How Hollow Extrusions are Made

Aluminum extrusion is the process of creating unique aluminum shapes through the use of machining and metal dies. Most manufacturers choose the extrusion process over other metal forming processes because it is easier to make complex or unique shapes through the use of aluminum extrusion. The aluminum extrusion process also makes it possible to create hollow extruded aluminum shapes, which are ideal for creating unique machine parts and other hollow pieces for a variety of uses.
Most hollow extruded aluminum has a fairly small hole in the middle, to increase the strength of the metal exterior. The larger the opening, the harder it is to keep the outside metal from collapsing. This makes creating hollow pieces with large interior diameters more expensive than hollow pieces with smaller interior openings.
Usually, a mandrel is used to create the interior hollow section of the extruded metal. There are two different methods that are commonly used to create hollow aluminum extrusion pieces. These two methods are known as the porthole and bridge extrusion methods.
Porthole: Porthole dies are the easiest hollow extrusion method, and therefore also the least expensive. In a porthole die, the die basically cuts open the two sides of the metal and uses a mandrel to scoop out the inside of the metal. The die then welds the two sides of the metal back together. The seam is nearly impossible to detect using this extrusion method, due to the way that the die and mandrel work together in heating the metal and shaping the design.
Bridge: The bridge die method is more expensive and a little more difficult to accomplish. Usually, this hollowing method is only used for high-pressure applications where cutting the aluminum piece open would damage the integrity of the metal piece. The main difference between a porthole and bridge die is that the mandrel is internal in a bridge die, and as the aluminum piece is formed, the die scoops out the inside as it goes along. Bridge dies are ideal for creating continuous hollow extrusions.