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Understanding extrusion process

The extrusion process is one of the most common processes in the production world today. It involves the use of a large variety of materials like metals, plastics, ceramics, etc. The process can be done in either hot or cold when better properties are required. Extrusion processes in metals may increase the strength of the material.

Today you’ll get to know the definition, applications, function, process, materials, equipment, types, and working principle of an extrusion process. You’ll also get to know the advantages and disadvantages of an extrusion process.

Extrusion processes

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What are extrusion processes?

Extrusion is a manufacturing molding process that involves forcing base material through a pre-shaped die to create objects with a specific shape and profile. This process is used to create objects of a fixed cross-sectional profile. In its working, the material is pushed through a die of the desired cross-section, its shape changes to reflect the die’s shape. Products of extrusion are generally called “extrudates”.

The extrusion ratio is defined as the starting cross-sectional area divided by the cross-sectional area of the final extrusion. One benefit of an extrusion process is that this ratio can be very large while still producing quality parts. This process has the ability to create very complex cross-sections and to work with brittle materials. This is because the material only faces compressive and shear stresses. The extrusion process also offers an excellent surface finish to parts and gives considerable freedom to form in the design process.

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Extrusion processes are similar to drawing, which uses the tensile strength of the material to pull it through the die. The drawing process is used to produce wire, metal bars, and tubes. However, it is limited to simpler shapes, and multiple stages are usually required, unlike extrusion that performs all in one step.

Extrusion is known to be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). Common materials used in the extrusion process are metals, polymers, ceramics, concrete, modeling clay, and foodstuffs.

Applications of an extrusion process

The application of extrusion is very common around us, since most materials used in our houses, offices, workplaces are extrudates. A good example of extrusion molding can be seen when you squeezed out a toothpaste, the shape at which the paste forms when it comes out. An icing bag is also a good example of when the icing is pushed out.

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Extrusion molding of plastics is used to make any long shape that has a constant cross-section. This process can be used to produce pipes, gutters, window sections, and decorative trims. Thermoplastic materials such as PVC (polyvinylchloride), LDPE (low-density polyethylene), HDPE (high-density polyethylene), and PP (polypropylene) can all be extruded. Below are the common applications of extrusion process or extrudates of thermoplastics:

  • Insulated coating for wires and cables.
  • Flat plastic sheets for signs, glazing, refrigerator interiors
  • Lighting applications.
  • Monofilament for rope, bristles, and synthetic textile fibers.
  • Pipes and tubing are used for hoses, water, gas, sewers, and drains.

Industrial applications of extrusion process include:

  • Production of tubes and hollow pipes.
  • Aluminum extrusion is used in structure work in many industries.
  • Automotive parts are widely made with this process.

See more applications of an extrusion process in the below section (materials used in extrusion process).

Note: the primary function of extrusion is to manufacture objects with fixed cross-sections and profiles.

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Extrusion process

Hot or warm extrusion processes begin by heating the stock material, which is then loaded into the container in the press. A dummy block is placed behind it where a ram presses the material to push it out of the die. The extrusion is then stretched to straighten it. The requirement of a specific or better property makes it to be cold worked or heat treated.

Hot extrusion process:

This is a hot working process, that is, it is done above the recrystallization temperature of the material. This keeps the material from work hardening and makes it easier to push the material through the die. The hot extrusion process is mostly performed on horizontal hydraulic presses, ranging from 230 to 11,000 metric tons (250 to 12,130 short tons). Their pressures range from 30 to 700 MPa (4,400 to 101,500 psi). Thus, lubrication is required, either oil or graphite can be used for lower temperature extrusions, while glass powder for higher temperature extrusions.

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Metal materials can require different temperatures in hot extrusion, materials such as magnesium, aluminum, copper, steel, titanium, nickel, refractory alloy, etc. All of these require different temperatures during the hot extrusion process.

Cold extrusion process:

Cold extrusion processes are performed at room temperature or near room temperature. Its benefits over the hot process include lack of oxidation, higher strength due to cold working, closer tolerances, better surface finish, and fast extrusion speeds. Material that is commonly used in cold extrusion processes includes lead, tin, aluminum, copper, zirconium, titanium, molybdenum, beryllium, vanadium, niobium, and steel. The applications of a cold extrusion process include collapsible tubes, firer extinguisher cases, shock absorber cylinders, and gear blanks.

Warm extrusion process:

Warn extrusion process is beneficial on both ferrous and non-ferrous metals and alloys. Warm extrusion is performed above room temperature, but below the recrystallization temperature of the material. These temperatures range from 800 to 1800 0F (424 to 975 0C). this process is usually used to achieve the proper balance of required forces, ductility, and final extrusion properties.

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Friction extrusion:

This process was introduced with intention of producing homogeneous microstructures and particle distribution in metal matrix composite material. This process is extremely different from conventional extrusion in the sense that the charge rotates relative to the extrusion die. An extrusion force helps to push the charge against the die. Both the die or the charge may rotate or they may be counter-rotating in some cases. This relative rotary motion between the charge and the die has several significant effects on the process, which include;

  • Large shear stresses due to the relative motion in the plane of rotation.
  • Plastic deformation in the layer of charge in contact with and near the die.
  • The plastic deformation is dissipated by recovery and recrystallization processes leading to substantial heating of the deforming charge.
  • The process does not require preheating of the charge by auxiliary means potentially resulting in a more energy-efficient process.

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Secondly,

  • The substantial level of plastic deformation in the region of relative rotary motion can promote the solid-state welding of powders. Other finely divided precursors, such as flakes and chips, effectively consolidating the charges (friction consolidation) before extrusion. Finally,

Micro extrusion process:

This process is a micro-forming extrusion process performed at a submillimeter range. Conventionally, metal is pushed through a die orifice, but the resulting product’s cross-section can fit through a 1mm square. In forward, ram and billet move in the same direction, while in backward, ram and billet move in the opposite direction.

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Materials used in an extrusion process

Just as earlier mentioned, extrusion materials can be metal, wood, plastic, and ceramic. Below are their explanations.

Metal:

Metals are one of the most common materials used in extrusion processes. The types of metals determine the working temperature and the property required is also a factor. Below are the various types of metals used in an extrusion process.

Aluminum is the most commonly extruded material and can be either hot or cold extruded, depending on how the operation is performed. It is heated to 575 to 1100 0F (300 to 600 0C). extruded products of aluminum include profiles for tracks, frames, rails, mullions, and heat sinks.

Brass is often used to extrude engineering parts, automobile parts, pipe fittings, and corrosion-free rods.

Copper is used for the extrusion of pipe, wire, rods, bars, tubes, and welding electrodes. This is done at a working temperature of 1100 to 1825 0F (600 to 1000 0C).

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Lead and thin are also used to extrude pipes, tubes, wire, and cable sheathing at the maximum temperature of 575 0F (300 0C). Molten lead may also be used in place of billets on the vertical extrusion process.

Magnesium is widely used to extrude nuclear industry parts and aircraft parts at a working temperature of 575 0F (300 to 600 0C). The level of extrusion of this material is almost about that of aluminum.

Zinc is widely used for fitting and handrails, hardware components, tubes, bar, rod, etc. at a temperature of 400 to 650 0F (200 to 350 0C).

Steel – used for rods and tracks at 1825 to 2375 0F (1000 to 1300 0C). Alloy steel and stainless steel can be extruded, but usually, plain carbon steel is extruded.

Titanium is also used for aircraft components including seat tracks, engine rings, and other structural parts. This is done at a working temperature of 1100 to 1825 0F (600 to 1000 0C).

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Plastic:

Plastics are another common material used for extrusion. They are commonly used as plastic chips or pellets, which are usually dried to remove moisture from them. in the extrusion equipment, the material is fed through a hopper, which is then heated to a molten state by a combination of heating elements and shear heating from the extrusion screw. The screw, or screws when its twin-screw extrusion, forces the resin through a die, giving the resin the desired shape.

The extrudate is cooled and solidified when pulled through the die or water tank. Improving the overall quality of the extrudate, a caterpillar haul-off is used to provide tension on the extrusion line. Pelletizes can also create this tension while pulling extruded strands to be cut, but the caterpillar haul-off provides a consistent pull. Else, vibration in cut length or distorted product will occur.

Diagram of plastic extrusion processes:

Watch the video below to learn how plastic extrusion process:

Ceramic:

Ceramic is another material that can be formed into shapes through extrusion. Many modern bricks are also produced using a brick extrusion process.

Extrusion equipment

Extrusion equipment is of different variations, varying by the following four characteristics:

  • Movement of the extrusion with relation to the ram, that is, direct or indirect extrusion. The direction extrusion is when the die is held stationary and the ram moves towards it. whereas indirect extrusion is when the ram is held stationary and the die moves towards the ram. This will be further explained.
  • Position of the press, either vertical or horizontal.
  • Either hydraulic or mechanical type of drive.
  • Types of load applied, either conventional (variable) or hydrostatic.

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In extrusion equipment, a single or twin-screw auger, powered by an electric motor, or a ram, driven by hydraulic pressure, oil pressure. Most modern direct or indirect extrusion presses are hydraulically driven, but small mechanical presses are still employed on some. These hydraulic presses are of two types: direct-drive oil presses and accumulator water drives.

Diagram of extrusion equipment:

Direct-drive oil presses are one of the most common types, they are reliable and robust. They can deliver over 35 MPa (500 psi). The billet in the system receives constant pressure. However, this extrusion equipment is slow, between 50 and 200 mm/s (2–8 ips).

Accumulator water drives are more expensive and larger when compared with direct-drive oil presses. They lose about 10% of their pressure over the stroke, but they work at a much faster rate, up to about 380 mm/s (15 ips). This is why they are used for extruding steel. Accumulator water drives can also be used on materials that require very hot temperatures for safety purposes.

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Types of an extrusion process

Different types of extrusion processes can be classified by the following:

According to the flow direction of materials

Direct extrusion:

These types of extrusion processes allow materials to flow in the direction of the feed of the punch. This punch moves toward the die during the process. Direct extrusion required higher force due to higher friction between billet and container. Direct extrusion is also known as forwarding extrusion, it is quite common in the industrial world. In its working, the billet is placed in a heavy-walled container which is pushed through the die by a ram or screw. A reusable dummy block between the ram and the billet help to keep them separated.

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The major limitation of using this process is that the force required to extrude the billet is high. This is because of the frictional forces introduced by the need for the billet to travel the entire length of the container. Due to the greater force required at the beginning of the process and slowly decreases as the billet is used up. Preceding the end of the billet, the force greatly increases because the billet is a thing and the material must flow radially to exit the die.

direct extrusion processes

Indirect Extrusion:

In an indirect extrusion process, working materials flows toward opposite directions of plunger movement. The die is fitted at the opposite side of punch movement. This material is allowed to flow through the annular space between punch and container. It is known as backward extrusion. In its process, the billet and container move together while the die is stationary. A “stem” that is longer than the container length help to hold the die in place. The maximum length of the extrusion is ultimately known by the column strength of the stem. The movement of the billet with the container eliminates frictional forces that may occur.

indirect extrusion processes

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Advantages of indirect extrusion:

  • Up to 25 to 30% of friction is reduced. This allows larger billets to be extruded, it can be performed at an increased speed. Also, the ability to extrude smaller cross-sections.
  • Less tendency for extrusion to crack, because no heat is formed from friction.
  • The container liner lasts longer due to less wear.
  • The billet is more uniformly used, resulting in fewer defects and coarse-grained.

Disadvantages of indirect extrusion:

  • Defects and impurities on the surface of the billet affect the surface of the extrusion.
  • The process isn’t as versatile as direct extrusions because of the limited cross-sectional area.

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Hydrostatic Extrusion:

These extrusion types use fluid to apply pressure on the billet. Friction is eliminated in this process because the billet is not in contact with the cylinder wall or plunger. There is a fluid between the billet and plunger, and the plunger applies force on fluid that is further applied on the billet. Vegetable oils are normally used as the fluid in hydrostatic extrusion. One effect of this process is the leakage problem and uncontrolled speed of extrusion. This process can be performed hot, warm, or cold, but the temperature is limited by the stability of the fluid used. A sealed cylinder containing the hydrostatic medium must be used to carry out this process.

The fluid can be pressurized in two ways; firstly, constant-rate extrusion, that is, a ram or plunger is used to pressurize the fluid inside the container. Secondly, constant-pressure extrusion – a pump is used, possibly with a pressure intensifier, to pressurize the fluid. It is then pumped to the container.

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Advantages of hydrostatic extrusion

  • Force requirement is reduced since no friction between the container and the billet.
  • Faster speeds, higher reduction ratios, and lower billet temperatures.
  • Ductility of the material increases when high pressures are applied.
  • No billet residue is left on the container walls.
  • Both large billets and large cross-sections can be extruded.

Disadvantages of hydrostatic extrusion

  • The billets must be prepared by tapering one end to match the die entry angle. That is, a seal needs to be form at the beginning of the circle.
  • The entire billet needs to be machined to remove any surface defects.
  • Fluid under high pressures can be difficult to attain.
  • A billet remnant or a plug of a tougher material must be left at the end of the extrusion to prevent a sudden release of the extrusion fluid.

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According to the working temperature

Hot Extrusion:

Just as earlier stated, hot extrusion process is performed above recrystallization temperature of the material. This usually above 50-60% of its melting temperature. In these types of extrusion, low force is required, easy to work on, and the product is free from stain hardening. Although high maintenance is required.

Cold Extrusion:

Cold extrusion takes place at room temperature or below the crystallization temperature of the material. It offers high mechanical properties, high surface finish, and no oxidation at metal surface. However, high force is required, and products are accomplished with strain hardening.

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Working principle of extrusion

The working of extrusion is easy and can be easily understand. With the various explanation on the types of extrusion processes, you are exposed to the different variations of extrusion processes. Well, in the conventional method, a piston or plunger is used to apply compressive force to the workpiece. In hot extrusion, the billet is heat or kept at room temperature in cold extrusion. It is then placed into an extrusion press, which is like a piston cylinder device, that is, metal is placed in the cylinder and pushed by a piston. The upper portion of cylinder is fitted with die.

This plunger fitted to the press pushes the billet towards the die that carries the shape of the desired object. The high compressive force applied allows the working material to flow through a die and convert into the shape. The part is removed from press and operation is achieved.

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Watch the video below to learn more on the working of extrusion process:

Advantages and disadvantages of extrusion

Advantages:

Below are the benefits of extrusion process in their various applications:

  • High mechanical properties can be achieved with cold extrusion.
  • A complex cross-section can be easily created.
  • Brittle and ductile materials can be work on.
  • High extrusion ratio.

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Disadvantages:

Despite the good advantages of extrusion process, some limitations still occur. Below are the disadvantages of an extrusion process in their various applications:

  • High initial or setup cost.
  • Require high compressive force.

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Conclusion

Extrusion process is one of the most common process in production world today. It involves the use of large variety of material like metals, plastics, ceramics, etc. The process can be done in either hot or cold when better properties are required. It is a manufacturing molding process that involves forcing base material through a pre-shaped die to create objects with a specific shape and profile. That is all for this article, where the definition, applications, function, process, materials, equipment, types, working principle, and advantages and disadvantages of extrusion processes were discussed.

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