The Benefits Of Emusion Stencil Post-Exposure
Post-exposure can be a useful technique for improving the resistance properties of a stencil. The benefits offered depend however on the type of emulsion used, and can be summarized as follows.
Diazo Emulsion
When a diazo emulsion is underexposed, the developed and dried stencil retains a yellow undercast from the unused diazo. This partially exposed diazo does not wash out of the stencil during developing as it has already reacted with, and become attached to, the polymers and resins that make up the stencil. So after drying it is possible to re-expose the screen, bleach out the remaining diazo and further cross-link the stencil to improve its solvent or water resistance. However, it should be noted that depending on the degree of initial under-exposure, the final stencil, although fully chemically cross-linked, may only be a thin skin stuck to the substrate side of the screen mesh. It will not be as durable and resistant to pinholes as a correctly exposed stencil, where the screen-mesh has been physically encapsulated, front and back, with hardened emulsion.
There is absolutely no benefit to post-exposing a screen made with correctly exposed diazo emulsion, since all the diazo is already used up.
Dual-Cure Emulsion
When underexposed, the situation is the same as for a diazo emulsion in that the unreacted diazo can further cross-link the stencil on post-exposure and improve its solvent and water resistance. However, the difference is that even correctly exposed dual-cures can benefit from post exposure. The reason is that the secondary cross-linking system can be made to polymerize further, even after all the diazo is used up. This usually improves only the solvent resistance, and can also result in easier reclaiming.
Pure Photopolymer Emulsion
Photopolymer emulsions benefit most of all from post-exposure. Unlike diazo, which can be used with 100% efficiency if the exposure time is long enough, photopolymer molecules can be very stubborn. Only a proportion of the reacts very fast, and are responsible for the short exposure times of photopolymer emulsions. The rest of the photopolymer molecules are not aligned correctly and can cross-link only with difficulty. In this case, increasing the exposure time causes a loss of resolution and detail with little payback in terms of improved stencil durability. However, the potential of this unused photopolymer can be realized with a post-exposure. The reason is that during development, when the stencil is wet, some of the unreacted molecules will re-align and be available for crosslinking the second time around, thus resulting in improved solvent and water resistance. In addition, the post-exposure can be made much longer than the original imaging exposure in order to maximize the cross-link density.