Extrusion Process: Types, Parameters, and Defects

Posted on Mar 31, 2025 in Electromechanical Engineering

Extrusion: A Comprehensive Overview

Extrusion: A cylindrical billet is forced through a die. A wide variety of solid or hollow cross sections may be produced by extrusion. A characteristic of extrusion: large deformations can take place without fracture. Depending on the ductility of the material: room or elevated temperatures. At room temperature: combined with forging operations, in which case it generally is known as cold extrusion.

The Extrusion Process

  1. Direct or forward extrusion: the most common process.
    1. A billet is placed in a chamber (container).
    2. Forced through a die opening by a hydraulically driven ram (pressing stem or punch).
  2. Indirect extrusion (also called reverse, inverted, or backward extrusion): the die moves toward the unextruded billet. Advantage: no billet-container friction, since there is no relative motion (used on materials with very high friction).
  3. Hydrostatic extrusion: the billet is smaller in diameter than the chamber (which is filled with a fluid), and the pressure is transmitted to the fluid by a ram.
  4. A less common type of extrusion is lateral (or side) extrusion.

Geometric Variables in Extrusion

  • The die angle, α
  • The ratio of the cross-sectional area of the billet to that of the extruded product, A0/Af, called the extrusion ratio, R.

Process variables in direct extrusion: the die angle, reduction in cross-section, extrusion speed, billet temperature, and lubrication all affect the extrusion pressure.

Metal Flow in Extrusion

The metal flow: influence on the quality and the mechanical properties of the extruded product.

Types of Metal Flow in Extruding with Square Dies

  1. Flow pattern obtained at low friction or in indirect extrusion.
  2. Pattern obtained with high friction at the billet–chamber interfaces.
  3. Pattern obtained at high friction or with cooling of the outer regions of the hot billet in the chamber.

Process Parameters

  • Extrusion ratios, R, usually range from about 10 to 100.
  • Less than 7.5 m long, but they can be as long as 30 m.
  • Ram speeds range up to 0.5 m/s.
  • Dimensional tolerances: from ±0.25 to 2.5 mm, and they increase with increasing cross section.
  • Extruded products, with small cross sections, require straightening and twisting.
  • In coaxial extrusion, or cladding, coaxial billets are extruded together provided that the strength and ductility of the two metals are compatible.

Hot Extrusion

Oxide film can be abrasive and can affect the flow pattern of the material.

Die Design

Square dies (shear dies) are used in extruding nonferrous metals, especially aluminum. Square dies develop dead-metal zones. The dead-metal zones produce extrusions with bright finishes.

Tubing

Tubing is extruded from a solid or hollow billet. Wall thickness is usually limited to 1 mm for aluminum, 3 mm for carbon steels, and 5 mm for stainless steels.

Die Materials

Die materials for hot extrusion usually are hot-worked die steels. Coatings (such as partially stabilized zirconia) may be applied to the dies to extend their life.

Lubrication

Glass is an excellent lubricant for steels, stainless steels, and high-temperature metals and alloys. Cold extrusion has the following advantages over hot extrusion:

  • Improved mechanical properties resulting from work hardening.
  • Good control of dimensional tolerances.
  • Improved surface finish.
  • Production rates and costs that are competitive with those of other methods.

Hydrostatic Extrusion

  • The pressure required in the chamber is supplied via a piston through an incompressible fluid.
  • Pressures are typically on the order of 1400 MPa.
  • The high pressure in the chamber transmits some of the fluid to the die surfaces, where it significantly reduces friction.
  • Hydrostatic extrusion usually is carried out at room temperature.

Extrusion Defects

  1. Surface Cracking
  2. Pipe
  3. Internal Cracking

Extrusion Equipment

Horizontal hydraulic press: The stroke and speed of the operation can be controlled. They are capable of applying a constant force over a long stroke.

Vertical hydraulic presses: For cold extrusion. They have less capacity than those used for hot extrusion, but they take up less floor space.

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