Meaning
Proportional dimensions of a cylindrical protrusion designed to receive a mechanical fastener define the feature. In thermoplastic part design, boss geometry determines the flow of molten resin and the subsequent structural integrity of the assembly.
Moulding Factor
Mold filling dynamics dictate that the wall thickness of the protrusion should not exceed sixty percent of the nominal part wall. This constraint prevents excessive local shrinkage and the formation of internal voids during the cooling phase.
Mechanical Resistance
Resistance to hoop stress during fastener installation relies on the ratio of the outer diameter to the inner hole diameter. If the outer diameter is too small, the tensile stress generated by a thread-forming screw will split the cylinder. A larger outer diameter increases resistance but raises the risk of cosmetic sink marks on the opposite cosmetic surface of the part.
Failure Mode
Catastrophic splitting during assembly or subsequent field service represents the primary risk of inadequate dimensioning. When the inner diameter is too narrow, the drive torque required to thread the fastener induces high radial stress that exceeds the yield strength of the polymer. This localized stress can lead to immediate fracture or initiate slow crack propagation under environmental loading, especially when the part is exposed to elevated temperatures or chemical agents that promote environmental stress cracking.
