Extrusion, hydrostatic

A corresponding pressure effect on the flow stress was observed on hydrostatic extrusion of conical specimens of the same alloy through a die of 5 mm dia at 220°C. The basic alloy was extruded at P = 12 kbar up to the area ratio equal to 4.1 while the alloy hydrogenated to a = 0.20 extruded to the area ratio of 7.6 even at a lower pressure, P = 11 kbax . Similar pressure/hydrogen effects on the flow stress were also observed on hydrostatic extrusion of ZrH,c, VH,c and Nb,c alloys with x = 0 and 0.1-0.2 wt.%. 

Hydroplastics, 29 Hydrostatic compression, 385 Hydrostatic extrusion, 739 Hydrostatic pressure, 384 Hyper-polarisability, 351 tensor, 351... 

In this review article, primary attention will be given to the results of hydrostatic extrusion measurements on polymers by the author and his colles es at Leeds University. This work will be described in the context of major developments which have taken place on both hydrostatic extrusion and ram extrusion by other groups, notably Takayanagi and his colleagues in Japan, Porter and his colleagues in... 

U.S.A. Keller and colleagues in U.K. and more limited studies of hydrostatic extrusion elsewhere... 

Early studies of the hydrostatic extrusion of polymers were undertaken by Buckley and Long , Alexander and Wormell , Nakayama and Kanetsumaand Williams Most of the results obtained for enhancement of properties were somewhat disappointing, although Williams obtained oriented polypropylene extrudates with Young s moduli up to 16 GPa. 

The full potential of the hydrostatic extrusion technique became apparent in 1974, when the production of ultra high mudulus polyethy lenes with stiffnesses up to 60 GPa were reported The main process parameter in hydrostatic extrusion is the nominal extrusion ratio Rj, the ratio of the billet cross-sectional area to that of the die exit (assuming deformation occurs at constant volume, which is a very good approximation). Because polymers can exhibit die swell in extrusion, it is convenient also to define an actual extrusion ratio R, based on the ratio of the initial and final billet cross-sections. R is, of course, direcUy comparable to the draw ratio in tensile drawing (assuming plug-flow) and in practice R R for all but the lowest reduction ratios. 

It was considered initially that because there is no net tensile stress in a hydrostatic extrusion process it might be possible to impose very large plastic deformations without incurring fracture. It was indeed shown that Rj, of 30 can be imposed in polyethylene comparable to the draw ratios of 30 adiieved in a tensile drawing process, and that both processes are limited by the strain hardening behaviour of the material, which is determined solely by the total plastic strain imposed. This led to an important... 

The studies of hydrostatic extrusion at Leeds University have used two... 

An upper bound analysis of the hydrostatic extrusion of linear polyethylene has been given by Gupta and McCormick This followed the approach outlined above for the flow stress (Eq. (4)) taking results frcnn Coates and Ward to describe the increase in strain rate sensitivity by a reduction in the shear activation volume. Reasonable fits were obtained for experimental data up to comparatively low deformation ratios (R = 9), giving further support for the more extensive treatments presented by Ward and coworkers. 

The Young s moduli of the small-diameter extrudates were uniquely related to the extrusion ratio R to a very gocxl approximation. As shown in Fig. 19, this relationship does not depend on the molecular weight of the polymer, consistent with the second principle enunciated above. In fact, it appears from extensive studies of the structure and properties of oriented LPE, PP and POM that comparable materials are produced in large section by hydrostatic extrusion to those produced as fibres or tapes by tensile drawing. 

Hydrostatic extrusion can also be applied to polymers filled with short fibres, so that products are obtained where the fibres are aligned in addition to the polymer matrfat. Excellent results have been obtained with glass reinforced POM where the Young s moduli of the extruded products were about twice those of unfilled polymer with a comparable deformation ratio. A theoretical modellii of the development of mechanical anisotropy in these materials using the aggregate model was shown to be successful provided it is assumed that... 

As in the case of hydrostatic extrusion, the die drawing studies at Leeds University have required the small scale rig shown in Fig. 23 for initial studies and a large scale rig for the production of substantial size rods, tubes and sheets. 

The operation of die drawing is illustrated in Fig. 24 which shows results for polypropylene As for hydrostatic extrusion there is the nominal deformation... 

Fig. 24a shows results for R, = 5 and 7. It can be seen that for low imposed draw speeds R = Rn, i e. the polymer remains in contact with the dife wall throughout. When the imposed draw speed is increased the polymer leaves the die wall and R increases quite dramatically. In contrast to hydrostatic extrusion higher deformation ratios are obtained at higher production speeds, in this case reaching 50 cm/min. It... 

It is only in the case of high modulus polyethylene produced by tensile drawing or hydrostatic extrusion that extensive structural studies have been undertaken. For these materials it was found in agreement with previous studies by Peterlin... 

The equilibrium sorption and diffusion coefficient were determined for toluene and tetrahydrofuran in oriented linear polyethylene pipe produced by hydrostatic extrusion The results, which are summarised in Table 4, show that the sorption is about a factor of ten less than that for isotropic material, and the diffusion coefficients are a factor of a hundred less. It is clear from these results, taken in conjunction with those for gases discussed above, that the improvements in barrier properties increase with molecular size. This can be attributed to the increased difficulty of penetrating the structure as the molecular size increases. 

Most of the experiments described in the literature, including the pioneering work by Pugh, were performed as hydrostatic extrusions. For materials like HOPE, PP, and POM this method is considered to be the only one which allows cxmtinuous preparation of extrudates, due to the reduced friction between billet and barrel and the decrease in strain hardening. The apparatus also used for hydrostatic extrusion is schematically depicted in Fig. 1. 

Capaccio, G., Gibson, A. G., Ward, I. M. Drawing and hydrostatic extrusion of ultra-high modulus polymers, in Ultra-High Modulus Polymers (eds.) Ciferri, A., Ward, I. M., p. 1, London, Applied Sdence 1979... 

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