Rosson offers advice on tracking your tension culprit.
Heat: As mentioned previously, heat makes wood frames more prone to beam deflection. Heat is also debilitating to polyester fabric. When exposed to extreme heat, polyester gets softer, and consequently, stretches more easily. In most cases, the heat screens are exposed to in a typical production environment falls far below polyester's temperature limit. However, if your production system involves flash or inline curing, the bed or shuttle system on your press may pick up enough heat to affect the mesh.
On press: Most screen-printing applications involve printing off-contact, where the screen is held slightly above the item being printed. As a result, the squeegee must push the fabric down into contact with the part in order for ink to transfer. The squeegee is pulled along the length of the screen, which means it eventually pushes down and stretches every part of the screen within its path. As you might have guessed, over the course of thousands of squeegee strokes during a print run, the screen's tension will drop.
The dilemma is that the screen should be as tight as possible since it will perform better under high tension. But if you overdo the tension, it won't hold for an extended print run. Additionally, the press should be set up to provide enough off contact to force the screen to snap off the print surface immediately behind the squeegee.
To meet all these criteria, you must compromise. Use the least possible off-contact that will permit instant snap off and use the maximum tension that a particular fabric will hold over a long run. The suggested tension level for your particular mesh will be specified by the manufacturer.
Also make sure that off-contact distance is consistent over the entire print surface and the path of the squeegee during the print stroke is parallel to the press bed or platen. If either one of these conditions is not satisfied, it's likely that you'll have to use excessive squeegee pressure, which puts extra stress on the screen and hastens tension loss.
Squeegee pressure is another parameter you must carefully control to maintain tension levels on your screens. As a general rule, use minimal squeegee pressure, which not only helps preserve the life of the fabric but produces a superior print. If the press or press operator is forced to use a lot of pressure, it indicates that something else is wrong. Platens may be warped or out of parallel with the plane of the screen. The squeegee may be hopelessly dull. Or the squeegee contact surface may not be perfectly straight.
Even if you carefully control your screenmaking and press setup, your screens will lose tension. The fact is, gradual tension loss is a natural occurrence. However, you can improve the overall performance of your screens and their ability to resist tension loss by using low elongation (LE) fabrics. These fabrics, available from all mesh manufacturers, cost slightly more than standard fabrics, but the improved performance they provide more than offsets the price difference. LE fabrics also tolerate slightly higher tensions than conventional mesh.
Now that we've identified frames and mesh as the two main sources of tension loss and pointed out specific causes of this phenomenon, you can see that controlling screen tension is not a hopeless challenge. You are now ready to pinpoint factors leading to tension loss in your own shop by taking a closer look at the materials and methods you employ.
Scrutinize your wood frames. If any of them are waterlogged, retire them for a while. And educate employees to avoid exposing the frames to water or heat. With rigid and retensionable metal frames, take care to ensure that the mesh is properly affixed to the screen. Tension your fabrics to the highest levels recommended and allow them to relax for a period before you affix them to frames. Finally, make sure your press operators use minimum off-contact distance and squeegee pressure. Follow these recommendations, and you'll realize better print quality, less downtime, and longer screen life.
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