Screen E-Book (.PDF) - MacDermid Autotype

Incidentally, note that the ratio of TPI/LPI (355/71) is exactly 5, yet the modellershows that at the other angles there is nosignificant moiré – so an integer ratio isnot what’s important.There has in the past been much puzzlingabout how accurate the mesh needs to beto avoid mesh moiré. We now see wherethe confusion has arisen. Exactly the sameprocess can give huge differences in sensitivityto the mesh. When you add on theother factors (thread diameter, ink colour,Rz/EOM of the stencil, type of ink) no wonderthe situation has seemed so unclear.Figure 6a. A change to a 380(150) mesh lets you print the37.5° with no problem, butthe 67.5° would be a disaster.By using such a moiré calculator it’s possibleto have a good idea which, if any,colours are going to give you problems.What are you going to do about it?Sticking with the maths, there’s one simplething you can do (and many printers arealready doing it). If (as in the case above)you have a bad moiré just for one colour,use this mesh/lpi combination for the otherthree colours and change your mesh forthis colour.A quick test on the modeller shows thata change to a 380 (150) mesh solves theproblem for the 37.5° colour (but don’tchange them all over as the 67.5° colournow has a strong moiré).If you don’t want to fight the maths thenyou have to work with the physics.The maths only says if you might get amoiré. The visibility of it partly dependson what you do about it. If you use a thinthread, if you have a low Rz and low EOMstencil, if you use a relatively free-flowingink, if you print the yellow at the worstangle for moiré then you will probablyget away with it. You might see the moiréon the single coloured print, but it might112