In fibre reinforced polymers, the material properties of the compounds are determined by the reinforcing fibres. In many cases, the strength of the fibres exceeds the strength of the matrix materials. Often the strength is 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.
Other fillers usually offer no or only minimal technical advantages. They are mainly used to reduce cost or weight: Ensinger offers, for example, lime, talc or hollow glass beads.