Many screen printers assume that proper off-contact distance alone is the key to successful printing.
When the lift-off angle is too low, the elasticity of the screen mesh is incapable of overcoming the lift-off counterforces. The result is that an area of the mesh exhibits delayed lift-off as it sticks to the substrate. This sticking area is immediately behind the squeegee during the print stroke. If the lift-off force is less than the counterforce of ink sticking to the mesh and substrate in the sticking area, the length of this sticking area is described by the ratio of these forces. Such delayed lift-off always leads to printing faults.
Parallel off-contact distance
Parallel off-contact distance is a technique printers often use. When using parallel off-contact, the lift-off angle near the beginning of a print stroke is always larger than the lift-off angle further along the print stroke. The lift-off angle decreases over the length of the squeegee stroke. In fact, every time you double the length of the squeegee stroke, you reduce the lift-off angle by half.
One way to minimize the discrepancy between screen angles when printing with parallel off-contact is to increase the size of the screen relative the size of the image area. Increasing the distance between the top inside edge of the screen and the beginning of the image area will help you avoid the high initial lift-off angle and reduce the difference between the angles at the beginning and end of the image area. However, you might also have to increase off-contact height, which could cancel out the benefit of using the larger screen by again making the lift-off angle too steep at the beginning of the image area.
Printing with parallel off-contact leads to lift-off problems and, consequently, to poor print quality, which is why many press manufacturers offer sophisticated peel systems on their equipment. These systems lift the rear of the screen as the squeegee moves along the print stroke. Peel systems maintain a constant lift-off angle over the length of the print stroke by raising the end of the screen during the squeegee stroke. The use of an automatic peel system requires the press operator to set minimum off-contact and maximum screen-lift heights. The optimum heights are those that cause the lift-off angle to remain constant over the full length of the squeegee stroke.
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