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
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
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