Testing Inks & Pens, Contact Angle Measurements – Surface Tension Testing for Metal Cleanliness or Plastic Substrates. Surface energy is one decisive criterion for the adhesion of printing ink, glue, varnish, etc. on many plastic and metal surfaces. Apart from some exceptions the general rule is: The higher the surface energy of a material, the better the adhesion of anything meant to be fixed to its surface. Reasons for a low surface energy might be material specific, such as low surface energies thus needing chemical or physical treatment to achieve a good adhesion. Also, contamination of the surface might be a reason for a low surface energy such as grease, oil, fingerprints or migrated additives in plastics. All the same, testing the surfaces for surface tension with these inks makes for a quick and easy method of inline operator testing, catching these discrepancies which can impact the quality of the products.