Four-screw extruder

According to a US patent [46], cracking or liquefaction of waste plastics is realized in a sequential three- or four-screw extruder system with increasing process temperature. Similarly, the catalyst (if used) is discharged with coke and mineral residue. At the end of the process, the distillable hydrocarbon fraction is separated from solid residue and coke. 

Multiple-screw extruders can involve either two screws (twin-screw design) or four screws. A typical four-screw extruder is a two-stage machine, in which a twin-screw plasticating section feeds into a twin-screw discharge section located directly below it. The multiple screws are generally sized on output rates (Ib/h) rather than on L/D ratios or barrel diameters. 

Extruders incorporating more than two screws in parallel are not in general use in the plastics industry. Occasionally 2-stage twin screw machines are referred to as four screw extruders (see Fig. 3). 

Figure 1.2 Schematic of a typical plasticating single-screw extruder. The extruder is equipped with four barrel heating and cooling zones and a combination belt sheave gearbox speed reduction drivetrain (courtesy of William Kramer of American Kuhne)...

An extrusion symposium (El) contains papers which deal extensively with the mathematics of viscous flow in screw extruders but which are limited to Newtonian materials. An extension of this work to materials which may be assumed to be Bingham plastic in behavior has been reported in Japan (M18, M19). The first of these papers deals with a screw extruder with a uniform channel the second with an extruder for which the depth of the channel decreases linearly with channel length. The mathematical results are shown graphically in terms of four dimensionless groups ... 

Screw extruders usually are built with a single screw as shown, but may have as many as four screws and the die may have multiple holes of various cross sections. An 8 in. dia screw can have a capacity of 2000 Ib/hr of molten plastics. Tubing can be extruded at 150-300 ft/min. To make pellets, the extrudate goes to cutting machines in which parts as small as washers can be made at rates as high as 8000/min. The extrusion of plastics is described at length by Schwartz and Goodman (Plastics Materials and Processes, Van... 

Mours, Flecke and Kohlgrueber compared four commercially available twin screw extruder modeling programs [5] ... 

Figure 8.3 illustrates the grid in the cross-section of single- to four-flighted screw elements. A mesh with a million nodes takes less than a minute to generate. It requires only a fraction of the time taken to calculate a flow in the twin screw extruder. 

Paul Troester started manufacturing single-screw extruders (SSE) in 1892, but only in 1935 the first SSE for thermoplastics was introduced. Four years later, Paul Leistritz built electrically heated, air cooled SSE, having L/D = 10, automatic temperature control, variable screw speed, and nitrided barrel. The machine is considered prototype of the modem extmders. The SSE offered continuous processing capability, but it was notoriously poor as a mixer. Furthermore, under the standard processing conditions, a significant non uniformity in the shear history had resulted in large temperature differences of the melt, AT = 60°C. To alleviate the... 

Ceramic feedstocks for extrusion can be produced either in a batch process, or continuously. Which approach is taken depends on the details of the application and the material to be prepared. In the ceramic industry at present, four different types of machines are generally used High shear mixers, roll mills, screw extruders and shear rolls. 

Different types of screw extruders are available such as single or multi-screw extruders. Multiscrew extruders can involve either two screws (twin screw design) or generally up to four. 

EPSIM II [41] is 30-mm diameter, 32 L/D twin-screw extruder with a specially constructed barrel mounted on the moving lathe. Screw elements and a centerline distance correspond to a Model ZSK-30 extruder by Werner Pfleiderer Corp. The machine has a clam-shell barrel, rapid cooling capabilities, four heating zones, and seven ports for feeding, venting, or process measurements. The unit, powered by... 

An example of grafting maleic anhydride onto polypropylene in a twin-screw extruder was described by Wong (14). Polypropylene was coated with 1.1 wt% maleic anhydride and fed to a 53-mm twin-screw extruder at 30 kg/hr. Four temperature control zones were used, and styrene monomer at 1.4 wt% was injected into the second zone. They obtained 0.8 wt% bound maleic anhydride on the product, and the melt flow index was 17 dg/min. When 500 ppm Lupersol 130 catalyst was added and no styrene was injected, the melt flow index increased to 258 dg/min, indicating signiflcant degradation. No grafting was obtained when an antioxidant was added to the feed. 

Here N is the rotation rate of the screws, m is the number of thread starts per screw, and V is the volume of a single chamber. In reality the output of a twin-screw extruder is, of course, smaller than the theoretical throughput because the chambers are not completely closed. Four different kinds of leakage flows can be distinguished (4) (see Fig. 2.8) ... 

The reactive extrusion was performed in an APV Baker MPF50 fully wiped corotating twin-screw extruder. The maximum screw length used was 14 D. The extruder utilized five heating zones and the barrel wall was kept at a uniform temperature of 130°C in the last four zones. The first zone, closest to the feed port, was kept at a temperature of 120°C. The amount of monomer-initiator mixture fed to the extruder ranged from 1.5 to 4.5kg/h. The pressure in the die varied between 0 and 13 bar. After every adjustment of the extrusion parameters samples were collected when the extruder had reached steady state. In order to stop the reaction immediately the samples were directly frozen in liquid nitrogen at the outlet of the extruder. 

A series of articles was published by Utracki et al. [318-322] on the modeling of mixing of immiscible fluids in a twin screw extruder. The fourth paper in the series [321] incorporates several refinements of the earlier model, one of the most important refinements being the incorporation of the effect of coalescence. The model considers two breakup mechanisms, both based on the micro-rheology. One breakup mechanism is the drop fibrillation and disintegration into fine droplets when the Weber number is greater than four times the critical Weber number. The second mechanism is drop splitting that occurs when the Weber number is below four times the critical Weber number. 

The primary difference between the various types of screw extruders is in the extrusion zone. An axial or dome extruder transports and extrudes the wet mass in the same plane. Axial extruders force the wet mass through a flat, perforated end plate, typically prepared by drilling holes in a plate. The thickness of the plate can be more than four times the hole diameter, resulting in high die length to radius L/R) ratios. An axial extruder is shown in Figure 6(A). 

Xylans were coextracted from wheat straw and wheat bran with a BC 45 twin screw extruder (Clextral). The 1.4 m long barrel is composed of seven different sections (figure 1), each cooled by cold water and four of which are heated by induction belts. Straw and bran were introduced at the beginning of the first section of the barrel and water in the fourth section. The corotative intermeshing screws are composed of small modular screw elements and present... 

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