Twin screws

A twin-screw extmder is used to reduce residual monomers from ca 50 to 0.6%, at 170°C and 3 kPa with a residence time of 2 min (94). In another design, a heated casing encloses the vented devolatilization chamber, which encloses a rotating shaft with specially designed blades (99,100). These continuously regenerate a large surface area to faciUtate the efficient vaporization of monomers. The devolatilization equipment used for the production of polystyrene and ABS is generally suitable for SAN production. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) 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 faciUtate 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 extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

D. Mueller, "The Continuous Processing of Gun Propellants by the Twin Screw Extmder," in Proceedings of the Third International Gun Propellant Symposium, Dover, N.J., Oct. 1984. 

Extrusion. In general, extmsion is the process of forcing a polymer melt through a die (104,105). Typical extmsion appHcations include initial resin pelletization after manufacture and production of film, sheet, pipe, tubing, and insulated wire. The HDPE extmsion temperature is around 150°C, the pressure 40—50 MPa (5800—7250 psi). An extmsion production line usually consists of an extmder (mono- or twin-screw) with a die at the end, a cooling and shaping device, a pulling device (a roUer), and a cutter. 

J. L. White, Twin Screw Extrusion Technology and Principles, Hansei Publisheis, Munich, Germany, 1991. 

The three most common diffuser configurations are a vertical cylinder in which the semifluidized cossettes ate scrolled upward (tower), a pair of upward-moving inclined twin-screw scrolls with cascading juices (slope), and a horizontal rotating dmm equipped with offset compartments which allow the cossettes to fall forward as the dmm turns (Raffinetie Tirlemontoise (RT) horizontal). Residence time within all of these diffusers is typically 45 to 60 minutes. 

Other methods also exist for handling the reaction mixture. For instance, self-wiping, twin-screw extmder using an intermeshing has been proposed (271). The advantages claimed include higher saponification temperature (less catalyst), increased water solubiUty (271), and fewer impurities (272). 

As with pipe, siding manufacturing is highly optimized for efficient production at high rates and minimal costs. Siding uses powder compounds, twin-screw extmders, and vacuum sizing/cooling. 

As compression occurs concurrently in both halves of the compressor, radial forces are oppositely directed, resulting in negligible net-radial loads on the rotor bearings (uuhke twin screw compressors), but there are some loads on the star wheel shafts. 

FIG. 18-50 Intermeshing corotating twin screw extruder (a) drive motor, (h) gearbox, (c) feed port, (d) barrel, (e) assembled rotors, (f) vent, (g) barrel valve, (h) kneading paddles, ( ) conveying screws, (/) splined shafts, (k) blister rings, (APV Chemical Machinery, Inc.)... 

The throughput rate of intermeshing twin-screw extruders (Fig. 18-56) and the Farrel continuous mixer (Fig. 18-51) is scaled up with diameter to about the 2.6 power. 

Oilseeds Cocoa butter Single screw Twin screw Screw 20% oil recovery 94% oil recovery... 

Twin-screw extruders now dominate the extrusion field, particularly because of their positive pumping action which is so important with PVC in powder form. 

Comparison of single-screw, co-rotating and counter-rotating twin-screw extruders... 

Type Single screw Low speed type High speed type Counter-rotating twin screw... 

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