Blow molding is a plastic forming technique in which a heated tube of molten plastic (called a parison) is inflated with compressed air inside a mould cavity. The air pressure forces the soft plastic outward against the mould walls, creating a hollow, seamless shape that cools and solidifies into the finished product.
Process types
There are three main blow molding methods used in industry. Extrusion blow molding is the most common: a continuous tube of molten plastic is extruded downward, the mould closes around it, and air is blown in to expand the plastic against the mould walls. Injection blow molding starts with an injection-moulded preform that is reheated and blown to its final shape — this method produces more precise neck and thread details. Stretch blow molding adds a mechanical stretching step before blowing, which improves the strength and clarity of the finished part.
All three methods share the same core principle: air pressure shapes a softened thermoplastic into a hollow form. Common materials include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and polyethylene terephthalate (PET).
Application
In the construction industry, blow molding is used to manufacture a range of hollow plastic products. Drainage pipes, water tanks, septic tanks, ventilation ducting, cable conduits, and hollow insulation panels are all produced using this technique. Blow-molded products are lightweight, corrosion-resistant, and cost-effective compared to metal or concrete alternatives.
Rainwater harvesting tanks, underground attenuation crates, and modular drainage cells are increasingly manufactured by blow molding due to the ability to produce large, complex hollow shapes in a single operation. The seamless construction eliminates joints and potential leak points, which is a significant advantage for liquid containment applications.
Outside of construction, blow molding produces everyday items like bottles, jerry cans, automotive fuel tanks, and playground equipment.
Related terms
- Injection molding
- Extrusion
- Thermoplastic
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.