BLOW MOULDING

Extrusion is used to push a molten tube of plastic, called a parison, through a die and into a bottle-shaped mold (a). Compressed air is blown into the parison, forcing it against the cold walls of the mold (b). The molded piece is then cut and removed from the mold (c).

Blow Molding

Blow Molding is a highly developed molding technology developed back in the late 1800's to produce celluloid baby rattles. It is best suited for basically hollow parts (such as plastic bottles) with uniform wall thicknesses, where the outside shape is a major consideration.

The first polyethylene bottle was manufactured using the blow molding process in December of 1942. This was the real beginning of a huge industry which currently produces 30 to 40 billion plastic bottles per year in the U.S. alone.

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The Basic Process

1. A thermoplastic resin is heated to a molten state
2. It is then extruded through a die head to form a hollow tube called a parison.
3. The parison is dropped between two mold halves, which close around it.
4. The parison is inflated.
5. The plastic solidifies as it is cooled inside the mold.
6. The mold opens and the finished component is removed.

Variations

There are basically four types of blow molding used in the production of plastic bottles, jugs and jars. These four types are:

1. Extrusion blow molding
2. Injection blow molding
3. Stretch blow molding
4. Reheat and blow molding.

• Stage 1
  Raw material resin is provided to the extruder. It is then transferred to the Die/Head of the extruder as it is melted by the frictional heat of the screw and the surrounding temperature.
• Stage 2
  The Die/Head exit of the extruder is a cylindrical shape from which cylindrical extrusion is issued. This cylindrical extrusion is called a parison.
• Stage 3
  Outside the Die/Head, generally a two-part mold is on standby. In the divided left and right molds is an empty space with the shape of the product to be molded, indicating that a cavity (mold) is formed.
• Stage 4
  In a state where the parison is extruded into the proper length at the Die exit, insert the parison between the two divided cavities.
• Stage 5
  Blow compressed air into the parison after insertion between the cavities so it fills the cavity.
• Stage 6
  When the parison is fully enlarged to in the shape of the cavity, it is cooled in that state. For our products, the temperature of the parison and its temperature after the cooling vary slightly by grade. The parison temperature of our company’s product is approximately 180-240 °C, whereas the appropriate temperature for the cavity coolant is about 5-20 °C.
• Stage 7
  Once the parison inside the cavity becomes solidified from cooling at the room temperature, open the cavity and take out the product.
• Stage 8
  Product molding is completed by removing unnecessary parts attached to the product.