The stencil has absorbed excessive light which is now cross-linking the photoemulsion behind the positive image.
Establish proper exposure times using the Chromaline Exposure Guide.
Light undercutting
The light source is too close to the screen. The angle of light is able to reach behind the positive and cause excessive cross-linking. If possible, move the light source back and readjust exposure time.
Mesh color
White mesh easily transfers light along it's threads. Excessive light will cross-link photoemulsion along the threads, even behind the positive image. This will result in a jagged image. Instead, use yellow or orange mesh, or a higher mesh count.
Mesh count too low
Higher resolutions require higher mesh counts. If a mesh thread is as thick as a halftone dot (50-99%) on the stencil, and the thread runs through the dot, the ink will not print that dot. In general, higher mesh counts have thinner threads which allow for greater resolution and greater halftone ranges.
Positive image
A stencil cannot produce greater resolution than the positive image. For example, film positives render better resolutions than do vellum positives.
Pre-exposed stencil
If the stencil has been subjected to stray light before exposure, cross-linking may occurring.
