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Extrusion blow molding

Plastic forming process where an extruded plastic tube is inflated with air inside a mold to form the final product.

Illustration of Extrusion blow molding

The process

Plastic pellets [1] are supplied via a hopper into a rotating screw [2] whose spiral shape moves the pellet forward through a heated chamber [3].The pellets are heated gradually and turns into a homogeneous plastic melt as it approaches the end of the helical screw.

The molten mass is extruded through a die [4] into a tubular profile, in which a cavity is created by the center core comprising a tube.

A: The profile then passes between two separated mold halves.

B: When the profile reaches the mold halves the bottom of the molds are closed which pinch the lower end of the profile and the mold clamps the center of the die core. Then the extruded profile is pressurized from the inside through the center core allowing it to expand and finally conform to the mold walls.

After the mass solidifies the mold halves are separated and the part is removed and the process is repeated.

Advantages and disadvantages
In comparison with alternative methods
The tool costs less than the injection molding
Low cost of equipment
Easy to create large components with open ends
Continuous process
Longer cooling time than the injection molding
Only uniform circular cross-section
Good tolerances only at the outer wall