Understanding and Controlling Reusability, Aging Kinetics, and Sustainability of Polyamide 12 in Selective Laser Sintering

Abstract

Capable of building high-quality, complex parts directly from digital models, selective laser sintering (SLS) additive manufacturing is a core method of agile manufacturing. The semi- crystalline polymer polyamide 12 has been extensively used in SLS thanks to its superior mechanical property, stable thermal property, broad sintering window for ease of processibility, low melt viscosity to reduce porosity, and high melting enthalpy to minimize secondary sintering. However, low utilization of the costly feedstock hinders the long-term sustainability of SLS in industry, and the feedstocks undergo complex thermal and chemical degradations in SLS. Understanding and controlling on reusability, aging kinetics, and sustainability of degraded polyamide 12 in SLS remain largely unexploited.This dissertation proposes (1) a new interlayer heating based process control method to maximize reusability of aged and extremely aged polyamide 12 powders and create parts with improved mechanical properties, (2) a novel process control approach with post-heating for SLS with (extremely) aged polyamide 12 powders to improve surface quality and build interrelations between process parameters and surface quality, (3) a combined theoretical and experimental approach to build a first-instance kinetic model for polyamide 12 degradation considering both the oxygen and laser effects in SLS, identifies (4) quantitative influences of successive reuse on thermal decomposition, molecular evolution and elemental composition of polyamide 12 residues in SLS, proposes (5) a process-oriented and mass-transfer based approach to model and predict volatile organic compound emission in SLS, and conducts (6) an in-process monitoring of temperature profiles from infrared images in various locations when sintering different polyamides in SLS. Results show (1) the proposed method can yield printed samples with 18.04% higher tensile strength and 55.29% larger elongation at break using as much as 30% of extremely aged powders compared to the benchmark sample, (2) parts 3D-printed using the 30%-30%-40% new-aged- extremely-aged powder mixtures exhibit the smoothest and flattest surface with no unmolten particles and nearly zero porosity, (3) the laser effects are 4-time stronger than oxygen effects on polyamide 12 degradation, and the predicted oxidation matches on average 89.53% with the actual SLS degradation rates, in contrast to a 34.48% accuracy from a basic autoxidation model, (4) laser and heat lower the material onset decomposition temperatures, making the material more labile at a decreased temperature after reuse, and the carbon deposit and degradation raise the atomic percentages of C, (5) the proposed method has an average accuracy of 85.32% to the experiment results in predicting VOC emissions, and the mass-transfer coefficient, diffusion coefficient, and partition coefficient have different influences on emission, and (6) reclaimed powders absorbed more energy during laser on periods at all positions, and the thermal conductivity for extremely aged powders decrease compared to new powders.