Classification of the Extrusion Process

Temperature at work

• Cold extrusion
• Warm extrusion
• Hot extrusion

By performance technique

• Continuous extrusion
• Discrete extrusion

By physical configuration

• Forward or Direct extrusion
• Backward or Indirect extrusion

The hot metal billet is put into a container in direct extrusion (forward extrusion).

• The material is compressed by a ram, pushing it to flow through perforations in a die at the container’s opposite end.
• The metal is plastically deformed, slides along the container’s walls, and is forced to flow through the die aperture.
• A little piece of the billet lingers as the ram reaches the die and cannot be driven through the die aperture.
  

• The butt, or superfluous piece, is detached from the result by cutting it slightly beyond the die’s exit.
• The high friction that arises between the work surface and the walls of the container as the billet is forced to move toward the die hole is one of the challenges with direct extrusion.
• In direct extrusion, this friction generates a significant increase in the ram force required.
• The existence of an oxide layer on the surface of the billet exacerbates the friction problem in hot extrusion.
• This oxide layer has the potential to induce flaws in the extruded product.
• A fake block is frequently utilised between the ram and the work billet to alleviate these issues.
• The dummy block has a diameter that is somewhat smaller than the billet diameter.
• As a result, the container is left with a thin cylindrical shell (skull) made mostly of the oxidised layer.
• The oxides are thus removed from the extruded result, and the skull is afterwards removed from the chamber.
• The procedure allows for hollow portions (such as tubes) via direct extrusion.
• A hole parallel to the axis of the beginning billet is drilled.
• This allows a mandrel attached to the dummy block to pass through.
• The material is forced to flow through the clearance between the mandrel and the die aperture when the billet is squeezed.
  

• The cross section that results is tubular. In most cases, semi-hollow cross-sectional shapes are extruded in the same manner.
• In direct extrusion, the beginning billet is normally spherical in cross section, but the shape of the die opening determines the final shape.
• Obviously, the die opening’s biggest dimension must be smaller than the billet’s diameter.
• Backward Extrusion/Reverse Extrusion (Indirect Extrusion).
• Metal is forced to flow in the opposite direction of the ram’s motion through the die.
• Because the work billet metal is not moving relative to the container wall, the extrusion force is reduced.
• The die is attached to the ram rather than the container’s opposite end.
• The metal is forced to flow through the clearance in the opposite direction of the ram’s motion as it penetrates the work.
• There is no friction at the container walls since the billet is not forced to move relative to the container, hence the ram force is smaller than in direct extrusion.
• The decreased stiffness of the hollow ram and the difficulty in sustaining the extruded product as it exits the die limit indirect extrusion.
• Hollow (tubular) cross sections can be produced by indirect extrusion.
• The ram is forced into the billet in this process, causing the material to flow around the ram and form a cup shape.
• The length of the extruded part that may be manufactured with this method is practical limited.
• As the length of the job grows longer, the ram’s support becomes a concern.
  

Extrusion (Hot)

• Prior to hot extrusion, the billet is heated to a temperature above its recrystallization temperature.
• This reduces the metal’s strength while increasing its ductility, allowing for more extreme size reductions and complicated shapes to be created in the process.
• Metals and alloys with insufficient ductility at room temperature can use this method.
• Reduced ram force, increased ram speed, and improved grain flow characteristics in the finished product are all positives.
• Die wear can be considerable in this extrusion, and cooling of the hot billet in the chamber can be an issue, resulting in very non-uniform deformation.
• Extrusion dies can be preheated to reduce billet cooling and extend die life, similar to how hot forging dies are prepared.
• Isothermal extrusion is occasionally utilised to alleviate the problem of the billet cooling when it contacts the container walls.
• For certain metals (e.g., steels), lubrication is crucial in hot extrusion, and special lubricants have been created to work under the extreme conditions of hot extrusion.
• The following issues arise while using a hot billet:
• Unless heated in an inert environment furnace, the billet acquires an oxide film because it is hot. This film can be abrasive, and it can alter the material’s flow pattern.
• It also results in an extruded product with a poor surface polish, which may be unsatisfactory in some instances.

Extrusion(Cold)

• Extrusion took place at room temperature. Frequently used in conjunction with forging processes.
• The need for heating the beginning billet is also eliminated with cold extrusion at ambient temperature.
• Extrusion, both cold and heated, is commonly used to make discrete components that are finished (or almost finished).
• Impact extrusion is a word that refers to high-speed cold extrusion.
• Increased strength due to strain hardening, tight tolerances, enhanced surface polish, the absence of oxide layers, and fast production rates are just a few of the benefits of cold extrusion.