Simulation of a 500 mm Diameter Melt-Fed Extruder

Large diameter, melt-fed extruders are commonly used for the final devolatilization and pelletization of LDPE and PE copolymers in resin manufacturing plants. A full description of this type of extruder and process is provided in Section 15.3. Simulation of these processes is complicated by the multiple flights used in the design and the high H/W aspect ratios of the channels. The processes can be simulated from the feed hopper to discharge, however, since they are not required to convey solids and melt resin. This section will show the requirements and difficulties for simulating these processes. 

500 mm. The flight widths tor all sections were 20 mm and the flight clearance was 0.5 mm. The H/W for this double-flighted screw was about 0.12 in the metering channel. A pressure transducer was positioned in the barrel at 5.6 diameters downstream of the start of the screw. For these conditions and the 0.8 Ml LDPE resin, the pressure at this location was experimentally measured at 6.4 MPa. The barrel temperature for this data set was at 190 °C. 

A three-dimensional simulation method was used to simulate this extrusion process and others presented in this book. For this method, an FDM technique was used to solve the momentum equations Eqs. 7.43 to 7.45. The channel geometry used for this method was essentially identical to that of the unwound channel. That is, the width of the channel at the screw root was smaller than that at the barrel wall as forced by geometric constraints provided by Fig. 7.1. The Lagrangian reference frame transformation was used for all calculations, and thermal effects were included. The thermal effects were based on screw rotation. This three-dimensional simulation method was previously proven to predict accurately the simulation of pressures, temperatures, and rates for extruders of different diameters, screw designs, and resin types. 

The three-dimensional FDM technique provided an excellent prediction of the pressure at 5.6 diameters from the start of the screw, as shown in Fig. 7.16. The method, however, is difficult to use and requires relatively long computational times on a fast computer. This example is an excellent test case for determining the acceptability of a simulation code. 

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