Thermal expansion of semi-crystalline polymers: Anisotropic thermal strain and crystallite orientation

Abstract

Performance demands on injection- and blow-molded parts are steadily increasing. As a dimensional stability depends largely on shrinkage and warpage during/after processing, the process-induced changes should be taken into account in a mold design. To predict shrinkage, this study introduces an approach to model the thermal expansion based on an elementary volume unit cell consisting of stacked crystalline and amorphous layers. Its validation is performed with the help of the thermal expansions of injection- and blow-molded polyethylene parts measured with respect to the process directions by a dynamic mechanical analyzer in a tension mode. Differential scanning calorimetry measurements are carried out to obtain crystallinity as a function of temperature of the PE parts as an input parameter to calculate the thermal expansion. The additional utilization of the phase specific coefficients of thermal expansion (CTE) and Young's moduli taken from literature for the model showed that the measured thermal expansion lies between the calculated ultimate coefficient of thermal expansion. To adjust the ultimate CTE, the process-dependent tilting and rotation angles are fitted, and it seems that relaxation processes at elevated temperatures (which are not considered in the model yet) cause a deviation. Thus, the model yields a good agreement with the measured data up to a temperature of 70 °C. © 2020 Elsevier Ltd