Screen E-Book (.PDF) - MacDermid Autotype

With a larger dot, there is much more ink ready to be printedWhen the stencil is nicely in contact withthe substrate, a certain amount of ink isprinted. When the stencil is held above thesubstrate by the presence of a precedingdot then more ink must be printed.The definitive study of this effect was carriedout in 1999-2001 by Eifion Jewell’steam at Swansea University and Autotypereported on the implications of that workin the 2001 SGIA magazine. Their studyshowed that the amount of gain dependedon the amount of dot underneath.A small dot gave a small dot gain, a largedot (i.e. something approaching a solid)also gives a small gain, and intermediatedots give the largest gain. This means thatthere is no simple correction process thatcan be provided to a print to compensatefor the effect. So we can guarantee that wewill have dot-on-dot problems and that noprint sequence can be perfect.Now we know we aren’t looking for perfection,can we at least find a good compromise?That was the function of our study.Basic rulesThere is no point in doing such a study ifyou don’t have the basics in place. Theseaspects of printing 4-colour jobs are entirelyunder your rational control. If you don’tdo these then your nightmare with thedot-on-dot effects will simply be worse.1.- Use a low EOM, low Rz stencilIt’s now widely appreciated that a low EOMgives you the smallest dots and thereforethe minimum dot-on-dot gain. High EOMregularly gives “skipping” in addition to terribledot-on-dot gain. As “skipping” oftengets confused with moiré a printer with ahigh-EOM stencil can get very confusedin trying to sort out the various problems.The rule is simple – never do 4-colourwork with a high-EOM stencil! It is alsoappreciated that a low Rz (stencil roughness)is needed to stop squeegee-inducedgain. This gives the second rule – neverdo 4-colour work with a high Rz stencil! Ifyou have a stencil with both low EOM andlow Rz then your mono-colour prints areunder excellent control (dot gains typically< 10% and with a total immunity to squeegeesettings, print speeds etc.) and yourdot-on-dot problems are minimized. Weused Capillex CP with a 3µm EOM and a53