Drying Blockout Retouching

Drying

Indirect photostencil films are dried after mounting. Direct/indirect stencils need two drying stages - after lamination and after washout. To avoid dimensional changes, edge lifting, edge shrinkage and reduction in adhesion, stencils should be dried with cold air; i.e., at ambient temperatures. Slightly warm air is sometimes necessary when time is limited or when working in high relative humidities. Indirect gelatine photostencils will contract and become brittle if there is excessive moisture loss. Their moisture content determines their flexibility, because during washout most of the film’s plasticizers are washed away.

After drying, the base support film can be stripped off. If resistance is felt, dry further. It is suggested that the base support film not be stripped until blockout is completed. Drying must be even over the entire stencil surface. Fan drying of large screens can cause overdrying and localized dimensional changes.

Thermostatically controlled drying cabinets give the best results and factory-built units usually have good air circulation. Drying cabinets can be constructed in-house from very simple materials. They save space, avoid damage to drying screens and are more efficient than fan heaters alone.

Edge curling - caused by excessive temperatures during drying. The direct/indirect stencil is not susceptible to the adhesion problems of overdrying, but note the following points:

1. Avoid temperatures above 30° C (86° F) when drying the emulsion/film after laminating; the thermal sensitivity of the dichromate or diazo sensitizer can cause premature insolubility of the stencil.
2. Slight resistance will be felt when stripping off the base support after drying the laminated film. If the film appears to be pulling off the mesh, dry further.
3. After washing out the exposed stencil, blot the excess water with a chamois leather or clean newsprint before drying. This will insure that any emulsion traces are removed.
4. Use cold air for drying the processed stencil. Temperatures in excess of 40° C (104° F) can cause edge shrinkage, giving poor edge definition to the printed image.

Blockout

When working to fine dimensional tolerances, apply the filler before drying the stencil. This reduces the forces acting on the mesh because both stencil and blockout dry at the same time. If you prefer to blockout after drying the stencil, do not strip the base support film, because it protects the stencil detail from inadvertent damage.

When printing solid areas of color, prepare a handcut stencil or use a filler. Mark out the edges of the area with self-adhesive tape and fill in the space between the frame and print area with a screen filler. This method is quick and cheap and permits long runs to be printed using flashdry fillers. If there is any question of the screen filler affecting the dimensional stability of the mesh and stencil, make the stencil as large as possible to reduce blockout to a minimum. Or blockout and dry the screen to within one or two inches of the stencil before adhering it to the screen.

Applying gummed paper tape along the inside edges of the frame to stop the ink from creeping between the frame and mesh is time-consuming and can affect the dimensional stability of synthetic and steel meshes. So proof the joint between the fabric and the frame with a screen mounting adhesive or solvent-resistant varnish.

Retouching

The retouching of a typical photostencil is illustrated. A fine, good-quality brush is used to apply the filler (thinned with water if necessary) to those parts of the stencil which are very thin or have opened up because of dust or tape edges. Apply the filler to the underneath of the stencil (the mesh will interfere with accurate retouching if the filler is applied from the mesh side). If retouching a stencil during the print run, you may have to wipe away carefully any solvent residue around the area to be retouched, to insure good adhesion to the filler. Retouching is usually necessitated by multilayer positives or by dust. It is often impossible to avoid multilayer positives, but retouching can be reduced by careful attention to cleanliness, particularly of the positive and the vacuum frame glass.

1. To reduce retouching:
   • Select the most appropriate film.
   • Optimize exposure conditions.
   • Pay attention to cleanliness during exposure.
   • Use clean positives.
   • Do not over-wash or use excessive water temperatures during washout. Both factors will affect the very thin layers of emulsion along tape edges.