THE EXTRUSION PROCESS
The aluminum extrusion process begins with the design process.
The aluminum extrusion process really begins with the design process, for it is the design of the product — based on its intended use — that determines many of the ultimate production parameters. Questions regarding machinability, finishing, and environment of use will lead to the choice of alloy to be extruded. The function of the profile will determine the design of its form and, hence, the design of the die that shapes it.
Once the design questions have been answered, the actual extrusion process begins with billet, the aluminum material from which profiles are extruded. The billet must be softened by heat prior to extrusion. The heated billet is placed into the extrusion press, a powerful hydraulic device wherein a ram pushes a dummy block that forces the softened metal through a precision opening, known as a die, to produce the desired shape.
This is a simple diagram of a typical horizontal hydraulic extrusion press; the direction of extrusion here is from left to right.
That is a simplified description of the process known as direct extrusion, which is the most common method in use today. Indirect extrusion is a similar process, but with some important differences. In the direct extrusion process, the die is stationary and the ram forces the alloy through the opening in the die. In the indirect process, the die is contained within the hollow ram, which moves into the stationary billet from one end, forcing the metal to flow into the ram, acquiring the shape of the die as it does so.
The extrusion process has been likened to squeezing toothpaste out of a tube. When pressure is applied at the closed end, the paste is forced to flow through the open end, accepting the round shape of the opening as it emerges. If the opening is flattened, the paste will emerge as a flat ribbon. Complex shapes can be produced by complex openings. Bakers, for example, use a collection of shaped nozzles to decorate cakes with fancy bands of icing. They’re producing extruded shapes.
But you can’t make very many useful products out of toothpaste or icing and you can’t squeeze aluminum out of a tube with your fingers.
You can squeeze aluminum through a shaped opening, however, with the aid of a powerful hydraulic press, producing an incredible variety of useful products with almost any shape imaginable.
As suggested by these toothpaste tubes, the shape of the extrusion (profile) is determined by the shape of the opening (die).