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Many polymers are processed and compounded in co-rotating, fully intermeshing twin screw extruders. A typical compounding process consists of multiple unit operations including feed introduction, solids transport, transitioning from the conveying zone to the kneading block melting zone, melting, a downstream feed zone, a mixing zone, a devolatilization region, and a pressure generating discharge section. This paper will focus on the kneading block melting zone which typically includes a reverse pitch element so the kneaders remain full. Mechanisms for flow and energy input to pellets in kneading elements, including friction heating, pellet compression, energy diffusion, and viscous dissipation to form a melt phase: followed by viscous dissipation in a solid pellet/melt slurry, and heat transfer; are developed and implemented in a novel melting model. The model is validated with extrusion measurements and visualizations using low density polyethylene (LDPE). The predictions of the model are also compared with a classical set of experiments using high density polyethylene (HDPE). This paper describes the physics and engineering concepts that the authors feel are inherent in the melting section of the twin screw extruder where a large pressure peak is calculated using the friction and compression of the polymer pellets. The modeled increase in pellet bulk/surface temperature is due to the inclusion of four energy sources, pellet compression, thermal diffusion, friction energy dissipation, and viscous energy dissipation. This combined thermal dynamics based melting model results in a novel melting protocol. The melting mechanisms are coupled to flow regimes in the kneading blocks as melting progresses. The effects of throughput, Q, and screw rotation speed, N, are also examined. Highlights * Novel 2.5D melting model for corotating, intermeshing twin-screw extruders. * Energy for melting; friction, deformation, diffusion, and viscous dissipation. * The model helps elucidate mechanisms during melting in a twin-screw extruder. KEYWORDS melting, melting model, twin-screw extrusion, 1 | INTRODUCTION The co-rotating, fully intermeshing twin-screw (TS) extruder is primarily made up of screw conveying elements and kneading blocks (KBs) along with some specialty elements, such as gear [...]

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 [...]

Twin-screw extrusion processes are commonly refined on laboratory-scale extruders then scaled-up to manufacturing systems. When using twin-screw extrusion to compound filler into a polymer, the dispersion of the filler must be considered during scale-up. In this work, two scale-up methods are evaluated for how accurately they scale dispersion as measured by the Residence Stress Distribution, an experimental method that quantifies stress developed in a twin-screw extruder. The first scale-up method evaluated is the industry-standard scaling based on maintaining equivalent volumetric flow rate across extruder sizes. Volumetric scaling is compared to a second, novel scale-up method, the percent drag flow rule, which maintains the same degree of fill in the strongest dispersive screw elements on all extruder sizes. Both scale-up rules have been used to scale between three extruder sizes and have been evaluated for how accurately the larger extruders recreate the dispersive mixing of the smallest machine. Results indicate that the percent drag flow scale-up more accurately maintains dispersive mixing behavior than the volumetric scaling. Furthermore, percent drag flow scale-up resulted in all three extruder sizes behaving similarly to changes in operating conditions. These results indicate that percent drag flow scale-up is a valid technique to scale real industrial processes., INTRODUCTION Twin-screw extrusion is commonly used to compound additives into molten polymers. When introducing a solid into the polymer melt, the mixing of the additive within the polymer has a [...]

The co-rotating fully intermeshing twin-screw extruder is the primary production unit for compounding of polymer based materials. It also has had a long term presence in processing material in the [...]

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