Wrinkling in polymer film‐polymer substrate systems and a technique to minimize these surface distortions

Thermally induced wrinkling during thermoforming of a commercial multi‐layered polymer film/substrate laminate has been reported. The differential thermal expansion of component layers coupled with phase transition of the substrate with increasing temperature, determined the critical conditions for wrinkling with a specific wavelength and amplitude. An effective technique to minimize wrinkling by biaxially stretching the samples at high temperature before the forming operation, has been proposed. The samples were biaxially stretched by inflating the samples using a specially designed blowing unit retrofitted to a conventional vacuum thermoformer. This method involved heating, inflation and forming, together to provide stretch‐assisted thermoforming. During biaxial stretching the stored compressive stresses in a wrinkled sample were relieved before the forming step, producing a decorative part without losses in surface appearance. POLYM. ENG. SCI., 57:31–43, 2017. © 2016 Society of Plastics Engineers