Many screen printers assume that proper off-contact distance alone is the key to successful printing.
Many screen printers assume that proper off-contact distance alone is the key to successful printing. It’s true that off-contact permits the mesh and stencil to release from the ink printed onto the substrate, but it is just one component of a larger influence on print quality: screen lift-off. Read on to find out more about this important facet of print quality.
Lift-off force and behavior
Unlike off-contact distance, lift-off force can’t be measured accurately or set on a screen-printing press. You must rely on experimentation and experience to help you balance off-contact and a number of other variables to achieve the ideal lift-off force. A screen’s lift-off behavior is determined by the lift-off force of the screen and a counterforce. These forces, however, are influenced by particular variable in the printing process, including off-contact distance, screen size, mesh elasticity and tension, and size of the image area relative to the screen size (inside dimensions).
The counterforce to lift-off force is determined by the following variables: ink viscosity, amount of ink deposited on the substrate, size of mesh openings, adhesive attraction of ink to the screen mesh, adhesive attraction of the ink to the substrate, absorption capacity of the substrate, and squeegee speed.
Lift-off speed and angle
Under ideal lift-off conditions, the screen releases the ink immediately behind the squeegee during the print stroke (Figure 1). In other words, the lift-off velocity of the screen (Va) is identical to the velocity of the squeegee (Vs). By adding the off-contact height (h) and length of the print stroke (L), we can describe the printing process according to the following equation: Va/Vs = h/L. However, this equation alone is still an incomplete representation of the influences on lift-off, including screen elasticity (E) and the lift-off angle (θ) formed by the off-contact height and length of the print stroke.
When you use a relatively steep lift-off angle and high squeegee speed, the ink tears off because it has insufficient time to transfer onto the substrate. The result can be incomplete image transfer or, in extreme cases, no image transfer at all. In general, excessively steep lift-off angle and/or relatively high squeegee speed lead to poor printing results. Be aware that the cohesive nature of an ink—the tendency of the ink to stick together—and its adhesive affinity for the mesh and substrate also affect ink release.
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