Characteristic gradient structures in polyamide 12 parts fabricated by material extrusion additive manufacturing

The crystalline structures and their spatial distributions of polyamide 12 (PA12) parts processed by material extrusion additive manufacturing (MEAM) were studied in this work by means of microfocus wide-angle X-ray scattering combined with transmission electron microscopy. There are two types of gradient structures observed in the PA12 parts. One of the gradient structures is found in the topmost layer, where the total crystallinity drops gradually from the free surface to the core region further to the weld region. Moreover, the coexistence of alpha and gamma crystals is found in all regions. The gradient structure in the topmost layer arises from partial melting and chain relaxation occurring during the welding of the free surfaces between adjacent layers. Another gradient structure is that the crystallinity in PA12 parts demonstrates a gradual variation with decreasing the distance from the deposited layer to the platform, especially at a platform temperature above 60 degrees C. Specifically, the bottom layer has much higher crystallinity than the top layer, with the middle layer in between, since the annealing effect of the hot platform gradually weakens from bottom to top due to the low heat conduction of PA12. This work provides a detailed insight into the crystallization mechanism of polyamide materials during MEAM process, which is meaningful for the manipulation of crystal structures in MEAM-printed semicrystalline polymers.