Continuous Extrusion Machines. The shuttle and the wheel types of machines have been adapted so that multiple screw-extruders feed into a common extrusion head to continuously form a parison tube having three, five, j seven, or more concentric layers of materials. All processing generates some scrap material. Multi-layer scrap is difficult to recycle because it is a mixture of materials which cannot be separated. Some applications will allow this mixed scrap to be reused as one of the noncritical layers. Careful control of reprocessing of scrap is required because reprocessing into the wrong layer of a container could be a calamitous situation.
Proprietary wheel-type machines have been used for the majority of barrier bottles manufactured to date. However, wheel-type and shuttle-type are now commercially available. The choice of type of machine basically depends on the rate of production needed, or the flexibility. Choose the wheel machine for high-volume production. Choose the shuttle machine for short production runs where a single parison tube is adequate. Shuttle machines, with their greater flexibility, will become more competitive with the wheel machine in the near future when extruder heads are available to simultaneously extrude two or more multi-layer parisons.
Coinjection Blow Machines. Nissei ASB Co. (College Park, GA)，the only coinjection stretch-blow machine commercially available today. It is used to manufacture narrow-neck bottles. The machine is similar to their standard stretch-blow machine, but it is equipped with two extruders. One extruder is for the structural material (such as PET) and the other extrudes the barrier material (such as EVOH, or nylon). A special manifold accepts the feed from the two extruders, but keeps the material separated until they are sequentially combined at parison nozzles. The PET material is the first material to be injected into the parison mold. It is followed immediately by the barrier material which flows into the hot interior section of the parison wall, between its cooling interior and exterior walls, to form a three-layer parison with the barrier material sandwiched between the PET layers. An adhesive layer is not used in this process where only three layers are the end result. Heat fusion is relied upon to produce a wall which will not delaminate in service. Coinjection-blow molding has been used with molds up to ten cavities. More cavities are theoretically possible, but costs and molding problems increase in some geometric ratio after the economical point.
A proprietary process and machine (American Can Co., Barrington, IL and reported in Feb. 1985, Plastics World) has been developed to produce wide-mouth can-shaped containers to package retortable food products. (“Retortable” means food subject to heat of cooking or sterilization after packaging). The process involves a five-layer tubular parison having an inner and an outer layer of poly olefin，two tie (or adhesive) layers, and an EVOH barrier. The tie-layers incorporate a desiccant to absorb moisture out of the EVOH barrier as the container cools after the retort operation, EVOH loses oxygen barrier properties as it absorbs water moisture，thus, the desiccant restores the barrier to full effect. A study reveals the precise timing sequence needed to ensure total encapsulation of the barrier material. It also shows only a three-layer operation, but it should not be difficult to visualize the tie-layer introduced to both sides of the barrier layer as it is injected. One advantage of coinjection-blow molding is the elimination of scrap formed at the neck and base of containers blown by other processes.