Stretch-blown bottles

The workhorse polyester is polyethylene terephthalate) (PET) which is used for packaging, stretch-blown bottles and for the production of fibre for textile products. The mechanism, catalysis and kinetics of PET polymerization are described in Chapter 2. Newer polymerization techniques involving the ring-opening of cyclic polyester oligomers is providing another route to the production of commercial thermoplastic polyesters (see Chapter 3). 

PET is known for its clarity and toughness when it is used for the manufacture of oriented film or stretch-blown bottles. It is also a good barrier to gases, such as oxygen and carbon dioxide. The good oxygen... 

There are limits to how much the plastic can be stretched. For engineered parts, IV2 to 2 is a comfortable blow ratio with a maximum of 4 to 5 depending on the material. However, much greater blow ratios are used for blown bottles, particularly those stretch blown from injection-molded parisons in thermoplastic polyester. The lowest blow ratios are... 

The design of the preform in injection blow molding is critical. The preform should be designed to have a wall thickness in the body of the preform anywhere from approximately 0.035 in. ( 1 mm) to approximately 0.200 in. (5 mm). The preform length is designed to clear the inside length of the bottle in the blow mold by approximately 0.005 in. (0.125 mm). Thus there is minimum stretch in the axial direction of the preform when the bottle is blown. The diameter of the core rod is in all practicality determined by the maximum inside dimension (I-dimension) of the finish of the desired container. In determining the wall thickness of the preform in the main body, it is necessary to know what wall thickness is desired in the final blown article plus the maximum inside diameter of the desired blown article. The ratio of the inside diameter of the blown bottle (Dj) to the inside diameter of the preform (D ) is known as the hoop ratio. 

Barrier properties will be lower than a two-stage produced container as the single-stage stretch blown bottle is blown at a higher temperature. Thus, top load and tensile properties are lower. 

The two-stage process consists of the injection-molded preform being heated via metal sheath heat or quartz lamps to the orientation stretch blow molding temperature for the material being processed (PET— 195-240°F). The preform exits the healing area and passes to the equilibration area. From this area it is then transported to the blow mold area where air or other gases enters heated preform at pressures from 200 to 600 psi. The blow air is exhausted and the blow mold opens and the stretch blown bottle is transported to the eject station. The bottles are ejected, then sent downstream for case packing or labeled and then palletized. 

The orientation process takes place while the bottle is blown from the preform into its final shape. The stretch ratio from the preform to the bottle, the rate of stretching, and the temperature affect the level of orientation achieved. The stretch leading to orientation is associated with generating residual stresses in the container wall. Unless the temperature is lowered rapidly, relaxation will take place and the orientation will dissipate. The relaxation rate is high above the... 

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. 

If the preheated preform is stretched and blown into a hot mold, for example, 100-110°C and held there for a short period, the resulting Tg is in the range of 88°C, allowing for hot fill capabilities of such bottles, increasing the crystallinity to the 28-30% level and further decreasing the O2 permeability to one-third that of an nonoriented amorphous... 

A newer development is the manufacture of bi-axially oriented bottles, in particular for PETP. The pre-form is injection- moulded and rapidly cooled in the mould it remains amorphous (PETP crystallises very slowly) by heating above its Tg (65°) it passes into the rubbery state, and can then be blown-up and, simultaneously, longitudinally stretched. The biaxial orientation thus obtained, accelerates the crystallisation, and, at the same time, results in a very fine crystalline texture, so that a thin-walled, strong, transparent and heat-resistant bottle is obtained. 

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). 

Blow moulding describes any shaping process in which air is used to stretch and form plastic materials and can be compared to inflating a balloon. A hot thermoplastic tube, usually made by extrusion, can be inflated with compressed gas while inside a cooled split mould. Hot thermoplastic tubes or parisons can also be blown into free shapes without the aid of a mould (Figure 3.33). Plastic bottles, drums, car fuel tanks and other containers are often made using blow moulding (Figure 3.34). 

In stretch blow molding, a heated pre-formed melt is positioned in the blow mold (Figure 1.5). A center rod extends which stretches the preform with axial orientation. Blown air then expands the preform in the mold, fcwming a bottle with radial orientation. The stretch process takes advantage of the crystallization behavior of the resin and requires the pre-form to be temperature conditioned and then rapidly stretched and cooled. PET soft drink bottles are formed using the stretch method. PET bottles are also formed using extrusion blow molding. 

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