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
Of all the variables that can change and cause problems on the press, those relating to the screen pose the most trouble. When your screen changes during the course of a print run, so do your prints.
Screen tension is the greatest variable you’ll notice on the press. Should the tension of the screen drop during production, the following issues can occur:
1. Hue shifts in process printing caused by increased dot gain, which is created by increased squeegee drag.
2. Insufficient coverage caused by a decrease in the screen’s shearing capability.
3. Image shadows at the stroke-end of the image, also caused by squeegee drag.
4. Loss of registration caused by screen instability.
Slight changes in tension when printing flat color can have little to no effect on the quality of the print. But in process-color printing, changes in screen tension are easily identifiable in the hue shifts of the print. Changes in memory colors, such as flesh tones, wood grains, aluminum, or glass, are the easiest to identify. Any light color shades made up of primarily highlight dots readily show any change in screen tension. In order to compensate for low tension, you must increase the squeegee pressure and/or increase off-contact. Or you could try to prevent the problem up front by stabilizing the screen’s tension through the use of a retensionable frame.
Identifying and troubleshooting moiré
Moiré is one of the most frustrating aspects of dealing with the printed dot pattern, whether printing a single halftone or multiple halftones in a process-color job. Moiré can appear in three forms:
• Primary moiré An interference between two or more halftone patterns as they appear on the positives.
• Secondary moiré This appears as an interference between the halftone line count and angle in relation to the mesh count being used.
• Tertiary moiré You can easily spot tertiary moiré as a conflict between the halftones produced on the screen and the knit pattern of the garment.
Primary moiré is rather straightforward to identify by a simple check of the positives prior to screen exposure. If you align the positives and get any kind of pattern other than the desired rosette arrangement, you will know right off that the undesired pattern will reproduce on the screens and, consequently, on the garments.
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