Single-and twin-screw extruders

The screw speed does not influence the physical properties, provided that there is constant torque utilization. Additionally, the number of filler additions has only a negligible effect, giving the compounder good flexibility and permitting adjustment of rate to suit market demands. 

A new disposable mixer that is claimed to reduce costs of production of reactive resins has been developed by Sulzer Chemtech, Switzerland. The design 

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Single- and twin-screw extruders with customised screw profiles are used to produce low density PETP-PP foams in the presence of reactive compatibiliser and coagents. Low density PETP foam is also produced from... 

Table 7.16 Comparison of Single- and Twin-Screw Extruder Screw Characteristics...

Other common continuous mixers involve substantial modification of single and twin screw extruders, aimed at improving distributive mixing capability in particular, and leading to the development of continuous mixers such as the Transfermix (50) and the Buss Ko-Kneader (51). Another approach in continuous mixer development is to transform batch mixers into continuous ones. Thus, the roll-mill can be converted into a continuous mixer by feeding raw material on one side and continuously stripping product on the other side. In addition, the Banbury mixer was imaginatively transformed into the Farrel Continuous Mixer (FCM) by Ahlefeld et al. (52), and, later, two similar continuous mixers were developed by Okada et al. (53) at Japan Steel Works and by Inoue et al. (54) at Kobe Steel. 

Calculate the time it takes to melt PP in both the single- and twin-screw extruders, under the condition, indicated. 

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. 

Table 10.17. Differences between single and twin screw extruders...

Polymer devolatilization can also be accomplished by the use of single and twin screw extruders [61]. Extruders are well suited for high viscosity materials and, when equipped with appropriate vents, allow the discharge of volatiles. 

Screw extruders are primarily developed for the polymers and plastics industry, and a large variety of designs and sizes are consequently available. Mainly single and twin-screw extruders are used for thermoplastic ceramic body extrusion. 

Cross-section of single and twin screw Extruder barrel... 

FIGURE 11.2 Single and twin screw extruder. (From Particle Sciences, Technical Brief, 3, 2011.)... 

Commercially, styrene can be grafted onto styrene-butadiene rubber to obtain high impact polystyrene, or HIPS. More than 800 thousand metric tons of HIPS was produced in 1991. Grafting reactions can be done in batches or in continuous processes. For continuous processes, single and twin-screw extruders are preferred. 

In most compounding operations, single- and twin-screw extruders are involved at some stage to melt, convey, compress and mix different compounds such as additives, colourants and different polymer types. 

There may be significant differences between single- and twin-screw extruders in how they achieve the above functions. 

Thiele (18) stressed the utility of continuous-single and twin-screw extruders compared with Banbury batch type mixers on the basis of the working volume, that is, small working mass in the extruder channels in continuous process equipment versus a large amount of blended material per cycle in batch type mixers. With a relatively smaller working volume in an extruder, there is more enhanced intimate mixing on the microscopic scale in a shorter period of residence time. 

The authors found that the depression of glass transition temperature (Tg) due to the addition of plasticiser is substantially reduced by the loading of wood flour. In addition, various wood-plastic composites were compounded into different colours, and several pairs of the compounds with different rheological properties were extruded in single and twin-screw extruders to see whether any wood-patterns are developed. When the differences in the shear viscosity and the Tg of the two compounds were too large, the incomplete plasticisation of the higher viscosity component was observed due to the lower viscosity component. It was found also that distinct wood-patterns were only developed both inside and on the snrface of the extruded prodncts for the pairs of the composites with optimal differences in both viscosity and plasticiser content. 

Table 10.9 Comparison of Various Single and Twin Screw Extruders ...

C. Rauwendaal, New Dispersive Mixers for Single and Twin Screw Extruders, Annual Meeting of the Poiymer Processing Society, Yokohama, Japan, June 8-12 (1998)... 

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