Inverse screw extruder

Fig. E6.2 The synthesis of an inverse screw extruder from building block 1. (a) The building block (b) a rotating solid cylinder forms the moving surface (c) the inner surface of a hollow cylinder forms the stationary surface. The result is a single roll processor without the channel block (d) the shallow channel is wrapped around the rotating shaft. The maximum length of the channel is set by the circumference of the shaft (e) a twisted channel relaxes the length constraint (f) cross section of an inverse screw processor.

Collins et al. (1983) Polybutene-Freon 0.6-4.5 Twin-screw extruder Atmospheric pressure, T = 20°C Varies inversely as... 

Among the formulation variables that control extrudability of a product, several studies (25,26,33-35) point to the fluid or moisture content of the wet feed material being more critical than others. The force or torque of extrusion and power consumption are often inversely proportional to moisture content of the extrudate as seen from Figures 11 and 12 (26,36-38), while the particle size of pellets increases linearly with water content when extruded with gravity feed basket and roll extruders (33). Using instrumented gravity feed and radial screw extruders, a three- to fourfold decrease in the force has been noted, with a 10% increase in water content of feed material (37). 

If a linear rubber is used as a feedstock for the mass process (85), the rubber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete rubber particles are formed. This is referred to as phase inversion since the continuous phase shifts from rubber to SAN. Grafting of some of the SAN onto the rubber particles occurs as in the emulsion process. Typically, the mass-produced rubber particles are larger (0.5 to 5 JJ.ni) than those of emulsion-based ABS (0.1 to 1 Jim) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to facilitate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extruders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

The validity of the theoretical assumptions was evaluated by comparing the two models predictions with the experimentally measured drop diameter at different axial positions in the twin screw extruder. Experimentally, after the extrusion reached steady state, the screw rotation was stopped and the molten blend was quenched within a specially designed extruder barrel. It was estimated that the PS/PE blends were quenched within 7-10 sec. The second model predicted well the morphology evolution of non-compatibUized blends of PS in PE, and their inverse of PE in PS (see Figure 9.16 in Chapter 9). 

It is observedthat the mixing efficiency in microchannels is about 3-4%. The efficiency is even lower in conventional reactors, for example, <1% in twin-screw extruders [72]. As the mixing time is inversely proportional to the efficiency, an increase of the energetic mixing efficiency, say by a factor 3 (from 3-10%), has the same influence on the mixing time as an increase of 1 order of magnitude (10 times) power dissipation. 

As discussed in Fig. 8.38, an in situ-formed inverse-Y shaped copolymer is hardly pulled out of the matrix. However, pull-out into the dispersed particles (pull-in) takes place in reactive blending by the use of an extremely long (L/D = 100, L screw length, D screw diameter) twin screw extruder (Sato et al. 2007). Under the intensive shear fields in the extruder, the dispersed particles can be highly deformed, as shown in Fig. 8.40. The deformation to ellipsoids and the recovery to spherical particles would be repeated in the extruder, which implies that, from the shear fields in the dispersed particles, the in sim-formed graft copolymers would pull into the dispersed particles. 

Processing conditions or chemical reactions occurring in one or both phases of the blend can strongly affect the phase inversion. Of course, these two parameters have a direct effect on the viscosity ratio of the components. The same blend of polyamide/styrene-acrylonitrile copolymer developed phase morphology where PA6 is the matrix when processed using a single-screw extruder, whereas the inverse situahon occurred when the blend was mixed several times in a laboratory mixer. 

Extrusion of compact and expanded (foamed) PE takes place on the same type of extruder using the operational parameters given in Table 6.4. The recommended screw profile for the feed zone is 2D, for the compression zone is 13D, and for the homogenisation zone is 5D LID > 18 (L is the length and D the diameter). The compression ratio is 2.5-3. A laminar flow through the die is essential for good-quality foam. The expansion degree is inversely proportional to rotation speed and residence time. 

---