The authors discuss the most commonly overlooked variables in the screen-printing process and how to control them.
Companies that use screen printing as part of a broader manufacturing sequence tend to treat it as an engineering process. Their aim is to understand where problems are likely to occur so that the problems can be avoided. As consultants, we encourage this approach to screen printing, because without it, printers tend to forget the fundamentals and lose control of the process. This month, we'll look at several important fundamentals as they relate to key variables in screen printing.
Ink has perhaps the greatest influence on success or failure in screen printing, namely because of the number of ink-related variables printers face. Some of these variables include viscosity, shear, surface tension, pigment particle size, speed of solvent evaporation, and temperature. Of these variables, one that printers often overlook is the evaporation speed of solvents in the ink, which, among other things, determines the likelihood of ink drying in the stencil.
If you print with conventional UV-curable ink, evaporation is not as important an issue, but shielding the ink from excessive amounts of ambient ultraviolet radiation (from sunlight, fluorescent tubes, etc.) is important. Water-based UV ink is best treated as a solvent-based ink system, since on-screen evaporation of water and co-solvents is constantly occurring, and it can alter your ability to print the ink successfully.
Regardless of what type of ink you use, a simple but effective rule is to add little ink to your screen at any one time, but add it often to maintain the correct working conditions in the ink. Besides concerns about ink drying in the stencil, large amounts of ink on the screen can lead to deflection of the screen, altering its off-contact height, and consequently, its snap-off characteristics. It is also very expensive to put too much ink on the stencil, as unused ink will lose solvents or pick up contamination.
The purpose of the squeegee is to bring the stencil into contact with the substrate and cause the ink to release from the mesh. Any other effects of the squeegee are likely to have a negative impact on the printing process.
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