Reduction of internal stresses in injection molded parts by metallic fillers

The build‐up of internal stresses in injection molded parts is due to the combined effect: of a low thermal conductivity (diffusivity) of normal polymer melts and their rapid cooling in the mold cavity. The present paper shows that increasing the thermal diffusivity of the polymer by the addition of relatively small amounts of metallic fillers resulted in a substantial reduction in the internal stress level. The filler content was of the order of a few percent by volume. The average internal stress (σ i ) was determined by a stress relaxation method. The modulus, tensile strength, and other mechanical properties of the samples were not influenced by the filler addition. The impact strength, however, decreased sharply, as no adhesion pro‐motors were used. The observed improvements in mold shrinkage, thermal shrinkage, hardness, and stress‐cracking tendency could be related to a decreased σ i level. In comparative experiments with other fillers (MgO, glass powder), a good correlation was found between the σ i level and the thermal diffusivity of the filler.