Aluminum extrusion is a manufacturing process where heated aluminum alloy is forced through a die to create objects with a specific, continuous cross-sectional profile. This method is valued because it allows for the creation of complex shapes while maintaining aluminum’s beneficial properties: lightweight structure, durability, and natural resistance to corrosion. The resulting profiles are widely used across industries, from structural components in construction and automotive frames to heat sinks for consumer electronics.
Preparing the Material and Tooling
The process begins with preparing the aluminum billet, a solid, cylindrical block of alloy. The billet is preheated in an industrial oven, typically between 800°F and 925°F (427°C and 496°C), to make it malleable. Heating the aluminum to this range softens it into a plastic state, allowing it to flow under pressure without becoming molten. Temperature control is important as it dictates the metal’s flow characteristics and influences the final product’s properties.
The tooling, known as the die, is typically machined from hardened H13 tool steel and meticulously designed to form the desired profile. The die is also preheated, often to temperatures between 750°F and 900°F (400°C and 482°C). Preheating the die prevents thermal shock when it contacts the hotter billet and ensures a smooth, consistent flow during pressing. Once the die and billet reach their specified temperatures, the billet is transferred to the extrusion press container.
The Extrusion Press Operation
The actual shaping occurs within the extrusion press, a powerful hydraulic ram system. The preheated billet is loaded into a container, and a ram pushes it against a dummy block toward the die opening. Immense force, often ranging from 100 to 15,000 tons, is applied. This pressure forces the softened aluminum through the shaped opening of the die, transforming the solid cylinder into a continuous profile matching the die’s cross-section.
The profile emerges from the die as a long, hot length of metal, often measuring between 950°F and 1022°F (510°C and 550°C). The extrusion speed must be carefully monitored, as it affects the final surface quality and mechanical properties. Faster extrusion requires a higher temperature but can lead to defects if not precisely controlled.
The most common method is direct extrusion, where the ram and the profile move in the same direction. An alternative is indirect extrusion, where the die is mounted on a hollow ram that moves toward a stationary billet, causing the aluminum to flow oppositely. Indirect extrusion significantly reduces friction between the billet and container walls, lowering the required force and allowing for a more consistent flow. While direct extrusion suits larger profiles, indirect extrusion provides better dimensional accuracy and a more uniform microstructure.
Post-Extrusion Handling
Immediately after exiting the die, the profile moves onto a runout table for cooling, or quenching. This rapid cooling stabilizes the aluminum’s shape and sets the metallurgical properties that determine its strength. Cooling is achieved using forced air from fans or a water bath, depending on the alloy type and the desired mechanical properties.
Once the profile has cooled sufficiently, it is transferred to a stretcher. The profile, which often has twisting or warping from the extrusion process, is mechanically gripped at both ends. It is then pulled in a controlled manner to straighten the length and relieve internal stresses developed during extrusion and cooling. This stretching ensures the profile meets required dimensional tolerances and enhances its mechanical strength.
After stretching, the continuous profile is cut into manageable sections using a finish saw. These lengths are often standardized, such as 20 to 50 feet, but can be customized based on handling capabilities or customer requirements. This initial cutting prepares the profiles for subsequent treatment and finishing stages.
Final Finishing and Treatment
The final stages involve enhancing the material’s strength and applying surface treatments for protection and aesthetics.
Heat Treatment (Artificial Aging)
Heat treatment, often called artificial aging, is performed in a furnace to achieve specific temper designations like T5 or T6. This process involves heating the aluminum to a set temperature, such as 350°F (177°C), for a defined period. This speeds up the natural aging process and significantly increases the aluminum’s hardness and yield strength.
Surface Finishing
Surface finishing improves corrosion resistance, durability, or visual appeal. One prevalent method is anodizing, an electrochemical process that thickens the naturally occurring, protective oxide layer. This porous layer can be dyed to various colors and offers enhanced resistance to abrasion and corrosion. Alternatively, a profile may undergo powder coating, where a dry powder is electrostatically applied and cured under heat. This process creates a uniform, hard finish available in many colors and is highly resistant to chipping and UV exposure.