Mixing Extruders With Granulator

There is a mixing principle in single-screw extrusion that always holds true: The better the mix entering the screw the better the final mix. Providing a better mix into the screw requires two things: A mixing procedure that first uniformly disperses the additives through the base polymer(s) and secondly prevents them from segregating before or even as the mix enters the screw.

The second requirement is particularly true of certain solids that cannot be sheared to take advantage of the dispersive-mixing effect that single screws provide. These include various mineral or metallic fillers that typically cannot be sheared into the base polymer because they do not melt or soften and tend to pass through the extruder with no changes in shape or size. Significant axial redistributive mixing of such materials is difficult to achieve on single screws because these extruders are surprisingly plug-like in flow, meaning the flow is very much “first in-first out.” This is related to the laminar flow of polymers in the single screw, and the predominant down-channel shear distribution in the channel, which results in restricted mixing in the axial direction.