In most cases, designers search to lighten parts for the majority of plastic and rubber applications. However, in a few cases concerning special uses they search high densities to obtain special features such as damping, imitation of heavy conventional materials. Sometimes, high densities are not desired but dictated by the research of other properties such as electrical conductivity, magnetic behaviour.
High-density fillers can be metals, mineral salts, oxides, carbides etc. Figure 3 shows an example of density distribution for a panel of minerals. The curve is very different from that of thermoplastics with a peak for roughly 3 and 95% of minerals with a density inferior to 10, which limits the possibilities to make heavy compounds. Among the heaviest materials some are harmful or/and expensive reducing the choice.
For rubbers, typical filler volume fractions are in the range of 0.05 up to 0.35. Higher volume fractions of fillers are rarely used and can be considered as heavy compounds,