It is useful to consider the overall mixing process in a solid/liquid system as consisting of three stages
which are quite distinct in nature but overlap in practice: namely; wetting of the powder,breakdown of
agglomerates ( and aggregates ) and finally stabilisation of the mixture by distribution of the solid particles .
The primary particle size of solid fillers and pigments is usually small enough for further
breakdown during compounding to be unnecessary. However, when dry, powders can often contain
some aggregates and agglomerates or indeed these may be formed during premixing or conveying in
the solid state. The initial mixing process in the extruder involves melting the resin and wetting out
the solid particles. Once wetted, mechanical energy input is required to bring about their complete
separation or dispersion. The finely divided particles and other formulation ingredients are then
distributed uniformly within the molten resin. The dispersion is usually stabilised by the relatively
high viscosity of the molten resin and can be assisted by surface active chemicals.