Stretch-blow molded containers

In PVC and PET stretch blow molded containers, molecular orientation plays an important role in the mechanical, barrier, and optical properties of the container. Orientation improves the strength and barrier capability of the container, and enhances its clarity. 

Stretch blow-molded containers of AN, PVC, and PET may have an average wall thickness of 0.23-0.64 mm (0.009-0.025 in.) Normal blow molding would call for an average wall thickness of 0.46-0.5 mm (0.018-0.020 in). 

Another unique property of PET is that it can be heat set. Heat set is achieved by either of two methods. Normally, a PET stretch blow-molded container cannot be subjected to heat above approximately 54°C (130°F) because of distortion. This relates to the fact that the glass transition of PET is approximately 68-71°C (155-160°F). 

From a zero number in 1977, in just over 28 years, the number of (PET) soft drink containers produced in the United States has made it the largest single item produced per year. In 2(X)8, the number of PET soft drink stretch blow-molded containers passed over 30 billion. 

Soft-drink bottles made from poly(ethylene terephthalate) (PET) are usuady made by stretch-blow mol ding in a two-step process. Eirst, a test-tube-shaped preform is molded, which is then reheated to just above its glass-transition temperature, stretched, and blown. Stretching the PET produces biaxial orientation, which improves transparency, strength, and toughness of the botde (54,56). A one-step process is used for many custom containers that are injection stretch-blow molded. 

Single-screw extmsion in general is the most important conversion process for biodegradable plastics. Only injection-molded and injection-stretch blow molded rigid containers like closures and bottles are not produced by a continuous extmsion process. 

Different processes take advantage in applying orientation to gain certain properties in certain products. Major product lines include stretched blow molded bottles/containers (Chapter 6), thermoformed oriented containers (used for such products as fruits, vegetables, and baked goods) (Chapter 7), tapes, etc. 

To get even wall thickness throughout the container, just like for extrusion blow molded containers, the preform must be programmed to put additional polymer where the most stretch will occur. Since the core and the cavity of the injection mold are fairly permanent, one cannot make changes on the fly as one can in extrusion biow molding. Therefore, the designing must be done upfront. 

In either extrusion or injection blow molding, the polymers in the containers are oriented because of the radial stretching that takes place in the blowing step. If desired, the mechanical or barrier properties of the material can be maximized by producing biaxial orientation, stretching the preform vertically as well as horizontally, using stretch blow molding. 

Stretch blow molding begins with the production of a preform, either by injection molding or extrusion blow molding (or rarely, extrusion alone). The container finish is formed in this step. The preform is shorter than the final height of the container. Next, the body of the preform, on a stretch rod, is conditioned to an accurate, consistent temperature, usually just above its Tg, while the bottle finish is kept cool to avoid distortion. Then, the preform is placed in the container mold and is stretched by a vertical movement of the stretch rod while air is blown through the rod to expand the bottle into its final shape, stretching it axially (Fig. 12.13). 

Stretch blow molding produces a biaxially oriented container with improved impact strength, gas barrier, stiffness, clarity, and surface gloss. Consequently, containers can have thinner walls, saving money through light-weighting. 

Figure 12.13 Injection stretch blow molding, (a) injection molding of parison, (b) stretch blow molding of container (reprinted with permission from [1 ])...

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. 

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. 

Orientation consists of a controlled system of stretching plastic molecules to improve their strength, stiffness, optical, electrical, and other properties. This process has been used for almost a century, and became very prominent during the 1930s for stretching fibers up to ten times later it was adapted to stretching film/sheet and more recently blow molded containers. Many other products take advantage of its benefits (tape, pipe, profile. 

Sulfonated aliphatic-aromatic copolyetheresters may also find utility as plastic containers. The preferred method for molding a container is stretch-blow molding, which is generally used in the production of PET containers. Subsequently, a method of preparation is illustrated (61). 

The counter-rotating die of Figure 11.5 can be used for film, tubing, and blow-molded containers, as well as multilayer coextruded products. The machinery, processing, and applications are described in subsequent sections of this chapter. Specifically, section 3 will describe the importance of stretching, combined with shear, to control orientation. 

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