Variables found in the garment-printing process have produced many a gray hair for apparel decorators ever since the first time a screen was lowered onto a T-shirt. Printers spend countless hours in their careers attempting to identify variables and deal with their effects. Variables plague both manual and automatic press setups.
Successful garment screen printing requires the skill to troubleshoot the process quickly and determine what changes take place during printing. Developing that ability is one big challenge new printers face.
Press operators are also under pressure to meet certain production requirements and, in some cases, are paid based on that productivity. This makes the need to troubleshoot and resume production promptly even more critical.
The ability to troubleshoot the printing process effectively is an acquired skill, but one that needs to be developed early in your production staff. Here we’ll consider troubleshooting at the press and review tips for resolving a few of the potentially major issues that are likely to turn up as a result.
The complexity of finding and fixing issues
In order to properly troubleshoot the on-press printing process, you must first have a good knowledge of the potential variables—the shifts or changes that can take place during production. At least 50 known variables affect the garment-printing process, and nearly every one of those variables can affect the quality of the print during production.
One of the greatest reasons that troubleshooting the garment-printing process is so complex is largely due to the fact that we have no standards by which to live. With the exception of the recommendations made by product suppliers and technical publications, the printer is own his own to perfect the process and learn what works.
The first step in troubleshooting on-press issues is to develop the ability to determine that something has changed since the start of the production run. This is best accomplished by keeping the approved strike-off at the unload position of the press and at the end of the dryer. You’ll readily detect an issue once a variation appears between the strike-off and the prints you’re removing from the press.
Troubleshooting screen tension
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.
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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