Using Quality Control to Manage Misprints
If high rates of seconds are increasing your costs and creating unwanted downtime, have a look at the quality control remedies Davis discusses for each stage of the garment-printing process.
Misprints—often called seconds—are a fact of life in screen printing. They’re even experienced by large, highly automated garment-printing facilities that use standardized operating procedures. However, while many shops throughout the industry are able to maintain a misprint factor near or below 3% (typically considered the industry norm), standardized operations typically enjoy misprint rates of 1% or less.
Regardless of the misprint level you face in your shop, it is still a good idea to conduct system-wide quality-control audits to determine which areas of the operation need improvement, what the specific defects are that lead to misprints, and what required procedures are needed to eliminate those issues in the future. This month, I’ll discuss how routine quality control can improve your shop—from prepress to finishing.
No matter how proficient your employees are or how solid your workflow is, you’re bound to encounter a job run that challenges your normal printing parameters. When you suspect that the artwork is the culprit, it is best to have the artwork reviewed by the production management for guidance. They may know the best way to engineer the artwork for a productive printing run. Although many graphic artists resist seeking additional input on their handiwork, getting a second opinion is key should there be any doubt about the art.
The ways in which art is engineered may expose issues you have in press maintenance. Can the printing equipment hold the artwork’s required registration tolerance? If you properly maintain your equipment, the press should hold registration to ±0.001 in. Facilities that know their limitations may choose to compensate for press deficiencies by modifying the artwork with traps, spreads, and gaps. Although you can engineer the artwork to bypass certain press issues, the true solution in this case is to eliminate the variable through proper press maintenance.
Facilities that have art staff who understand how the artwork is separated and then reproduced on the press are able to print the difficult graphics on a regular basis with few problems. If the people in the art department have a solid grasp of the influence of mesh counts, inks, squeegees, and substrates, they’ll be able to prepare and separate graphics specifically for the garment screen-printing process.
The key to consistently producing screens that deliver the quality and durability you need is to set the proper screenmaking procedures in stone and ensure that they’re always adhered to. This applies to procedures for tensioning, cleaning, reclaiming, degreasing, coating, drying, exposing, and developing your screens.
In the statistical process-control audits I’ve conducted in the past, I found that a large percentage of the seconds that were produced resulted from a defect of one kind or another in the screens. Although a great number of these defects were actually pinholes that could be repaired, the fact remained that seconds were generated and production time was lost.
Pinholes are most often generated by one of two causes: poorly degreased and rinsed screens or lint contamination. In the first case, the emulsion’s ability to adhere properly to the mesh is impaired, resulting in breakdowns on the press. In the second scenario, lint settles on the screen, which results in pinholes that are invisible during the screenmaking process but open up under the stress of printing. Again, both situations are controllable.
Bubble entrapment generated by improper coating procedures is another contributor to pinhole formation. Most bubbles that are caught within an emulsion coating result from coating the screen too fast, which allows air to be trapped between the emulsion coatings on the coat and print side of the screens. Once a bubble is trapped within the emulsion coating, it is a matter of time before the wear and tear of the squeegee on the screen’s surface will pop the bubble and produce the pinhole. This is simply resolved by slowing the speed at which you pass your coater over the screen during the coating process.
Finally, improper screen tension can really boost your seconds rates. Although most large garment screen-printing facilities use retensionable frames, some are too quick to neglect the retensioning process. The result is an expensive retensionable frame that holds a screen tension just higher than that of a wooden frame. Improper screen tension also leads to slower production speeds, excessive ink consumption, and poor registration. Some facilities simply do not realize the value of screen tension and chalk up their high seconds counts as part of the process. The issue is simply resolved by routinely following the appropriate retensioning procedures.
Controlling variables on press can sub-stantially reduce reject rates in your operation. As odd as this may sound to some, one of the most important press variables to control is squeegee durometer. Using squeegees with a standard durometer will play a big role in ensuring quality prints and timely execution of a production run. Press operators need to have a thorough understanding of the effects that different squeegee durometers can have on the finished print.
Facilities that make use of multiple squeegee durometers often mix and match different squeegee types on the same job. Doing so results in slower setup times and actually adds more variables to the process, which all leads to a higher seconds rate. The simple solution is to select the best squeegee durometer, blade profile, edge shape, and so on, for each of your most common applications and stick to these squeegee parameters. When you attempt to use a variety of squeegees in the same job, you lose production time.
You should use similarly standardized practices when setting off-contact and when registering screens. Additionally, keeping your equipment clean, lubricated, and well maintained will go a long way to preventing problems that lead to misprints.
Although the dryer is seldom considered to be the tool that generates a significant number of seconds, quality problems do occur—sometimes on a much greater scale than you anticipate. When you fail to properly maintain the dryer and carefully adjust its temperature and belt speed for each job, you’ll notice more and more scorched or undercured prints coming into the picture.
Conducting a wash test will determine whether the prints are undercured. Garments with this problem can usually be cured again at the proper levels to overcome the problem. But once a garment is scorched, it is toast.
A properly maintained dryer will not require much adjustment for standard wet-on-wet printing, as long as the appropriate and regular maintenance on the unit takes place. In many cases, this is little more than a routine cleaning of the air filter and air knives on forced-air drying units.
Seconds to spare
Too many printers react to trouble by making random adjustments to their equipment in hopes of producing fewer seconds on a particular job. You can exert a much greater deal of control over the quality of your work by implementing quality-control procedures and eliminating the variables they bring to light.
Rick Davis is the president of Synergy Screen Printing in Orlando, FL. A 27-year veteran of the textile-printing industry, Davis is a member of the Academy of Screen Printing Technology 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.