Extruder scale‐up in a corotating twin‐screw extrusion compounding process

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Abstract
The effects of processing conditions and extruder scale are examined for a compounding process involving fully intermeshing corotating twin‐screw extruders. A systematic experimental study involving a matrix of two independent variables (extruder throughput rate and screw rotational speed) was conducted on two different size twin‐screw extruders using a styrene copolymer and carbon black. Several process parameters were examined including melt temperature at the die, motor power, residence time distribution, and carbon black concentration in the extrudate. The residence time distribution data were fit to a three‐parameter plug flow/stirred tank model to obtain a time delay, a time constant, and a mean residence time for each extrusion condition. At low screw speeds, as utilized in many compounding processes, the mean residence time dropped only slightly with increasing screw diameter (for comparable screw length/screw diameter ratios). The carbon black concentrations as measured in the extrudate pellets were within ±5% of mean value, indicating that the extruder damped out the high frequency noise caused by feed rate fluctuations in the individual polymer and carbon black feeders.

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