Melt extraction screws

The output rate of the extruder is a function of screw speed, screw geometry, and melt viscosity. The pressure developed in the extruder system is largely a function of die resistance and dependent on die geometry and melt viscosity. Extrusion pressures are lower than those encountered in injection molding. They are typically 500 to 5000 psi (3.5 to 35 MPa). In extreme cases, extrusion pressures may rise as high as 10,000 psi (69 MPa). Variants on the single screw include the barrier or melt extraction screw and the vented screw (Chapter 3). 

Key Words Melting, screw designs, barrier screw, melt extraction screw... 

A mixture of 2 g. of potassium chlorate, 30 g. of commercial sodium hydroxide, 10 g. of finely powdered sodium /3-anthraquinone-sulphonate ( silver salt ), and 40 c.c. of water is heated for twenty hours at 170° (oil bath) in an autoclave or in an iron tube with screwed-on cap. The cooled melt is repeatedly extracted with hot water and the extracts, after being combined and filtered, are acidified while hot with excess of hydrochloric acid, which precipitates the alizarin. When the mixture has cooled the precipitate is collected at the pump, washed successively with dilute hydrochloric acid and water, and dried. 

Fig. 1. Process flow sheet for the continuous conversion of latex in a counterrotating, tangential twin-screw extruder as it might be arranged for the production of acrylonitrile-butadiene-styrene polymer (Nichols and Kheradi, 1982). Polystyrene (or styrene-acrylonitrile) melt is fed upstream of the reactor zone where the coagulation reaction takes place. Washing (countercurrent liquid-liquid extraction) and solids separation are conducted in zones immediately downstream of the reactor zone. The remainii zones are reserved for devolatilization and pumping.

The intake behavior is also substantially influenced by the resistance of the downstream plastification section. If discharged at atmospheric pressure, e.g., via screw feeders, the throughput rate is considerably higher than if the same feed section is succeeded by a plastification section (Fig. 11.4). For instance, if metering a product with a bulk density of 0.35 g/cm3 and a melt phase density of 0.7 g/cm3, it is clear that considerable quantities of gas will have to be extracted upstream through the intake hopper. 

Identification Place about 150 mg of melted sample into a 16- x 125-mm tube equipped with a screw cap having a Teflon liner, and add 4 mL of absolute methanol, 4 drops of a 25% sodium methoxide solution in absolute methanol, and a boiling chip. Cap the tube, reflux for 15 min, and cool to room temperature. Extract as follows Add 8 drops of a 15% potassium acid sulfate solution, 4 mL of water, and 4 mL of n-hexane cap the tube shake for 1 min and centrifuge for 30 to 60 s. Decant and discard the M-hexane layer, and repeat the extraction with three additional 4-mL portions of M-hexane, discarding each extract. Transfer the aqueous alcoholic phase from the tube into a 50-mL round-bottom, glass-stoppered flask place the flask in a water bath at 50° to 55° and evaporate to near dryness (about 0.5 mL of residue) in a rotary film evaporator under full water aspirator vacuum. 

Table 5.3, extracted from the European Patent application repolymerization [28], shows the beneficial effects of titanates on most properties of ABS, PC, PP, and PS thermoplastics. As an additional example. Figure 5.6 shows the repolymerization effect of 0.2% LICA 12 on a 50/50 blend of LDPE and PP after six thermal cycles through a twin-screw extruder. The melt index of the control blend without titanate climbs from 17 to 38, while the value for the blend with titanate is only 24 this indicates a significant decrease in chain-scission due to the titanate. 

The polymer melt is forced from the metering section into a bypass flow channel by incorporating a multi-flighted screw section with reversed pitch and shallow channels between the metering section and the extraction section. The bypass channel has one or more adjustable restrictions to control the rate of flow into the extraction section. The polymer flows from the bypass channel into the beginning of the extraction section. Maddock and Matzuk concluded that bypass venting allows a wider range of operation, is less susceptible to instabilities, and is less operator-sensitive. 

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