Visualization study on the dynamic mixing quality during single-screw extrusion

An empirical method was developed to study the dynamic mixing characteristics of a single-screw extrusion process. The method basically involved a modified version of the visible extruder developed by Wong et al. (Plast Rubb Comp Proc Appl 26:78, 1997). 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 in bitmap file format. Subsequent statistical analysis (largely variance) on the pixel intensity of the cropped films had made quantification of mixing quality possible. The higher the average variance of the pixel intensity, the poorer the mixing quality. The focus of this investigation was on the effects of temperature and screw speed. For the selected single-flighted conventional screw, it was found that, in general, the higher the screw speed, the shorter the residence time and, hence, poorer mixing quality. High screw speed also generated erratic average variance data. 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 50 and 80 rpm. However, unlike screw speed, melt temperature alone appeared to have relatively little effect on the mixing quality during extrusion.