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Dies

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Dies --- Aheadmold

Of all the tooling components the extrusion die is of paramount importance to the extrusion process, since its design defines the extruded product as regards accuracy and consistency of shape and dimension of the cross-section, and linearity and freedom from twisting of  the extruded length. The design must also take account of any tendency for apart of the section not to “fill” the die orifice, and also the variations in dimensions implicit between front and back of one extruded length and between successive extrusions. Thin ends of limbs and thin parts of a section adjacent to thicker parts will not “fill” even at relatively slow extrusion speeds unless the die design corrects this tendency.
Only for copper sections, which are subsequently heavily cold drawn, is detailed consideration of this problem not so important. For all other extruded metals the control of “differential flow” is vital.
In general, solutions to the above problems have been developed empirically, by trial- and-error methods, over decades, for basically flat plate dies. Dies based on flat plates but with profiled or bell-mouthed entry to the shape defining die orifice are only widely used for high temperature shape extrusion, e.g. steel, nickel or titanium alloy.
1.    Die Layout and Container Size
Experience gained by commercial extruders has to a large extent defined which sections can be extruded under given conditions of temperature and available press power.
Of the various expressions formulated to relate minimum extruded area to available press power, the following empirical form seems easily usable
P/A = k (b ln (A/a) + c)    (5.1)
where “P” is the press force, “A” is the cross-sectional area of the billet, “a” is the extruded cross-sectional area and “k”, “b”, “c” are constants.
This form ignores the billet/container friction losses and any effect of conicity of die entry. For relatively high extrusion ratios the expected minimum in extrusion pressure for a flat die is confirmed, and except for very large values of billet length to diameter ratio, in direct extrusion the above equation is adequate for simple copper and brass sections.
For a single hole die the circumscribing circle center would normally be placed at the die center, although highly asymmetric sections are sometimes displaced with a thick part of the shape out towards the edge follower section (e.g. a round) on the other side of the die to balance flow.
The orifices in multihole dies should be positioned so as to minimize flow control problems. Where possible, sections should be positioned with their centers of gravity on the same diameter to equalize the exit speeds of separate strands (compare Figs. 5-4 (a) and (b)). Where a section includes a large tongue, it can be of value to position the orifices with their centers of gravity furthest from the die center, to overcome a tendency for the metal to extrude faster over the tops of tongues. Thus the layout in Fig. 5-4 (e) is preferred to that in Fig. 5-4 (c). The requirement for aluminium extrusions that one surface of the product shows the best possible surface finish may, however, alter the design to ensure that this  surface  does  not  touch  the  runout table during extrusion (Fig. 5-4 (d)).
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Fig. 5-4   Examples of die layout (W-Wrong, C-Correct)

2.    Design of the die orifice
An extrusion dimension can be up to 3% smaller than the die orifice for reasons including effects of thermal expansion and deflections of the die under load. Even for simple shapes the differences between die orifice and the extrusion dimensions are not totally accounted for by thermal expansion, and a “shrinkage allowance” must be determined for each extruded alloy, the form of which is best decided for each alloy and extrusion temperature from measurements on a wide variety of extruded sections and their dies, taking into account elastic deflection and/or plastic collapse of the die orifice and variations during extrusion of a series of billets.

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