Compounding extrusion Twin screw extruders

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. 

The first twin-screw compounder (ZSK) went into production at Werner Pfleiderer in 1957, marking the beginning of a success story for this type of machine. The first major applications were in the chemical industry. Today, the machine is predominantly used in the plastics industry, e. g., in extrusion and compounding. These screw machines are therefore also known as extruders and the twin-screw is known as the twin-screw extruder. 

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. 

Compounds with 2% to 10% (vol.) filler and an antioxidant additive (Irganox N225 0.2wt%) were prepared with a twin screw extruder (ZSK), at a temperature of 190°C. In case of silica, a first compound was prepared with a high shear mixer (Brabender) before extrusion, to obtain a good dispersion. 

In a typical formulation, an ethylene-n-butyl-acrylate-carbon monoxide (60/30/10) terpolymer (60 wt%) is melt compounded with plasticized PVC (40 wt%) in a twin-screw extruder and the ethylene terpolymer dispersion cured in situ during the mixing by catalytic amounts of a suitable peroxide (0.3%) and a bismaleimide crosslink promoter (0.2%). The extruded pellets of the elastomeric blend can be used in conventional melt fabrication processes such as profile extrusion, extrusion coating, milling and calendering of sheets, injection and/or compression molding. 

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. 

Co-Rotating Twin Screw Extruder (CTSE). This system is commonly used for compounding and profile extrusion in high output applications. CTSE with its high shear is, however, harder to control than the counter rotating system (see below). 

Most twin-screw extruders used in ceramic production were chosen either to produce the high requisite extrusion pressures or to introduce the high requisite or desirable shear forces for kneading and homogenizing ceramic compounds. For decades now, Werner Pfleiderer - now COPERI-ON - have been supplying such extruders to well-known producers. Werner Wiedmann s and Maria Holzel s contribution describes the operation and technical makeup of Twin-screw extruders in ceramic extrusion . 

Not all ceramic compounds are put through single-screw and twin-screw extruders. If the idea is to achieve very high extrusion pressures, minimal contamination, short series, etc., an intermittent piston extruder can be the machine of choice. 

A peroxide-induced reactive compounding of PP/GRT blends in a corotating twin-screw extruder was carried out to prepare thermoplastic elastomer alloys (Wiessner et al., 2012). The extrusion experiments showed that the width of the residence time distribution was the key parameter determining the mechanical properties of elastomer alloys, whereas the mean residence time had little effect. This was confirmed by performing both quasi-static and cyclic tensile tests. 

Dry blending may be used to produce pellets by melt compounding. Machines of single- or twin-screw extruders are normally used. The pellets produced from compounding are processed by conventional routes such as extrusion, calendering or injection moulding. 

Coperion Werner Pfleiderer (2009) ZSK twin screw extruder (and personal communication with Dr. Paul Andersen), CTO, Coperion Werner Pfleiderer, Ramsey, NJ 2009. Brochure available at http //www.coperion.com/en/ compounding-extrusion / machines-systems/zsk-series/ (accessed 21 February 2011). 

The modular co-rotating twin-screw extruder is the most widely used of all twin-screw extruders. There is probably no application for twin-screw extruders to which it has not been applied. Applications include all aspects of compounding and blending of thermoplastics with particulates, oils, and other polymers. The machine is also widely used for removal of liquids, that is, devolatilization and for reactive extrusion. The modular co-rotating twin-screw extruder has also been used for both polymerization and for grafting reactions. 

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