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Abstract

Biaxially oriented low cost polyolefin bottles of improved optical clarity, gloss, and strength, and useful for packaging materials such as liquid detergents, shampoos, pharmaceuticals and intravenous solutions, are prepared by stretch blow molding polypropylene homopolymer co-polymer and allomer parisons or preforms which are formed from low temperature melts at temperatures just above the melting points of the polymers and allomers, and are injected at constant rates against cold polished mold surfaces at relatively low pressures and at slow rates to avoid melt fractures. Melt temperatures of from 490.degree. to 700.degree. F. are preferred for polypropylene blow mold type polymers and allomers. Polished beryllium copper mold surfaces are desired for high luster gloss finishes. Mold temperatures of from 30.degree. to 80.degree. F. are preferred. Injection molding with a high compression ratio of 3 to 3.5 for insuring a homogeneous melt is preferred with a relatively slow injection rate to fil the mold cavity of about 3 to 10 seconds. The low temperature molding limits the size and increases the number of spherulites that form as the melt cools and solidifies thus minimizing crystallinity in the parison. The low mold temperatures provide faster molding cycles, increasing production and reducing costs but the injection rate is sufficiently slow to avoid melt fracture. The strength blow molding is preferably conducted at temperatures just below the amorphous flow temperature of the parison with temperatures of about 295.degree. F. being sufficient for the polypropylene copolymer and about 325.degree. F. below satisfactory for the polypropylene homopolymer.

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