Auger geometry

An extrusion auger is characterised by the so-called auger geometry, the parameters of which are summarized in table 5 (Fig. 7). 

Fig. 7 Diagrammatic view of augers with designation of the auger geometry...

In addition to that the body becomes heated as a result of friction, which is absolutely undesirable. Useful physical models and practice-orientated methods have been established to calculate the optimum auger geometry with the aim of producing a high enough flow pressure. 

In a second test using a special extruder barrel the development of the pressure in the auger for different speeds, auger geometry and nozzle cross sections, respectively the influence of deformation conditions, are established. Simultaneously the torque figures as well as the axial and radial pressure rates are measured during both tests. 

All data obtained from the tests with the ESM (table 2) are processed in a mathematical model. By adopting similitude models, the data can be used as a design basis for extruders, e.g. the L/D ratio of the auger, the auger geometry, the gearbox torque, and the size of the thrust bearing, etc. 

The calculations reveal one striking difference between Cu and Ag it is found that it requires only 4 kcal/mole for the Cu atoms to move into the plane of the surface Si atoms whereas for Ag this geometry is 53 kcal/mole higher than the ground state - even when the nearest Si atoms are allowed to move away from the noble metal atom. Thus, Cu is seen to penetrate fairly easily into the Si lattice whereas Ag stays above the surface. These theoretical findings are substantiated by thermal desorption and Auger spectroscopy measurements (48) showing that at elevated temperatures Ag desorbs into the gas phase whereas Cu remains in the solid phase. 

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