Blow molding is a manufacturing process that’s designed to create hollow parts. It’s called “blow molding” because it typically involves blowing air or inert gas into the center of a heated object. As the air or inert gas accumulates, it creates an enlarged cavity inside the object. The object is then allowed to cool and harden, after which it will feature a hollow interior. Like with other manufacturing processes, though, there are different types of blow molding, each of which works in a different way. In this post, you’ll learn more about the most common types of blow molding, including extrusion, injection and injection stretch.
Extrusion Blow Molding
Perhaps the most common type of blow molding process is extrusion. With extrusion blow molding, plastic is heated until it turns from a solid state to a liquid state. The liquefied plastic is then forced into an extrusion machine. The extrusion machine also forces air into the melted plastic, essentially inflating it to create a hollow interior. Once finished, the plastic is cooled — typically at room temperature — and the newly created hollow object is ejected and removed from the mold.
Extrusion blow molding can be classified as either continuous or intermittent. The former involves the continuous, nonstop extrusion of the parison, whereas the latter involves interval-based extrusion of the parison.
Injection Blow Molding
Another common type of blow molding process is injection. As the name suggests, injection blow molding involves the “injection” of material, including heated glass and plastic, into a mold. The heated material is typically forced into a core pin, after which the pin rotates around a molding station where the heated material is inflated and then cooled.
Injection blow molding isn’t as popular as extrusion. Nonetheless, it’s a viable molding process for small, hollow objects. Injection blow molding is used to make everything from medical devices to soda bottles.
Injection-Stretch Blow Molding
There’s also injection-stretch blow molding. Injection-stretch blow molding is a variant of standard injection blow molding. It can be performed in either one step or two steps. Both types of injection-stretch blow molding, however, involves stretching the parison. The heated material is literally stretched so that it creates a hollow interior.
To recap, there are three main blow molding processes: extrusion, injection and injection-stretch. Extrusion blow molding involves extruding heated material, injection blow molding involves injecting heated material into a mold, and injection-stretch blow molding involves stretching heated material.