What is Metal Extrusion Process?
Metal extrusion is a metal forming process in which a work piece usually a round billet is forced to flow through a die of a smaller cross sectional area, thus forming the work to the new uniform cross section of a certain length. The different type of cross-section is possible to produce by this forming method. The total length of the part will change depends upon amount of billet material. The basic principle of metal extrusion is illustrated below.
How does the Extrusion Process Work?
In Extrusion machine, the ram will push the round billet through the reduced diameter of die opening. As ram continuous to force the billet, a uniform and continuous length of component will be formed at the other end of the die. The component is supported as it is extruded from the die. When the ram reaches the maximum stroke, some amount of billet stock cannot be pushed any further through the die opening. This portion of billet stock is called the butt end. Usually the product will be saw cut to the desired length at end of the operation.
Materials plays a major role to determine the process capability. Soft material like Aluminum, Zinc Copper, Magnesium Tin etc. are well suited for extrusion process. Tough material like Steels, Titanium etc is difficult and not recommended.
As other metal forming processes, Extrusion causes geometric change like extended grain structure and deformation to the component.
Type of Metal Extrusion Process
Metal extrusion can be performed either hot or cold. It is depends upon the factors like material (difficult material need to be hot extruded), tolerance, required cross-section complexity etc.
Similar to other type of manufacturing process, hot extrusion involves heating the metal above its recrystallization temperature and them performs extrusion. As you know, when you heat the metal it offers numerous advantages during any manufacturing process.
It improves of the mechanical properties of the part by distributing solid impurity inclusion throughout the mass of metal. It also improves impact resistance, ductility and strength. Along with improved mechanical properties, hot extrusion also have advantage of easily manipulating difficult material than a cold metal. When metals are worked above their recrystallization temperature, it allow us to make a large amount of shape change.
Yes, of course it do have disadvantages like oxidation problem on part surfaces, loose tolerances poor surface finish compares to cold working etc.
How to Choose Between Cold working and Hot working process?
- How difficult is your material? If material is tough, you can choose hot extrusion.
- For large sized parts, prefer hot extrusion as it allows more complex geometry changes. For smaller parts, less complex cross-section and soft material, you can prefer cold extrusion process.
- It is obvious that cold extrusion do not required to heat the work piece so it offers good surface finish, strong parts due to strain hardening and can maintain tight tolerance.
Direct Extrusion and Indirect Extrusion
Depends upon the application of force and many operational factors, metal extrusion process can be classified into Direct Extrusion and Indirect Extrusion.
What is Direct Extrusion?
In Direct Extrusion also known as Forward Extrusion, the billet is placed inside a container. The ram applies force on the side of the billet to push through the die. The component is extruded from the die in the same direction that the applied force on ram.
Forces required to extrude the component and billet material flow during direct extrusion is largely affected due to the friction between the billet and the container walls. Mostly during hot extrusion method, oxide formation on the outer surfaces of the billet can adversely affect the operation. To avoid this scenario, it is good manufacturing to have a dummy block between the ram and the billet. This dummy block which is of slightly smaller diameter than the container and billet. As the extrusion happens, the outer surface of the work will not be extruded and remains in the container. This results in formation of Skull ,thin material film around the billet.
How are Hollow Extrusions Made?
To make the Hollow or semi hollow extrusion components, a mandrel which will be attached to the dummy block is used. A axis of hollow will be in parallel to the axis of the ram which pushed the mandrel. The Hollow profile is controlled by the mandrel, and the outer profile is controlled by the extrusion die.
What is Indirect Extrusion?
Indirect and Direct Extrusion method differs by the direction of extrusion to the direction of applied force. In Indirect extrusion process the billet is placed in a completely closed off container. The extrusion die are attached on the end of the ram, which applies force from the open end of the container. As ram pushes the die against the billet, the component will get extruded in the opposite direction that the die is moving. The Hollow ram provides the path for extruded component to move out of the mold. Indirect extrusion don’t require the dummy block as no frictional forces exist between the billet and the container walls. But as ram is hollow, they are not strong as in direct extrusion. Also, Tooling and machine set up are difficult.