In a hot extrusion the metal billet is heated up to plastic state and put into a cylinder and it is forced to flow through the opening of a die. On the basis of direction of flow of the metal with respect to direction of the force applied, hot extrusion is divided into two classes—forward extrusion and backward extrusion.
In forward extrusion, plunger moves in forward direction and the metal is forced through the die opening on same direction but in backward direction the metal is forced through the die opening in plunger itself in backward direction as shown in Figure 21.6.
Figure 21.6 Forward and Backward Extrusion
Backward hot extrusion involves no friction between the metal billet and chamber walls because the billet does not move in the chamber compared with forward hot extrusion. Total force required due to low friction in the backward extrusion is less but equipment used is more mechanically complicated in order to accommodate the passage of the extruded shape through the centre of the plunger.
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