Compounding extruder barrel screw

A Uniroyal patent originated the principle of using pins protruding into the extruder barrel to disrupt the laminar flow of compound, and this principle was further developed by Troester in their QSM (cross-flow mixing) machines. The protrusion of pins into the barrel necessitated a modification to the screw design to accommodate them. Many papers can be found in the literature dealing with this type of pin extruder. 

Fig. 25. Ko-kneader intensive compounding extruder [Courtesy Buss. AG] (a) Schematic diagrams of slotted screw and barrel pins, (b) ko-kneader with barrel in open position...

In a single-screw extruder, a screw is rotated inside a cylindrical barrel. The feed may be in the form of a pre-blend or dry blend, pellets or in the form of viscous melt in melt extrusion. The extruder acts as a pumping and mixing device. The extruder performs the additional functions of conveying and melting and also performs compounding and devolatilisation functions. 

In this investigation compounding and molding are integrated in order to minimize the exposure time of the natural fibers to temperatures above 200°C. The specific process entails twin screw extrusion with fiber addition half way down the extruder barrel, formation of a hot log and immediately compression molding a test plaque. The material also is coarsely ground and injection molded for comparison. 

A Leistriz co-rotating twin screw extruder, with a diameter of 34 mm and an L/D ratio at 25, has been employed for the compounding of blends. A 4-hole strand die was attached at the exit of the extmder. Identical processing conditions were used in the compounding with different screw configurations. The temperature profiles along the barrel are Ti= Tg= l = 230°C and Ta= Ts= T4= Ts= 240°C. The screw speed is 30 rpm and the throughput at 5.7 kg/hr. 

WPC samples were compounded using a Leistritz Micro-27 co-rotating twin-screw extruder with K-Tron powder and pellet loss-in-weight feeders. The PP-g-MA or SEBS-g-MA pellets were hand mixed with PP pellets in a bag before loading into the feeders. The wood flour was pre-dried in an oven at 100°C for 7 hours to ensure that the moisture content was less than lwt%, while clay was pre-dried at 80°C for at least 16 hours before compounding. The barrel temperature was controlled at 165 - 180°C from hopper to the die, while the screw rotation speed was fixed at 150 rpm. The total feeding rate was controlled at 4.8 kg/hr, and all of the samples contained 50 wt% wood flour the experimental matrix is listed in Table 1. 

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. 

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