Basic Principles of Metal Extrusion 

Metal Extrusion Process

Extrusion is a metallurgical deformation process used to produce long, straight metal semi-finished products, such as tubes, bars, solid and hollow profiles, and cables. Extrusion is performed at high temperatures using preheated billets, with the working temperature depending on the alloy and method.

The process involves forcing a metal billet, previously encased in a container, through a die or matrix to reduce its cross-sectional area via compressive force.

The extrusion force is hydraulically transmitted through a piston to an intermediate ram, which then transfers the force to the billet. The axial load is ultimately applied to the tooling assembly.

Direct Extrusion

In this process, the billet first expands within the extrusion container. Once subjected to compressive force, it expands to the inner diameter of the container sheath, reducing its length. It is then pushed through the die by the extrusion ram. During the process, there is relative motion between the billet and the container. The primary characteristic of direct extrusion is the friction between the billet and the container.

Axial Force in Hot Extrusion

In direct extrusion, the axial force applied to the billet consists of two components: one to overcome friction between the billet and the container, and the other to deform the material in the deformation zone. In reverse extrusion, there is no friction, so all applied force is used solely for deformation.

During the extrusion process, the force-stroke curve typically follows the shape shown below:  The billet is pre-compressed, and the pressure rapidly increases to a peak as extrusion begins.

The second stage, known as “steady-state extrusion,” involves a decrease in force in direct extrusion due to reduced friction between the material and the container.

In reverse extrusion, the force remains constant.  

Finally, a sharp increase occurs until the “discard” or “shorts” are compressed.

Reverse Extrusion

In indirect (or reverse) extrusion, the billet is first pre-compressed in the extrusion container.

As the piston rod moves forward, the die is pushed toward the stem, which is then forced into the die, and the billet is extruded through a strip passing through the hollow stem.

During the process, the billet and container move together as a single unit, with no relative motion between them.

Reverse extrusion is characterized by the absence of friction between the billet and the container.

Additionally, the uniform material flow in reverse extrusion prevents quality defects, such as extrusion end defects (also known as “coring” or “piping”).

Specific Pressure

Specific pressure is defined as the pressure within the container liner and is calculated using the formula:

Ps = Fp / Ac

Where:  Fp is the force applied by the press  

Ac is the area of the container sheath bore

Extrusion Ratio

The extrusion ratio is one of the main variables affecting the force required for extrusion and is defined as:

ER = Ac / Ae

Where:  Ac is the area of the container sheath bore  

Ae is the cross-sectional area of the extruded product