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Screen printing is one of the leading printing processes.  In this process, which was previously known as silk screen printing, an image is produced on a flat screen by blocking the pores in the nonprinting areas.  This can be done manually by cutting out templates and mounting them on the screen or it can be done photomechanically by coating the screen with a photosensitive material, exposing a positive and developing out the image areas.  Previously, flat screens were made essentially using bichromated compositions.  Environmental concerns led to development and use of diazo as well as photopolymer systems.

 

Generally, screens are made of polyester, nylon or stainless steel filaments.  Silk fabric is hardly used. The mesh count usually ranges from 17 to about 500 per inch.  The actual printing operation involves placing the surface to be decorated under a stencil and drawing a mass of ink across the stencil surface by a rubber squeegee.  The ink is forced through the open areas of the stencil and deposited on the surface to be printed.  Automated multi-color presses are available.

Screen-based stencil systems fall into two broad categories :  1) Indirect, wherein the image and nonimage areas are fully formed on the stencil material before it is adhered to the screen mesh and 2) direct, in which the image and nonimage areas are formed after the stencil material is on or in the mesh.  Indirect materials are usually in film form and the direct systems may be either liquid or film type.  The liquid systems are generally referred to as emulsions.

Indirect films are either gelatin- or polyvinyl alcohol based films sensitized with inorganic or diazo compounds.  Direct emulsions are supplied in either unsensitized or presensitized forms.  Conventional emulsions are mixtures of polyvinyl alcohol (solvent-resistant but water-sensitive) and polyvinyl acetate (water-resistant but solvent-sensitive) resins.  The "dual-cure emulsions" incorporate diazo as well as photopolymer technolo­gies.  These usually consist of polyvinyl alcohol/diazo sensitizer/acrylic monomer/photoinitiator compositions.  The diazo sensitizer is usually added to the emulsion just prior to the screen preparations.

            Pure photopolymer stencil systems are referred to as PVA/SBQ.  The abbreviation SBQ stands for stilbazole quaternary.  The system is based on photodimerizable resin, which is produced by reacting a styrylpyridine salt possessing a formal or an acetal group  with polyvinyl alcohol.  These polymers are water-soluble in uncrosslinked state. 

            Photopolymer systems generally offer higher solids content, faster photospeed, better resolution and increased shelf life compared with diazo systems.  Applications of screen printing range from T-shirts to electronic circuits to CD/DVD disc printing.  Newer processes such as pad printing and inkjet digital printing now compete with screen printing.

MANUFACTURERS

Chromaline Screen Print Products (Ikonics)

Kiwo

MacDermid Autotype

Murakami Screen

Saati Group

Ulano

 Copyright © 2007-2008 Savla Associates

 

 

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