The main disadvantage of the contact angle method used in a production line is that it takes time to get a result. As a
rule, a piece of the foil has to be cut out and brought to the contact angle device. For preliminary tests of the
treatment effect it is suitable to use the test inks. These are fluids with known surface tensions. Dependent on the
behaviour of a drop on the surface the surface tension can be estimated very quickly close to production. If the fluid
which may be applied to the surface with a brush or a felt pen forms drops on the surface, surface tension of the ink
is higher than that of the surface. On ther other hand, if the ink spreads on the surface, surface tension is higher than
the surface tension of the ink.