Analysis of the insertion and overtightening torques of plastic nuts depending on the air humidity

Plastic nuts are already being frequently utilised as auxiliary joining elements in automobile lightweight construction. When these injection- moulded plastic nuts are used, attention must be paid to the ambient conditions (especially to the relative air humidity) during the storage since thermoplastics can absorb and release water. The mechanical properties of the plastic alter in this case. In turn, this exerts significant influences on the screwing and joining properties of the plastic nuts. In order to prevent any overtightening of the nuts, the assembly tightening torque must be chosen depending on the air humidity. The investigations show that the possible process window for the assembly tightening torque becomes significantly smaller as the water mass fraction rises and, in addition, is dependent on the glass fibre content of the thermoplastic. In this respect, a particularly decisive influence may be assigned to the glass transition temperature.