SPRAY POWDER

Spray powders are regarded as being all the same. However, this assumption can be the cause of many problems at the delivery end of the press. Powders can be into three main groups: regular, hydrophobic/coated and water soluble/vanishing. Each is designed to do aspecific job under certain conditions.

Within each group there are powders with different grain sizes; this is necessary to achieve sufficient separation of sheets in the stack and support a sufficient weight of stock, while maintaining the correct distribution of grains for different

jobs. The correct choice of powder is also affected by the ink film coverage, whether the sheet is to be passed through the press more than once and whether there are any subsequent jobs to be carried out on the sheet.

CHOOSING THE CORRECT POWDER

Regular powders are ideal for jobs which are to be passed through the press once and have no subsequent operation. These powders are usually vegetable starch based, which result in a soft feel on the sheet. Also, they do not wear the blanket as much as some other types. The alternative to a vegetable starch based regular powder is a mineral based one. This type is generally more suited to letterpress printing, consisting of hard grains which do not fly around the press room, but do tend to wear plates.

If more than one pass is required hydrophobic/coated powders are better, as these will not build-up on the blanket and are more tolerant of subsequent applications of materials. These powders do not dissolve in water, and demand

less critical press settings than soluble powders.

Vanishing/soluble powders are ideal for multi-pass work, as they are dissolved by the damping system and therefore leave the sheets clean each time. Of all the powders, these will affect gloss levels of varnishes and laminates the least.