The premixing step homogenizes the raw materials mix so that a consistent feed can be introduced into the extruder. Moreover, large resin flakes are fractured, which helps facilitate a mix that can be fed more easily. A high-intensity mixer can achieve acceptable blending in a relatively short duration (2 to 5 minutes). Adequate mixing can also be accomplished with a slower, less intense process that typically can take 10 to 25 minutes.
Consistency can also be enhanced by the order that raw materials are added to the premixing vessel. “Sandwiching” minor components (tints, additives, etc.) between bulk items (resins, extenders, etc.) helps minimize these minor additives from being flung inside the premixer onto the vessel wall and thereby compromising distribution.
Some mixers employ detachable bowls that offer versatility in managing multiple mixes with the same mixing head. This design also allows for ease in cleaning. Premixing equipment is designed for an optimal range of vessel filling. Charges too small will be poorly mixed because the mixing blades will be incompletely covered. Excessively large charges will be poorly blended due to lack of free space to transport 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 electrical drives.