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 deposited along the vertical direction. 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 parts, obtained with one and two deposited layers, 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 warpage for all materials and layer counts. This phenomenon is mainly attributed to a slower crystallization kinetics and a more rapid and extended relaxation process, which diminishes internal stresses, thereby limiting bending. The parameter ΔTB = TC − TB (difference between the crystallization temperature TC and the bed temperature TB) 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 TB lay on two “mastercurves”, which are functions of the number of printed layers. It is thus convincingly shown that minimizing ΔTB leads to reduced warpage, due to the slowing down of the crystallization kinetics and relaxation of internal stresses.