Extrusion is a kind of batch forming process.In this process, the workpiece metal is forced or compressed through the die hole to achieve a certain cross-sectional shape.
In short, extrusion is a metal processing process that includes forcing metal through a die hole under increased pressure to compress its cross-section.
Thanks to the development of extrusion technology, the world has begun to rely on extrusion to produce bars, pipes and hollow or solid profiles of any shape.
Because this operation involves pushing or pulling the blank through the die, the force required to extrude the blank is quite large. Hot extrusion is the most commonly used method because the deformation resistance of metal is lower at high temperatures, while cold extrusion is usually only performed on soft metals.
History:
Although the concept of extrusion was born from the molding process. According to records, in 1797, an engineer named Joseph Bramah applied for a patent for the extrusion process. The test included preheating the metal and then forcing it through the die cavity to manufacture pipes from the blank. He used a manual plunger to push the metal.
Bramah invented the hydraulic process after inventing the extruder. Then, Thomas Burr combined various technologies using hydraulic press technology and basic extrusion technology to produce pipes (hollow). He also obtained a patent in 1820.
This technology then became a basic need for the constantly evolving world, and this process is not suitable for hard metals. In 1894, Thomas Burr introduced the extrusion of copper and brass alloys, bringing about the development of extrusion technology.
Since the invention of extrusion technology, this process has developed into multiple technologies capable of producing products with various complex structures at the lowest possible cost.
Classification or types of extrusion processes:
1. Hot extrusion process:
In this hot extrusion process, the blank is processed at a temperature higher than its recrystallization temperature. This hot processing can prevent the workpiece from work hardening and make it easy for the punch press to push it through the die.
Hot extrusion is usually carried out on a horizontal hydraulic press. The pressure involved in this process can range from 30 MPa to 700 MPa. For intact high pressure, lubrication is adopted. Oil or graphite is used as lubricant for low-temperature profiles, and glass powder is used for high-temperature profiles. Provide heat between 0.5 Tm and 0.75 Tm for the blank to obtain high-quality operation.
Hot extrusion temperatures for several commonly used materials are as follows:
Material temperature (°C):
aluminum 350 to 500, copper 600 to 1100, magnesium 350 to 450, nickel 1000 to 1200, steel 1200 to 1300, titanium 700 to 1200, PVC180 nylon290.
Advantages:
● Deformation can be controlled as required.
● The billet will not be strengthened due to work hardening.
● Requires less pressure.
● Materials with premature cracks can also be processed.
Disadvantages:
● Poor surface finish.
● Dimensional accuracy will be affected.
● Reduce container life.
● Possibility of surface oxidation.
2. Cold extrusion:
This is the process of shaping metal by hitting metal with a bullet. This knocking is done by a punch or punch in a closed cavity. The plunger forces the metal through the die cavity, transforming the solid blank into a solid shape.
In this process, the workpiece is deformed at room temperature or slightly above room temperature.
For too much force required, a powerful hydraulic press is used in this technology. The pressure range can reach 3000 MPa.
Advantages:
● No oxidation.
● Increase product strength.
● Tighter tolerances.
● Improve surface finish.
● The hardness is increased.
Disadvantages:
● Requires greater force.
● More power is required to run.
● Non-ductile materials cannot be processed.
● Strain hardening of the extruded material is a limitation.
3. Warm extrusion process:
Warm extrusion is the process of extruding blanks above room temperature and below the recrystallization temperature of the material. This process is used in cases where microstructural changes in the material must be prevented during extrusion.
This process is important for achieving the proper balance of required force and ductility. The temperature of any metal used in this operation can range from 424 degrees Celsius to 975 degrees Celsius.
Advantages:
● Increased strength.
● Increased hardness of the product.
● Lack of oxidation.
● Very small tolerances can be achieved.
Disadvantages:
● Non-ductile materials cannot be extruded.
● In addition, there is a heating device.
4. Friction extrusion:
In friction extrusion technology, the blank and the container are forced to rotate in opposite directions. At the same time, the blank is pushed through the die cavity during operation to produce the required material.
This process is affected by the relative rotational speed between the charging and the die. The relative rotational movement of the charging and the die has an important influence on the process.
First, it will cause a large amount of shear stress, resulting in plastic deformation of the blank. Second, a large amount of heat will be generated during the relative movement between the blank and the die. Therefore, there is no need for preheating and the process is more efficient.
It can directly generate basically consolidated wires, rods, pipes and other non-circular metal geometries from various precursor charges such as metal powders, flakes, processed waste (chips or shavings) or solid blanks.
Advantages:
● No heating required.
● The generation of shear stress can improve the fatigue strength of the product.
● Any type of material can be used as a blank, making this process economical.
● Low energy input.
● Better corrosion resistance.
Disadvantages:
● Expected oxidation.
● High initial setup.
● Complex machinery.
5. Micro extrusion process:
As can be understood from its name, this process involves the production of products in the sub-millimeter range.
Similar to macro extrusion, here the blank is forced through the die hole to produce the expected shape on the blank. The output can pass through a 1mm square.
Forward or direct and reverse or indirect micro extrusion are the two most basic techniques used in this era for the production of micro-components. In forward micro extrusion, the plunger drives the blank to move forward. The movement direction of the blank is the same. In reverse micro extrusion, the movement directions of the plunger and the blank are opposite. Micro extrusion is widely used in the production of absorbable and implantable medical device components, ranging from bioabsorbable stents to drug-controlled release systems. In the mechanical field, applications in manufacturing micro gears, micro pipes and other aspects can be widely observed.
Advantages:
● Very complex cross-sections can be made.
● Tiny elements can be made.
● Improved geometric tolerances.
Disadvantages:
● Manufacturing a small die and a container to meet our needs is a challenge.
● Skilled workers are required.
6. Direct or forward extrusion:
In the direct extrusion process, the metal blank is first placed in a container. The container has a forming die hole. The plunger is used to push the metal blank through the die hole to make the product.
In this type, the direction of metal flow is the same as the movement direction of the plunger.
When the blank is forced to move towards the die opening, a large amount of friction will be generated between the blank surface and the container wall. Due to the existence of friction, the plunger force needs to be greatly increased, thereby consuming more power.
In this process, it is very difficult to extrude brittle metals such as tungsten and titanium alloys because they will break during this process. The tension throughout the process promotes the rapid formation of microcracks, leading to fracture.
It is difficult to extrude brittle metals such as tungsten and titanium alloys because they will break during processing. The tension causes microcracks to form rapidly, leading to fracture.
In addition, the presence of an oxide layer on the surface of the blank will aggravate friction. This oxide layer may cause defects in the extruded product.
To overcome this problem, a dummy block is placed between the gate and the working blank to help reduce friction.
Examples are pipes, cans, cups, pinions, shafts and other extruded products.
Some parts of the blank always remain at the end of each extrusion. It is called the butt. Cut it off from the product immediately at the die exit.
Advantages:
● This process can extrude longer workpieces.
● Improved mechanical properties of the material.
● Good surface finish.
● Both hot and cold extrusion are possible.
● Able to operate continuously.
Disadvantages:
● Brittle metals cannot be extruded.
● Large force and high power requirements.
● Possibility of oxidation.
7. Indirect or reverse extrusion:
In this reverse extrusion process, the die remains stationary while the blank and the container move together. The die is mounted on the plunger instead of the container.
Metal flows through the die hole on the side of the plunger in the opposite direction to the movement of the plunger when the blank is compressed.
When the blank is compressed, the material will pass between the mandrels and thus through the die opening.
Since there is no relative movement between the blank and the container, no friction is recorded. Compared with direct extrusion, this improves the process and results in less plunger force being used than in direct extrusion.
To keep the die stationary, a "rod" longer than the length of the container is used. The column strength of the rod determines the final and maximum extrusion length. Since the blank moves with the container, all frictions are easily eliminated.
Advantages:
● Requires less extrusion force.
● Can extrude smaller cross-sections.
● 30% reduction in friction.
● Increase operating speed.
● Very little wear is recorded.
● Due to more consistent metal flow, extrusion defects or coarse-grained ring zones are less likely.
Disadvantages:
● The cross-section of the extruded material is limited by the size of the rod used.
● Possibility of residual stress after extrusion.
● Impurities and defects can affect the surface finish and affect the product.
8. Hydrostatic extrusion:
In the hydrostatic extrusion process, the blank is surrounded by fluid in the container, and the fluid is pushed towards the blank by the forward movement of the plunger. Due to the frictionless fluid inside the container, there is very little friction at the die hole.
When filling the hole of the container, the blank will not be disturbed because it is subjected to uniform hydrostatic pressure. This successfully produces blanks with a huge length-to-diameter ratio. Even coils can be extruded perfectly or have uneven cross-sections.
The main difference between hydrostatic extrusion and direct extrusion is that there is no direct contact between the container and the blank during the hydrostatic extrusion process.
Special fluids and processes are required when working at high temperatures.
When the material is subjected to hydrostatic pressure and there is no friction, its ductility increases. Therefore, this method may be suitable for metals that are too brittle for typical extrusion methods.
This method is used for ductile metals and allows a high compression ratio.
Advantages:
● The extruded product has excellent surface polishing effect and accurate dimensions. ● There is no problem of friction.
● Minimize force requirements.
● There is no residual blank in this process.
● Uniform material flow.
Disadvantages:
● When operating at high temperatures, special liquids and procedures should be used.
● Before working, each blank must be prepared and tapered at one end.
● It is difficult to control the liquid.
9. Impact extrusion:
Impact extrusion is another main method for producing metal extruded profiles. Compared with traditional extrusion processes that require high temperatures to soften materials, impact extrusion usually uses cold metal blanks. These blanks are extruded under high pressure and high efficiency.
During the traditional impact extrusion operation, a properly lubricated block is placed in the die cavity and struck by a punch in a single stroke. This causes the metal to flow back around the punch through the gap between the die and the punch.
This process is more suitable for softer materials such as lead, aluminum or tin.
This process is always performed in a cold state. The backward impact process allows very thin walls. For example, making toothpaste tubes or battery cases.
It is performed at a faster speed and with a shorter stroke. Instead of applying pressure, impact pressure is used to extrude the blank through the die. On the other hand, impact can be performed by forward or backward extrusion or a mixture of both.
Advantages:
● Significantly reduced size.
● Fast process. Processing time is reduced by up to 90%.
● Increase productivity.
● Improve tolerance integrity.
● Save up to 90% of raw materials.
Disadvantages:
● Requires very high compressive forces.
● The size of the blank is a limitation.
Factors affecting extrusion force:
● Working temperature.
● Equipment design, horizontal or vertical.
● Extrusion type.
● Extrusion ratio.
● Deformation amount.
● Friction parameters.
Extrusion process applications or uses:
● Widely used in the production of pipes and hollow pipes. And also used in the production of plastic items.
● The extrusion process is used to produce frames, doors and windows, etc. in the automotive industry.
● Metal aluminum is used for structural work in many industries.
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