Blow molding is a manufacturing process used to form hollow plastic parts. The process of blow molding begins with the melting of plastic and forming it into a parison which is a piece of plastic with a hole in one end. The parison is clamped into a mold and compressed air is blown into it. The air pressure inside the parison pushes the plastic out to match the mold. After the plastic has been allowed to cool and harden, the mold opens up and the plastic part is ejected.

The two main variations of blow molding are continuous and intermittent. In continuous blow molding the parison is extruded continuously and the individual parts are cut off. In intermittent blow molding, an accumulator gathers the melted plastic. When the previous mold has cooled and enough plastic has accumulated, a rod is used to push the melted plastic to form the parison.

The production of large quantities of hollow glass and plastic objects utilizes injection blow molding. In injection blow molding the polymer is injected onto a core pin. The core pin is then rotated to a blow molding station where it is inflated and cooled. This process is typically used to make small, single serve bottles. This process produces an injection molded neck for accuracy but only suits small capacity bottles because it is difficult to control the base center during blowing.

In the injection stretch blow molding process the plastic is molded into a preform using the injection molding process. These preforms contain threads on one end and are typically used as the necks on bottles. The preforms are packaged and after cooling they are fed into a reheat stretch blow molding machine. The preforms are heated then blown using metal blow molds. This process yields extremely high volumes with very little restriction on bottle design.

Plastic jars and other plastic containers typically have an excess of material from the molding process. A spinning knife is often used to trim around plastic containers and the excess plastic can then be used to create new moldings.

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