Conical twin screws

Composite formulations were prepared as follows The straw samples as received from INEEL were ground to 0.69 mm in a hammer mill and oven dried to 1.1% moisture. The dried straw samples were then blended with various amounts of high-density polyethylene (HDPE), lubricants, and maleated polyethylene blends (MAPE) (see Table 2). The mixed formulations were then extruded with a 35-mm Cincinnati Milacron Model CMT 35 counterrotating conical twin screw extruder (Cincinnati Milacron, Batavia, OH), which produced a 9.525 x 38.1 mm2 solid cross-section. Flexural strength, density, and water sorption were measured for the extruded samples according to ASTM Standard Methods (13,14). 

The use of conical twin screw extruders for the extrusion of wood filled PVC has also been presented (156). The characterisation of apparent viscosity for PVC wood fibre extrusion has been carried out and quadratic models used to establish tolerances within which the dimensional and physical specifications were achieved (69). 

Counter-rotating twin screw extruders are used almost exclusively for the production of uPVC pipes, profiles and sheet because they treat material more gently than single screws. Parallel screw and conical twin screw extruders are compared. Cincinnati Milacron Austria introduced its Argos parallel twin screw extruders last year. These machines are intended to complement the company s Titan conical screw range. Coextrusion of foamed core pipes is also described. 

Fig. 10 Screw profile for Conical Twin-screw. (Courtesy of Cincinnati Milacron.) fView this art in color at www.dekker.com.)...

With good temperature control and the low shear, these extruders are well suited for compounding and for extrusion of rigid poly(vinyl chloride). Typically, high-speed (200- to 500-r/min ) extruders are employed for compounding, whereas low-speed (10- to 40-r/min ) machines are used for profile extrusion. Conical twin-screw extruders with their tapering screws (Fig. 5.31 ) are utilized almost exclusively for chlorinated polyethylene and rigid poly(vinyl chloride). 

Figure 5.31 Screws for a conical twin-screw extruder. ...

The components of a pipe extrusion line include an extruder, annual die, calibration system, cooling system, puller, wind-up unit or stacker, and auxiliary equipment. The extruder typically has anL/D of 24 1 or greater and the screw design depends on the material. Conical twin-screw extruders are often used for poly(vinyl chloride). The diameter of the annular dies is 25 to 100 percent of the extruder diameter. However, the output of a pipe extrusion line is limited by the ability of the downstream equipment to cool the product and hold tolerances. Since extruder capacity is often underutilized, the same die may be used for a range of pipes of differing diameter and wall thickness. IVpically, a number of adapters and replaceable components are used with the same rear section of die. Dies with multiple outlets are employed when large-diameter extruders are used for small-diameter pipe. 

Conical twin-screw extruders have the advantage that the space provided for bearing is large. Moreover, by moving the screws axially it is possible to compensate for wear of the screws. A disadvantage of this type of machine is that screw elements with different geometries are not easily interchangeable. 

Figure 12.27 Screws from a fully intermeshing conical twin-screw extruder.

Parallel twin-screw extruders have more surface area compared to conical twin screws. However, the flight flanks in a conical have more surface area than in a parallel extruder and can transfer more heat to the material in the channel. The screw surface area is 40% larger compared to a parallel extruder. Due to the larger channel volumes, conical extruders generate less shear heat and more conductive heat compared to parallel twin-screw extruders, making them better for processing shear-sensitive materials such as PVC. 

Conical twin screws have a distinct advantage in being able to transfer torque to the screws because of their one-piece design. Since much of the energy to melt the material is supplied by the motor and screw rotation, conical extruders have more strength in areas of the screw where it is required. Most screw wear will occur in the feed, plasticating, and restrictive areas of the screw. 

In conical twin-screw extruders, a restrictive zone, follows the plastication or melting zone. The restrictive zone seals off the plastication zone from the devolatilization zone. Mixing will occur in the restrictive zone compressing the polymer in the narrower... 

The questions of the most advantageous mixing system for the production of silica mixes, optimum mixing process and the use of conical twin-screw extruders for processing of silica-containing tread mixes are discussed. Articles from this journal can be requested for translation by subscribers to the Rapra produced International Polymer Science and Technology. 

PEN resin in pellets form (X-10 PEN homopolymer, intrinsic viscosity 0.79 ml/g) was obtained from Excell LLC and dried at 150°C for more than 24 hrs. It was mixed with 0, 1, 3 and 5 wt% of graphite (surface enhanced flake graphite, Asbury Carbons) and transferred into recirculating conical twin screw extruder (Microcompounder, DACA Instruments) or 16 mm corotating twin screw extruder (Prism TSE 16TC) controlled... 

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