Extrusion is one of two forms of blow molding used for hollow enclosed products. An extruder, or ram, is used to force melted plastic through a tube forming die. The extruded tube, called a perform or parison, descends vertically between a split cavity mold, made of either aluminum or high temperature epoxy. The cavities shape the final part. The mold closes on the parison. The tube is trapped inside the mold. The top and bottom of the tube is sealed as the mold closes. At some point on the mold, the parison is punctured with a small air tube. Air pressure is injected into the soft parison causing it to expand against the sides of the mold. The mold cools the plastic and forms the hollow shape. The mold opens and blown hollow objects is ejected by an upper ejector plate. The pinched ends can be removed by machining. Typical products made by extrusion blow molding are 55 gallons drums, bottles, trash containers, double walled tool and instruments cases, arm rests, portable hot drink containers, and other hollow objects.

Blow molded containers have irregular wall thicknesses induced by the stretching of the plastic parison during the blowing process. To overcome the irregular wall thickness the parison is extruded with irregular thickness. The opening in the die that forms the parison is changed to form different wall thickness. To control the thickness a computer with a parison control program is connected to the extruder die. The programs automatically opens and closes the die to increase the amount of material to be formed in the parison around threaded areas and the weaker bottoms sections. The pinch-off mark left at the bottom of hollow containers identifies extrusion blow molded from injection blow molded product.