It is not just plastic sheets that are non-absorbent. Foil and plastic lined boards are widely used and it is normally the foil or plastic surface that is printed. There are occasions where it is necessary to print on previously varnished or laminated surfaces and these are generally non-absorbent. Metallic ink surfaces present drying problems of drying and adhesion when overprinted, so care in necessary. Some of the high gloss paper coatings have very poor absorbency and need special care when being printed.

Why should non absorbent or poorly absorbent surfaces present problems?

The majority of sheet feed litho inks dry by a ‘quick-setting’ process which starts when a low viscosity petroleum distillate is absorbed into the surface of the paper or board, leaving the remainder of the ink in a ‘set’ state on the surface. Drying is completed by the remaining resin/oil component of the ink combining with oxygen in the air to oxidize and change to a hard, dry film. If the petroleum distillate does not leave the ink film it inhibits the oxidation process. Most companies that regularly do work on foil or plastic surfaces have moved over to the use of UV curing inks that do not in any way rely on absorption. Then they have no problems so long as the ink maker knows the surface that the inks are to be printed on. However, UV curing is unlikely to be a practical option for the average sheet fed commercial printer, who only deals with these materials occasionally. What should he do?

Specialized inks

The first thing to do is to use inks that have been specifically formulated for non-absorbent surfaces. These are fully oxidation drying vegetable oil based inks that have been reinforced with resin and are free from the petroleum distillate that needs to be absorbed into the substrate. It is important that they have been formulated for the application. An increasing number of quick-setting inks are mineral oil free as the distillate has been replaced by esterified vegetable oils. These esterified oils are non-drying and, like the petroleum distillate, need to be absorbed into the substrate. Therefore, such inks are unsuitable for non-absorbent surfaces.

The problem with fully oxidation inks is that they are slower drying on the print, but have a tendency to dry on the press. As a result, there is a constant battle to balance these two opposing factors.

Care on the Press

The first thing to do is add a water-soluble drier, such as cobalt acetate to the fountain solution. This has the effect of increasing the drier content in the ink with the dampening solution, whilst not increasing it further back up the inking train where it is not wanted. Additives such as cobalt acetate are available from all ink makers. Care needs to be taken with the other fount additives to ensure that the pH does not get too low as increased acidity is likely to retard drying on the print. Drying on the roller ends can be reduced by lubricating the ends of the rollers with small quantities of linseed oil, or some other drying oil. Petroleum distillate or other non-drying oils should not be used as they can interfere with the drying on the print.

Whatever actions are taken, the drying of these inks will always be slower than modern quick-setting types. As a result, it will be necessary to use more anti-set-off spray of a larger particle size than would normally be used on coated papers and boards and to keep stacks low. It is frequently beneficial to air the sheets during drying as the oxidation process requires access to the oxygen in the air. This can be done by simply ruffling through the sheets in the stack after a few hours.

Test the Substrate

The final point of caution is to test the ink on the substrate before proceeding with the production run. This is especially important when printing on plastic surfaces as they can vary from batch to batch and with time during storage. Many plastics contain plasticizers, anti-oxidants anti-static agents that may migrate to the surface. They then form a very thin surface layer which can stop the ink from wetting the surface properly so that good adhesion becomes impossible. There is no reason why any litho printer should not take on printing onto non-absorbent substrates as long as the correct inks are used and care is taken.