Pre-treating plastic bottles or containers prior to printing is sometimes required in order to get good ink adhesion as often measured by the tape test. Several methods are used for improving the surface tension of the substrate, including open flame exposure, corona discharge, and chemical application. Generally, flame treatment gives the best result that will last the longest. Some substrates do not have the right surface tension to allow the ink to soak in and bond correctly. Both low- and high-density polyethylene are most often treated after the molding process. Normally, in the case of HDPE bottles, the manufacturer will pass them through a treatment process of some kind.