What Is Blow Molding?

Three plastic orb water bottles.

Blow molding is a molding process used in the manufacturing industry to create hollow objects made of plastic. Like other molding processes, it involves the use of heated, liquid material that’s forced into a mold cavity under pressure. Blow molding is a special type of molding process, however, that leverages the properties of traditional glassblowing.

Overview of Blow Molding

Blow molding, also known as blow moulding in the United Kingdom, is a molding process in which heated plastic is blown into a mold cavity to create a hollow object. The defining characteristic of a blow molding is that it’s used to create hollow objects. Raw plastic is first heated, after which it’s formed into a parison. Next, the plastic parison is secured to the top of the mold. Finally, air blown down onto the plastic parison, thereby stretching it across the interior walls of the mold cavity.

Blow molding follows the same principle as glassblowing. With glassblowing, a glassblower blows air across heated glass, thereby creating a hollow glass object. With blow molding, a machine blows air across heated plastic that’s placed on top of a mold cavity. The air forces the heated plastic to expand across the interior walls of the mold cavity.

The Different Types of Blow Molding

There are several different types of blow molding, one of which is extrusion. Known as extrusion blow molding (EBM), it lives up to its namesake by extruding heated plastic into a parison. It’s a common molding process used in the manufacturing industry because of its ability to mass-produce a large volume of objects in the same size and shape.

Another common type of blow molding process is injection stretch blow molding. Using either one or two stages, injection stretch blow molding is typically used to create plastic bottles. It’s specifically effective for creating preforms of plastic bottles, which are then either sold to bottling companies or used to manufacture a bottle.

Blow Molding vs Injection Molding: What’s the Difference?

The terms “blow molding” and “injection molding” are often interchgeably when referring to molding processes. While similar, though, they aren’t necessarily the same. Both blow molding and injection molding involve the use of liquid material — typically plastic — that’s forced into a mold cavity. The difference is that blow molding is used to create hollow objects, whereas injection molding is used to create solid objects. For hollow objects, only blow molding offers a fast and effective solution for manufacturing companies.

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