Abstract

Fused Deposition Modelling (FDM) of semi-crystalline polymers is widely affected by residual stress build-up and warpage due to the crystallisation-driven volumetric shrinkage and the non-homogeneous thermal history experienced during layer-by-layer deposition. Although the influence of processing parameters such as raster pattern, ambient temperature and nozzle speed on part distortion has been extensively investigated, the influence of geometrical parameters particularly increasing component height on the distortion behaviour in both amorphous and semi-crystalline polymers has received limited attention. In this study, a coupled thermo-mechanical finite element model incorporating crystallisation kinetics was employed to investigate the influence of increasing build height on the development of thermal residual stresses and warpage in the bottom layer of PP (semi-crystalline) and ABS P400 (amorphous) samples. Samples comprising 4 layers (PP-4, ABS-4) and 8 layers (PP-8, ABS- 8) with constant in-plane dimensions of 50×50mm were simulated and experimentally validated using 3D scanning of FDM-printed specimens. The results show that doubling the number of layers increases the accumulated residual stress in the bottom layer by 19.9 % in PP and by 34.9 % in ABS. In contrast, the bottom-layer warpage exhibited a different response between the two polymers: PP showed a counterintuitive decrease of 14 %, whereas ABS showed a significant increase of 98.7 %. The reduction in PP warpage with increasing height is attributed to the in-situ annealing effect of repeated reheating from successive layer depositions combined with the mechanical constraint imposed by overlying layers, which outweighs the additional crystallisation induced shrinkage. The simulated warpage results were in good agreement with the experimentally measured values. These findings demonstrate that build height should be considered alongside processing parameters when optimising dimensional accuracy in FDMprinted semi-crystalline polymer components..

Keywords:Part distortion; Semi-crystalline polymers; Amorphous polymers; Crystallisation; FDM; Simulation; Residual stress

Abbreviations: PP: Polypropylene; ABS: Acrylonitrile Butadiene Styrene; FDM: Fused Deposition Modelling; FEA: Finite Element Analysis