Screen E-Book (.PDF) - MacDermid Autotype

The lower the solids, the higher the Rz. If wehad a 25% solids emulsion in the exampleabove then in the holes it would shrink to25µm below the mesh surface and in thesolid areas it would shrink to 12.5µm, givingan Rz of 37.5µm.The advantage of wet-on-wet coating isthat it is quick and easy. The disadvantageis that it doesn’t do all that much. As youput on subsequent coats you drag awayplenty of the previous coats. The benefitsto both Rz and EOM are modest.Wet-on-dry coating involves much morework. Each drying stage takes up precioustime. But the results speak for themselves.You can get much closer to the ideal of alow-Rz and low-EOM if you are preparedto take the time to do multiple wet-on-drycoats.A low-solids emulsion gives a high RzA 3D view of high RzHere’s a 3D view of how Rz is caused byshrinkage above the holes in the mesh:Our own experience is that by far the bestway to achieve a low-Rz, low-EOM stencil isto use a capillary film especially designedto give that balance of properties. It’smuch faster and more reproducible thanall those wet-on-dry coatings!DryingIt’s amazing how many printers don’tunderstand that you can’t dry a stencilwithout some warmth and some flow ofair that isn’t already saturated with watervapour. The theory of drying says that airflow is usually more important than meretemperature. Just think of the differenceof drying your hair with a cool hair dryer(plenty of air flow but little heat) and withan electric heater (plenty of heat but littleair flow). The hair dryer wins every time.And remember that overheating the stencilwill cause it to fuse and give poor reclaim.77