The authors discuss right and wrong ways to clean screens on press.
Successful screen printing involves doing the simple things well. Cleaning a screen during production is one such procedure that's easy to master, yet many printers do it very badly. And the result is excessive waste, poor print quality, and reduced stencil life.
The normal scenario begins when an image area becomes contaminated with dirt or dried ink. The printer sees the resulting print flaw and stops the press. The screen frame is lifted and the printer assaults the underside of the stencil with a solvent-soaked abrasive cloth. The squealing of mesh can be heard across the shop--it's stencil language for "Stop rubbing so hard! You're tearing my emulsion." Occasionally a fatal "crack" is heard, followed by the sound of a ripping screen and the cry of a press operator: "Curse that #@*~! weak mesh again!"
A truly expert printer rarely rubs the print side of a stencil because he knows that print definition requires a sharp emulsion shoulder at all image edges. Abrasion removes that necessary shoulder, and further abrasion scores channels in the emulsion. Continued abuse actually removes the emulsion, leaving only bare mesh. Any further abrasion at this point results in that familiar shriek from both the screen and the press operator.
When printing fine- or high-line process-color images, the film of emulsion below the threads can be as little as 5-6 microns thick and the mesh itself may have a thread diameter of just 30 microns. While this combination of thin mesh and emulsion is remarkably tough, it will not withstand the violent physical abuse wrought upon it by unthinking printers. So the real key to avoid abusive cleaning is to prevent contamination of the stencil in the first place.
The greatest cause of contamination is poor ink-management practices that lead to dried ink in the mesh. With solvent- and water-based inks, the problem may result from improperly mixed ink, thinner, and/or retarder. Always use the recommended amounts of additives to suit the application. Also, maintain the condition of the ink during the run by frequently adding pre-mixed ink in small amounts. And when using UV-curable ink, take steps to shield the printing screen from all UV light sources--including sunlight shining through adjacent windows.
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