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.

A preform-molding device has a nozzle section including three cylindrical layer-forming flow channels, which include an inner flow channel and an outer flow channel for forming the substrate layers of the main resin, and a middle flow channel for forming the intermediate layer, and also includes a predetermined number of vertical blocking rib pieces disposed at a downstream end of the middle flow channel, in parallel in the circumferential direction, so that the intermediate layer would be segmented in the circumferential direction into a predetermined number of vertically running streams, wherein flow behavior of the intermediate-layer resin flowing through the middle flow channel is adjusted by positioning the vertical blocking rib pieces in the circumferential direction so as to minimize a difference in height between a maximum height point and a minimum height point that appear in a leading edge of the intermediate layer of the preform.

A method for manufacturing a container includes injecting a first material into a first mold to form a top portion of a preform. The first material and a second material are injected into the first mold to form a bottom portion of the preform. The preform is disposed in a second mold. The preform is blow molded into a finished container.