Screen E-Book (.PDF) - MacDermid Autotype

As exposure time increases more of thelight emitted by the lamp passes throughthe stencil as less is used in the crosslinkingreaction. If only 418nm light is consideredthen the reaction can be seen tobe complete when the light reaches it’smaximum:Definition is the reproductive quality asregards, for example, the straightness ofedges of print. There are no real gains instencil durability from over exposure.The following diagrams outline the effectsof under-exposure at 1/4, 1/2 and 3/4 ofthe full exposure for a direct stencil.1/4 full exposureEFFECTS: During washout the image wouldappear and the stencil would probablystart to come away during washout.In this example the light reaches a maximum at 92seconds and so this is full exposure.1/2 full exposureIf exposure is insufficient and the wholelayer is not penetrated by UV light, thestencil may wash completely off the meshduring developing, or the stencil may beintact after washout but break down, orbecome tacky during the print run.The resolution and definition of the imageare compromised by over exposure as excesslight is scattered under the positive.Resolution is taken to be the finest limit ofreproduction achievable.EFFECTS: The stencil would probably bedamaged by a strong washout spray. Theemulsion appears soft on the squeegeeside.63