The quality and accuracy of the extruded products in the multi-hole extrusion process depend on many factors such as extrusion ratio, die land length, lubrication, location of the holes and metal flow pattern in the multi-hole die. Die pockets design is an important process parameter which has been studied by many researchers to obtain balanced flow of metal during extrusion. Pocket volume, stepped pocket and eccentricity of pocket have also been investigated. In the present work, the die pockets are produced on the holes of multi-hole dies to study their effect on extrusion load, length of the extruded products and bending of the extruded product.
The details of multi-hole dies fabricated to carry out experiments are explained in Section 3.6, Chapter 3. The extrusion load, bending of the extruded products and variation in extruded product lengths are discussed here.
4.12.1 Comparison of Extrusion Load Obtained from Experiments for Dies without and with Die Pockets
Due to the different number of holes on die, the location of holes and pocket geometry, the variation in extrusion load is observed as shown in Table 4.12. The extrusion load is found to be less when the peripheral holes are located nearer to the centre. The holes away from the centre experience higher effective extrusion ratio, due to which the extrusion load increases. This is observed experimentally as well as in FEM simulations that will be discussed in Chapter 5. The least extrusion load is observed for the die pocket depth of 2 mm. Die pocket depths of 3 and 4 mm provide an increased load compared to pocket depth of 2 mm. This may be due to the increase in dead zone height in die pocket.
For a particular die design, the optimum die pocket depth helps in reducing the extrusion load. Similar observations also can be seen with the results obtained from finite element modelling explained by Li et al. [2003]. For a particular profile width, there exits an optimum pocket width beyond which the effect of pocket on metal flow control is ineffective. The die pocket depth can be considered as an important parameter to reduce the extrusion load. The pocket design in the aspect of
pocket depth, pocket width and pocket angle can produce an optimum extrusion load along with quality product in multi-hole extrusion process.
Table 4.12. Comparison of extrusion load Extrusion load (kN) Die pocket
depth (mm)
Die IV Die V Die VI Die VII
0 108 116 102 106
2 103 107 100 104
3 106 112 104 108
4 110 114 107 110
4.12.2 Bending of the Extruded Products from the Dies without and with Die Pockets
The different process parameters and their effect on the bending of the extruded products are not known exactly. In this investigation, the cross section of the extruded product is solid and circular. Radii of curvature of the extruded products are calculated for the extruded products obtained from experiments. The bending of the extruded products is also observed in simulations. However, simulation assumes a homogeneous material and perfectly co-axial movement of the punch, which may not be possible in actual practice. Table 4.13 shows the radii of curvature of the extruded products coming out from the centre hole of the different dies obtained from experiments.
The dies with high pocket depth produce more bent products. The proper guiding of the extruded parts is found to be more important than making metal flow easy in multi-hole extrusion in order to minimize bending. In this regard, die land length is the important parameter. As the pocket depth increases, the die land decreases in the present work. The reduced die land length is the main factor for the decrease of the curvature.
Table 4.13. The radii of curvature of the extruded products coming from centre hole (Values in bracket are standard deviations)
Radius of curvature (mm) Die pocket
depth (mm)
Die IV Die V Die VI Die VII
0 29.4 (2.35) 31.43 (1.37) 28.84 (1.09) 31.13 (1.2) 2 23.87 (1.24) 27.13 (1.23) 25.52 (1.69) 30.82 (1.28) 3 21.95 (1.24) 24.35 (1.52) 25.5 (1.83) 27.98 (1.21) 4 21.16 (2.29) 22.46 (1.68) 22.46 (1.70) 25.6 (1.81)
Table 4.14. The radius of curvature of the extruded products coming from peripheral hole (Values in bracket are standard deviations)
Radius of curvature (mm) Die pocket
depth (mm) Die IV Die V Die VI Die VII
0 29.75 (2.97) 41.73 (2.54) 27.94 (2.1) 34.7 (0.92) 2 27.8 (1.49) 37.65 (1.38) 25.41 (1.85) 33.61 (1.04) 3 27.5 (1.28) 36.46 (2.11) 25.49 (1.82) 24.65 (1.37) 4 23.5 (1.78) 26.49 (1.89) 25.05 (2.28) 24.3 (2.26)
Table 4.14 shows the average radius of curvature values of the extruded products coming out from the peripheral holes of different dies. It is clear that the die having no die pockets produces less bent products as compared to the dies with different pocket depth. As the flow of metal is not uniform with multi-hole extrusion, it is difficult to achieve an optimal pocket design in order to reduce the bending of the extruded products. The die land lengths get reduced when pockets are made. To minimize the bending, more attention is needed on die land length than the pocket design. The location of the holes must be taken care of as the exiting profiles coming out from the holes located near the centre influence each other and more bending and/or distortion occurs. This has been observed from the experiments as well as FEM simulations that will be described in next chapter.
4.12.3 Variation in Length of the Extruded Products from the Dies without and with Die Pockets.
The extruded products lengths obtained from experiments are measured and the variation among them is studied. The coefficient of variation for length of the product is calculated using the following formula,
Coefficient of variation s 100%
= x× (4.2)
where is standard deviation and is average values x .
It is observed that increase in die pocket depth helps in balancing the non-uniform metal flow and as a result, lesser variation in the length of the extruded products is observed as shown in Figure 4.24. With higher die pocket depth, less difference in length of the extruded products from both centre and peripheral holes is observed.
The die pocket geometry must be selected properly to obtain less variation in lengths of the extruded product.
10 12 14 16 18 20
0 1 2 3 4
Die pocket depth (mm)
Coefficient of variation (%)
Die IV Die V Die VI Die VII
Figure 4.24. Coefficient of variation of the extruded product length