Greetings from The Mechanical post ! Today we'll see What is extrusion ? its process, advantages and limitations So let's start with the definition of extrusion.

Definition of extrusion

Extrusion may be defined as the manufacturing process in which a block of metal enclosed in a container and is forced to flow through an small opening of a specific shape and size called a die .



Extrusion related terms you must know 

Billet                                                                                                                       

A billet are uniform blocks of the material to be extruded (take for example aluminium extrusions). These blocks sizes vary depending upon the machinery used for extrusion. These blocks are then used to extrude the desired product.

extrusion die
picture showing extrusion die

Die 

Extrusion dies are circular steel plates or disks with considerable thickness and having one or more openings to create the desired profile. They are made from H-13 die steel and heat-treated as they have to withstand high pressure and temperature caused by forcing the hot billet through the die.

Ram extruder 

The ram is a type of piston or plunger and heavy in mass which is used to push the billet through the die by applying huge amount of force. An example of ram is the doctor's syringe, as the doctor applies force onto the ram the fluid in it is forced to flow out.

Extrusion process 


The extrusion process starts with the heating of the billet. The billet are usually heated upto a temperature such that the material becomes easily malleable and ductile. 

The billet is then loaded onto the container. When the ram forces the metal to exit through the die opening , the metal is subjected to plastic deformation and it undergoes reduction and elongation during extrusion.

The section of the product will depend upon the shape of the die opening. This process could be considered an adaptation of closed die forging, the difference being that in a forging, the main body of the metal is the product and the flash is cut away and discarded; in extrusion, the flash ( metal flowing out of the die) is the product and the slug remaining in the die is not used.

Extrusion is more widely used in manufacturing of solid and hollow sections from non-ferrous metals and their alloys ( aluminium alloys, copper, brass and bronze etc.), but steel and other ferrous alloys can also be successfully processed with the development of molten-glass lubricants. 

The initial material in extrusion is cast or rolled billets. The range of extruded items is very wide: rods from 3 to 25mm in diameter, pipes of 20 to 400mm in diameter and wall thickness of 1mm and above, and more complicated shapes which con not be obtained by other mechanical working methods. Besides metals extrusion of plastics is also possible.

Types of extrusion:

Direct extrusion:

The heated billet is placed in the container. It is pushed by ram towards the die. The metal is subjected to plastic deformation, slides along the walls of the container and is forced to flow through the die opening.

 At the end of the extrusion operation a small piece od metal called butt-end scrap, remains in the container and can not be extruded. To obtain a hollow section, a hollow billet is often used around mandrel. 

In some instances, the billet is solid and is pierced by the mandrel or an axial hole drilled in it by means of a drilling machine before the extrusion cycle begins. The mandrel may be fastened to the ram. As the ram moves in the direction towards the die, the metal is forced out through the annular clearance and from a tube.

Indirect extrusion:

In an indirect extrusion process, the metal is extruded in the direction opposite to the ram motion. For this purpose a hollow ram is used (As shown above). 

The hollow ram contains the die in the shape of desired product. For the production of the part, the die is mounted on the  end of the hollow ram whereas on the other end of the container is closed. 

As the ram travels the die applies pressure on the billet and the deformed metal flows through the die opening opposite to the ram motion and the product is extruded through th hollow ram.

Hot extrusion :


Hot extrusion is a process in which the billet is heated before extrusion. Hot extrusion is done above the material's recrystallization temperature this not only helps the material from work hardening but also it becomes easier to push the material through the die. 

Most hot extrusions processes are done on horizontal hydraulic presses that range from 230 to 11,000 metric tons (250 to 12,130 short tons). Pressures range from 30 to 700 MPa (4,400 to 101,500 psi), thus this process requires lubrication such as oil or graphite (for lower temperature extrusions), or glass powder (for higher temperature extrusions). Hot extrusion process 

Cold extrusion:

Opposite to hot extrusion process the cold extrusion process works at room temperature. The cold extrusion is advantageous over hot extrusion as it doesn't cause oxidation,  provides higher strength due to cold working, can be worked with closer tolerances, better surface finish, and fast extrusion speeds.
Materials that are commonly cold extruded include:
  • lead,
  • tin,
  • aluminum,
  • copper,
  • zirconium,
  • titanium,
  • molybdenum,
  • beryllium,
  • vanadium,
  • niobium, and steel.

Examples of products produced by this process are: collapsible tubes, fire extinguisher cases, shock absorber cylinders and gear blanks.

This process is similar to the hot extrusion except that the metals worked possess the plasticity necessary for successful forming without heating them. 

Usually, these metals have a high degree of ductility. Cold extrusion is also done to improve the physical properties of a metal and to produce a finished part.

Advantages of extrusion:

  • The range of extruded items is very wide. Cross- sectional shapes not possible by rolling can be extruded, such as those with re-entrant sections.
  • No time is lost when changing shapes since the dies may be readily removed and replaced.
  • Dimensional accuracy of extruded parts is generally superior to that of rolled ones.
  • Automation in extrusion is simpler as items are produced in single passing.
  • Extrusions are lighter, more sound and stronger than castings.
  • Extrusion are more accurate than castings and require less post-extrusion finishing processes.

Limitations of extrusion process:

  • Process waste in extrusion is higher than rolling, where it is only 1 to 3%.
  • Relatively high tooling cost, being made from costly alloys steel.
  • In productivity, extrusion is much inferior to rolling, particularly to its continuous varieties.
  • Cost of extrusion are generally greater as compared to other techniques.
  • Service life of extrusion tooling is shorter because of high contact stresses and slip rates.
  • Only shapes with constant cross-section can be produced.

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