Extruders twin-screw

There are two main areas where twin-screw extruders are used. One is in the processing of polymers that are difficult to process, because they don t flow easily, and they degrade 

FIGURE 8.6 Various types of twin-screw extruders (a) counterrotating intermeshing elements, (b) corotating intermeshing elements, and (c) counterrotating nonintermeshing elements. 

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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. 

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... 

The following table compares the single screw extruder with the main types of twin screw extruders. 

Several commercial products of PVC/TPU blends are available. The BF Goodrich Chemical Group has a PVC/ TPU blend based on their Estane series TPUs. For example, their Estane 54620, a polyester-based TPU with a °ShA 85 hardness, shows excellent compatibility with flexible PVC. The blends are produced by mixing PVC, TPU, plasticizer, stabilizer, and lubricant in a twin-screw extruder. These polymeric blends show intermediate mechanical properties between PVC and TPU. 

A twin-screw extruder fitted with a stuffer box. The twin-screw extruder continues to blend the compound as it passes through although with the dryer. 

In another case where the twin-screw extruder was used, the rubber and plastic were melt mixed with all ingredients in a similar manner as described in blend compositions for static vulcanizations. The product was then dumped, cooled, and granulated. The premixed granules were then fed into a twin-screw extruder where a very narrow temperature profile was maintained with a relative high compression (2 1), and the screw speed was adjusted depending on the final torque and the flow behavior of the extruded stock. The stock was cured by shear force and temperature enforced by the twin-screw extruder. The dynamically crosslinked blend was taken out in the form of a strip or solid rod to determine the... 

During 1989-1990, the Research Institute for Petrochemistry, Czechoslovakia developed extrudable PVC-wood (25 wt.%) composites using a twin-screw extruder, which produce 442 Ib/h and about 0.33 million Ib/year hardboards with approximately 0.25-0.85 cm ( /lo- /s in) thickness and 101.6 cm (40 in) wide (D. Maldas, personal communication, 1992.)... 

Prior to blending, the LCP was dried at 155°C for 5 h. The melt blending of the materials was carried out with a Berstorff ZE 25 x 33D corotating twin-screw extruder at a melt temperature of 290°C, with a screw speed of 200 rpm, and an output of 6.4 kg/h. The extrudate was immediately quenched in a water bath and repelletized. 

The blends prepared by twin-screw extruder with two different draw ratios were extruded with the Brabender single-screw extruder at temperatures ranging from 180 280°C, The sample designation and specific processing conditions are given in Table 1. 

Figure 3 Twin-screw extruded PP-LCP blend processed at a melt temperature of 290°C with low- (left) and high-draw ratio (right). Upper micrographs are taken from the core and lower ones from the skin region.

Blends of polypropylene (PP) and liquid crystalline polymer (LCP) processed without melting the LCP were compared with conventional melt processed blends. In a first stage, PP was blended with 20 wt% of LCP in a twin-screw extruder with the take-up speed varied to achieve blends with different LCP fiber dimensions. In the second stage, these blends were processed both below and above the Tm of the LCP by extrusion and injection molding. 

The materials were dried in a vacuum oven at 115°C for 24 h. They were then melt blended by using a domestic twin-screw extruder ( 35) [screw diameter = 35 mm]. The weight ratios of PES-TLCP were 90 10 and 70 30, respectively [12]. 

Example 3. Hydrolysis in an Extruder.69 PET reactive extrusion experiments were carried out on a 25-mm Berstorff ZE25 corotating twin-screw extruder with a barrel length-to-screw ratio of 28 1. The extruder consisted of six barrel sections equipped for heating, cooling, and controlling the temperature of each section of the extruder. Initially reaction extrusion of PET and water was performed with cold water at room temperature injected into the extruder. Typical operating conditions were reaction temperatures of 230-265° C, extruder speeds... 

Most of the compounds were extrusion compounded in a conical, partially intermeshing, counter rotating twin screw extruder (Haake Reomix TW-lOO). The extruder speed was set at 50 rpm and the barrel temperature profile was set to produce a melt temperature of 260°C at the die. Samples were injection molded in a 31.8 MT Battenfeld press with a 59 cc shot size. Where noted, samples were compounded in a 60 cc Brabender internal mixer and compression molded. 

Details are given of ways of obtaining energy from the pyrolysis of disposable syringes. The waxy product was chemically modified with maleic anhydride and tested in the processing of PP/LLDPE blends in a rheometer or twin-screw extruder. Compatibility of components in binary or ternary blends and DSC results are discussed. 35 refs. 

Post-consumer PMMA plastics can be depolymerised back into their starting components. Degradative extrusion in twin-screw extruders can be used for this process. 10 refs. Translation of Kunststoffe, 87, No.2, Feb.1997, p.183-8... 

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