Temperature-Dependent Preload Force Relaxation of Direct Screw Joints in Sheet and Bulk Moulding Compounds

Direct screw joints in thermoset moulding compounds, such as those used in Sheet and Bulk Moulding Compound applications, are of interest for assemblies with sealing and functional requirements. Chemical crosslinking, as present in Bulk Moulding Compound, restricts the stress relaxation of the polymer chains and suggests higher long-term preload force stability than in thermoplastic bosses. In the present study, the preload force relaxation of screw bosses made from Sheet and Bulk Moulding Compound materials is investigated and compared with thermoplastic reference specimens made from polyamide 12. In addition to measurements at room temperature, thermal cycling between −40 °C and +80 °C is carried out. The results show a significantly lower relative decrease in preload force for the thermoset materials compared with the reference material, as well as lower temperature dependence. From this, indications for the design of direct screw joints can be derived.