Rosson offers advice on tracking your tension culprit.
Your ability to control beam deflection is largely dependent on the type of screen stretcher you use. One variety simply stretches the mesh, holding the desired tension level until the mesh is attached to the frame. When the mesh is cut loose from the stretcher, the tension load is transferred to the frame, which can bend under the stress and allow the center of the screen to relax.
Retensionable frames also fall into this category. However, with retensionable frames, the frame itself serves as the tensioning mechanism. So the frame is subject to deflection throughout the tensioning process. In most cases, retensionable frames only exhibit severe deflection when the mesh is tensioned far in excess of normal tension levels or when the frame is extremely large.
The other stretcher type pre-bends the frame as it stretches the mesh. This way, when the mesh is affixed to the frame and cut loose from the stretcher, the frame will not bend further, allowing it to maintain the screen tension level provided by the stretcher.
Although the framing system you use can play a part in screen tension loss, the majority of tension problems occur due to the mesh fabric itself. Mesh elasticity: All materials have limited ability to stretch and return to their original shape and size. That includes polyester, rubber, and steel--yes, even steel will stretch when enough force is applied. If you stretch any of these materials beyond their elastic limit, they will not return to their original shapes. Material elongation below this stress-limit point is called elastic deformation. Any material stretched below its elastic limit will snap back to original size. But if stretched above the stress-limit point--a condition known as plastic deformation--the material will not return entirely to its original shape.
To illustrate this concept, consider a balloon. If you blow it up, hold it in its inflated state for a few minutes, and then release the air, you'll notice that the uninflated balloon is now larger than it was before you inflated it. It has been stretched beyond its elastic limit and cannot return to its original size. The same holds true for mesh threads tensioned to typical screen-printing levels.
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