Effect of flow rate during injection molding on crystallization kinetics and ultimate properties of PEEK and its short fiber composites

A miniature injection molding device has been utilized in conjunction with in-situ wide angle x-ray scattering, using a position sensitive detector, to determine the kinetics of strain-induced melt crystallization of poly(ether ether ketone) (PEEK) and its short fiber composites under a range of processing conditions. It has been demonstrated that increased flow rate of the melt in the mold and, consequently, increased shear rate accelerates the crystallization process of PEEK as well as its composites. Short glass fiber composites of PEEK crystallize slower than the resin under identical processing conditions, while short carbon fiber composites crystallize faster than the resin, except at the highest mold temperatures and the lowest flow rates.;A model based on the Avrami equation has been proposed to fit the kinetics data obtained experimentally. The Avrami coefficient has been calculated and Arrhenius plots have been used to predict the crystallization kinetics at temperatures lower than those at which experimental data have been obtained here.;The effect of the process parameters of injection molding on the ultimate crystallinity, elastic modulus and fracture toughness has been determined in order to optimize these parameters: flow rate, mold temperature, molding time, melt temperature and soak time at melt temperature. It has been shown that the crystallinity of the resin and, hence, the elastic modulus increase with increase in crystallization temperature and/or flow rate. Fracture toughness has been shown to decrease with increasing mold temperature. The elastic modulus of the composites is, as expected, substantially greater than that of the resin and is not significantly affected by variation of the process parameters. The fracture toughness of the composites is also higher than that of the resin; the matrix of the composites undergoes a more ductile failure in the regions of greater fiber content than in the fiber-free regions. Chain orientation has been shown to be absent in the bulk of injection molded PEEK, under the present molding conditions; however, extensive fiber orientation is observed in the composites.;In summary, crystallization kinetics data of PEEK as well as its short fiber composites under melt-strain conditions have been obtained for the first time and have been correlated to a model based on the Avrami equation. This will enable minimization of the processing time for injection molding of these materials. Processing-property relationships have also been developed so that the ultimate mechanical properties of these materials under each set of processing conditions can be predicted. This will enable the injection molder to optimize the process parameters for any desired set of mechanical properties in the final product.