In this installment of “Controlling the Variables,” I will review the effects of the frame and screen tensioning on the screen printing process. It is the failure to control these variables that cause many screen printers the greatest headaches and, in some cases, much more.
The frame is the base tool for the entire textile screen printing process. There are three basic frame types that can be used in the process: wooden, tubular aluminum and retensionable. My basic philosophy is: “Quality in, quality out.” The objective of this article is to identify and control the variables of the process. You will see by the end that the more control you have over the screen, the more control you will have over the quality of your prints.
Wooden Frames: This frame type is popular due to its inexpensive cost. Although a new wooden screen will work well for a one-color or multicolor print with loose registration, the screen eventually will lose enough tension after a few thousand impressions and the mesh will have to be removed and restretched. In time, once the wooden frame has absorbed enough water and chemicals, it inevitably will have to be replaced. Wood also is susceptible to deflection from the inward stress of the mesh. In time, the frame will have a “memory” from the stress of the mesh and possess a permanent inward bow.
Wooden frames provide an easy way for newcomers to start out on a tight budget. The best part is that they can upgrade to a higher quality frame as the company grows.
Tubular Aluminum Frames: This frame type offers a far more stable platform than wooden frames. Aluminum frames will not give in to the effects of the stress of the screen mesh or the repeated exposure to water and chemicals. Tubular aluminum frames will not wear out over time, although the screen mesh will have a limited lifespan as the tension of the screen drops over time. Although the tubular aluminum frame will produce a screen with a much greater life-span than that of wood, the screen inevitably will have to be restretched.
Retensionable Frames: This frame type offers the most stable and reusable platform for textile screen printing. Impervious to chemicals and water, retensionable frames and screens allow printers to retension the screen following every few thousand impressions. This will drastically increase the life of the mesh and screen, and allow for complete control over the tension of the screen, which is the most uncontrolled variable in the process. For this reason, I will address the benefits of printing with high tension throughout the remainder of this article.
SCREEN TENSION EFFECTS
Screen tension is one of the most disregarded variables within the textile screen printing process. Although it is addressed in countless articles and seminars, many printers do not implement the procedures needed to ensure consistent and proper screen tensions. This includes printing facilities that use retensionable frames.
At some point in the process, the established procedure for retensioning the screens is bypassed for any of a number of reasons. Once this procedure is neglected, it is many times never reestablished properly. From here, the general quality and tension of the screens in the facility starts to drop. Many people think that as long as a degree of “commercial acceptability” is achieved, the standardized procedure of proper screen retensioning in unnecessary. The result is typically greater than many believe. Of the effects of screen tension listed in this article, the off-contact distance has a direct effect on a number of those variables. Proper off-contact distance increases as screen tension decreases, and is needed to ensure a clean snap-off of the screen from the surface of the garment. It is this snap-off that determines the sharpness or resolution of the image being printed. As the screen’s off-contact distance increases, so does the squeegee pressure required to depress the screen to the garment surface, which is required for the proper ink transfer.
Proper screen tensions will allow for minimal screen off-contact, which, in turn, allows for minimized squeegee pressure. This enables maximized squeegee speed.
Having these variables of the screen controlled will result in minimal ink film penetration and thickness. It is here that the hand of the print is minimized, since you have now minimized the ink film thickness.
OTHER VARIABLES
1. Minimized ink consumption is the result of the previously mentioned variables. The thinner ink films will now allow for faster flashing of an underbase or multicolor ink film. Although not previously mentioned, once you have controlled your tension and minimized your off-contact, you can consider the concept of possibly degreasing your overall mesh counts. When properly depositing your ink films onto the surface of the fabric (as opposed to driving it into the fabric), you can easily minimize ink film thickness, which will minimize the overall ink consumption.
2. Screen tension plays a critical role in the uniformity of the stencil thickness across the overall image area of the screen. Lower tension screens will allow for “emulsion pooling” in the center of the screen. This is a result of the emulsion’s weight pushing the screen mesh away from the coating edge. The result is a thicker emulsion coating in the center of the screen’s image areas, as opposed to the edges . Tighter and more rigid screens will result in a more even and consistent emulsion coating across entire image area of the screen. The benefit here is consistent image and halftone reproduction, decreased emulsion consumption, and decreased reclaiming chemical consumption and reclaim time.
3. Maximized screen and stencil life through proper screen tension is a result of minimized squeegee pressure and decreased off-contact. This minimizes screen abrasion caused by the squeegee, as opposed to employing greater squeegee pressure due to lower screen tension.
4. Few people consider the effects that the screen’s tension can have on the opacity of the ink film being printed onto the garment. The truth is that the screen’s tension has a direct effect on the opacity and color intensity of any ink. As lower-tension screens require greater squeegee pressure, the resulting ink film will be depressed further into the fabric, as opposed to being deposited onto the fabric. As for color intensity, consider printing an all-purpose color onto a white underbase. A lower-tension screen will deposit a thicker ink film, resulting in the appearance of a darker color. A thinner ink film being printed through a higher-tension screen will deposit a more even and accurate ink film and true color.
5. Thinner underbase ink films enable faster flashing. The aforementioned thinner ink film being deposited onto the surface of the fabric will require less heat in order to be properly flashed, as opposed to a thicker ink film that has been depressed into the fabric. In such a situation, the fabric also insulates the ink film. The result is an increase in required flash times in order to properly gel the underbase or color. The rule is simple: Thinner ink films require less heat to flash and cure. The benefit here is not only the softer hand, but also an increase in productivity.
6. Since the higher-tension screens allow for the production of thinner and consistent stencil thicknesses, they also will reclaim in less time and decrease the generation of a ghost image in the screen. Let’s simplify it with an analogy: Using a pressure washer to remove ink from a metal post is easy, as the post is rigid and the ink has nowhere to hide. The force of the pressure washer makes ink removal easy. Attempting the same effect on a loose piece of fabric simply will not work, as the fabric is blown out of the way due to the force of the washer and the ink remains on the fabric. The same applies to tight and loose screens. The tighter the screen, the cleaner and faster it will reclaim.
Of all the variables that require control in the textile screen printing process, none carry the weight that the screen does when attempting to control quality. The quality of your press-ready screen is directly proportional to the quality of your finished product.
Rick Davis is southeastern regional sales manager for Triangle Ink Co. He is a 35-year veteran of the textile screen printing and apparel manufacturing industries. His background includes plant design, management, troubleshooting, apparel manufacturing and consulting. He also is a member of the Academy of Screen Printing Technology. For more information or to comment on this article, email Rick at rdavis@triangleink.com.
Tensioning Tips
When I was in college in the 1970s, we were taught to check screen tension by bouncing a quarter off the center of the screen. If the quarter only jumped 5 or 6 inches into the air, the mesh was too loose. If it bounced 12 to 18 inches high, we were good to go.
A lot has changed when it comes to checking screen tension. But one thing hasn’t: Time and again, printers of all skill levels ask me, “What tension should my screens be?”
This is a loaded question. With so many variables to consider, there’s no one-size-fits-all answer. It would be easy for me to throw out an arbitrary number — say 25 or 40 N/cm — but that won’t work because each shop is at a different skill level.
I usually answer the tension level question with a question of my own: “What is your current screen tension?” Tests have proven that mesh needs to be stretched to a minimum of 16 N/cm to prevent movement caused by squeegee pressure.
My recommendation is to gradually increase that tension level by 5 N/cm or so until the ideal tension level is achieved for your shop’s conditions and printing type.
If you’re currently printing with wood frames at tension levels below 10 N/cm, ask your frame supplier for tighter screens, or switch to stretch-and-glue aluminum frames in the 15 to 20 N/cm range (or higher).
If you’re using retensionable frames tensioned at 20 N/cm, take them up to 25. If you’re at 25 N/cm, raise it to 30. Just be sure to do it gradually. Reaching high screen tension levels is a journey that each shop takes until the optimum screen level is found.
— James Ortolani, Contributing Writer
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