Coudray walks through screen and stencil preparation, identifiying techniques for producing accurate and durable screens that will last beyond 100,000 impressions.
Complete exposure is critical. A low-output UV light source will not get the job done. Long-run screens require the efficiency of metal-halide or trimetal-halide lamps. The rich UV output of these bulbs, coupled with their high watt density, result in complete polymerization of the diazo/photopolymer sensitizer and resin of the emulsion. The long molecular bonds that result are very tough and will hold up to much abuse.
The length of exposure is also critical. Any premature or underexposure of the emulsion will cause the thin emulsion coating on the squeegee side to wash off. When this happens, the surface is now extremely vulnerable to squeegee/flood friction and ink/solvent penetration. These factors will cause a screen to fail quickly, sometimes in less than 500 impressions.
With the screen washed out, blotted (or vacuumed,) and dried, it is time to apply blockout. Two very effective application techniques can dramatically lengthen the life of the screen. The first is to use emulsion as the blockout agent, which results in a very strong final stencil.
Technique number one uses sensitized emulsion as the blockout. We card on the emulsion as we would normal blockout. Next we apply double and triple thickness coatings up and down the frame where the ends of the squeegee and floodbar will track. We are reinforcing these typical failure points to increase resistance. And we must make sure to dry the screen in the yellow safelight of the coating room--it is important to have a sensitized image when the emulsion is dry.
When the emulsion is completely dry, we take the screen outside and post expose it to the sun for a minimum of 2 min. The longer the screen is left outside, the better off we will be. If we cannot use the sun due to weather or climate, we can post-expose the screen for a minimum of 300% of the normal exposure in the exposure unit. These post exposures will greatly strengthen and improve the friction resistance of the screen.
Did you enjoy this article? Click here to subscribe to the magazine.