a method for controlling the hang time of a tubular parison formed from a thermoplastic resin is provided with a second higher melting thermoplastic resin as an additive , and blow molding the resulting resin at a temperature below the glass transition temperature of the higher melting component . in the case where the lower melting base resin is pbt and the higher melting additive resin is pet , the temperature of molding is below the tg of the pet additive .