Manufacturing Processes - MFRG 315 - 3.4.3 Extrusion and 3.4.4 Drawing

3.4.3 Extrusion

In extrusion the workpiece is pushed against the deforming die while restrained in a container. As the workpiece is in compression, large deformations are possible. Extrusion processes can be classified in a number of ways:

Direct or forward
Indirect or reverse

Shapes produced may be solid or hollow - in which case a die insert that forms a cavity in the extruded part is needed. A mandrel or spider may be used.

The engineering strain in extrusion is: e = (Ao - A1)/Ao

The extrusion ratio is a useful descriptor: Re = Ao/A1

The true strain = ln(Re) = ln(Ao/A1)

The mean strain rate is given by:

where alpha is the semi angle of the die. In unlubricated extrusion, with a dead metal zone, it may be taken to be 45o

In cold extrusion work hardening will probably occur and a mean flow stress must be determined, sigmafm

Deformation in extrusion is inhomogeneous and the extrusion pressure is given by:

pe = sigmafm Qe = sigmafm(0.8 + 1.2 strain)

Limitations: For a small extrusion ratio and large alpha, the centre of the extrusion is not directly deformed, but dragged along by the stretching outer surface material. this generates tensile stresses in the core which can lead to 'arrow-head' failure or centre-burst defects.

3.4.4 Drawing

To manufacture long slender products (wire, tube) material is drawn through a die. The material is deformed by compression, but the deformation force is supplied by pulling on the deformed end of the wire or rod. This is termed 'indirect compression'. Most drawing is done cold. The calculations of stresses and forces is similar to those for extrusion.

For drawing a rectangular slab through a wedge shaped die the factor is:

... = 0.8 + 0.2h/L

where L is the length of the contact zone and h is taken as the mean diameter or mean thickness.

The drawing force must not exceed the strength of the drawn wire. Typically this means that the maximum reduction (as area, not diameter) attainable is less than 50%. In practice reduction is usually limited to 20 - 30% to avoid frequent breakage.

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David J Grieve, 16th October 2008.