Extrusion screw intermeshing

The industrial use of twin-screw extruders for this purpose revolves extensively, but not exclusively, around intermeshing co-rotating variants. Closely in-termeshing counter-rotating designs are widely used for profile extrusion of UPVC dry-blends since they permit close temperature control and exhibit a high conveying efficiency due to the positive displacement of material where the screws intermesh [150]. 

Different polymer blends like PE (polyethylene)/PS (polystyrene) [10-11] and PMMA (polymethylmethacrylate)/PS [12-13] have been produced using supercritical C02-assisted extrusion. Fully intermeshing twin-screw extruders have been used in these studies. A decreased shear thinning behavior on dissolution of supercritical CO2 into blends was observed. The obtained reduction in viscosity ratio resulted in a finer dispersion of the minor phase, which is desirable to create a good polymer blend. The effect of supercritical CO2 on the dispersion of the minor phase for a PMMA/PS blend can be seen clearly in Fig. 12.5. 

Twin Screw Extrusion Counterotation/Intermeshed Screws 150 rpm, 100-180°C in Sooking and Pressure-die Barrel Zones, Slit Die Orifices... 

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. 

A. C. Y. Wong, F. Zhu, and T. Liu, Qualitative Study on Intermeshing Co-rotating Twin Screw Extrusion Using Novel Visual Technique, Plast. Rubber Comps. Proc. Appl., 26, 271 (1997). 

L. Zhu and X. Geng, Visual Research of Melting Mechanism of Polymer Pellets in Intermeshing Co-rotating Twin-screw Extrusion, SPE ANTEC Tech. Papers, 55, 3684 (1997). 

T. Sakai and N. Hashimoto, Application of Novel Counter-rotating Intermeshed Twin Extruder for Degassing Operation, SPE ANTEC Tech. Papers, 32, 860-863 (1986) also T. Sakai, N. Hashimoto, and N. Kobayashi, Experimental Comparison between Counterrotation and Co-rotation on the Twin Screw Extrusion Performance, SPE ANTEC Tech. Papers, 33, 146-151 (1987). 

T. W. McCullough and B. T. Hilton, The Extrusion Performance of Co-rotating, Intermeshing Twin Screw Extruder Screw Elements-An Experimental Investigation, SPE ANTEC Tech. Papers, 39, 3372-3379 (1993). 

The twin-screw injection molding extruder is an injection molding machine that is capable of both blending/compounding and extrusion in one step. Because it is a one step process, the fibers never go through the entire extrusion process as well as the pelletization that limits the fiber size, but are blended into the molten plastic before injection. The screw part of this machine is based on a non-intermeshing, counterrotating twin-screw extruder (Chapter 5). One of the screws in this machine is capable of axial movement and has a non-return valve on the end. This action enables the screw to inject and mold parts. 

Almost exclusively used for extrusion of PVC-U into pipe profile and sheet, extrusion equipment consists of a cylindrical barrel containing two close fitting, intermeshed, Archimedean screws. PVC dry blend is fed in via a hopper at the start of the screws and is conveyed forward, softened, sheared and finally gelled by rotation of the screws and the heat of the barrel. The resulting homogeneous viscous melt is forced through a heated die fitted to the exit end of the barrel. Thereafter the downstream equipment sizes or calibrates, cools and pulls off the product which is also cut to desired lengths. 

Another modular system is often called laboratory equipment it can be used for small scale production and for the laboratory evaluation of small samples. Fig. 11.26 depicts the design and some of the accessories. In the most simple execution a hopper feeds a pair of rollers which are driven by a hand crank. The rollers can be solid and may be equipped with compacting or briquetting surfaces (see Section 8.4.3) or two perforated, geared, intermeshing pelleting rolls (see Section 8.4.2) are installed to accomplish medium pressure extrusion. In a modular fashion the rollers can be motorized, screw feeders can be added, and the rolls may be oriented vertical or horizontal or in any other direction. As shown in the photographs of Fig. 11.27 the roller frame can be totally enclosed for dust control if toxic or hazardous materials are processed. A panel includes controls and instrumentation for data display and collection. 

To really control the mixture and dispersion of additives, or to do reactive extrusion, a twin-screw extruder is necessary. Twin-screw extruders are available in several different configurations, but for the work of compounding the most often used is a closely intermeshing co-rotating extruder, with a specially designed screw for the particular task. In order to accomplish this with the least amount of machining. 

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