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Blow molding, also called blow forming, is a manufacturing process which is conducted by using thermoplastics by which hollow plastic profiles are formed.
The blow molding process begins with applying heat energy to the resins thus which converts them into a molten state and forming them into a parison or perform. The prison is a tubular piece of plastic with a hole in one end in which compressed air is released. The compressed air expands the parson into the female mold cavity, while the pressure of the compressed air is maintained for a pre determined period of time, thus allowing the parson to expand and take the shape of the mold cavity. Once the plastic has cooled the mold is opened and the part is ejected.
In general, there are three main types of blow molding systems which are as follows:
Extrusion blow molding can be either continuous (where the parson is extruded continuously and separated by a hot cutter or knife) or intermittent (where the screw turns, stops and pushes the material out. If there is an accumulator head it will gather the melted material and form into a parson when enough material is accumulated).
Injection blow molding consists of two stages, injection molding of thermoplastic material into hollow tube shape called a preform and blow molding where the preform is transferred on to a metal shank called the core rod which rotates to a blow moulding station where it is inflate and cooled. This process is suitable for producing large quantities.
Stretch blow moulding is the process where a plastic container is made from a preform or parison that is stretched in both directions when the preform is blown into desired container.