Blow Molding

General

Blow molding is a process for shaping thermoplastic into a hollow product. A hot thermoplastic tube is stretched with air pressure and cooled against a mold surface.

A variety of blow molding techniques exist for different applications:

• Extrusion blow molding
• Injection blow molding
• Stretch blow molding

In extrusion blow molding, a hollow tube is extruded. The tube is then positioned in a mold and compressed air causes the tube to press against the cool walls and solidifies to the mold surface shape.

Extrusion molding exists as continuous extrusion and intermittent extrusion. For PVC, the continuous process is normally used.

Injection blow molding and stretch blow molding involves first forming of a preform, by extrusion or injection molding. This is later blown to the desired shape.

Mold material requirements

Mold material requirements are of course totally different if talking about the injection molding or extrusion of preforms or if talking about the blow molding itself.

Blow molding is a low-pressure method where strength and wear demands are very moderate. Some parts of the mold, like shot off areas and/or neck ring, may require higher strength material. PVC is a common material for bottles and may cause corrosion problems. Blow molding is a very high productivity method where cycle times are of great importance.

Mold material properties

Important properties are:

• Moderate strength demands
• Corrosion resistance
• Heat conductivity