What is injection blow molding?

Hollow plastic parts are formed through the injection molding process. The process utilizes three steps.

1. Plastics are injected into a mold, forming a plastic tube.
2. The tube is blown into a cavity mold, forming a hollow part.
3. The part is ejected.

In the first step, a split half mold closes on a mandrel, which is a steel rod, forming the center of the tube. Plastic is forced into the mold and around the mandrel. The formed plastic tube is referred to as a parison.

At the second station, the mandrel and parison are subjected to a second mold. The shape of the final product is reflected by the cavity of the second mold. As the mandrel is held at the center of the closed mold, air is injected through the mandrel. This process develops a hollow plastic part as the plastic parison is blown into the shape of the mold. After the hollow part has cooled off, the mold opens. The plastic part is then sent to the third station where the completed part is ejected off the mandrel using a stripper bar.

In the extrusion molding process, a ram, or extruder, forces melted plastic through a die that forms a tube. The extruded tube is referred to as a parison or perform. The parison drops vertically between a split cavity mold that can be made of either high temperature epoxy or aluminum. The finished part is shaped by the cavities. After the mold closes on the parison, it traps the tube inside. As the mold closes, the top and bottom of the tube is sealed.

The parison is then punctured with a small air tube. Air pressures than injected into the parison that forces it to expand against the mold all sides. The hollow shape is formed as the mold cools the plastic. As the mold opens, and upper ejector plate ejects the blown hollow object. Machining may be used to remove the pinched ends and burs.