Ink length is a more traditional, but still widely used, quantity to describe the viscosity of an ink. The most simple instrument to assess the length of an ink is a broadblade spatula.
A more scientific definition (ifra) is
ink length =
= viscosity / apparent yield value
Viscosity and yield value are of crucial importance whether an ink feeds from the duct or not.
Ink profiles in a duct:
(A) low yield value, low viscosity - feeds;
(B) high yield value, high viscosity - feeds;
(C) high yield value, low viscosity - hangs back
Tack may be described as the ability of the ink to act as an adhesive. It is by definition the force required to split an ink film between two rollers.
Representation of an ink film splitting.
Tack can be judged from a bit of ink between the finger-tips.
less tacky tacky short long
To measure tack precisely, tackmeters are employed. There are several instruments, but they all resemble the inking system of a printing machine.
Typical tackmeter (drag movenment).
The more tacky the ink is, the greater the force pulling the rider roller will be.
Tack is given in arbitrary units, which depend on the instrument and the method used. Tack is an important property in the inking system as well as in the interaction between ink and paper and between the inks. High tack is generally desirable, but too high tack will result in picking, that is: fibres are pulled out of the paper. Tack also plays an important role in ink trapping in four-colour printing. As a rule, the ink printed first should have a higher tack than the next one. Both tack and viscosity strongly depend on temperature.
Fineness of grind
Fineness of grind for paste inks is usually assessed using the "grindometer". Liquid inks are examined under the microscope.
Abrasion occurs on a gravure cylinder, because the ink contains pigment agglomerates; this must not exceed a preset value.
Colour and colour strength
Colour and colour strength must meet the requirements. There are several test instruments of different levels of sophistication. For paste inks, the printablility tester is quite popular.
Drying time is tested especially for paste inks containing drying oils.
Gloss is determined optically. Glossy inks reflect incoming light more or less like a mirror. This property depends strongly on the printing stock.
Rub resistance is tested by pressing and rubbing some standard material against the ink film and given in arbitrary units. There are several standardized instruments for this purpose.
Packaging engineers and advertising people are very demanding folks to printers and inkmakers.
Among others, the following properties are assessed by more or less standardized tests:
- sealing ability
- sealing strength
- adhesion of printed films after laminating
- residual solvent
- bleeding of colourant
- migration of other constituents
- resistance to packaged material
- odour transfer to packaged material
- heat resistance
- deep freeze resistance
etc., etc., etc.