Surface Tension and Printing on Film

All film printers have, from time to time, encountered printability problems such as pinholing, poor adhesion or lack of colour strength. Very often, upon contacting their ink supplier, they are confronted by the question, "What is the treatment level of your film?"

The treatment level, or Dyne level, or, more precisely, the surface tension of the substrate has a strong influence over the quality and durability of the printed film.

Generally, a substrate surface tension of 38 dynes/cm2 or higher is required for the successful printing of solvent based inks. Water based inks require a minimum surface tension of 40 dynes/cm2.

Normally, only one side of the film is treated. The main reason for this is to ensure that the ink has more affinity for the printed surface, to avoid blocking onto the reverse side. For this reason, it is important to be sure that the treated side of the film is the one in contact with the ink.

Though a film may have a good surface tension at the time it is extruded, that surface energy will diminish with time. As well, movement of the substrate through the press will reduce the treatment level by several dynes. Contact with idler rollers at higher speeds will inhibit film treatment or entrap ionized air on the surface of the film, reducing print quality. For this reason, a surface tension of 42 dynes/cm2 is recommended before the substrate runs through the press.

How can this be achieved, particularly for printers who buy their film from an outside supplier? The most common method is in-line Corona Treatment. With this method, the surface energy of the film is increased through the application of an electrical discharge just after the unwind of the press.