Quantification Of The Dynamic Mixing Quality During Twin-Screw Extrusion

Mixing is an important component in most processing operations in polymer processing. Generically, mixing refers to a process that reduces composition nonuniformity. In this work two empirical methods were developed to study the dynamic mixing characteristics of metering zone during twin-screw extrusion using a "transparent" extruder.First, the specimens collected from the metering section of the extruder during the process were pressed into flakes which were then photographically captured under a microscope. Second, digitized films of the molten polymer web emerging from the slit flow cells attached to the "windows" of the visible extruder's barrel were captured. The analysis was made on the measurement of color homogeneity of the grayscale images, using variance and Shannon Entropy, which enabled quantification of mixing quality possible. The focus of this investigation was on the effects of screw speed. For the selected single-flighted conventional screw, it was found that, the two empirical methods mentioned above went into the same result. In general, the higher the screw speed, the poorer mixing quality. The results suggested that a critical screw speed for optimized mixing might have been present. For the equipment used in the current project, this critical screw speed lied between 80-120rpm. A complex stress and velocity field and filling rate resulted from the effect of the high screw speed were thought to be responsible for the results.