New 2.5-D twin screw extruder model for the feed section of a co-rotating twin screw extruder with comparisons to data

Many polymers are processed and compounded in co-rotating, fully intermeshing twin screw (TS) extruders. The typical compounding process consists of multiple unit operations including feed introduction, conveying solids away from the feed zone, transitioning from the conveying zone to the kneading block (KB) melting zone, melting, a downstream feed zone, a downstream mixing zone, a devolatilization region, and a pressure generating discharge section. This paper will focus on the first of these unit operations, the conveying of pellets in a partially filled screw leading to compaction and pressure generation in the full screw channel prior to the KB melting zone. Mechanisms for pellet conveying, for heating the polymer pellets, and for developing pressure at the end of the solids conveying screw elements are first presented for low-density polyethylene (LDPE). The predictions of the model are also compared with a classical set of experiments on high-density polyethylene (HDPE). The model is then tested using published characteristics of a polyamide-type polymer. This paper focuses on the physics and engineering concepts that are inherent in the feed section of the TS extruder where the pressure is calculated using the friction of the polymer pellets. The effects of throughput, Q, at a constant rotation speed, N, are examined. Low and high Q/N ratios have significantly different axial pressure profiles. KEYWORDS heat transfer, solids conveying, solids transport, solids transport model, twin screw extrusion
1 | INTRODUCTION The twin screw (TS) extruder is primarily made up of conveying elements, kneading blocks (KBs), and some specialty elements, such as gear mixers. The mathematical description of [...]