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| Posted on 3/15/06 at 11:54 AM | |
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Hello Susi, It is always difficult to give short answers to questions on the message boards, as there are so many variables that may contribute to such problems, and to accurately assess the problem we often need more information from you than what is supplied in the question posted to the boards. Often there are a few uncontrolled variables occurring which contribute to the problems. With that said, let me try to address them. First, lets discuss the ink "sweating" through the emulsion. I believe this is caused by under exposure. Other possible contributing factors include improper emulsion drying, using expired emulsion, and coating the emulsion too thinly. When printing with water-based ink systems, it is imperative to dry the coated emulsion in low humidity environments i.e. below 50% RH and preferably at 30-40%. Then keep the screens dry until they’re exposed to avoid re-hydration. The next prerequisite to printing with water-based inks is to expose the screens for as long as you can, and develop them with a pressure washer (1,000-1,500 psi) on fan setting at about 18 inches from the substrate side of the screen. A short final wash from the squeegee side will remove any residual emulsion. Exposing the screens as long as possible assures complete exposure and maximum water resistance. Developing with a pressure washer assures uniform washout and opens small details. When Poly-Plus SRX, as with many emulsions, is dried properly and exposed fully there is no risk in developing with a pressure washer. An exposure calibration test using a commercially available exposure calculator, or a series of stepped exposures using your own artwork, will help you determine where your proper exposure times truly need to be. Fully, or even over exposed screens, contrary to popular belief, actually reclaim easier than under exposed screens. They also leave less ghost haze images, don’t break down on press, and result in far fewer pinholes. Once sensitized Poly-Plus SRX has a pot life of 3-4 weeks. If kept in a refrigerator, you can extend it by several more weeks, however its water resistance degrades with time. You mentioned that the emulsion has been sensitized for four or five months. This is way past its usable life and should be discarded. This is also contributing to your inability to reclaim the emulsion. Finally, in case you haven’t fallen asleep yet, if your emulsion coating is very thin and even slightly underexposed (not fully hardened) it is susceptible to premature softening, especially on the squeegee side of the screen, which may allow the ink to permeate through the screen causing the “sweating” problem you describe. Now are you sorry you asked the question? In any case, thank you for the inquiry and I hope I’ve been able to shed some light on the subject. Dave Dennings KIWO Inc. Phone: +1 (800) 549-6872 Email: daved@kiwo.com | |
  
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