Plastic often contain other added material called fillers。Fillers are employed to increase bulk and to help impart desired properties. Plastic contaning fillers will cure faster and hold closer to established finished dimensions,since the plastic shrinkage will be reduced. Wood flour is the general-purpose and most commonly used fillers. Cotton frock ,produced from cotton linters,increases the mechanical strength. For higher strength and resistance to impart,cotton cloth chopped into sections about 1/2 inch square can be processed with the plastic. Asbestos fiber may be used as filler for increased heat and fire resistance ,and mica is used for molding plastic parts with superior dielectric characteristics.Glass fibers ,silicon ,cellulose ,clay or nutshell flour may also be used .Nutshell flour is used instead of wood flour where a better finish is desired.Plastic parts using short fiber fillers will result in lower costs,while those with long fiber fillers having greater impact strength are more expensive .Other materials ,not desired as fillers,such as dyes,pigments,lubricants,accelerators and plasticizers may also be added.
Plasticizers are added to soften and improve the moldability of plastics.Filler and moldifying agents are added and mixed with the raw plastic before it is molded or formed.