The premixing step homogenizes the raw materials mix so that a consistent mixture can be fed into the extruder and comminutes large resin particles so that the mix can be fed more easily. A high-speed impeller can achieve premixing in a relatively short duration (two to five minutes). Adequate blending can also be accomplished with a slower, less intense process. Some mixers use detachable bowls that offer versatility in managing multiple mixes with the same mixing head. This design also allows for ease in cleaning. An impeller design can make a difference in the consistency of the final mixture.
Premixing equipment is designed for an optimal range of vessel filling. Charges that are too small will be poorly mixed because the mixing blades will be incompletely covered. Excessively large charges will be inconsistently mixed due to a lack of free space to agitate the contents of the vessel. Excessively large charges can also tax the drive mechanism of the mixer to the point of tripping circuit breakers and damaging motors and gear drives.