Thermal Ratcheting

Deformation Mechanism

When a plastic component is locked in place against a metal frame, temperature rises cause the polymer to expand at a much higher rate than the metal. Since the expansion is constrained, compressive stress builds up and exceeds the elastic limit of the polymer, leading to plastic flow. During the subsequent cooling phase, the material cannot return to its original shape and remains permanently elongated or warped.

Repeating this cycle causes the plastic strain to pile up, causing gradual lengthening or distortion of the component over time.

Resin Property

Susceptibility of a polymer to this failure mode depends on its coefficient of thermal expansion and its high temperature mechanical strength. Amorphous polymers like polycarbonate generally resist this effect better than semi crystalline materials because they do not have a sharp melting transition, although their glass transition temperature must exceed the maximum cyclic temperature. Sourcing resins with glass reinforcement increases the modulus and lowers the thermal expansion coefficient, which reduces the potential for cyclic deformation.

Moulders must ensure that materials are processed with low residual stresses to prevent early thermal distortion.

Design Solution

Mitigating this issue requires careful allowance for thermal movement in the assembly design, such as using slotted holes or flexible clips rather than rigid fasteners. Specifying materials with matched thermal expansion coefficients prevents the generation of excessive stress at the joining points. Process engineers can also optimize moulding parameters, such as holding pressure and cooling time, to minimize the molded-in stresses that exacerbate thermal deformation.

Regular thermal cycling tests on prototype assemblies verify the design before starting mass production.