A bit of history
First hydraulic extrusion presses were tested by J. Bramah in 1810 in the UK and designed for the lead extrusion. Later on, A. Dick applied the same principle to higher melting point alloys in Germany in the 1890’s. He placed a separate dummy block in front of the extrusion stem, allowing the discard and the dummy block to be ejected together, which was a decisive invention for the development of the extrusion together with the improvements of the mechanical construction, tooling and application of hot working steels.
The extrusion process
Extrusion is a deformation process used to manufacture long, straight, semi-finished metal products such as tubes, bars, solid and hollow sections (profiles) and wires. Extrusion is carried out at high temperature with preheated billets, depending on the alloy and the method.
The principle is to squeeze a billet closed in a container through a die to give a reduction in size by means of a pushing high force. The extrusion load is transmitted by hydraulic driven ram through an intermediate dummy block to the billet. The axial load is applied to the die stack in the press platen.
Direct extrusion
In this process the billet is at first upset within the container, so as far as it is compressed it assumes the diameter of the container bore becoming shorter. Then it is pressed by the stem through the extrusion die. During this, relative movement takes place between the billet and the container. The main characteristic of direct extrusion is the friction between the billet and the container.
Indirect extrusion
In indirect extrusion as in the case of direct extrusion the billet is first upset within the container. A short closing plug seals off the container on the main ram side, and on the other side, the extrusion die is supported at the end of a fixed hollow stem. While the main ram is moving forwards and pushing the container against the stem, this is going inwards into the container and the billet is then extruded through the die and the hollow stem. During extrusion the billet and the container are displaced together with no relative movement between them. The characteristic of indirect extrusion is the absence of friction between the billet and the container.
On the other hand, the constancy of the material flow over the entire extrusion process excludes product defects such as back end defect or ‘piping’
Axial forces during hot extrusion
The axial force of the direct extrusion has two components, first to overcome the billet against container friction and the second is deforming the material of the billet in the deformation zone. Friction between billet and container does not exist In the case of indirect extrusion, hence the full applied force is for deformation of the metal.
During extrusion the load-displacement curve most commonly has the form shown in figure.
• The billet is upset, and pressure rises rapidly to its peak value.
• The pressure decreases, and what is called “steady state” extrusion proceeds. Force reduces as stroke advances in direct extrusion. Force is constant in indirect extrusion.
• The pressure reaches a sharp rise as the ‘discard’ or ‘butt’ is compacted.
Specific pressure
It is defined as the inner pressure in the container liner and is given by:
Ps = Fp/Ac
where:
Fp is the force applied by the press
Ac is the area of the container bore
Extrusion ratio
It is one of the principal variables which influences the force required for the extrusion and it is defined as:
ER = Ac/Ae
Where:
Ac is the area of container bore
Ae is the area of extrusion
The extrusion ratio of a shape is a clear indication of the amount of mechanical work that will occur as the shape is extruded.