Pocono Mt. Screen Supply, Inc.

Determining Optimum Exposure

Question

Why is optimum exposure important?

Solution

Screen printing is dependent on optimum exposure time at the beginning of the process as a means of guaranteeing quality and performance at the end. Since probably 99% of incorrect exposure of direct photostencils is caused by underexposure, taking a little extra time and care during screen exposure can help you eliminate:

Question

What is optimum exposure?

Solution

Conventional and dual-cure emulsions contain a photosensitive ingredient, the diazo, that strongly absorbs blue and UV light. During exposure it decomposes, losing its absorption and causing the stencil to cross-link. This happens incrementally through the thickness of the stencil. In order for the emulsion in and behind the mesh to be properly cross-linked, it is necessary for the highly absorbing diazo in the emulsion in front of the mesh (i.e., closest to the light source) to be "bleached-out" by having a long enough exposure.

Unlike diazo, emulsions pure photopolymer products contain light-absorbing ingredients that react together to cause cross-linking. Although these products are photographically faster than diazo containing systems, the surface layers still need to be exposed first in order to properly cure deeper parts of the stencil. Remember, the cross-linked emulsion that encapsulates the mesh fibers gives direct stencils their superior durability, and this vital "inner" part of the stencil is most affected by underexposure.

As most of you know, underexposing stencils on purpose as a way of guaranteeing high resolution for fine-detail printing is a fairly common practice. While this may make a noticeable difference in resolution capability under adverse conditions (i.e., a low-resolution emulsion on white fabric), today's higher quality emulsions, when used on a correctly dyed fabric, are capable of resolving finer details than most inks are capable of printing. As long as you use high quality materials in the screen-making process, resolution should not be a factor in choosing exposure time. The optimum exposure time is always the point where the stencil has been fully cross-linked.

Question

What affects optimum exposure?

Solution

If you asked 100 screen makers to list the variables that affect exposure time, chances are they would all come up with basically the same answers. If, however, you asked them to arrange the items in order of importance, you would probably end up with 100 different versions. Basically, you should examine six different variables, listed in the approximate order that they affect screen exposure time:

Question

How do I quickly and accurately predict optimum exposure time?

Solution

While there are several options to predict optimum exposure, (i.e., the "color-change" method), there is a new method that eliminates lengthy and less accurate testing procedures that are difficult to interpret. By measuring the output of an exposure lamp in a narrow spectral band (i.e., violet/blue) where the photoemulsion is at its most sensitive, it is possible to predict what the required exposure time will be. Through extensive testing, manufacturers of stencil materials are able to compile the necessary data that relates the exposure dose required by a product when used under almost any condition. The quality control device used in this method is a digital radiometer.