A parison for blow molding is applied to a core rod by successive injectionoperations in different injection molds. The material for the first layer is injected into the end of the mold cavity remote from the neck portion of the core rod, in the usual manner. The second layer is injected into the second injection moldcavity adjacent to the neck of the core rod to flow over the first layer in a direction from the cooler part of the first layer toward the hotter part. The twolayer parison is then transferred to a blowing mold where it is blown in the usual way.
This invention applies a two-layer parison to a core rod in successive injectionmolds and then transfers the two-layer parison to a blowing mold. The practical difficulty, in the prior art, to the injecting of two layers of plastic on a core rod before blowing has been that the second layer damages the first layer.
The usual way of applying a parison to a core rod, in an injection mold, is to inject the plastic material into the mold cavity at the end of the cavity remote from the neck of the core rod. Thus the plastic material is applied to the end of the core rod which is furthest from any contact with the metal of the mold; and the injected material highly heats the end of the core rod since the only way in which heat can flow from the end of the core rod is lengthwise toward the neck and through the relatively restricted cross-section of metal.