High modulus/high strength polyethylene obtained by high‐pressure injection molding

Increasing the nominal injection pressure up to 500 MP a improves the mechanical properties (modulus and strength) of injection‐molded high‐molecular‐weight high‐density polyethylene substantially. By a proper combination of barrel and mold temperature, the modulus of the molded parts (test bars) may be increased at least eight times compared to parts molded at 100 MPa. This improvement is partly due to the formation of high‐strength crystalline modifications of the polyethylene induced by flow and pressure. The extent to which these structure modifications occur in the samples molded at various conditions has been determined by thermal analysis (DTA). When increasing the thickness of the samples, a sharp reduction of the modulus and strength was observed, even though the concentration of the crystalline high‐strength phase was higher. An explanation of this effect in terms of the relaxation of the tie‐molecules connecting the crystallites is suggested.