Most of the damage to the screen mesh occurs during tensioning due to insufficient tensioning and stabilization times, incorrect tensioning values, and stressed screen corners.
Polyester screen fabrics are one of the most expensive supply items in screenmaking. Besides being costly, they are also delicate and easy to damage. Careless handling, poorly designed storage, and accidents on the press account for 20-30% of ruined screens. Although some of these problems are preventable, they also represent the unavoidable cost of doing business.
The majority of screens, however, are damaged during screenmaking, which is a problem that can be avoided and corrected by training the screenmakers properly and providing them with necessary instrumentation and equipment. The leading causes of screen damage are poor tensioning practices, lack of instruments tools and properly maintained tensioning devices, and poorly prepared frames. Most of the damage to the screen mesh occurs during tensioning due to insufficient tensioning and stabilization times, incorrect tensioning values, and stressed screen corners.
Although some meshes, notably those under 100 threads/in., can be tensioned to their maximum tension level in one stretching step, the safest approach is to stretch screens incrementally. The screen should first be stretched to its initial tension level, which is a fraction of its final tension. The mesh should then rest for 20 min before the next tensioning step. Each subsequent stretching should increase the tension level by 1/3-1/4 of the difference between the initial and final tensions.
For example, if the initial tension is 15 N/cm and the final (maximum) tension is 24 N/cm, then the next three tensioning steps should increase the tension by 3 N/cm each with 20-min rest periods in between. These rest periods should be not less than 15 min, and not more than 30 min since there seems to be no practical advantage for longer rest periods.)
Once the screen is stretched to its maximum tension, it should be left to stabilize for a minimum of 24 hours and preferably for 48 hours. During this time, the screen will lose 3-6 N/cm tension due to cold flow, which is normal for polyester material. If the screen is processed without this stabilization period and used for printing, the images will be significantly larger than they should be, and the registration of subsequent colors will be difficult to maintain.
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