Extrusion ratios

Materials Minimum Section Thickness, (in.) Extrusion Temperature (°F) Extrusion Pressure (ksi) Extrusion Exit Speed (fpm) Extrusion Ratio InlAo/A,)"... 

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. 

For different polymers the results can be more readily appredated by examining the change in pressure with extrusion ratio R for a constant extrudate velocity. Results for different polyethylenes are shown in Fig. 17, where the rapid upturn occurs at comparatively low extrusion ratios. For different polymers results are shown in Fig. 18, together with the best analytical Gts based on modifled Hoffman-Sachs analysis, which incorporates the strain, strain rate and pressure dependent flow stress according to Eq. (4) and the Avitzur strain rate field of Eq. (5). Figure 17... 

Fig. 18. Best analytical fits to experimental extrusion pressure — extrusion ratio Rj, data...

Fig. 19. Room temperature modulus as a function of extrusion ratio R for 2.5 mm diameter polyethylene extrudates (T, = 100 °C)...

The theory did not model accurately the rise in pressure with increasing extrusion ratio, due to the omissicHi of the effects of strain rate and pressure on the flow stress. 

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. 

Fig. 49. Variation of thennal conductivity at 100 °K for various values of extrusion ratio >. ( ) small-diameter extrudates (O) specimens cut from laige-diameter extrudates...

The nominal extrusion ratio or nominal draw ratio (Rn) is determined by the ratio of the billet cross-sectional area to that of the die bore. As it will be discussed below, die swell is often observed in solid-state extrusion experiments Therefore, the actual draw ratio (Ra) is often smaller than the nominal value. 

As shown in Fig. 2 for two homopolymer samples and one copolymer, the polymer grade had a strong influence on the extrusion behavior. At an extrusion temperature of 110 C the highest extrusion pressure was needed for the high molecular weight homopolymer sample. The difference in extrusion pressure increased with increasing draw ratio. The copolymer was more easily extruded than the homopolymers. Consequently, the authors observed a significant variation of the maximum extrusion ratio... 

In Table S the experimental conditions for the extrusion experiments are compiled. In all cases die swell occurred. The largest die swell (40%) was observed for material A at an extrusion temperature of 48 °C. In all cases the die swell increased with decreasing temperature. As shown in Table 5 for material C, the degree of die swell was independent of the extrusion ratio. This is in contrast to the results on copolyethylenes (see Chap. 4). The die swell depended on the extrusion rate, however. This led the authors to the conclusicm that the amount of reversible deformation of the extruded material in the die was determined by the volume fiaction of the soft matrix. This volume fiaction of the soft matrix increased with decreasing hard s ment lengths and falling extrusion temperature. 

Fig. 10. Stress-strain curves of extrudates for poly(ether ester)s A, B and C. Initial sample length 25 mm, strain rate 1 mm/s, 25 °C, nominal extrusion ratio 4, undercooling 60 °C (for sample description see Table 4) >...

Al-Zn-Mg-Cu alloys with additions of Fe, as well as with combinations of Fe, Mn, Zr and Sc, were produced by induction melting using ceramic crucibles and supplementary methods of melt purification, which included ceramic filters and blowing the melt with argon. Rods of 6 mm in diameter were manufactured by extrusion at 400 C with extrusion ratio A.=84. The T6 treatment consisted in water quenching from 465 °C and aging at 120 °C for 24 h. The alloy compositions and the properties of rods in the T6 condition are given in Table 1. 

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