PURPOSE: To prevent that partially non-uniform tensile force is applied to the parison in a mold by integrally providing a coating layer of a thermoplastic resin of which the melt viscosity at blow molding temp. is specified on the outer wall of a preformed object composed of a thermoplastic resin and introducing the coated preformed object into a blow mold to blow air into the preformed object.
CONSTITUTION: The thermoplastic resin A charged in an accumulator 3 is extruded by a ring piston 4 and the extrudate passes through an annular passage 5 to reach a passage 8. Next, a thermoplastic resin B of which the melt viscosity at the blow molding temp. of the thermoplastic resin A is 300-2000P is extruded by an in-line extruder 6 to be passed through an annular passage 7 and the coating layer of the thermoplastic resin B is formed on and unified with the outer wall of the thermoplastic resin A to form a parison having a two- layered structure. Next, the two-layered parison is introduced into a pair of left and right opening and closing blow molds and air is blown into the hollow part of the parison while the coating layer composed of the thermoplastic resin B is brought into contact with the surfaces of the blow molds to expand the parison.
HARADA KOHEI
ISHIDA KEIICHI
