Extrusions is a process of forming parts with a constant cross-sectional shape by forcing a metal through a die or a set of dies. The metal undergoes plastic deformation as it is squeezed through the opening of the die, which determines the shape and size of the final product. Extrusions can be used to produce rods, tubes, wires, and other complex shapes from metals such as aluminum, copper, steel, and titanium.

There are three main methods of extrusion: direct, indirect, and hydrostatic.

In direct extrusion, also known as forward extrusion, the metal billet is placed in a container and pushed by a ram through the die. The container and the die move in the same direction, creating friction between the billet and the container walls. This friction increases the force required to extrude the metal and reduces the quality of the surface finish. Direct extrusion is the most common and economical method of extrusion.

In indirect extrusion, also known as backward extrusion, the metal billet is stationary and the die moves towards it. The die is attached to a hollow ram that fits over the billet. As the ram pushes the die against the billet, the metal flows backwards through the opening in the ram. Indirect extrusion eliminates the friction between the billet and the container walls, reducing the force required to extrude the metal and improving the surface quality. However, indirect extrusion requires more complex equipment and is less suitable for producing long products.

In hydrostatic extrusion, the metal billet is placed in a chamber filled with a pressurized fluid, usually oil. The fluid acts as a cushion that prevents direct contact between the billet and the container walls. A ram pushes the fluid against the billet, forcing it to flow through the die. Hydrostatic extrusion reduces friction and increases ductility, allowing for higher extrusion ratios and lower temperatures. However, hydrostatic extrusion is more expensive and requires special seals and pumps to maintain the fluid pressure.

Extrusion can be performed at different temperature ranges: cold, warm, or hot. Cold extrusion is done at or near room temperature, resulting in high strength, high accuracy, and good surface finish. However, cold extrusion requires high forces and is limited by the ductility of the metal. Cold extrusions is limited in the complexity of the shape and the maximum reduction from blank to extruded shape that can be achieved without heating the metal. Warm extrusion is done at temperatures below the recrystallization temperature of the metal, which increases its ductility and reduces its strength. Warm extrusion allows for lower forces and higher speeds than cold extrusion but may compromise surface quality and dimensional accuracy. Warm and cold extrusion can also be done quickly and with less environmental impact than hot extrusions. Hot extrusion is done at temperatures above the recrystallization temperature of the metal, which reduces its strength and increases its ductility to a maximum level. Hot extrusion requires low forces and allows for high extrusion ratios and complex shapes. However, hot extrusion may cause oxidation, grain growth, and surface defects.

Designing for and Using Extrusions

A very large variety of aluminum (and other metal) extrusions can be purchased from suppliers that can often meet design needs for building things like structural support members. See: T-Slot Extrusions from 80/20. These types of solutions can also include standardized components for connections and attachments, simplifying the design process.

For custom extrusion, you will work with a supplier to develop die design to meet your part specification needs. The die and the design time to develop them are high in cost and only justified if standard extrusions will not satisfy requirements and production volume is high.