Tips & Tricks: Mottling

When speaking of "mottling" or "back trap mottling", printers are referring to a cloudy print output. The problem originates in paper that is partially uneven and the resulting uneven absorption and back split characteristics.

If the paper is spotted, or if a certain amount of cloudiness is already present in the coating, then the printing ink will be only partly absorbed. In four color sheetfed offset printing, the printing result will back trap on the printing blankets of the subsequent printing units. When an uneven penetration of printing ink is especially pronounced, this will become visible after the back trap process occurs, as an uneven print reproduction.

The problem of a "cloudy printout" is most prevalent in offset printing. In order to isolate the printing paper as the culprit, one must first determine if the visual appearance of the cloudy printout matches up with the paper structure. This can be done using backlighting, or under UV light. The absorption characteristics of the printing ink on the paper in question can also be made visible by resorting to a simple wipe test using special wipe-test ink. In this way, proof that paper is the root cause may be confirmed.

Remedy:
In cases where the cloudy printout is purely the result of the paper being used, the solution is to change to an alternative type of paper or contact your supplier.