Screen E-Book (.PDF) - MacDermid Autotype

Undercutting from a rough stencilSome brilliant detective work by Dr Eifion Jewell showed that high Rz stencils suffer seriousproblems during exposure. He used sophisticated equipment to measure “tone gain”(i.e. size of exposed dot compared to expected dot size) in the dots on the stencil itself.The scattering from the high Rz stencil made dots smaller and smaller as exposure wasincreased. Here is a comparison between a capillary film and a typical emulsion:Effect of exposure time on tone gain for an 85lpi test positive in a capillary stencil. There isvery little change from increased exposure times because there is very little scattering fromthe smooth stencil surface.The same experiment using a 1+2 DirectEmulsion (high Rz). With increasedexposure time, the scatter from the Rzcaused dramatic filling-in of the dots,i.e. severe undercutting.68