Compounds are defined as mixtures of pure basic elements in which fillers, reinforcements and/or additives (stabilisers, masterbatch) are added and processed into an end product without having lessened the molecular concentration in the process. Additives include granules, powder or liquids. There is no one set solution for the individual basic elements. "Compounding" is the name of the production process. The purpose of compounding is to modify the elemental properties to a specific application. There are certain challenges involved such as attaining 100% homogeneity and avoiding decomposition or disintegration over time.

Polymers are mixed in one extruder (usually twin screw) with fillers and additives to form a homogeneous mixture. The twin screw extruder is outfitted with main and side inlets on which a side feeding is installed so that aggregate (fillers and eventually additives) can flow downstream into the already-plasticised polymer and then homogeneously blended. The main component (polymer) along with secondary components (additive) may be fed using a loss-in-weight dosing unit at the main inlet. With an additional loss-in-weight dosing unit, the appropriate amount of filler is sent through a side feeding the number of which is based on proportion of filler material in the compound. The homogeneous compound, except in in-line processes, solidifies to granules by means of strand, water-ring or underwater granulating procedures.

These granules are then conveyed to a silo-mixer where the compounded material is again mixed into homogeneous batches prior to packaging. In an additional procedure, the compound is further processed in plastics processing machines such as injection or blow moulding, cable or fibre extruders.