What was the root cause? Improper screw design!
The following image shows the Pressure along the screw, as predicted by the VEL™ Extruder module simulation.
The above image indicates that an extremely high pressure was being created, near the start of the screw, due to a very high amount of material being delivered by the grooved feed section. This resulted and very large forces (pressure and shear stress) acting on the screw surface, and particularly, in the primary/solids channel. These forces were so high that they discolored the screw and, ultimately, eroded the surface.