Upsetting of Billet

In order to facilitate insertion of billets in the container, the former are made 5 to 10 mm smaller in diameter than the container bore. A sickle-shaped slit is thus formed between billet and container wall (see also Fig. 120). If the billet were pierced in this state and then extruded, the mandrel would be subjected to an up-acting pressure, which would cause the mandrel to deflect and result in tubes of non-uniform wall thickness. 

Prior to piercing, the billet must therefore be upset so that it fills the container space completely in order to do that the face of the dummy- 

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