Ink-test-method to define surface tension or freedom of grease, respectively
You can determine the surface tension and, at the same time, the freedom of greaseof the surface of metal or plastic parts with the help of test inks. A coloured ink with adefined surface tension is applied to the surface to be tested with a brush. If the surface will be coated without being contracted the surface tension of the tested surface area is equal or bigger than the test ink. On the other hand, if the stroke of the brush contracts or shorten itself the surface tension of the tested surface area is smaller than the one of the test ink.

Therefore, with the test ink you can directly determine the grade of contamination of a metal or plastic surface. Oily or greasy surfaces do have a value of around 30 mN/m. Clean or greaseless surfaces have a value above 40 mN/m. High performance bonded parts out of rubber, plastic and metal request a freedom of grease with a surface tension of at least 38 mN/m.