Extrusion is a continuous process which is used to create an object with required cross sectional area. During the process force is applied on the molten metal to flow in to a closed shaped die in one direction under high pressure.
Extrusion is classified in to several types based on process, equipment and direction. They are:
- Hot extrusion
- Cold extrusion
- Warm extrusion
- Forward Extrusion or Direct Extrusion
- Backward extrusion or Indirect Extrusion
- Hydrostatic extrusion
- Impact Extrusion
- Tube extrusion
It is a hot working process. Extrusion process is carried out at above recrystallization temperature. The material should keep at strain hardening to easily flow inside of the die. In most of the cases the horizontal hydraulic presses are used for the hot extrusion process.
The range of the hydraulic press varies from 230 metric tons to 11, 000 metric tons and the pressure is varies from 30 MPa to 700 MPa. During the extrusion process lubrication is required for high temperature extrusion process glass power is used and for low temperature extrusion process graphite or oil is used.
The biggest drawback of the process is cost of machinery is high. Due to the high pressures and high temperatures life of the die is decreases. Good lubrication is required for the process of extrusion.Advertisement:
Cold Extrusion Process
Compare with the hot extrusion cold extrusion process gives better results. Cold extrusion process is done at the room temperatures or below the recrystallization temperature.
It is used to produce separate parts. By using the cold extrusion process we can get high production rate, close tolerance, strain hardening, improved surface finish, and absence of oxide layers.
- Good mechanical properties
- Proper use of lubricants results in good surface finish
- Dimensional tolerances
Warm extrusion is done above the room temperature. The temperature varies from 424 to 975 centigrade at below the recrystallization temperature.
Forward extrusion is also known as direct extrusion. In the forward extrusion process the flow of the metal and ram are moving in the same direction. Ram was tightly fits the die which does not allow the metal to backward flow.
Loose fitting pin is arranged to create a cavity in the material.
Backward extrusion is also known as indirect extrusion. In the backward extrusion process the flow of metal and the direction of the ram both are in opposite direction. The ram creates a mirror image in the material as required.
In the hydrostatic extrusion it consists of billet it is totally surrounded by a pressurized liquid, but where the billet connects the die.to obtain a hydrostatic medium sealed cylinder is used.
In the hydrostatic extrusion cold, hot, warm processes are done. Depends on the stability of the fluid the temperature must be limited. Hydrostatic extrusion is divided in to two types they are
- Constant rate extrusion
- Constant pressure extrusion
- Large cross sections and billets can be extruded
- Material flow is even
- During high pressure ductility of the material is also increases
- In between the container and billet no friction is required
- Seal arrangement is done at that beginning of the process
- Handling of high pressure fluids is difficult
Impact extrusion is used to produce hollow shapes. For this type of process the required material are copper, tin, aluminum and lead. High speed mechanical presses are used for the impact extrusion process.
In this process mandrel is attached to end of the ram to produce tubes by extrusion. The thickness of the tube is determined by the clearance maintained between the die wall and the mandrel.
Extrusion ratio is defined as the ratio between the internal cross section area and the billet final cross sectional area after the extrusion.
For hot extrusion of steels R is nearly 40:1
For aluminum R is nearly 400:1
Fractional reduction in area (r):
r = 1 –
where, = billet product area
= extruded product area
There are three main defects in extrusion they are
- Internal cracking
- Surface cracking
- Pipe defect
- Internal cracking is also called as center burst cracking
- At the centerline of distortion zone the hydrostatic stress are acting
- If the die angle and the impurities are increased at that case the internal cracking is also increases
- Surface cracking is also called as hot shortness
- Inter granular cracks are occurred because of high speed , friction and temperature
- In zinc alloys magnesium and aluminum surface cracks are common
- Pipe defect is also called as fishtailing or tailpipe
- Surface oxides and impurities are draws towards the center of the billet to metal flow
Material used for the extrusion process
Most commonly used material is aluminum, brass, copper, lead and tin, magnesium, steel, and titanium.
|S.No||Material Type||Maximum TemperatureFahrenheit||Minimum TemperatureFahrenheit||Uses|
|1||Aluminum||1100||575||Tracks, frames, rails, heat sinks|
|2||Brass||Pipes, engineer parts|
|3||Copper||1825||1100||Pipes, rods, wire, tubes, welding electrodes, bars|
|4||Lead and Tin||575||Tubes, cable, pipes, wires,|
|5||Zinc||650||400||Tubes, rods, bars, hardware, fitting, hand rails|
|6||Magnesium||1100||575||Nuclear parts, aircraft parts|
|7||Steel||2375||1825||Tracks and rods|
|8||Titanium||1825||1100||Engine rings, aircrafts parts, seat tracks|
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