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==== ==== Authority information on the specalized subject of Blow Molds ==== ====

In a Blow Molding process a hot tube of molten plastic is extruded through a die. The hot tube of molten plastic is suspended in mid air before the mold closes, while the tube is in mid air it is stretching and cooling. With Injection molding the plastic is contained in the barrel of the machine and the runner of the mold, the process is controlled precisely until the part is ejected from the mold. With Blow Molding there is precise control in the machine but the process has elements that can be difficult to control. The big difference on the molded part is the fact that Injection molded parts molded in one shot have a single wall while Blow molded parts need to be a complete shell with an air gap on the inside. A simple blow molded part would be for example a shampoo bottle. A more technical blow molded part could be a bottom shell lawnmower seat hollow on the inside, this type of lawnmower seat would have many shapes and cut out's with some performed in the mold. When designing an Injection Molded part the general rule is to maintain a uniform wall thickness as much as possible, this is controlled by the mold and mold design. Wall thickness on a blow molded part is controlled by the machine while also the mold design does play a part in the wall thickness of the finished part. When designing a blow molded part many shapes are possible but blow ratio's have to be maintained. A hole is required on a blow molded part as the plastic is expanded in the mold with air pressure although some parts can be made using trapped air to blow. Apart from a sprue in a typical Injection mold there is no waste in fact on a Hot Runner mold there is no waste. A blow molded part does have waste because the plastic trapped between the mold does have excess outside the molded edge. This molded edge on a blow mold is called pinch edge or nip edge. It has a cutting effect to separate the waste plastic from the molded part. Designing a blow molded part needs many considerations and an understanding of the process is vital. Generally a shape can not be blown with reasonably uniform wall thickness unless the width of the cavity is equal to or less than the depth of the cavity. Another consideration is sharp corners, the only place a sharp corner is allowed on a blow molding is the parting line. Usually Injection Mold designers design only Injection Moulds and the same usually applies to Blow Molds because the mold designs are so different. A product designer working on plastic parts should know each process and be capable of desigining a part that will work for the process chosen. It is not possible to have a part design that can be blow molded or Injection Molded. It has to be one or the other and a good designer will know which process to choose.

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==== ==== Authority information on the specalized subject of Blow Molds ==== ====

Blow Molding process and Injection Molding process.