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 th