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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.

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By Carol Swift, Peter Kiddell

When selecting a dot shape, use elliptical dots, which tend to produce crisper details than round dots. We have also experimented with stochastic or random dots, and the results have been fairly satisfactory. Registration accuracy is not quite so critical when using a stochastic dot pattern, but poor registration can give the print a sandy look.

The film-output device you use should produce positives with a 5-95% tonal range. However, because no trim area exists on the printed part, it's impossible to print targets, gray scales, and color bars that are normally used for measuring registration accuracy, dot gain, and tonal range. That's why it is always a good idea to experiment with your system by creating independent test prints to measure these print characteristics under specific conditions. Once you've done this, it may be possible to use the information in prepress to compensate for the changes that occur on press.

Ink Ink conditions can influence your success when pad printing process colors. The first aspect you need to consider is the amount of pigment in the ink. If your ink supplier provides you with standard process-color inks (yellow, cyan, magenta, and black) and can guarantee that the inks have a consistent pigment density, then you can turn your attention to the other main ink variable--the solvent level.

Because pad-printing inks must have a particular solvent content to ensure proper transfer from the pad to the part being printed, it's critical to keep the solvent level stable. If your press features an open ink trough, you can maintain the ink's solvent level by adding measured amounts of solvent at regular intervals.

Closed ink cups certainly make it easier to control the solvent level, but over an extended period, you still have to replace or adjust the ink. Since pad presses use relatively small volumes of ink, we recommend replacing the ink daily, both in closed-cup and open-trough systems.

Pad specifications Whenever possible, use identical pad shapes for printing process colors; variations in pad shape may cause image distortion. Make sure the pads are the same hardness, and when you order pads, make sure the manufacturer sends pads that were produced in the same batch.

Controlling variables


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