Grafica News - Oct 2014
Proper Screen Tension Guidelines
By Dawn M. Hohl-Nowlin, USA - Uncommon Enterprises LLC
Dawn Hohl-Nowlin is a Technical Trainer and Consultant on screen printing. Her industry experience includes 18 years with SPTF/SGIA, USA, overseeing screen printing workshops, developing training resources and conducting processrelated research. She is a member of the Academy of Screen and Digital Printing Technology (ASDPT). She can be reached at email@example.com
In screen printing the quality of the finished product is largely determined by the quality of the screen. It is the foundation of the process and the root of many problems in the screen printing process. Without proper screen tension, screen printing is next to impossible to control.
To deal with tension at all, you need a calibrated tensionmeter Figure 1. But a tensionmeter won't help if it sits in a box. This tool should be used as part of a daily routine of managing tension on all production screens. Tension should be measured on every screen - every time you stretch, retension, or select a screen to enter into the production cycle. Let's examine three critical areas to manage tension on an ongoing basis; tension level, tension uniformity and screen to screen consistency.
It is a proven fact that higher tension levels produce better print results, improve registration and increase production speeds. Low tension screens can cause problems like poor print detail and edge definition, incomplete ink transfer, inconsistent ink transfer, mesh marks, slower printing speeds, excessive elongation of the image size, poor registration and greater manual printing fatigue. With halftones, moiré is often worsened or created from poorly tensioned screens.
Appropriate tension levels for a particular mesh count vary based on the fabric strength (based on thread diameter), frame type and in some cases stretching device. Recommended tensions are obtained from the mesh manufacturer, and can be used as target values during the stretching process. Often they are given in a range, or in three levels dictated by the printing application, frame size, tensioning device and screen making expertise.
The bottom part of the range should targeted when low end tensioning equipment is being used, when screen making experience is minimal and when stretching large format frames (127 cm or larger). However, higher tensions may be possible on large format frames when proper tensioning procedures are used.
The middle part of the tension range, is appropriate for advanced screen makers using state-of-theart stretching equipment and stable frames. The highest part of the tension range is reserved for expert screen makers using stable frames and high quality tensioning systems. Proper tensioning procedures become critical in the second and third tension range. Bringing screens to higher tensions will require a few adjustments in your operation. High tension screens have less resistance to knocks and pokes and will break more easily requiring extra care when handling Figure 2. Off
contact distances must be reduced when higher tension screens are introduced to the press. Failure to lower off-contact accordingly can result in the mesh breaking on press. When raising tension levels, it is a good idea to establish timely communication with the pressroom so appropriate adjustments are made.
High tension should not be considered a goal unto itself. Tension levels should be chosen that can be consistently achieved and maintained under your shop conditions. Printing application also dictates screen requirements. For specific tension recommendations, contact your mesh manufacturer or distributor and speak with a technical representative.
Screen tension uniformity refers to tension consistency within one screen. Maintaining tension uniformity is important for preventing image distortion and registration problems. Uneven warp and weft tension can create inconsistent registration, unpredictable ink deposit, and moiré. For best results variations should be within +/- 1.0 N/cm.
Strictly reading tension in the center of the screen is an incomplete picture of a screen's tension profile. Measurements should be taken in at least five places on the screen. This is known as a five-point measurement method. To establish your measurement locations divide the screen into four equal quadrants, each with a corner joining at the center Figure 3. Measurements are taken in the center of the four quadrants and in the center of the
screen. Warp and weft measurements should be taken in all five points. For larger screens, (over 100cm) measurements should be expanded to include nine points. Four additional points can be selected in between the four quadrant measurements to come up with a total of nine points.
Screen to Screen Consistency
For multicolor prints, selecting screens with similar tensions will help ensure consistent printing results. This issue is especially important when printing fourcolor process work or on any job where the quality requirements are high. Tension differences screen to screen can result in inaccurate registration as well as different printing effects on the image. As a rule of thumb, variations of no more than 1.5 N/cm screen to screen will help ensure registration and print quality on multicolor work.