The art of troubleshooting relies heavily on the science of identifying and managing variables. Find out how to develop the skill to spot problems with screen tension, moir
By Rick Davis
Many shops have all but eliminated secondary moiré through the use of proper screen tension and mesh counts. If you’ve produced the separations with the respective halftones at the proper angle and use the proper mesh at the correct (stabilized) tension, the issue of secondary moiré should virtually disappear from your garment-printing work.
Tertiary moiré is most commonly seen during production and is the hardest to identify. This is especially true if you’ve missed any aspect of managing the primary or secondary moiré in your workflow. The first aspect of on-press moiré to consider is the repeatability of the irregular pattern. If the pattern remains consistent from platen to platen, odds are good that the cause is the screens and/or separations. Should the pattern shift in appearance from platen to platen, the effect is most likely caused by a clash between the knit of the fabric being printed and the halftone angles of the separations. This scenario assumes that the platens are level and screens are parallel to all platens.
The variables involved at this point include the line count of the art, halftone angle, and the knit of the garment. If you have come this far into a process-color production run, your best bet is to look into an alternate garment that has a higher fabric mass or stitch count in order to resolve the problem.
Selecting the proper fabric for process-color printing is crucial, because there are as many garment constructions available as there are mesh counts. Garments with high stitch counts per inch (1000 or more) work best when printing process-color work, because the fabric offers minimal free space on its surface to interfere with the halftone or line count.
Other resolution issues
Resolution and registration issues are also some of the more common concerns that hamper productivity for printers during the job run. Those who print with a more-is-better mentality are apt to experience shadows at the squeegee stroke-end of the design where squeegee drag on the image is the greatest. This is to say that the squeegee will pull the screen in the direction of the stroke and cause the image at the furthest point of the squeegee stroke to shadow in the direction of the squeegee action. When this happens, the first, most obvious, thing the printer realizes is that the squeegee pressure is too great and/or the screen tension is too low.
The solution is to maximize screen tension, minimize off-contact, and minimize the squeegee pressure. This situation is common when printing on an automatic press with wooden frames. Wooden frames are dimensionally unstable and, therefore, lend the mesh minimal support. Excessive squeegee pressure is required to pass the ink through the mesh under the resulting low-tension conditions.
Registration variations can come from a number of sources, including unlevel platens, lack of screen-to-platen parallelism, press misalignment, and the easiest of all: loose screens in the press.
As I mentioned at the beginning of this column, the art of troubleshooting is an acquired ability. Learning from experience builds the knowledge base required to properly and efficiently find and address problems that arise on your press and throughout your facility.
Rick Davis is the president of Synergy Screen Printing in Orlando, FL. A 27-year veteran of the textile-printing industry, Dave is a member of the Academy of Screen Printing Technoloy and has a background that spans production management, artwork engineering, application testing, and industry consulting. He is a frequent contributor to trade publications and a speaker at industry trade events.
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