If you want to print on plastic, it’s challenging – most plastics are chemically inert and non-porous. That means you can apply all the ink you want; it’s not going to stick. In the 1950’s, a Danish engineer named Verner Eisby experimented with various techniques to overcome these challenges. He found that exposing the surface to be printed on to gas flame or sparks modified the surface to improve adhesion with the ink. It did so, though, in a crude & uneven manner, leaving imperfections & inconsistencies in the printed product. He then tried applying a high frequency corona discharge in a linear array. The plasma (gas in an ionized state) generated left a homogeneously treated surface on which to print, smoothly & evenly.
This has become the “industry standard” for many of the labels we see on commercial products, from shampoo & wine bottles on the grocery store shelf, to pennants & banners at public events. It also leaves the surface even more prone to picking up a static charge from rolling or unrolling, stacking, sliding, etc.
One of our customers makes a great many labels for all kinds of these commercial products, and uses an EXAIR Gen4 Ionizing Bar immediately prior to the printing operation: