Overview of 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. 

For successful orientation, the preform must be brought to a temperature within its orientation temperature range, below its crystalline melting point but above its Tg. For PET this temperature range is 88 to 116°C, forPVC 99 to 116°C, and forPP 104 to 127°C. The temperature and amount of stretching affect the degree of orientation obtained. The cuial ratio is the ratio of the length of the stretched part 

Extrusion stretch blow molding begins with an extrusion blow molded preform injection stretch blow molding begins with an injection molded preform. Injection stretch blow molding is the method of choice for most PET bottles, including all soft drink bottles. It is also used for PEN bottles and some PP bottles. Extrusion stretch blow molding is used for some PVC bottles. 

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