Screen E-Book (.PDF) - MacDermid Autotype

SymptomsOnly seen when you print the 15° colour.CauseThe clash between dots at 15° to eachother.IdentificationIf you don’t have a 4-colour print, then it’snot 15° moiré (though if you are printinga duotone, make sure the separations areat 30°) With a loupe or microscope look atthe moiré area of your print. If you see regularrepeats every 3.8 (i.e., roughly 4) dots,then it’s 15° moiré. Sometimes you see itas a pattern of dots, sometimes as lines. Itsintensity will vary across the print.CureIdeal. Replace the 15° component witha different frequency (dangerous unlessyou know what you are doing) or, as somemodern RIPs do, replace it with some sortof stochastic equivalent.OK. Reduce its visibility by switching the15° component to a less visible colour(usually yellow).OK. Reduce its visibility by printing the15° component last. With luck it will finda more random surface because of thevariety of dots underneath and the moiréwon’t be so intense.OK. If the moiré is seen as lines rather thandots, make your printed dot-shape rounder.This will make the lines weaker and hopefullythe dots won’t be so visible.OK. If the moiré is seen as lines ratherthan dots the other possibility is that thesqueegee is amplifying the problem in thedirection of the squeegee stroke. Reducesqueegee pressure and/or use a low-EOM,low, controlled Rz film. This reduces the intensityof the moiré. This is not just Marketinghype. It really helps at the fundamentallevel.OK. If you have a low Rz stencil, use a thinnerink.OK. If you have a high Rz stencil, try to reducethe Rz without going too thick withthe stencil, then use a thinner ink.3 Moiré look-alikesAnything which looks like an unwantedregular pattern in parts of a print gets labelledas moiré. But there are plenty of‘moirés’ that aren’t. But you still need tosolve them, so here they are.3a. SkippingSymptomsAn unpleasant ‘dotty’ area, usually in darkerregions of the print. Often found whenyou back off squeegee pressure to avoidflooding in darker areas.CauseInsufficient squeegee pressure (or, equivalently,ink too thick) when printing one dot101