Analysis of the effect of pressure on compression moulding of UHMWPE

Ultra-high molecular weight polyethylene (UHMWPE) powder is effectively processed by compression moulding due to its very high melt viscosity. Compression moulding involves application of temperature and pressure as a function of time. The pressure applied during processing has a significant influence on the part properties. The effect of pressure applied during compression moulding was studied by moulding parts at different pressures. Increase in the applied pressure causes increase in the melting and recrystallization temperatures. An increase in the pressure applied at the melt temperature ( approximately 140 degrees C) from 7.8 MPa to 15.6 MPa caused the crystallinity to increase from 54% to 61%, the stiffness of the moulded part to increase from 257 MPa to 435 MPa and oxidative index to increase from 0.055 to 0.059. Further increase in the pressure applied at the melt to 23 MPa caused the crystallinity to fall to 49%, the modulus to reduce significantly to 302 MPa and the oxidative index to change to 0.063. Increase in the pressure applied at the recrystallization temperature ( approximately 91 degrees C) from 38 MPa to 78 MPa increased the crystallinity from 54% to 65%, increased the modulus from 257 MPa to 279 MPa and increased the oxidative index from 0.055 to 0.065. Further increase in the applied pressure to 97 MPa, caused the crystallinity to drop slightly to 61% the modulus to reduce to 269 MPa, and the oxidative index to reduce to 0.057. The experiments showed that for obtaining maximum crystallinity and stiffness, the applied pressure should be within a narrow range. The highest recrystallization pressure (97 MPa) indicated the formation of extended-chain crystals in addition to the chain-folded crystals. The change in pressure applied at the melt temperature had a significantly greater effect on Young's modulus, as compared to change in pressure applied at the recrystallization temperature. Fourier transform-infrared spectroscopy analysis of the samples moulded at different pressures revealed that the increase in crystallinity and stiffness was accompanied by increase in oxidation within the part. By filling the die in a nitrogen atmosphere instead of air, the oxidation level in the moulded parts was reduced by almost 60%, without adversely affecting the crystallinity and the modulus.