Cryptographics, Jefferson Holland, Silkscreen Posters, Wallstreet, Colorado, www.cryptographics.com
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Silkscreen Signs
Jeff Holland - November 1984

There are two basic methods for small scale screen printing. Photo-Emulsion is used most often for sign and marker applications. The other technique, Lacquer Film Stencil, is commonly used for textiles and quick graphics.

Photo-Emulsion
In this method you are effectively making a "negative" from a photo or hand drawn positive. The result is a screen with the black areas of your drawing or "positive" washed out to allow ink to pass through. There are a number of ways to prepare the "positive". If your copy is solely text, a printer can prepare the text, typeset it, and produce the copy for you. If a photograph or very fine line drawing is involved it must be reproduced in "half tones", a dot grid pattern producing black and white patterns similar to newspaper photos. A photo or drawing can also be reproduced from a photo negative developed on "ortho" or Kodalith film, processed in the darkroom.

Another way to prepare a positive is to draw directly on acetate with opaque ink. There is a red block-out ink made specifically for this, but a high quality black india ink works as well. The acetate then becomes your "positive". Lettering can be accomplished with chartpak rub on lettering.

The emulsion is applied on the screen in even, thin coats. There are two commercially available ones, Speedball or Advance-Excello, both available at Art Supply houses. Both come as two part solutions- a glue-based resin, and ammonium bichromate to sensitize the emulsion. This is available in quart and gallon sizes. The choice is determined by economy. You only need I cup of resin to complete two 18" x 24" screens. With either kit, make up only as much as you need because, once sensitized, the emulsion has a very short shelf-life. Wash the silk first with mild soap and let it dry. Apply the emulsion in room light, with a piece of cardboard. A THIN even coat is best. Get screens to dry in a DARK place such as a cupboard, bottom side down (raise up with tacks or cardboard). Drying time takes 1-3 hours. The screen should be exposed within 24 hours.

Exposure is most easily carried out by using a #2 photo reflector bulb with a pie plate reflector sound it. Regular 150 watt household bulbs can be used but take one hour or more exposure time. A general rule of thumb to place the bulb over the screen is the light source should be equal to the diagonal cross-section of the original being exposed. The screen should be placed on dark or black paper. Place the "positive", then sandwich with a piece of glass or plexi-glass for good contact. A general exposure time for an 18" x 24" screen with a photo bulb is approximately 15 minutes. An exposure test similar to a darkroom timing strip is recommended for the first trial. Chemicals and emulsion thickness, light source and image density are all variables.

The emulsion is then washed out with a lukewarm-cold water spray. The image being printed should be completely "open" if done properly. The screen should be taped up on the sides and checked for pinholes. You can use "blockout" or tape or mask off areas you don't want to be printed.

Printing can be done on metal, paper, or plastic. There are plenty of Ink types to choose from, including water based commercial inks. One thing to keep in mind is that it's important not to let the ink dry out in the screen. If an extended run is planned, there is an ink "extender" that slows drying time down.

Set up the screen so that easy, repeatable results are possible. This usually means having it hinged on a backboard with the place for printing registered with masking tape and cardboard stops. Use a squeegee large enough to complete the pass in one stroke. More than one pass will result in bleeding of the ink beneath the screen. It helps to have the screen raised off of what you're printing by an 1/8th of an inch.

Water based inks clean up with a special mild soap solution. Enamel ink cleans up with kerosene or mineral spirits. USE PROPER VENTILATION (and common sense). The screen itself can be cleaned of the emulsion with a solution of 1 to 1 household bleach.

Lacquer Film Acetate Stencils
Lacquer film is a thin layer of lacquer on a plastic base that you cut away with an x-acto knife to produce a stencil. The stencil is adhered to the screen with lacquer thinner and acetone. The tricky part is not getting the film saturated with solvent as you draw it into the screen. You can blot with one rag barely damp with solvent and another dry one. Work from the center. Let the film dry before peeling away the plastic. This method is commonly used for quick graphics and textile printing because of the ability to use water soluble inks. Fine lines can be produced depending on how steady your hand is…

  Materials Needed

For the screen
- 2"x 2" lumber for the frame, routed to fit insert strips that hold and tighten the screen
- 120xx to 320xx mesh polyester/nylon screen - silk is expensive and doesn't last as long with photo-emulsion techniques. The mesh you choose is ink and resolution dependent with a general recommendation of 200xx
- corner staples for frame, "L" brackets, drywall screws for tightening the wood strips that hold the screen

For printing
- squeegee - size to fit approximately what you're printing
- ink - for specific project
- extender - to retard drying speed if printing a big run
- solvent - to clean up with

For preparing the screen
- glass or plexiglass to sandwich'positive" to screen
- #2 photoflood with pie plate
- emulsion and sensitizer or laquer film...
- cardboard - for carding emulsion into and on screen
- tape - to block off and mask screen
- blockout - to fix small irregularities in the screen
- spray bottle - to spray out emulsion in finest areas if washing out with the kitchen sink sprayer isn't enough

Assorted
- rags to clean with- DISPOSE of properly!
- paper for test printing
- plan ahead if you're printing a lot for where to put it all

Safety
Acetone, lacquer thinner, etc. are all extremely volatile (fire hazard) and the fumes produced are not good. Use adequate ventilation. Water based inks for commercial production have come a long way in the last few years. TW Graphics and Naz-Dar are good brands to experiment with.

Note: Store inks and photo emulsion where they will not freeze, or they will break down and be useless.
 
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