Modelling and characterising FFF process of semi-crystalline polymers: Warpage formation and mechanism analysis

Fused filament fabrication (FFF) is commonly utilised for 3D printing of semi-crystalline polymers, i.e., polypropylene (PP), while warpage deformation can often be observed in the printed products, with significantly reduced surface quality and mechanical properties. This study develops computational models for predicting rectangular boxes made of PP with different thicknesses under the FFF process to study the stress concentration and warpage mechanisms during 3D printing. Numerical models were established based on heat transfer, thermoelasticity, and crystallisation kinetics, with an activating elemental approach to calculate the FFF process of PP. The numerical models were validated with repetitive printing tests to study the mechanisms and relationships of stress conditions and warpage formation in PP boxes under FFF. The results show that the boxes with the thinnest thickness exhibited mostly severe warpage deformation (6.8 mm/5.9 mm in experiment and simulation, respectively), which is much more than that of the thickest box (1.3 mm/1.6 mm in experiment and simulation, respectively). The average crystallinity of the three boxes increases as the box thickness increases, but to a lesser extent. In terms of residual stress, the thinner box has a smaller residual stress (25.1 MPa, almost 45% of the thicker box).