Post 316 – by Gautam Shah
Screen printing is process of spreading a viscous colourant through partially occluded screen. The occluded screen represents a pattern where opaque sections do not allow any transfer of the colourant. This was a process once used for duplicating artwork sections with Cartoons.
Cartoons are in the form of an opaque plane created with a sized cloth, parchment or paper. The outlines of the figure or pattern were drawn and the outlines of it were pricked by pin. The cartoon was placed over the wall, canvas or drawing panel and powder or liquid colour was rubbed on it with a rag. The impression created by closely spaced pinholes were joined by charcoal lines. With this technique artist used to create copies of figures and pattern within the same painting, across several paintings and also across carpets, tapestries, ceramic tiles, etc. Such cartoons were sold to others or borrowed from others.
Screens for printing uses a process, similar to the cartoon-copying. For screen printing a fine mesh of silk fabric (in earlier ages) was tautly held over a frame to form the screen. Screen blocking was once done with non water soluble medium such as bone glue or paint. Actual printing occurred by placing the screen over fabric, paper or ceramic tile, and rubbing the colourant paste with rag, flat brush, squeegee or wide spatula. For running patterns like borders or lengths of fabrics, the screens were sequenced. It was possible to create many different colour and pattern combinations. Stenciling is another process of pattern printing, used since prehistoric cave painting.
The Silk screens were very fragile, and the screen blocking materials were even more delicate. Screens could be used for very few repeat operations. This problem was solved with the production very fine and durable fabrics of Nylon and later polyesters. Today screens are made with synthetic fine gauze fabrics, wire gauze (phosphor bronze, stainless steel, nickel) or of combinations (nylon-copper, nylon-bronze). These are called bolting cloths (of 200 to 800 mesh).
The modern day screens could be one for each of the colour to be printed. The colours of the image to be printed are section-separated as black and white transparency image, where the Black represents area to be printed with the particular colour. The transparency in hard copy or as digital-photo image is projected over a light sensitive chemical coated screen. The black area does not sensitize the chemical coating, so can be washed off, leaving the light reacted area intact. The screen is then coated with a screen-paint a two-pack formulation, commonly of amine resins. This screen paint is tough, and can take frequent rubbing of colour spatula or strike plate. A screen often can print up to 100,000 times.
The fabrics are printed on a long printing tables which have screen registration stops, ensuring accurate pattern overlapping and fitting. Screen tables are of many types such as plain rubber felted, vacuum suctioned, warmed or heated, etc. For single page print-work such as letterheads, visiting cards, invitations, envelopes etc. are printed with lifting a screen table. For bottles, tins, etc. the product is rotated under the screen or rotary screens with round printing facilities are used.
Colours for screen printing are pigment paste colours mixed with high viscosity acrylic binders. Often mordants (certain metal compounds), warmed wax, gums, are printed on fabrics by the screen process. The mordant printed fabric is reacted with various dye stuffs, waxed fabrics are over printed with dyestuff (in a batik like format), and gum prints are covered by finely chopped staples of fibres (flocking) or metallic dust or flakes.