Pigments give the ink the desired color and opacity. They are normally
supplied in powder form and incorporated into the ink by a mechanical
dispersion process. Dyes are sometimes used instead of pigments, for instance
in sublimation inks, and for transparent coatings.
Solvents
or carriers enable inks to be applied in the liquid state. Inks can be divided
into two main groups: conventional inks with an organic solvent and waterborne
inks. Inks based on an organic solvent generally have better adhesion to
substrates than waterborne inks, but solvents may attack the substrate and
cause stress cracking. Generally, no single solvent has all the desired
properties and a mixture of solvents is therefore used. Waterborne inks have
superior properties in relation to environmental, health and safety matters.
Special additives are used to give the ink the desired flow properties in the
application phase or an improved flexibility after curing. Some additives
enhance adhesion and appearance.
Pad printing inks have a
formulations comparable to screen printing inks, but there are some
differences. Pad printing inks are formulated for rapid solvent evaporation,
whereas screen printing inks are designed to resist rapid evaporation so that
they don't dry in the screen. Furthermore, screen
printing inks are sometimes applied as a very thick film, unlike
pad printing inks.
Different types of inks can be distinguished according to the way curing takes
place:
- Air-curing inks harden due to evaporation of the solvent,
while the resin polymerizes. They dry rapidly and are the most commonly used
ink type.
- Heat-curing inks require elevated temperatures for curing.
The use of these ink systems is limited by the high curing temperature that
the plastic must be able to withstand.
- Two-component inks have the
big advantage that no volatile components evaporate during curing. Pot-life
after mixing is however limited.
- UV-curing inks are widely used for
screen-printing. The curing process is fast and environmental problems are
smaller than for solvent-based systems. Small changes in ambient conditions
have little influence, which makes the printing process very stable.
-
Oxygen-curing inks have limited use, as they dry slowly. The polymerization
takes place under the influence of oxygen absorption.
- Sublimation
inks are heated to a temperature of about 200ºC (392ºF) during the application
process, so that dyes in the ink sublime and are absorbed by the polymer
surface while they are in the gas state. Sublimation inks are in the solid
state at ambient temperature, like a wax, and become fluid when raised to 80ºC
(176ºF) in the ink reservoir and cliché.
Ink systems should always be tested on prototype parts over an extended period
of time to establish the compatibility of the ink.
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