Coinjection Blow Molded Bottles

Technology to successfully produce multilayer injection blow molded bottles is much newer than that to produce multilayer extrusion blow molded bottles. Heinz Inc. s ketchup bottle was the first U.S. example (Fig. 12.22). The use of PET bottles made by coinjection blow molding has grown rapidly. As is the case for coextrusion blow molding, the key is production of the parison. Once the multilayer parison is produced, the remainder of the process is essentially the same as for single layer materials. Even stretch blow molding can be used. 

Two techniques are used for producing the multilayer injection molded preforms. The technology used for the Heinz PET/EVOH ketchup bottle involves simultaneous injection of two or more resins into the mold cavity in such a way that they remain in distinct layers. The second process involves a multiple set of preform molds, with a single resin injected in each step. 

As illustrated in Pig. 12.22, the coinjection stretch blow molded PET bottles actually have a five-layer structure, containing two layers of EVOH and three layers of PET. Presumably, the reason is that two thin EVOH layers provide better oxygen barrier than one thick EVOH layer, because the effects of any defects in one layer are diminished by the presence of the second. No tie layers are used in the structure, to facilitate recycling of the containers in existing PET recycling facilities. When the bottle is granulated and washed, most of the EVOH is removed. The small amount that remains does not significantly impact the quality of the recyclate. 

As was the case for ketchup bottles, most uses of coinjection blow molding have the aim of improving the barrier characteristics of the container, to extend the shelf life, and to better preserve the flavor, aroma, or other characteristics of the product. Reheat stretch blow molding is the most common process, with three to five layers of resin forming the parison. The skin layers, both inside and outside. 

One development for coinjection equipment was the inclusion of separate hot-runner temperature control systems for the different resins, for example, in order to allow an EVOH core layer to be processed at a temperature 70 C lower than the PET skin layers. Another development was the use of computer simulation to design tooling that manipulates the barrier resin distribution to fortify critical areas such as wall sections. Both wide mouth and narrow neck coinjection stretch blow molded bottles are available. 

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A similar problem is presented by vehicle tires and certain blow molded bottles, which must be virtually impermeable to air and other gases. An example of the use of a very impermeable elastomers is butyl rubber. Because of its impermeability to gases, butyl rubber is used as a roof coating. With plastic bottles, different layers of both coinjected and coextruded plastics (Chapter 8) can be used to fabricate the bottle to make it impermeable to different vapors and gases depending on the barrier plastic included. 

Figure 12.22 Coinjection stretch blow molded bottle (reprinted with permission from [1])...

In 1997, Coca Cola introduced a high-barrier 12-oz PET/EVOH coinjection stretch blow molded bottle that was reported to double the shelf life of the beverage in hot and humid conditions. It also permitted the use of post-consumer PET content in the center layer of the container. In 1998, Miller test-marketed a five-layer PET bottle with a nylon barrier layer in several U.S. cities. Anheuser-Busch also test-marketed plastic beer bottles. In 2000, Miller introduced plastic PET beer bottles in the 16- and 20-oz sizes on a wide scale, becoming the first U.S. brewer to do so [5]. In 2014, MillerCoors expanded use of plastic bottles to the 32- and 40-oz sizes, reportedly reducing the weight of each bottle by 1 lb [6]. However, beer in plastic bottles remains rare. 

Plastic users have made a market for recycled plastic by modifying machinery for two and three layer coextrusion heads. Coextrusion is a method applied primarily to HDPE and sandwiches a recycled plastic layer in between virgin resins. It is used because it produces a uniform appearance of bottle exteriors and a market safe container. Coinjection stretch blow molding of PET is another fabrication method being looked at. It is capable of producing multi-layer bottles with recycle PET sandwiched in between. Examples of... 

Single-stage blow molding machines are predominately used to produce PET bottles. There are installations, however, that are using crystal polystyrene, and coinjected materials as PET/nylon/PET, and PET/EVOH/PET. 

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