Control Points for Quality Screens, Part 1
Davis reviews the material and procedural variables that have the greatest influence on screen quality.
My favorite screen-printing proverb is, "The quality of your printed products is only as good as the quality of your screens." In order to achieve high-quality screens, you must control the variables involved in creating and maintaining those screens. In this installment, we will review the first five of ten critical parameters that dictate the quality of textile-printing screens. 1. Mesh selection A broad range of mesh fabrics are available today, and every mesh salesman will tell you why his are better than the rest. What this means is that you have to carefully evaluate the brands and types to determine which fabrics work best on the types of jobs you print and in your production environment. Except in a few specialty applications, polyester monofilament mesh is the fabric of choice for garment printers. When you experiment with a new mesh brand, follow the manufacturer's tensioning and usage guidelines, then run it through your normal production sequences. Measure tension before and after printing and reclaiming, and evaluate the print quality delivered by the fabric. When possible, compare your findings with the performance of similar mesh fabrics that your facility has already found to be acceptable and uses regularly. You may end up paying a little more for a higher-quality mesh, but the benefits it brings to production and print quality will be worth it. 2. Frames and mesh tension Frames serve as the support structure for the entire print area of the mesh. The highest level of tension that the screen will be able to maintain--and, thus, the reproduction quality it will provide--is influenced greatly by the type and quality of the frame used. In textile printing, consistency and repeatability are, in my opinion, best supported with retensionable frames, which offer excellent stability and adjustability. Only retensionable frames allow you to retension screen mesh once a production run is completed and return the fabric to the optimum tension level. Since mesh is typically glued to rigid aluminum and wood frames, such screens cannot be retensioned. As a result, each time you reuse the screens for another production run, you'll experience a drop in the tension level and a corresponding drop in print quality. Your only option is the costly solution of remaking screens more frequently, tearing off the old mesh and replacing it with new fabric. Only on retensionable frames can you return mesh to higher tension levels and maintain print quality as you reuse your screens for multiple production runs. Tension level also influences your ability to reclaim screens. Screens with lower tensions will give in to the force of pressure washing. The effect is much like spraying a loose hanging curtain. The result is that you'll remove less of the residual ink caught in the knuckles of the mesh. Higher tension screens help the mesh remain rigid during the pressure washing, exposing more of the residual ink in the fabric for effective removal with a pressure washer. 3. Degreasing Degreasing of the screen is perhaps the most understated procedure involved in screen preparation. Proper degreasing and rinsing of the screen is critical because it influences how well the stencil material will adhere to the mesh. With complete adhesion, you'll achieve better image resolution and accuracy and greater stencil durability during production. After application of a degreasing agent, both sides of the screen require vigorous scrubbing and a thorough rinsing prior to stencil application. Rinsing is particularly important because any residual degreasing agent will also deter from the emulsion's ability to adhere to the mesh. Most printers experience problems from improper degreasing because it is an easy process to "skimp" on. Because the process is relatively unexciting and repetitive, screenroom personnel can easily miss areas of the screen or frame during rinsing. Yes, frame rinsing is also important because any residual chemicals left on the frames will drip onto the screen and cause the stencil to break down during production. Once you have to stop the presses to correct such screen defects, the time, effort, and expense you invested to prepare the screen are lost, as is the profitability of an uninterrupted production run. 4. Manual pressure washers Many shops suffer from screen-quality issues simply because they use underpowered pressure washers during reclaiming. Larger shops may be more likely to have the proper unit for the job (or may even use automated screen cleaning/reclaiming systems), but they often fail to maintain their washers properly and can't ensure they're getting the proper pressure levels. Pressure washers range in size, power, and cost, but this is one area where being cheap definitely won't pay off. If you purchase a pressure washer based simply on low cost, you can expect to see screen-quality issues soon after. Pressure washers that are underpowered leave residual ink and stencil particles on the mesh during the reclaiming process. These residual particles will lead to poor resolution and ghost images when you attempt to create a new stencil on the screen. And they prevent the stencil from bonding correctly with the mesh, leading to premature stencil breakdown during production. If you use pressure washers, I recommend units that deliver pressures in the 2000- to 2500-psi range. Units that deliver less than 2000 psi may leave residual materials in the mesh. But this doesn't mean you should invest in Tim Taylor's favorite 10,000-psi Binford model, either. Most screens can't withstand water pressure above 3000 psi without tearing. Play it safe and find a system in the 2000- to 2500-psi range. 5. Screen-coating and drying As with so many other aspects of textile screen-printing, there are no rules or standards that spell out the right way to coat and process a screen. So screen printers are left to the trial-and-error method to find the procedures that work best in their operations. Many textile printers use the 2-1 coating method, which means they coat the print side of the screen twice, then finish with a third coat on the print side of the screen. They then dry the screen with the print side down. This process may vary somewhat from shop to shop, but the actual coating method is really not the key issue. More important is that each of your screens be coated the same way to deliver consistent and predictable results. In many shops, coating technique varies with every individual in the screenmaking department. Some larger shops avoid this problem by investing in automated screen-coating systems. These units remove the human variable from the coating process and ensure identical coating characteristics on every screen they process. The drawback is that automated coating equipment is often too expensive for smaller shops. For such operations, the only option for consistent results is to develop standardized coating procedures that all screenmakers follow. To be continued The care you take in preparing and maintaining screens not only impacts the quality of your prints, it also influences the productivity of your operation. So, to make sure screens meet your performance and quality requirements, standardize your screenmaking, cleaning, and reclaiming procedures. Next month, we'll continue this discussion by examining the five remaining screenmaking variables you must control.