Screen E-Book (.PDF) - MacDermid Autotype

A modest Rz gives amodest absolute spotgain and modest 13%dot gain…However, a typical low-Rz stencil is highEOM and a high EOM gives you a biggerink deposit which also spreads (slumps)more to give you dot gain.Worse even than that is that the thick dotsfrom a high-EOM stencil give you a roughsubstrate when you print subsequentdots, so you get even more dot gain thatis mostly out of control because the % ofprevious dots varies across the print.We’ve seen this situation time and timeagain. The only cure is a low-Rz, low-EOMstencil that reduces dot gain on the firstprint (no spreading via the Rz) and, by givingdots that aren’t so high, reduces theeffective roughness of the stencil for subsequentcolours.Because of the high Rz we have dot gain (shown in red). This happens to be30µm of absolute dot gain, giving a 13% dot gain for a 50% dot.… but for a high lpi printthe same spot gain givesdisastrous dot gainRemember that a small absolute increasein dot size will give you a small dot-gainon a low lpi image and a large dot-gain ona high lpi image. This is shown in the DotGain Modeller from the DSP:84