Types of Extrusion

Extrusion processes can be classified into four broad categories.  They are

Direct Extrusion
In Direct Extrusion, the die is stationary.  The billet is put into the die and the force is applied by a ram.  A dummy block is added between the molten metal and the die. As force is applied, the molten metal is forced through the shape of the die.  In direct extrusion, friction develops between the billet and the die.  This requires the greater force to move the billet forward.

Indirect Extrusion
In Indirect Extrusion, the billet is stationary.  The die is pressed against the billet by means of rams.  The force required is lesser as the friction is minimized.  However, long components cannot be made using the indirect extrusion as the die needs to be supported

The risk of cracking is minimized as lesser temperature is produced. 

Hydrostatic extrusion
In hydrostatic extrusion, the extrusion container is filled with a hydraulic liquid.  Unlike other forms of extrusions, where the force is transmitted by means of a ram, in hydrostatic extrusion, the force is transmitted hydraulically. 

The high pressure required can be generated by a pump or by a press.

Hydraulic extrusion does not generate friction as there no contact between the billet and the walls of the container.  It is used in the extrusion of brittle materials, such as ceramics. 

Impact Extrusion
This form of extrusion is used to create objects such as small cups and tubes.  Here, the billet is placed above the die.  A high impact force is applied using a piston or a hammer.  The billet is extruded through the die in the form of the desired shape.