Process-induced residual stresses in compression molded UHMWPE

Non-isothermal cooling during processing causes the development of residual stresses, which are analyzed for compression molded UHMWPE, and affects th e dimensional stability. The development of thermal residual stresses was p redicted using an incremental stress analysis that included temperature-dep endent material properties. Strain gauges were used to measure the residual stresses as layers were removed from a molded disk using a Process Simulat ed Laminate (PSL) approach. The PSL technique has not previously been appli ed to a compression molded neat polymer. For initial surface cooling rates of similar to 11 degrees C/min, the model predicted a compressive stress at the bottom surface of 14 MPa and a tensile stress near the center of 2.5 M Pa and matched the experimental distribution well. Because the compressive residual stress was 70% of the yield strength (similar to 20 MPa), a lower cooling rate was also tested (2.6 degrees C/min). The maximum tensile and c ompressive stresses for this cooling rate were, 0.91 MPa and 2.5 MPa, respe ctively. The model demonstrated its use for predicting thermal residual str esses in compression molded parts, instead of trial-and-error experimentati on. UHMWPE is shown to develop residual stresses continually from similar t o 120 degrees C to 23 degrees C.