01 Aug 14 Tips for Better Stencils
While everyone has their own thoughts about everything about screenmaking, there are Best Practices in place that will work for everyone if followed correctly.
by Bill Hood, ASDPT Fellow
Just like other variables in the screenprinting process, coating, drying, exposure, and development of the stencil must be controlled to attain the best tonal-range reproduction and print detail quality. Use a 21-Step Stouffer Scale on every stencil you make to assure that you have the right exposure every time!
1. The distance from the stencil to the lamp should be the diagonal of the emulsion area to be exposed. Douthitt has an exposure unit that has a proprietary reflector that is stated to be able to provide full coverage of the emulsion area at a much shorter distance, but I have yet to see it work in person, so I cannot recommend it.
2. The emulsion must be dried to a moisture level of 4% throughout the entire thickness. This requires the use of a non-destructive moisture meter.
3. The UV-A spectrum should be between 380 and 420 nanometers. And, yes it has to be UV-A to expose emulsion properly. Assure that your lamp is emitting UV-A.
4. Most work lamps do not emit UV-A. If you are using a lamp that is emitting UV-A and it has a glass cover you should be aware that the glass will block a percentage of the UV-A from reaching the stencil.
5. The higher the UV energy output, the shorter the exposure time, and the better the detail that can be attained.
6. You can lessen the development time and water usage by wetting the image area for a few minutes to pre-soften the diazo and dual-cure emulsions. You should never pre-soak an SBQ emulsion as they are much softer and will absorb the water too quickly. Likewise, you should not pre-soak thick film emulsions. Pre-softening can be done with a wet sponge applying moisture to both sides of the image area or by using a dip tank for a few minutes. However, if the cross-hardened emulsion is allowed to stay in contact with water for too long, it too can become softened as it absorbs moisture.
7. Water used in the development process should be between 60˚F and 90˚F (15˚C and 32˚C).
8. Use a 2000 psi or less for stencil development. You can use a 3,000 psi for reclaiming.
9. If the emulsion is fully cross-hardened, you can use a pressure washer with a wide fan spray nozzle (not a narrow nozzle), held at 12- to 18-inches (30cm to 45cm) from the emulsion, and still hold excellent detail. The higher the pressure the more distant the nozzle should be from the stencil. Never aim the wand at the emulsion and then pull the trigger. The initial blast may be more intense on some units and wands. Always start the spray off to the side of the frame and then move into the frame and mesh.
10. The screen should be developed from the substrate side using pressure to ensure that tiny areas of emulsion are not pushed away from the squeegee side.
11. Assure that the emulsion on the squeegee side is not slimy and if so, it was underexposed.
12. Finish by rinsing the squeegee side with only low pressure.
13. Dry the developed stencil in a dust-free environment.
14. Post-hardening of SBQ emulsions will provide a small advantage, but it is not effective on diazo stencils and provides little benefit on dual-cure emulsions, as with diazo and dual-cure emulsions the unused sensitizer will literally be washed away in the development process. With SBQ the post hardening needs to be 10 times longer than the exposure and the result will only provide 15% in improved resistance.