Blow molding

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Blow molding efficiently produces hollow items such as bottles, containers, and light globes.Design permitting, the process may also produce hollow shapes such as automotive air ducts and gas tanks. Wall thickness can vary throughout the part and may change with processing. Blow molding cannot produce features that project from the surface such as ribs and bosses. Part geometry determines mold and equipment costs, which can range as high as those for injection molding.
The two most-common types of blow molding are extrusion and injection. In extrusion blow molding, mold halves pinch the end of a hanging extruded tube — called a parison — until it seals. Air pressure applied into the tube expands the tube and forces it against the walls of the hollow mold. The blown shape then cools as a thin-walled hollow shape. A secondary step removes the vestige at the pinch-off area.
Injection blow molding substitutes a mold shape in place of the extruded parison. Air pressure applied from inside the still-soft molded shape expands the shape into the form of the hollow mold. This process eliminates pinch-off vestige and facilitates molded features on the open end such as screw threads for lids.