Screen Printing - December 2012/January 2013 - (Page 14)

SOLUTION SOURCE BOOK A LOOK AT SCREEN LIFT-OFF any 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 offcontact 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 offcontact 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 14 SCREENPRINTING M 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. 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 Squeegee speed Vs Screen elasticity E Offcontact h height Va Screen lift-off speed Lift-off angle (θ) (q L Length of print stroke FIGURE 1 Lift-off force is determined by the elasticity and tension of the screen mesh (E); the lift-off angle (θ) formed by the off-contact height (h) of the screen and the length of the print stroke (L); and other factors, such as squeegee speed and image size relative to screen size.

Table of Contents for the Digital Edition of Screen Printing - December 2012/January 2013

Screen Printing - December 2012/January 2013
Prepress for Special-Effect Garment Printing
Tips for Optimum Screen Exposure
Maximizing Image Size and Position
Understanding Screen-Tension Loss
A Look at Screen Lift-Off
Staging Garment-Printing Jobs
How to Control Dye Migration
Fine Tuning for Fine Details
Application-Specific Considerations for Graphics Printers
Upping the Ante at SGIA 2012
Statement of Ownership
U.S. & Canadian Directory
Opportunity Exchange
Advertising Index

Screen Printing - December 2012/January 2013