Troubleshooting the pad-printing process requires the identification and control of associated variables. The tips presented here will help you avoid some of the persistent problems you encounter on press.
By Julian Joffe
Chemistry plays an important role in pad printing. If it is a plastic, is it an inert material, such as a polyolefin, nylon, or polyester? Does it need to be pre-treated with a corona or flamer or post-treated in a heat tunnel? Familiarize yourself with the idiosyncrasies of the materials involved. Not all plastic is identical.
Treat metal, glass, ceramic, and other substrates with respect and find inks and thinner combinations that are specific to those substrates. Contamination control is just part of the process. There may be additives present that will leach to the surface, or the pad may have been sprayed with a mold-release agent.
If you are printing on a plastic part that is painted, you are not really printing directly onto plastic. The surface that contacts the ink is the substrate. Wood that is painted with polyurethane is a polyurethane substrate and ink adhes-ion is very different on raw wood versus urethane.
Always test print by producing a number of samples prior to production. Run tests as required by your customer to guarantee proper adhesion (Figure 4).
Cliche and support-table inspection and cleaning
Clean the underside of the plate as well as the plate support. Even tiny amounts of debris can affect print quality, eventually damaging your cliche. Consider the concealed areas—the underside of the cliche and the support on which the cliche sits. Even something as small as a grain of dust lodged under the cliche is enough to cause a bump on the surface. Bumps can lead to pooling marks, which transfer to your part.
One of the more common occurrences of pooling (Figure 5) stems from ink build-up on the cliche leaking over the edge and seeping into the gap bet-ween the cliche and support table. A typical symptom of this problem is phantom marks transferring to the printed parts. These marks return almost immediately, no matter how frequently the press oper-ator cleans the cliche’s surfaces. The operator should also check the underside of the cliche, as well as the corresponding support table, as built-up ink dries under the cliche surface and creates a speed bump, which results in unwanted ink on the surface. The offending debris is almost always discovered and, after removal, normal and smooth print quality returns.
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