Graphic Design & Printing TechnologyTack:- Tack or stickiness is a property of ink that must be controlled in order
to transfer images and deliver the sheets through the press. Tack can cause
paper (especially coated paper) to adhere or stick to the blanket of an offset
press. Ink that is excessively tacky may also pick the surface of the paper and
cause misfeeding. Tack increases as one colour is printed over another. An
inkometer is used to measure the tack of ink.
Opacity:- Opacity refers to the covering ability of an ink. High opacity means
very little light is transmitted. Most lithographic inks are transparent, so two
colours can be over printed to create a third colour. Process colour printing is
dependent on light passing through all the layers of ink. Screen printing inks
are highly opaque.
Ink stability:- Ink stability or thixotropy is the tendency of an ink to flow
more freely after being agitated by the ink rollers.
Drying time:- There are two stages in the ink drying process; first the ink
should instantly set or stick to the paper. The second stage in the drying process
is hardening. When ink has hardened the vehicle has completely solidified on
the paper surface. The time it takes for liquid ink to harden to a solid state is
called the drying time.
Manufacture of Lithographic Ink
Lithographic ink is manufactured by mixing vehicles, pigments and additives,
There are two stages in ink manufacture- mixing and milling
Mixing:- The pigment is introduced into and distributed throughout the vehicle
and the mixing process is performed as blades slowly rotate to steer ink inside
a metal container.
Milling:- It is a process used to crush and further blend the ink pigment into
the vehicle.
Ink drying methods
There are several methods for getting the inks to dry. Drying occurs when a
liquid becomes solid through one or more chemical methods. The most common
ink drying methods are oxidation, polymerisation, evaporation, penetration
(absorption), precipitation, radiation curing, etc.
Oxidation:-Oxidation drying ink dries by absorbing oxygen from the
surrounding air. By combining the ink’s drying oil, the oxygen of the air changes
the vehicles of the ink from a liquid to a solid. Letter press and offset printing
use oxidation drying inks.
Polymerization:-Polymerization drying inks dry by a chemical reaction that
causes molecules in the ink to combine. Oxygen thickens to a gel-like material