Rosson offers advice on tracking your tension culprit.
Screen printers exceed the elastic limit of polyester fabric all the time. And as the fabric threads stretch, tension drops. This condition is most pronounced when the fabric is stretched the first time. However, after initial tensioning, polyester threads actually get a little stronger and resist excessive stretching--in other words, their elastic limit increases.
To get the most stable tension level, mesh fabric should be tightened to the desired tension level and then left for a few minutes. Over a short period, the fabric will lose some of its tension. The fabric can then be retensioned and left to relax once more. Again, the mesh will lose some tension over time. But this time, the loss will be significantly smaller. This process can be repeated until the mesh reaches stability at the tension level you desire. This procedure can be easily accomplished using most mechanical stretchers and retensionable frames.
For screen makers who use pneumatic stretchers, the tensioning procedure is a little different. Pneumatic stretching devices continually adjust for tension losses until the desired tension level is achieved. Therefore, it's usually a good idea to leave the fabric in the pneumatic stretcher for a few minutes before bonding it to the frame. If the mesh is bonded immediately, more severe relaxation of the fabric will probably occur.
Regardless of the stretching system you use, giving the mesh time to relax and adjust to new tension levels is important. And while it may take longer to produce a printing screen in this way, the extra time you spend in stretching will help you avoid screen remakes and other costly problems on press.
Incidentally, printers used to believe that polyester fabric had to be stretched and held for several hours before it was adhered to the frame. But research conducted by the Screen Printing Technical Foundation disclosed that fabrics performed almost as well when given only a few minutes to adjust to new tension levels before they're affixed to frames.
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