Warpage of polypropylene and propene-ethylene random copolymers in material extrusion additive manufacturing

Material extrusion additive manufacturing (MEAM) exhibits promising capabilities due to its high customization options. However, warpage emerges as a primary challenge hindering the widespread adoption of MEAM for mass production. This study focuses on investigating warpage in selected grades of propene-ethylene copolymers and a polypropylene homopolymer during MEAM. The influence of ethylene content on crystallinity was assessed using thermal analysis, and warpage was evaluated for rectangular parts, made of either one or two layers. Wide-angle X-ray diffraction was employed to analyse polymorphism, confirming the presence of α-phase. In situ infra-red camera measurements provided insights into the cooling profiles of the printed geometry. For both one and two-layer prints, results show that increasing the ethylene content decreases crystallinity, and consequently, warpage is also decreased. Warpage substantially increases when printing thicker parts, primarily due to non-simultaneous crystallization between layers, as evidenced by comparing the cooling profile with DSC-measured crystallization temperature. Increasing bed temperature mitigates warpages for all materials and layer counts. This phenomenon is mainly attributed to a slower crystallization kinetics and to a more rapid and extended relaxation process, which diminishes internal stresses, thereby limiting bending. The parameter ΔT B = T C − T B (difference between the crystallization temperature T C and the bed temperature T B ) was introduced to account for crystallization kinetics in relation to processing conditions. This parameter is proposed as a unified descriptor for warpage behaviour, as all the data collected with different materials and T B lay on two “mastercurves”, which are functions of the number of printed layers. It is thus convincingly shown that minimizing ΔT B leads to reduced warpage, due to the slowing down of the crystallization kinetics and relaxation of internal stresses.