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SynopsisThe properties of injection-molded plastics parts are affected by resin properties, molding geometry, and molding conditions. The thermomechanical history, which results from the interactions between resin properties and molding conditions, controls the development of microstructure within the article. In turn, the microstructure controls the ultimate behavior of injection-molded parts. Morphology and orientation and their distributions are among the most important microstructural characteristics affecting the ultimate mechanical and physical behavior of molded articles. This work summarizes some of the results of an extensive experimental and theoretical program to study the factors affecting the development of morphology and orientation in injection-molded polyethylene. In particular, various experimental techniques are employed to study the distribution of morphology and orientation in simple moldings and to relate these characteristics to resin properties and molding variables. Furthermore, an attempt is made to employ mathematical models to explain and predict some of the observed phenomena especially in relation to the distribution of morphological zones in the molding. It appears that model predictions are in good agreement with experimental results.

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Two chemically modified chain extended/branched polyethylene terephthalate (PET) resins and one unmodified resin, considered to be linear, were characterized in terms of their melt flow, die swell, and viscoelastic properties. The three resins had reportedly similar nominal intrinsic viscosities but exhibited different viscoelastic behavior. The modified resins had lower melt flow index, higher die swell, higher complex viscosity and higher storage modulus than the unmodified one. The Cole-Cole plots of the resins were independent of temperature, and the data for modified resins formed a group that lay below the data group for the unmodified PET. The distribution of relaxation times was determined. The modified resins had higher relaxation strength, G i , especially at high relaxation times, i . The mean relaxation times of the chain extended/branched resins were approximately an order of magnitude higher than that of the unmodified resin, implying pronounced elastic character. The modified resins had better foaming characteristics in extrusion foam processing than the unmodified one owing to their elastic nature.

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