Blog 32: Metal Extrusion and its Different Types?

Introduction:

In the metal forming process known as extrusion, a material is subjected to plastic deformation by the application of a force that causes it to flow through a die or aperture. If the material has the right characteristics, it will take on the cross-sectional profile of the die and maintain that form in the final extrudate. In order to generate the force needed for this procedure, either a moving piston or a spinning auger in a barrel are typically used. The process is ongoing in the latter scenario, and distinct portions are produced by slicing the extrudate into pieces that are a specific length.

The manufacture of pipes and steel rods makes great use of this method. Compressive force is employed to extrude the work piece. This procedure is comparable to drawing, with the exception that drawing extends the metal work piece using tensile stress. When compared to drawing in one pass, the compressive force allows for significant distortion. Plastic and aluminium are the most widely used materials for extrusion.

Thomas Burr was the first to extrude lead pipes using the technique that Joseph Bramah first patented. The method has three essential components: a machine (extruder, piston, or screw) is needed to create pressure; a die is needed to shape the material; and a formulation is necessary to make the ceramic powder or precursor plastically malleable.

The beginning cross-sectional area divided by the cross-sectional area of the end extrusion is known as the extrusion ratio. The fact that this ratio can be quite high while still generating high-quality parts is one advantage of an extrusion method. This method may work with brittle materials and produce cross-sections that are quite complicated. This is so because the only stresses the material experiences are compressive and shear. The extrusion method also gives products a superb surface polish and a great deal of shape freedom during the design phase.

History of Extrusion:

Extrusion of ceramic materials can be dated to 1643 with the production of clay bodies, and the first commercialization for brick production was described at the start of the 19th century, according to Reh (2007). Building materials composed of clay continue to make up the vast bulk of the things produced today using this shaping technique in terms of manufacturing volume and mass. If the method produces the desired shape, almost any ceramic material can be extruded. Therefore, extruding ceramics is a straightforward procedure in theory, at least.

Extrusion procedures are comparable to drawing, which pulls material through the die using the material’s tensile strength. Wire, metal bars, and tubes are all produced through the drawing process. In contrast to extrusion, which completes the task in a single step, it is restricted to simpler shapes and frequently requires numerous phases.

Extrusion is known to be continuous or semi-continuous (theoretically producing indefinitely long material) (producing many pieces). Metals, polymers, ceramics, concrete, modelling clay, and food products are often utilised materials in the extrusion process.

Working Principle:

A straightforward compressive metal forming technique is extrusion. A piston or plunger is employed in this operation to apply compressive force to the work item. The following can serve as a summary of these steps.

  • First billet or ingot (metal work piece of standard size) is produced.
  • This billet is heated in hot extrusion or remains at room temperature and placed into a extrusion press (Extrusion press is like a piston cylinder device in which metal is placed in cylinder and pushed by a piston. The upper portion of cylinder is fitted with die).
  • Now a compressive force is applied to this part by a plunger fitted into the press which pushes the billet towards die.
  • The die is small opening of required cross section. This high compressive force allow the work metal to flow through die and convert into desire shape.
  • Now the extruded part remove from press and is heat treated for better mechanical properties.
  • This is basic working of extrusion process.

Types of the metal 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

Direct Extrusion

The most typical form of extrusion is direct extrusion, often known as forward extrusion. The procedure is started by placing a fake block behind a heated billet (only for hot extrusion, which will be detailed later) in a press cavity container. The material is then forced into the die by the mechanical or hydraulic ram. The portion is then stretched to straighten while it is still hot. Core Materials does a wonderful job of animating this process.

Molten glass is used as a lubricant in direct extrusion to alleviate the high friction brought on by steels at higher temperatures, while lubricants containing graphite powder are used for lubrication at lower temperatures. In hot extrusion, the dummy block is utilised to shield the pressing stem’s (punch or ram) tip. A little part of the billet known as the “butt end” cannot be forced through the die opening as the punch reaches the conclusion of its stroke.

Direct metal extrusion advantages

  • No billet modification required
  • Can be used for both hot and cold extrusion
  • Simple tooling compared to other extrusion processes

Direct metal extrusion disadvantages

  • High force requirement due to friction
  • Butt end left inside the cavity
  • The force required to push the ram changes as the punch moves

Indirect Extrusion

To force the material through the die during indirect extrusion, the die is moved toward the billet inside the cavity by the hydraulic ram, which is at the end of the device.

Because the static billet container creates less friction on the billet, this method uses less power. When the extrudate exits the die, it is challenging to support the extruded portion.

In-direct metal extrusion advantages

  • Less friction and less power used
  • Can be used for both hot and cold extrusion
  • Simple tooling compared to other extrusion processes

In-direct metal extrusion disadvantages

  • Difficult to support the extruded part
  • The hollow ram limits the load applied

Hydrostatic extrusion

As seen in figure 3, hydrostatic extrusion involves creating a chamber or cavity that is smaller than the billet and filling it with hydraulic fluid to transfer the force from the ram to the billet. Despite the fluid applying triaxial forces, the pressure enhances the billet’s ability to be formed. Early on, sealing the fluid must be considered in order to prevent leaks and lessen pressure problems.

Due to the specialised equipment needed, the hydraulic fluid’s use in the industry is constrained in comparison to other extrusion processes, despite the fact that it isolates the wall and billet to eliminate friction.

Hydrostatic metal extrusion advantages

  • 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

Hydrostatic metal extrusion disadvantages

  • 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

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.

Impact Extrusion

Impact extrusion, which is limited to softer metals like lead, aluminium, and copper, belongs to the cold extrusion group and is very similar to indirect extrusion. The punch is pressed down quickly and exerts a great deal of force on the slug to extrude backward, as shown in the diagram. The distance between the punch and the die chamber determines the extrude’s thickness. A stripper plate is used to slide the extrudates off the punch.

A mechanical press is frequently used for impact extrusions, and the part is created at a high speed and over a relatively short stroke.

Due to the extremely high forces operating on the punch and die, tooling must have adequate impact resistance, fatigue resistance, and strength in order to extrude metal by impact. The flow of the material allows impact extrusion to be categorised into the following three categories.

  • Forward
  • Reverse
  • Combination

In forward impact extrusion, the metal flows in the direction that the force is applied, but in reverse impact extrusion, it flows in the opposite direction. The metal flows in both directions as combined, as seen in the illustration above.

Impact metal extrusion advantages

  • 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 the 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 the manufacturing industry by backward impact extrusion.

Impact metal extrusion disadvantages

  • 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

Hot Extrusion

The most often used technique for creating items with a set cross-sectional profile is hot extrusion. By performing the extrusion process at a higher temperature, the materials are kept from processing hardening and the operation of forcing the material through the die is simplified.

In general, horizontal hydraulic presses with a tonnage range of 250 to 12,000 tonnes are used to accomplish large-scale hot extrusion. In this case, lubrication is necessary due to the pressure range of 30 to 700 MPa (4,400 to 102,000 psi).

For extrusions at higher temperatures, glass powder can be used as lubrication; for extrusions at lower temperatures, oil or graphite can be used. Hot extrusion has some limits due to the cost of the extrusion machinery and the upkeep of those devices, even if hot extruded material is highly desirable. Following are the two distinct types of hot extrusion.

Types of Hot Extrusion

  • Forward or Direct extrusion
  • Backward or Indirect extrusion

Cold Extrusion

The process of cold extrusion is carried out at room temperature or at temperatures that are relatively high with the aid of extruders and extrusion equipment. These specific devices/machines were created using cutting-edge extrusion technology.

The process of moulding cold metal by striking a slug is known as cold extrusion. The metal is pushed upward around the punch during this blow, which is made with a punch (in a confined cavity). Extrusion pressing, cold forging, cold pressing, and impact extrusion are additional names for this procedure.

Types of Cold Extrusion

  • Hooker extrusion
  • Hydrostatic extrusion
  • Impact extrusion
  • Cold extrusion forging

Extrusion Defects

Depending on the material condition and process variables, extrudates can develop many types of defects that could affect the quality of the end product.  These defects can be grouped under the following three defects.

  • Surface cracking
  • Piping
  • Internal cracking

Application:

  • 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 and Disadvantages:

Advantages:

  • 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 achieved by cold extrusion.
  • Low cost per part
  • Flexibility of operation
  • In hot extrusion, post execution alterations are easy because product is still in heated condition
  • Continuous operation
  • High production volumes
  • Many types of raw materials can be used
  • Good mixing (Compounding)
  • Surface finish obtained is good
  • Good mechanical properties obtained in cold extrusion

Disadvantages:

  • High initial or setup cost.
  • High compressive force required.
  • Variations in size of product
  • Product limitations because of only one type of cross section can be obtained at a time
  • High initial cost setup

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