A resin molding method includes a first molding step of molding a first molded part and a second molded part in a mold and a second molding step of joining the first molded part and the second molded part. In the first molding step, the second molded part is formed by injecting resin into an area where a first core movable in the first direction and a second core movable in a second direction are engaged. In the second molding step, the second molded part is joined to the first molded part by injecting the resin while the first core is in contact with the second molded part and the second core is removed from the second molded part. Areas where the first core and the second core are located in the first molding step become hollow areas connected to each other and part of ends of the first core in the first direction is not in contact with the second core.

Disclosed herein are methods and systems for the preparation of a co-injection molded multilayer plastic article. Some methods include co-extruding a combined polymeric stream having an interior core stream encased by an inner stream and an outer stream. One or more pulses in a thickness of the interior core stream create portions of an edge of the outer flow stream flowing along different streamlines. A first portion of an outer edge of the interior core stream flows beyond a second portion of the outer edge thereby forming one or more cohesion members that physically interlock the interior layer with the inner layer, with the outer layer or with both. Some embodiments reduce or eliminate a need for addition of an adhesive in the composite stream to prevent delamination of layers of the resulting multilayer plastic article. In some embodiments, cohesion members may be employed to produce a desired cosmetic effect in a resulting article.

Disclosed herein are methods and systems for the preparation of a co-injection molded multilayer plastic article. Some methods include co-extruding a combined polymeric stream having an interior core stream encased by an inner stream and an outer stream. A volumetric ratio of the inner stream to outer stream is adjusted during injection of the combined stream into the a mold cavity to alter a streamline along which the interior core stream flows thereby forming cohesion member that structurally interlocks the interior layer with the inner layer, with the outer layer or with both. Some embodiments reduce or eliminate a need for addition of an adhesive in the composite stream to prevent delamination of layers of the resulting multilayer plastic article. In some embodiments, cohesion members may be employed to produce a desired cosmetic effect in a resulting article.

A method of injection molding a test tube-shaped preform for biaxial stretch blow molding includes supplying a major material resin from outer and inner flow paths to a combined flow path for a predetermined time and rate. For a period of time within a range of the predetermined time period during which the major material resin is supplied, the intermediate layer resin is simultaneously supplied from the middle flow path to the combined flow path at a second predetermined supplying rate. A columnar laminated molten resin is injected into a cavity of a metal mold connected to a tip of the nozzle through a gate to fill the cavity, the columnar laminated molten resin being composed of the major material resin and the intermediate layer resin formed in the major material resin in a laminated manner that are combined into a columnar shape at the combined flow path.