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Process-Color Pad Printing

(January 2002) posted on Wed Jan 30, 2002

The authors explain machine tolerances, color-separation quality, ink characteristics, and pad specifications.

By Carol Swift, Peter Kiddell

As with any process-color printing, registering four-color pad-printing jobs is a challenge. Before you can achieve accurate registration, you must fully understand and control all aspects of the pad-printing process. We define printing as replicating original artwork by laying down a controlled thickness of ink. We use the term "replicating" rather than "copying" to emphasize the level of precision you must achieve in order to get a quality print.

While process-color printing of any type requires precise control, reducing variables in pad-printing is particularly important. The variables you must consider include machine tolerances, color-separation quality, ink characteristics, pad specifications, and the accuracy of cliche etching and positioning.

Machine tolerances Pad printing is a thin-ink-film reproduction process. Dried ink films for a single print are approximately 5 microns (0.0002 in.) thick. Because the pad delivers such a thin ink layer on the substrate, this is an ideal marking technique for process-color printing on objects where high ink buildup tends to be problematic.

With conventional single-color pad printing, the quality of the press is not critical--most machines deliver reasonable print definition. But with process-color work, the accuracy and consistency of the machine is of major importance.

Some presses position the object being printed as it moves from one print position to another. On other models, the object remains stationary while a single pad moves between the part and cliches for each color. In both cases, the way the machine positions the part relative to the cliche and pad determines registration accuracy between colors. So the first step you should take to ensure good registration is to purchase a dependable, well-engineered pad-printing press.

Film generation Now that you know it's possible to accurately register process-color pad prints, you need to look carefully at the color-separating procedures you employ. Specifically, you must consider three items when making separations for digital output: the halftone line count (measured in lines/in.), dot shape, and tonal range represented by the film positive.

The line count of a separation is the line ruling you want to print. With pad printing, this can be up to 200 lines/in. However, we recommend that you work at a lower count of 150 lines/in., which also provides a high resolution image but requires less stringent control of process variables.


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