Using the Alkemi Exposure Calculator - Part II
Reading the Results of the Exposure Test
I. Direct Stencils
The correct exposure for capillary films, direct emulsions and direct/indirect stencils is the point at which the entire thickness of exposed stencil is fully hardened by the UV light. This process of exposing/hardening is accompanied by a color change in the stencil.
EXAMINING THE STENCIL
The Stencil will show variations in color from one factor to the next. Follow the color change from the lightest to the darkest until it stops.
The factor where the color change stops is the column that represents optimum exposure
Once the correct factor has been chosen. multiply the factor by the test exposure time. This gives the correct exposure time (or number of units) for that particular stencil/mesh/light source combination. Example:
Correct Factor X Test Exposure = Correct Time
.7 X 10 Minutes = 7 minutes
Underexposure If there is still a color change between Factor 0.7 and Factor 1. this indicates an underexposed stencil. Double the original test exposure, and repeat the test.
Underexposure can result in a weak stencil with poor mesh adhesion and reduced resistance to printing inks, wash up solvents etc. It can also make the stencil more difficult to reclaim.
Overexposure leads to loss of fine detail. but will not compromise mesh adhesion. or solvent resistance.
In the below example, we see that the stencil is properly exposed because there is no color change difference between the 0.7 factor and the 1.0 factor. We see that the 0.25 factor and 0.33 factor are underexposed, leading to a distorted and inaccurate stencil.
II. Indirect Stencils
The process for testing indirect stencils is very similar to direct stencils, but the test is performed off the mesh.
Process the stencil as normal but dry unmounted.
There are two ways to read the results:
Carefully cut out solid areas of the emulsion from the columns. peel them from the base and measure with a micrometer.
Overexposure will lead to reduced mesh adhesion and loss of fine detail.
Underexposure will produce a weak stencil.
An alternative method is visual inspection of the resolution of the five columns. Inspect the centers of the targets for filling in of the image. As the exposure time increases. resolution will decrease. At optimum exposure. the films have different resolving powers:
Note: Resolution capability is diminished by the use of multi-point light sources (e.g. fluorescent tubes). or shorter than recommended lamp to glass distances.
|Film||Micron Resolution||Inches Resolution|
Part III provides screen printers with insights into reading fine lines and halftone dots following an Alkemi Exposure Calculator test.