Successful Packaging Printing on Plastics and Films : Adhesion problems when printing on films and other plastic packaging are a persistent issue. Why is it so difficult to maintain quality levels despite corona or flame treatment and thorough checking with test inks/dyne pens? We at KRÜSS often encounter two classic mistakes that occur time and time again:
1. Many of our customers used to work with a traditional guideline value of 38 dyne (38 mN/m) for the surface free energy (often referred to as surface tension) of the plastic. However, this value has been established for polypropylene with solvent-based printing inks. If you change the material and/or choose water-based printing inks but stick to the 38 dyne level without questioning it, you will most likely run into problems.
2. “More is better” is the wrong approach when it comes to surface activation, because overactivation is a thing. When after the activation the surface is more polar than the ink, the two will become incompatible and long-term instability of the print may occur.
Fortunately, these two common mistakes are easy to avoid.
Find the sweet spot for your substrate/ink combination
You will achieve the best printing results when the SFE value and polarity of your pretreated plastic match the ink. Use contact angle and surface tension measuring instruments to determine the degree of activation that really works for your substrate and ink. Depending on where you work in the process chain, you might also be able to adjust the ink formulation to achieve the best match.