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Design of Tools for Hot Extrusion

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Design of Tools for Hot Extrusion --- Aheadmold

Traditionally, in hot extrusion a billet of metal is forced through a shaped hole at one end of a container by the action of a ram pressing from the other end. The container bore is of uniform cross-section and the ratio of its area to that of the product is known as the extrusion ratio. In the most common form of the process, direct extrusion (Fig. 5-3 (a)), the ram moves through the bore of the container, the die being clamped rigidly against the end of the container. The extrudate exits in the same direction as the ram moves.
In indirect extrusion (Fig. 5-3 (b)) the die is fitted on to the ram which is forced against the billet held in a container closed at the far end. In this case the extrudate leaves the die through the hollow ram. Although the container bore is normally round it may also be rectangular to permit extrusion of very wide products.
Fig. 5-3   Hot extrusion

As an alternative to generating the required extrusion pressure by the direct action of the ram against the billet held, it is possible to transmit the pressure via a fluid medium to the billet. This is known as hydrostatic extrusion, but here “hot” extrusion is limited to temperatures at which a stable fluid can be found.
In direct extrusion, a dummy block is generally interposed between the hot billet and the ram end to minimize damage to the ram——the dummy block being a cheap, easily replaceable item. For extrusion of hollow sections or tubes the ram carries a mandrel, which serves to form the inner surface of a hollow extrusion and which on some presses may be driven independently of the main ram to permit piercing of solid billets.
The main tooling components for the hot extrusion process are therefore the container, the die assembly, the dummy block and the mandrel.
The container is the most expensive component and the design of the compound containers used today is properly left to the press or container manufacturer. In use the container has to withstand high cyclical, mechanical and thermal stresses and excessive thermal shocks. Prolonged exposure to high temperatures such as caused by “sticking” billet should be avoided.
Extrusion dies therefore should be designed in such a way that the extrusion pressures required is within the capability of the press and sticking billets do not occur.
The requirements on tools vary widely, depending on the extrusion temperature, pressure, extruded material and type of product, and the materials of construction of the tooling. To give an idea of this variety Table 5-1 lists some common parameters of industrial production for sections. This fragmentary list is clearly not definitive and is only intended to indicate the types of problems met with in tooling design and construction.
Table 5-1 Common Parameters of Industrial Production for Sections

Two important factors emerge from the table:
(1)    Minimum extrusion ratios less than 10 are seldom met except for forging feedstock. The theoretical approaches to die design should reflect this fact.
(2)    Where industry can use a flat die it does so. Only where the temperature of extrusion is so high as to create a gross instability of the die to creep or plastic collapse, are dies bell- mouthed or entry-profiled. Economic structures limit what is practical.
Details of tool design and manufacture of the various components in extrusion tooling are influenced by both economic and technical factors, which are often dependent upon how critical the particular component is in ensuring tile production of a saleable product.

Article come from AHEADMOLD,website is WWW.AHEADMOLD.COM.

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