Test pen for quick test of film surface tension.
To perform the test, a line of ink is applied to the film with the pen and the behaviour of the ink observed. If the ink forms a solid line, the film has a surface energy of at least 38 mN/m and can be used for printing and other converting steps. If the ink contracts and does not form a solid line, the film's surface energy is below 38 mN/m. This indicates that the film is insufficiently treated for most converting processes.