Offset printing on nonabsorptive surfaces is growing rapidly. Such printing occurs on polyethylene, polypropylene, polyester, polystyrene, polyvinyl chloride, and aluminum foil. Many printers successfully print on nonabsorptive substrates on a regular basis, but it must be done correctly to avoid a disaster.
For best wetting and ink adhesion to nonabsorptive surfaces, the “dyne level*” or surface free energy of the substrate should be about ten (10) dynes above the surface tension of the ink. This is measured with a series of liquids (dyne solutions) with known surface tensions in dynes/cm. Liquids will not wet surfaces with free energies lower than their surface tension. Poor wetting and adhesion, however, will also result if the surface free energy is too high above the surface tension of the ink. Thus, for good printing with normal offset inks, Printing Industries of America recommends that the surface free energy of the substrate be approximately 40-50 dynes.
The wettability of surfaces is easily tested with “dyne solutions.” When these solutions are applied and spread over the nonabsorptive surface to be tested, they will wet surfaces with surface energy that is somewhat higher than the surface tension of the liquid. If the liquid has a higher surface tension than the surface, it will bead and not wet the surface. These dyne solutions are usually strong solvents and may give false indications if they dissolve the surface being tested. They are generally unsatisfactory on absorptive surfaces. See below for dyne levels of some common materials.
Unfortunately, many plastics have surface energies that are too low for good printing (below 35 dynes) and should be treated to increase the surface energy to 40-50 dynes. A substrate with a surface energy between 35-40 dynes is considered to be marginal. Below 35 dynes, the material should probably be treated. Treatment is usually done with an adjustable corona treatment (electric field) or by applying a suitable primer coating. Corona treatment is fast and relatively inexpensive, but surface dyne levels are susceptible to loss or decay during prolonged storage and upon contact with some fountain solutions. Precautions must be taken to obtain a uniform dyne level that is not too high. Corona treatments can also dramatically increase troublesome static charges in the material treated.
Primers that adhere both to the substrate and to subsequently applied inks are also used. These coatings are especially useful on aluminum or other surfaces with high free energies.
Many of the problems encountered can be eliminated with good inks that are properly formulated for printing on nonabsorptive surfaces. It can be difficult to obtain these inks unless the ink company has considerable experience in formulating inks to print on specific nonabsorptive surfaces. With UV coatings, for example, it is difficult to reduce surface tension for proper adhesion over conventional inks. Primers such as aqueous coatings are often needed. All materials must be compatible, so the printer should furnish the ink supplier with samples of stock and request samples of the inks with coatings on these surfaces.
Even with proper wetting, there are printing challenges. A plastic will not absorb water present in the offset printing process. It is important that the plate and fountain solution be selected so that the plate stays clean with a minimum amount of fountain solution. Otherwise, water on the plastics surface may interfere with ink lay, resulting in severe mottle. Too much water on the first unit, for example, may cause a severe mottle in the image of the fourth unit.
The inks must also be formulated for low water pick up. Excessive water emulsification in the ink can cause problems such as snowflaking, dot gain, poor trap, poor ink lay, and slow drying.
To minimize ink drying and set-off problems, the inks must contain very high percent solids with a low amount of solvent. Some plastics require inks that have an aggressive solvent that attacks the surface of the plastic and binds to it. Others are more fragile and can be damaged by an overly aggressive ink. Thus, the ink must be formulated for the substrate to be printed. Quick-setting inks are not suitable for printing on nonabsorptive surfaces since there is nothing to absorb the ink oils.
Additionally, the inks used on plastics must be quite strong so proper color can be attained at relatively low ink film thickness. Otherwise, there will be greater tendencies for the ink to set off on the back of the next sheet in the delivery pile. Printing Industries of America also suggests using very fast drying inks and adding drying accelerators or stimulators to the fountain solution, as well as adding perborate-type dryers to the ink at press side. The latter should be done following the instructions of the ink maker.
To reduce setoff problems, keep the delivery piles small and use a well-distributed spray powder. After printing, the delivery piles must be left undisturbed until the ink has dried. Just applying excess pressure to the top of the pile could cause extensive setoff down through many sheets. Thus, the piles should be checked occasionally and left undisturbed until the ink dries.
UV or aqueous coatings applied and dried in line greatly reduce setoff problems and the need for antisetoff powders. These coatings also improve gloss and resistance to mechanical or transit damage.