Post-exposure, or re-exposing a screen after its dry from rinsing out from  the initial exposure  can be a useful technique for improving the Durability and performance properties of a Stencil. The benefits that in can offer however depend on the type of Emulsion used, and can be summarized as follows.

DIAZO EMULSION:

When a diazo emulsion is under-exposed, the developed and dried stencil retains a yellow hue from unused diazo. This is the basis for the color change method of determining optimum exposure. 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. After drying, it is possible to re-Expose the screen, bonding the remaining diazo and further cross-linking the stencil to improve its Solvent and 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 Squeegee 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 cross-linked, 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 under-exposed, 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-cure stencils can benefit from post-exposure. The reason is that the second cross-linking Photopolymer, the part that makes an emulsion dual-cure, 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 the most 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 them react 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, while the stencil is wet, some of the unreacted molecules will re-align and be available for cross-linking the second time around. Thus resulting in improved solvent and water resistance. In addition, the postexposure can be made much longer than the original imaging exposure in order to maximize the cross-link density.