Blow Molding

Blow molding is a manufacturing process that is used to create hollow plastic parts by inflating a heated plastic tube until it fills a mold and forms the desired shape. The raw material in this process is a thermoplastic in the form of small pellets or granules, which is first melted and formed into a hollow tube, called the parison. There are various ways of forming the parison, as explained below. The parison is then clamped between two mold halves and inflated by pressurized air until it conforms to the inner shape of the mold cavity. Typical pressures are 25 to 150 psi, far less than for injection molding. Lastly, after the part has cooled, the mold halves are separated and the part is ejected.

Parts made from blow molding are plastic, hollow, and thin-walled, such as bottles and containers that are available in a variety of shapes and sizes. Small products may include bottles for water, liquid soap, shampoo, motor oil, and milk, while larger containers include plastic drums, tubs, and storage tanks. Blow molded parts can be formed from a variety of thermoplastic materials, including the following:

  • Low Density Polyethylene (LDPE)
  • High Density Polyethylene (HDPE)
  • Polyethylene Terephtalate (PET)
  • Polypropylene (PP)
  • Polyvinyl Chloride (PVC)

As mentioned above, there are different methods used to form the parison which distinguish the following three forms of blow molding:

  • Extrusion blow molding - An extruder uses a rotating screw to force the molten plastic through a die head that forms the parison around a blow pin. The parison is extruded vertically between the two open mold halves, so they can close on the parison and blow pin. Pressurized air flows through the blow pin to inflate the parison. This is the most common type of blow molding and is used to manufacture large quantities of relatively simple parts.
  • Injection blow molding - The molten plastic is injection molded around a core inside a parison mold to form the hollow parison. When the parison mold opens, both the parison and core are transferred to the blow mold and securely clamped. The core then opens and allows pressurized air to inflate the parison. This is the least commonly used method because of the lower production rate, but is capable of forming more complicated parts with higher accuracy. Injection blow molding is often preferred for small, complex bottles, such as those in medical applications.
  • Stretch blow molding - The parison is formed in the same way as injection blow molding. However, once transferred to the blow mold, it is heated and stretched downward by the core before being inflated. This stretching provides greater strength to the plastic. Stretch blow molding is typically used to create parts that must withstand some internal pressure or be very durable, such as soda bottles.

Extrusion Blow Molding
Extrusion Blow Molding

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Typical Feasible
Shapes: Thin-walled: Cylindrical
Thin-walled: Cubic
Thin-walled: Complex
Part size: Envelope: Up to 105 ft³
Materials: Thermoplastics
Surface finish - Ra: 250 - 500 μin 250 - 500 μin
Tolerance: ± 0.04 in. ± 0.01 in.
Max wall thickness: 0.015 - 0.125 in. 0.01 - 0.24 in.
Quantity: 100000 - 1000000 1000 - 1000000
Lead time: Days Days
Advantages: Can form complex shapes with uniform wall thickness
High production rate
Low labor cost
Little scrap generated
Disadvantages: Limited to hollow, thin walled parts with low degree of asymmetry
Poor control of wall thickness
Poor surface finish
Few material options
High tooling and equipment cost
Applications: Bottles, containers, ducting

Disclaimer: All process specifications reflect the approximate range of a process's capabilities and should be viewed only as a guide. Actual capabilities are dependent upon the manufacturer, equipment, material, and part requirements.

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