Crystallization-induced residual deformation evolution in thermoplastics during material extrusion and subsequent annealing

Thermomechanical histories in material extrusion (MEX) and subsequent high-temperature service strongly influence the geometric accuracy of semi-crystalline thermoplastic structures. The crystallization-induced residual stress generated in MEX manufacturing process is partially released during subsequent annealing at moderate temperatures. However, high-temperature annealing induces further cold crystallization and additional residual deformation. Therefore, releasing and generating of residual stress in subsequent annealing would affect the high-temperature performance of structural components made by MEX. Tracing the crystallization history during manufacturing and subsequent annealing plays a key role in design and evaluation of components made by MEX throughout the entire lifecycle. To address this issue, we developed a process simulation method for MEX based on the element activation technology of the finite element method and a continuous phase evolution constitutive model of thermoplastic crystallization. In the simulation, the elements of the target object are sequentially activated following the real manufacturing path, and the thermomechanical boundary conditions are updated stepwise. In the constitutive model, crystal growth is modeled by a series of continuously formed crystal phases that are sequentially added to the initial amorphous medium to memorize the crystallization history coupled with the entire deformation history. Therefore, during subsequent annealing, the crystallization-induced bi-directional bending of the polylactic acid frame structure is observed in the simulation, and then verified by experiments. The square frame structure after MEX is observed first to bend downward and finally to bend upward during subsequent annealing. The initial downward bending is induced by the release of the residual stress generated by crystallization during MEX manufacturing, and the final upward bending is induced by the orientation difference of the crystals formed during manufacturing and subsequent annealing. Overall, the good agreement indicates that the developed method can accurately describe the inheritance of the crystalline state during manufacturing on the subsequent high-temperature service performance.