, 525 56, 525 61, 430270, 430281, 430286, 430287, 524 81, 524186, 524233, 524401, G03F 712, C08F 246

Patent

active

059940336

DESCRIPTION:

BRIEF SUMMARY
This invention concerns a screen printing stencil.
In screen printing a negative of a print design, known as a stencil, is produced on a mesh, those areas of mesh where printing is not to be effected (stencil areas) being covered with a material impervious to the ink to be used for the printing. Typically the mesh is of a polymer (usually a polyester) or of stainless steel, stretched over a wooden or metal frame.
Many methods have been proposed for the production of screen printing stencils. Stencils can be prepared independently of the screen, by the so-called indirect method, or directly on the screen itself by the so-called direct method.
In the indirect method, a layer of a photosensitive emulsion is coated on to a temporary support sheet, usually a transparent polyester sheet, and the coating is then exposed to actinic light. Thereafter, the layer is developed on the support sheet using water to remove unexposed regions of the photosensitive emulsion. The wet stencil on its support is then adhered to the screen using gentle, even pressure. After drying the stencil, the support sheet is removed, leaving the stencil on the screen mesh ready for printing.
In the direct method, a photosensitive emulsion is coated on to a mesh and dried to give a continuous, even film. A stencil is then produced by imaging the coating directly with a laser, or photographically through a line or half tone positive, the image being developed by removing unexposed areas of the film with water.
A further technique, the so-called capillary film method, involves the application of a photosensitive layer to a polyester sheet which is then transferred to a mesh prior to imaging. Imaging and development are then effected as described for the direct method.
Combinations of emulsions and films are also possible in which a direct emulsion is used to adhere a film to a mesh. Development of the stencil is then effected as with the direct method.
The photosensitive coatings used for all of these techniques are polymer based, and they are formulated to achieve chemical, thermal and mechanical resistance, both in use and during manufacture, dimensional stability, flexibility, maintenance of light stability, and impermeability to liquids.
Hitherto proposed compositions for preparing screen printing stencils are in general based on water soluble colloids, usually polyvinyl alcohols, which act as binder resins. Irradiation of the compositions with actinic light results in their becoming insoluble in water so that they remain on the screen after development, while areas protected from actinic light retain their water solubility and are easily washed away with water.
Image development with water requires the coatings of the water soluble colloids to be rendered insoluble to water as a consequence of exposure to actinic light. This can be achieved either by crosslinking the colloid or by forming a secondary polymer matrix within the coating structure, thereby preventing redispersion in the water used for development.
Crosslinking can be achieved by a number of methods using ultra-violet or visible radiation. In one method, colloidal polymer molecules having reactive groups along the polymer chain can cross-link with other polymer molecules via the reactive groups. Alternatively, some reactive groups require an intermediate crosslinking agent. In either case, the resultant increase in molecular weight of the colloid reduces its solubility in water.
Secondary polymer matrices can also be formed by dispersing photopolymerizable monomers and/or oligomers in the aqueous colloidal solution.
Desirable properties for colloidal binder resins for producing screen printing stencils are solvent resistance, decoatability and water solubility. Polyvinyl alcohol derivatives meet all of these requirements, as well as having good mechanical strength and heat resistance. Furthermore, decoating of insolubilized polyvinyl alcohol based coatings can be simply achieved by selectively oxidising the 1,2-diol groups on the polymer chain, for example by applying a soluti

REFERENCES:
patent: 4554240 (1985-11-01), Schulz et al.

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