Constant-temperature embossing of supercooled polymer films

In conventional hot embossing, a thermoplastic polymer undergoes phase transitions in liquid, semi-solid, and solid states through cyclic heating and cooling. This paper, in contrast, describes the development of a constant-temperature embossing process and compares its characteristics against standard hot embossing. The new process utilizes the crystallizing nature of supercooled polymer films to obtain the necessary phase transitions. By softening and crystallizing the supercooled polymer at the same temperature, the embossing and solidification stages can be carried out isothermally without a cooling step. PET, due to its relatively slow crystallizing kinetics, was chosen as a model material for this study. The embossed films with microgroove patterns of different sizes and aspect ratios were characterized for their replication fidelity and accuracy. For supercooled PET films, constant-temperature embossing with high replication quality and acceptable demolding characteristics was achieved in a large processing temperature window between Tg and Tm of PET. A parametric process study involving changes of the embossing temperature and embossing time was conducted, and the results indicated that the optimal process parameters for constant-temperature embossing can be derived from the crystallization kinetics of the polymer. The removal of thermal cycling is a major advantage of constant-temperature embossing over conventional hot embossing and represents an important process characteristic desired in industrial production. POLYM. ENG. SCI., 54:1100–1112, 2014. © 2013 Society of Plastics Engineers