the amount of material entering the mold finally when desired the displacement rod 18 can be rotated at a predetermined speed for enabling better radial flow around the channel formed between displacement rod 18 and preform 14 also contributing to coating evenness particularly since molten polymers are generally thyxotropic and reduce their viscosity with applied shear force the shear force exerted by the rotating displacement rod 18 in the molten barrier material 6 39 reduces the viscosity and enhances even flow additionally this reduces the hydraulic pressure needed to press the displacement rod 18 into the preform 14 with certain plastics this shearing action can also enable orientation of the plastic enhancing its barrier properties provided the molten barrier material 6 39 is kept at the appropriate temperature during its displacement when the displacement of the molten barrier material 6 39 is completed the preform 14 is reinserted into the cooling mantel 20 shown at position a of fig4 a and a cooling fluid is switched to flow in place of heating fluid in channel 19 the barrier material 6 39 now solidifies to form an inner layer 22 within preform 14 if the adhesion of the barrier material 6 39 of the inner layer 22 with the main material of the preform 14 requires an adhesive or a 34 tie layer 34 this can be introduced in the same manner as described before this results in an adhesive layer not shown between the material of the preform 14 and the inner layer 22 depending on the adhesive system used the need to melt.