THE EXTRUSION PROCESS

Extrusion is a plastic deformation process in which a block of metal (billet) is forced to flow by comparison through the die opening of a smaller cross - sectional area than that of the original billet. Extrusion is an indirect - compression process. Indirect - compressive forces are developed by the reaction of the work piece (billet) with the container and die results in high values. The reaction of the billet with the container and die results in high compressive stresses that many breakdowns from the billet. Extrusion is the best method because the billet is subjected to compressive forces only. Extrusion can be cold or hot, depending on the alloy and the method used. In hot extrusion, the billet is preheated to facilitate plastic deformation. Conventional Direct Extrusion: The most important and common method used on aluminium extrusion is the direct process. In this process, the principle of direct extrusion, the billet is placed in the container and pushed through the die by the ram pressure. Direct extrusion finds application in the manufacture of aluminium solid rods,aluminium bars, hollow tubes, and hollow and solid sections according to the design and shape of the die. In the same direction as ram travel. During this process, the billet slides relative to the walls of the container. The resulting frictional force increases the ram pressure considerably. During the direct extrusion, the load or pressure - displacement curve most commonly. Traditionally, the process has been described as having three distinct regions: 

• The billet is upset, and pressure rises rapidly to its peak value.
• The pressure decreases, and what is termed "steady state" extrusion proceeds.

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