EXTRUSION AND ITS TYPE

Extrusion Processes:

Extrusion is the process of confining the metal in a close cavity and then allowing it to flow from only one opening , so that the metal will take the shape of the opening. The operation is identical to the squeezing of toothpaste out of the toothpaste tube.

By the extrusion process, it is possible to make component which have a constant cross section over any length as can be formed by the rolling process. Some typical parts can be extruded are shown:

Compexity of parts that can be obtain by extrusion is more than that of rolling, because the die required being very simple and easier to make. Also extrusion is a single pass process,unlike rolling the amount of reduction that si possible in extrusion is large. Generally brittle materials can be very easily extruded. It is also possible to produce sharp corners and different angles. It is possible to gets shapes with internal cavaties in extrusion by the use of spider die. Large diameters, thin walled , tubler products with execellent concentricity and tolerance charasitic can be produced.

Types of extrusion:

1: Direct extrusion or forward extrusion

2: Indirect extrusion or backward extrusion

Direct Extrusion or Forward Extrusion

The equipment consists of a cylinder or container into which the heated metal billet is loaded. One end of the container, the die plate with necessary opening is fixed. From the other end plunger or ram compresses the metal billet against the container walls and die plate, thus the forcing it to flow of metal in the forward direction through the die opening.

Acquiring the shape of the opening the extruded metal is then carried by the metal is then carried b the metal handling system as it comes out of the die. A dummy block which is a steel disc of about 40mm thick with a diameter slightly less than container is kept between the hot billet and the ram to protect it form heat and pressure. In direct extrusion, the problem of friction prevalent because of the relative motion between heated metal billet and cylinder walls. To reduce this friction lubricants are to be used. To reduce the damage to equipment, extrusion is finished quickly and the cylinder is cooled before further extrusion.

Indirect extrusion or backward extrusion

In order to completely overcome the problem, the backward hot extrusion as shown in figure, in this process the metal is confined fully by the cylinder, the ram which houses the die also compresses the metal against the container , forcing it to flow backward to the die in the hollow plunger or ram.

It is termed backward because of the opposite direction of the flow of the metal. Thus the billet in the container remains stationary and hence produce no friction. Also the extrusion pressure is not effected by the length. In the extrusion press since the friction is not loss. The figure of the backward extrusion is shown;