How does blow molding make hollow plastic bottles?

Blow molding is a manufacturing process used to create hollow thermoplastic containers, such as bottles, by expanding a heated plastic tube within a mold.1

The process follows these key steps:

• Parison Formation: A hollow thermoplastic tube, known as a parison, is created using either injection molding or extrusion.1
• Expansion: While still heated, the parison is sealed at one end and inflated with air, similar to blowing up a balloon.1
• Cooling and Shaping: The tube expands into a split mold with a cold surface. As the plastic contacts the cold mold, it cools and becomes dimensionally stable, taking the shape of the container.1

Manufacturers can program the parison's wall thickness during formation to ensure the final product has controlled thickness at critical locations, such as corners. In some cases, the material undergoes "stretch blow molding," where it is expanded in both diameter and length, resulting in a biaxially oriented structure that provides enhanced strength and crystallinity.1