Many screen printers assume that proper off-contact distance alone is the key to successful printing.
The influence of ink, off-contact, and mesh tension
Off-contact distance and mesh tension are measurable, but ink viscosity as a counterforce cannot be measured. The behavior of the ink as a counterforce relates more to its tack (stickiness) than its viscosity. Ink viscosity manifests itself as a certain degree of tack during printing and varying degrees of tack in the printed ink deposit.
The tack characteristics of the printed ink are determined by the height of the ink deposit, the thixotropic state of the ink, drying rate, and adhesive attraction for the substrate. In general, a good rule to follow when considering the lift-off force required for a job is that medium- to high-viscosity inks require medium to high lift-off forces.
Keep in mind when setting off-contact that doubling off-contact distance quadruples printed-image distortion. For example, if you print an image with an off-contact distance of 4 mm, and the image becomes elongated by 0.1 mm, then doubling the off-contact distance to 8 mm will cause image elongation of 0.4 mm.
Also consider that doubling the off-contact distance or mesh tension doubles the lift-off force of the screen. Therefore, if you double off-contact distance and mesh tension, you get a four-fold increase in lift-off force. Table 1 shows how off-contact distance and mesh tension influence the lift-off force required to overcome the ink-tack counterforce. We can’t measure ink tack, so this table represents the force of ink tack on a scale from 1-5, where values of 1-2 are for no-tack to low-tack inks, 2.5-3.5 for medium-tack inks, and 4-5 for high-tack inks.
Accommodating close-tolerance printing
Off-contact distance should always be as low as possible so that you can meet requirements for close-tolerance printing without risking image distortion, poor registration, incorrect ink deposit, and short stencil life. But don’t exceed the screen’s maximum tension level, which is influenced by the size of the screen frame, the bending strength of the frame, the type of stretching device you use, and the tensioning method you prefer.
If you need to take ink tack into account for a job with strict requirements for image length and positional accuracy and the mesh tension is too low to provide the proper lift-off force, you may be able to assist the lift-off behavior by reducing the squeegee speed during the print stroke.
If reduced squeegee speed doesn’t solve the problem, your next option is to use a film positive or negative that is corrected to compensate for image distortion on press. Distortion positives or negatives are produced by entering the empirical distortion values into the software used to create the artwork or into the output device prior to generating the film. Another option is to use a larger screen size or a decreased image area, which reduces the width and length of the squeegee stroke relative to the screen size.
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