What is Extrusion?
Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex cross-sections; and to work materials that are brittle, because the material encounters only compressive and shear stresses. It also creates an excellent surface finish and gives considerable freedom of form in the design process.
Drawing is a similar process, using the tensile strength of the material to pull it through the die. It limits the amount of change that can be performed in one step, so it is limited to simpler shapes, and multiple stages are usually needed. Drawing is the main way to produce wire. Metal bars and tubes are also often drawn.
Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). It can be done with hot or cold material. Commonly extruded materials include metals, polymers, ceramics, concrete, modeling clay, and foodstuffs. Products of extrusion are generally called extrudates.
Also referred to as “hole flanging”, hollow cavities within extruded material cannot be produced using a simple flat extrusion die because there would be no way to support the center barrier of the die. Instead, the die assumes the shape of a block with depth, beginning first with a shape profile that supports the center section.
The die shape then internally changes along its length into the final shape, with the suspended center pieces supported from the back of the die. The material flows around the supports and fuses to create the desired closed shape.
The extrusion of metals can also increase their strength.
Characteristics of Extrusion
- Able to create complex cross-sections and will be uniform over the entire length of the extrudates
- Factors that affect the quality of extrusion are die design, extrusion ratio, billet temperature, lubrication, and extrusion speed. Check out the detailed design guide for metal extrusion, “How to design parts for direct metal extrusion” to understand the 5 key design variables of metal extrusion and design for manufacture (DFM) extrusion design tips.
- Similar to any other metal forming processes, it can be performed either hot or cold, although the process generally is carried out at elevated temperatures in order to reduce the extrusion force and improve the ductility of the material
- Low cost due to reduced raw material wastage and high production rate
- Brittle material can be deformed without a tear as it only exerts compressive and shear forces in the stock part
- Parts that are formed have an excellent surface finish which minimizes post-processing machining
- Metal extrusion tends to produce a favorable elongated grain structure in the direction of the material.
- The minimum wall thickness of ~1mm (aluminum) to ~3mm (steel) could be achieved
Types of Extrusion Process
Metal extrusion can be subdivided and grouped into the following categories depending on the direction of extrusion flow, the medium used to apply force, working temperature, etc.
- Direct Extrusion
- Indirect Extrusion
- Hydrostatic Extrusion
- Lateral or Vertical Extrusion
- Hot extrusion
- Cold Extrusion
- Impact Extrusion
1. Direct Extrusion
Direct Extrusion, sometimes called Forward Extrusion is the most common type of extrusion. Begins by loading a heated billet (only for hot extrusion, discussed later) into a press cavity container where a dummy block is placed behind it.
Then the mechanical or hydraulic ram presses on the material to push it out through the die. Then, while still hot, the part is stretched to straighten. This process is beautifully animated by Core Materials.
Under direct extrusion, the high friction caused by steels at higher temperatures is reduced using molten glass as a lubricant while oils with graphite powder are used for lubrication for low temperatures. The dummy block is used to protect the tip of the pressing stem (punch or ram) in hot extrusion.
When the punch reaches the end of its stroke, a small portion of the billet called “butt end” cannot be pushed through the die opening.
Advantages of Direct metal extrusion
- No billet modification required
- Can be used for both hot and cold extrusion
- Simple tooling compared to other extrusion processes
Disadvantages of Direct metal extrusion
- High force requirement due to friction
- Butt end left inside the cavity
- The force required to push the ram changes as the punch moves
2. Indirect Extrusion
In Indirect Extrusion, the die is located at the end of the hydraulic ram and moves towards the billet inside the cavity to push the material through the die. This is illustrated in figure 2 below.
This process consumes less power due to the static billet container causing less friction on the billet. However, supporting the extruded part is difficult when the extrudate exits the die.
Advantages of In-direct metal extrusion
- Less friction and less power used
- Can be used for both hot and cold extrusion
- Simple tooling compared to other extrusion processes
Disadvantages of In-direct metal extrusion
- Difficult to support the extruded part
- The hollow ram limits the load applied
3. Hydrostatic extrusion
In 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 tri-axial forces are applied by the fluid, the pressure improves billet formability on the billet. Sealing the fluid must be considered at the early stages to avoid any leaking and reduced pressure issues.
Although the hydraulic fluid eliminates the friction between the wall and the billet by isolating them, due to the specialized equipment requirement, the high setup time and low production rate limit its usage in the industry in comparison to other extrusion processes.
Advantages of Hydrostatic metal extrusion
- Low power/force requirement due to no friction
- Fast production rates & high reduction ratios
- Lower billet temperature
- Even flow of material due to the balanced force distribution
- Large billets and large cross-sections can be extruded
- No billet residue is left in the container
Disadvantages of Hydrostatic metal extrusion
- Billets need preparing by tapering one end to match the die entry angle
- Only cold extrusion is possible
- Difficult to contain the high-pressure fluid
4. Lateral Extrusion
In Lateral Extrusion, the container is in a vertical position as shown in the image, and the die is located on the side. This process is suitable for low melting point material.
5. Impact Extrusion
Impact extrusion is part of the cold extrusion category very similar to In-direct extrusion and limited to softer metals such as Lead, Aluminum, and copper. As the schematic illustrates, the punch is pushed down at high speed and has extreme force on the slug to extrudes backward.
The thickness of the Extrude is a function of the clearance between the punch and the die cavity. The Extrudates are slide off the punch by the use of a stripper plate.
For impact extrusions, a mechanical press is often used 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 have sufficient impact resistance, fatigue resistance, and strength, for extruding metal by the impact. Impact extrusion can be divided into the following three types by the flow of the material.
- Forward
- Reverse
- Combination
In forward impact extrusion, the metal flows in the same direction that the force is delivered while it flows in the opposite direction in reverse impact extrusion.
Advantages of impact metal extrusion
- Raw material savings of up to 90%
- Reduced machining times up to 75%
- Elimination of secondary machining operations
- Reduction in multi-part assemblies
- Improved mechanical properties for material strength and machining due to cold working of the material
- Significantly reduced total part costs up to 50%
- Hollow thin-walled tubes, closed on one end, are often produced in manufacturing industry by backward impact extrusion.
Disadvantages of impact metal extrusion
- Produced as long as the part is symmetrical over the axis by which it is formed
- Many of the parts formed by impacting, in industry, will require further manufacturing processes, such as metal forging, ironing or machining, before completion
Application of Extrusion
- Extrusion is widely used in production of tubes and hollow pipes.
- Aluminum extrusion is used in structure work in many industries.
- This process is used to produce frames, doors, window etc. in automotive industries.
- Extrusion is widely used to produce plastic objects.
Advantages of Extrusion
- High extrusion ratio (It is the ratio of billet cross section area to extruded part cross section area).
- It can easily create complex cross section.
- This working can be done with both brittle and ductile materials.
- High mechanical properties can achieve by cold extrusion.
Disadvantages of Extrusion
- High initial or setup cost.
- High compressive force required.
