Unit_8_-_Plastic_Packaging (8)

This is important when the finished bottle will be

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the extrusion blow molding process. This is important when the finished bottle will be used for a carbonated beverage or if it will be pressurized because the threads on the bottle and in the closure must fit tightly enough to provide a tight seal and prevent the loss of the carbon dioxide. The dimensions and the amount of plastic in the preform depend on the size of plastic bottle being produced in the second step of the process. The next step in injection blow molding is to transfer the preform, or parison, into the blow mold, where air pressure is used to form the final container shape. Injection blow molding is used for HDPE and PP bottles, especially those in small sizes, as well as PET bottles, though PET bottles actually use a variant of this process most of the time. One advantage of injection blow molding compared to extrusion blow molding is a lot like the advantage of press- and-blow for glass bottles compared to blow-and-blow; we have much better control over the distribution of the plastic in the final container if we form the parison by injection rather than extrusion. An additional, and very important, advantage is that we have better control over the finish dimensions of the bottle when the finish is made by injection molding, as it is in this process. That translates into more reliable sealing of the bottles, which is very important for sensitive applications such as pharmaceuticals. Another difference relates to the versatility of the process. We cannot use extrusion blow molding to make PET bottles, we have to use injection blow molding. PET will not hold the hollow tube shape of the parison because of its low melt strength. 8. 23
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  • Winter '08
  • HaroldHughes
  • Blow molding, injection blow

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