6. Expose Emulsion
Exposing PLUS Emulsions and Choosing the Right Exposure System
Under exposure is the single biggest cause of stencil breakdown on the press.
Using the Alkemi Exposure Calculator to calibrate your stencil is a good practice to optimize the exposure with Plus emulsions and Capillex capillary films.
Once the Plus emulsion has been coated on the mesh and thoroughly dried, it is ready for exposure. The objective with exposure is to penetrate the photo-stencil material all the way through to the mesh to ensure hardening around the filaments. Proper exposure of quality emulsions and capillary films provides good stencil adhesion while accurately reproducing the detail on the film positive.
If the exposure is not enough and the whole layer is not penetrated by the UV light, the stencil may wash completely away on washout. More often, the stencil may be intact after washout, but break down or become tacky during the during the screen printing process. Both of these symptoms indicate under exposure.
As a general rule, "direct" systems do not suffer from a lack of flexibility on over-exposure, in fact, the adhesion to the mesh is improved. However, the resolution and definition of the image deteriorate as the stencil is over exposed.
The following 3 diagrams on the left outline the effects of under-exposure at 1/4, 1/2 and 3/4 of the optimum exposure for a Diazo sensitized Alkemi Plus emulsion.
< Optimum exposure
At full exposure all the emulsion is fully hardened and the stencil will provide the optimum resistance and durability.
Pro Tip: Light Source
As a general rule, the minimum distance between the exposure lamp and the vacuum frame should be equal to the diagonal of the area to be exposed. This will ensure fairly even illumination over the exposure area.
SPECTRAL OUTPUT OF LIGHT SOURCES
In general terms a light source with a spectral output in the region of 340-440 nM (nanometers) is suitable for exposing photo-stencil materials. Some light sources have a continuous output, whereas others have a discontinuous or line emission. Modern light sources such as metal halide lamps have a line output and it is important that these emissions coincide with that portion of the sensitivity curve of the photo-stencil materials, giving the best results in terms of tanning and image contrast i.e. diazo bulbs or photopolymer bulbs.