What is metal extrusion?
Metal extrusion is a metal-forming manufacturing process in which a cylindrical billet is forced to flow through a die of the desired cross-section inside a closed cavity. Extrudates are fixed cross-sectional profile extruded parts that are pushed out using either a mechanical or hydraulic press. Thomas Burr used Joseph Bramah’s patented process to extrude lead pipes for the first time.
The materials that are extruded the most frequently are lead, copper, steel, magnesium, and aluminum. Plastics and ceramics are also widely extruded but are not covered in this article.
Properties of Metal extrusion
- Capable of producing complex cross-sections that are uniform along the entire length of the extrudates
- Die design, extrusion ratio, billet temperature, lubrication, and extrusion speed are all factors that influence extrusion quality. To learn about the 5 key design variables of metal extrusion and design for manufacture (DFM) extrusion tips, consult the detailed design guide for metal extrusion, “How to design parts for direct metal extrusion.”
- It can be performed either hot or cold, like any other metal-forming process, but it is typically performed at elevated temperatures to reduce extrusion force and improve material ductility.
- Low cost due to less waste of raw materials and a high production rate Brittle material can be deformed without tearing because the stock part only exerts compressive and shear forces.
- Formed parts have an excellent surface finish, which reduces post-processing machining.
- Metal extrusion produces a favorable elongated grain structure in the material direction.
Types of the metal extrusion process
Metal extrusion can be subdivided and categorized according to the direction of extrusion flow, the medium used to apply force, the working temperature, and so on.
-
Direct Extrusion
The most common type of extrusion is direct extrusion, also known as forwarding extrusion. The process begins by loading a heated billet (only for hot extrusion, as discussed later) into a press cavity container with a dummy block behind it, as shown in figure 2. The material is then pushed out through the die by the mechanical or hydraulic ram. The part is then stretched to straighten while still hot. Core Materials has beautifully animated this process.
Direct extrusion reduces the high friction caused by steels at higher temperatures by using molten glass as a lubricant, whereas oils with graphite powder are used for low-temperature lubrication. In hot extrusion, the dummy block is used to protect the tip of the pressing stem (punch or ram). When the punch reaches the end of its stroke, the “butt end” of the billet cannot be pushed through the die opening.
-
Indirect Extrusion
In indirect extrusion, the billet is located inside the cavity, and the die is at the end of the hydraulic ram, moving towards the billet to push the material through the die.
Because the static billet container causes less friction on the billet, this process consumes less power. However, when the extrudate exits the die, it is difficult to support the extruded part.
-
Hydrostatic extrusion
The chamber/cavity is made smaller than the billet and filled with hydraulic fluid, which transfers the force from the ram to the billet. Although the fluid applies tri-axial forces, the pressure improves billet formability on the billet. Sealing the fluid should be considered early on to avoid leaking and reduce pressure issues.
Although the hydraulic fluid eliminates friction between the wall and the billet by isolating them, the high set-up time and low production rate limit its use in the industry when compared to other extrusion processes.
-
Lateral Extrusion
In lateral extrusion, the die is placed on the side of the container, which is vertical as shown in the image. This method is appropriate for materials with low melting points.
-
Impact Extrusion
Impact extrusion is a type of cold extrusion that is similar to In-direct extrusion in that it is limited to softer metals such as lead, aluminum, and copper. As shown in the schematic, the punch is pushed down at high speed and exerts an extreme force on the slug, causing it to extrude backward. The clearance between the punch and the die cavity determines the thickness of the Extrude. A stripper plate is used to slide the Extrudates off the punch.
A mechanical press is frequently used for impact extrusions, and the part is formed at a high speed and over a relatively short stroke.
Since the forces acting on the punch and die are extremely high, tooling must be impact-resistant, fatigue resistant, and strong enough to extrude metal by impact. The flow of the material distinguishes the three types of impact extrusion.
Forward
Reverse
Combination
-
Extrusion flaws
Extrudates can develop a variety of defects that can affect the quality of the end product depending on the material condition and process variables. These flaws can be classified into one of three types.
Surface cracking
Piping
Internal fracturing
