Meaning
Measurement of the force required to maintain the integrity of a connection between two or more components defines this performance metric. High joint retention ensures that assembled parts do not separate or loosen under operational loads or vibrations. In polymer assemblies, this depends heavily on the friction between surfaces and the internal tension of the fasteners.
Fastener Engagement
Threaded screws or snap-fits rely on the modulus of the plastic to exert a constant clamping force. Lower stiffness in the base material reduces the initial grip of the joint.
Creep Response
Continuous load on a plastic boss causes the material to deform over time through a process known as stress relaxation. This movement reduces the tension in a screw, eventually leading to a loss of joint retention even if the fastener itself remains intact. Reinforced grades with glass or carbon fiber resist this flow much better than unfilled resins.
Specification of the long-term decline in clamping force remains necessary when setting torque values for assembly.
Thermal Stability
When the temperature drops, the joint becomes loose because the plastic has shrunk beyond its original dimensions. Fluctuations in temperature cause the plastic and the metal fastener to expand at different rates. If the polymer expands significantly more than the screw, it may reach its yield point, causing permanent deformation.
