The property profile of a plastic can be selectively altered by introducing additives or fillers designed to achieve specific characteristics. The resulting plastic compounds allow for different (usually increased) thermal, mechanical, electrical, optical or other characteristics to be enhanced beyond what is typically seen in the unmodified base polymer. The most common fillers and additives, besides colourants, are reinforcement fibres, stabilisers, and friction reducing additives.
In fibre reinforced polymers, the reinforcement fibres determine the compound's material properties. In many cases, the strength of the fibres is greater than the matrix materials - and frequently also higher than the tensile strength of metallic materials. The density of the most commonly used fibres, in contrast, is usually lower than that of aluminium, which allows for greater potential in designing parts for lightweight construction. In most cases, glass, carbon and aramid fibres are used in fibre reinforced plastics. From the designer's point of view, the mechanical characteristics of the material are particularly important.