Compression molding simulation of ultra high molecular weight polyethylene

Narasinha Chandrakant Parasnis, Purdue University

Abstract

Experimental investigation based on design of experiments approach was conducted for studying the effect of different processing variables on mechanical and physical properties of compression molded Ultra High Molecular Weight Polyethylene (UHMWPE). Soak times at melt and at crystallization temperatures, cooling rate from molten state and pressures applied during the molding cycle were most significant processing variables. Based on the preliminary experiments, further detailed investigation into the effect of pressure on the mechanical and physical properties of the molded part was conducted. The study revealed that for highest crystallinity and modulus, the pressure applied at melt should be ∼15 MPa and that at crystallization temperature it should be ∼77 MPa. Studies on non-isothermal crystallization of UHMWPE showed differences in ultimate crystallinity (∼11%) as the cooling rate was changed from 1°C/min to 22°C/min. The changes in crystallinity were more significant at lower cooling rates (<6°C/min) than at higher cooling rates (6 to 22°C/min). A compression molding simulation including crystallization kinetics and temperature dependent polymer properties was implemented. Three different processing cycles capable of producing different crystallinity distributions were modeled and verified. The model showed that a compression molding cycle with soak at crystallization produced high crystallinity (∼48%) parts. The model was modified to account for the effect of pressure on crystallization. The perfect contact assumed in the finite element model at all the interfaces led to low predictions of pressure within the polymer. Pressure-volume-temperature analysis was conducted at high pressures (20 to 160 MPa) to access the effect of pressure. The change in onset, peak and end crystallization temperatures was linearly related to the applied pressure with every 1 MPa increase in pressure accounting for an increase of 0.3$C in the crystallization temperatures. The temperature predictions from the finite element model were used in a residual stress model to predict residual stresses within compression molded parts. The model predicted that for a cycle with soak at crystallization the residual stresses would be low (∼−1 to 0.8 MPa) since the temperature and cooling rate gradients at the end of the crystallization are nonexistent.

Degree

Ph.D.

Advisors

Ramani, Purdue University.

Subject Area

Mechanical engineering|Materials science

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