The Stages of Blow Moulding

today virtually all cast plastic parts are manufactured using blow molding technology.

Blow molding is a process used to cast plastic parts that are hollow. Since 1942 when the first polyethylene bottle was produced using blow molding technology, blow molding has become the standard manufacturing process for making cast plastic parts. Blow molding involves the fabrication of a tube made of molten plastic called parison. The next stage of the process is to shape the tube into the desired form using one of several blow molding processes.

Heating The Resin

The first step in any blow molding process is to heat the resin. Thermoplastic resin is heated to about 400 degrees. Once the resin reaches its molten state, it is extruded over a die head. The die head forces the thermoplastic resin to flow around it, thus creating a hollow center in the resin. The resin has now formed a tube called parison.

Molding

The parison is then placed inside a pre-determined mold. The interior shape of the mold cavity is what will ultimately determine the shape of the finished casting. The mold is then closed around the parison. At this point in the process, the parison is still very warm. The mold however, is cooled with water.

Inflation

Compressed air is then blown into the mold. The air fills the center of the parison tube and forces the sides to expand, thus conforming to the shape of the mold interior. The parison is still very warm at this point to allow it to expand without breaking. As the warm parison expands to meet the cold walls of the mold, it freezes off and becomes rigid.

Cooling

Once the warm parison has cooled off, it solidifies inside the mold. Once properly cooled, the parison will maintain the form in which it was cast. The mold can now be opened and the finished plastic component removed.

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