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Direct extrusion

Direct extrusion is the process used most often. Here the billet to be extruded is pressed towards and through the die by the stem. The friction arising be­tween billet and liner limits the length of the container. Because the material to be extruded is always deformed at the die end of the container, not only does a higher thermal loading occur here but wear is also more extensive at this point, leading to a very one-sided loading situation.

Since this is the ­oldest extrusion process, some containers without controllable heating systems are still in operation. Modern direct extrusion presses are usually equipped with a zone-controlled resistance heating system and sometimes also with a cooling system, despite the comparatively short length of the container.

Indirect extrusion

The indirect extrusion process has gained significant importance in the last few years. In this process the container, with the billet, is pressed directly against the hollow stem, with the result that the friction between billet and liner becomes negligible thus lowering the necessary force. This enables the use of very long containers. The resulting long press times lead to higher ­thermal stresses in the container components.

Since the extruded material must pass through the bore in the stem, the diameter of this bore limits the dimension of the extrusion which can be produced. In order to be able to produce a wide range of products despite this, the hollow stem is usually ­designed with a large bore which in turn leads to a high compressive stress on the stem and so necessitates careful selection of materials. Most indirect presses are equipped with containers over 1200 mm in length, meaning that multi-zone heating systems and cooling systems are standard.

BW030_Special steels and tools for tube and rod extrusion