The thermal behaviour of polyamide-12 in the powder bed fusion process

Polyamide-12 (PA-12) powder is used in additive manufacturing (AM) techniques such as selective laser sintering (SLS) and Multi-Jet Fusion (MJF). MJF is a proprietary Hewlett-Packard technique that uses a fusing agent (FA) and detailing agent (DA). These agents are a fundamental and unique part of MJF in comparison to other AM techniques. PA-12 powder was characterised using a variety of techniques including Differential Scanning Calorimetry (DSC), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Particle Size Analysis (PSA), Thermogravimetric Analysis (TGA) and optical microscopy. DSC showed that the peak melting and peak crystallisation temperature of PA-12 were 186 and 148 ℃, respectively. TGA was used to measure the mass loss degradation of PA-12, where a 1% mass loss was displayed at 322 ℃. FTIR spectra showed characteristic peaks in line with literature for a part manufactured by MJF. PSA showed that the particle size varied between 11.96 – 164.32 μm. SEM was used to characterise the shape of the powder particles. The powder particles appeared to be elongated and non-uniform, where some particles were more spherical. Optical microscopy was used to observe the interaction of powder particles on melting, it was shown that the powder particles fused together to form a droplet shape. The isothermal crystallisation kinetics were studied between 162 to 168 ℃. It was demonstrated that as the isothermal crystallisation temperature increased, the time taken for crystallisation increased. This data was used to determine the equilibrium melting temperature (T$_M$$^0$) by the Hoffman-Weeks method using the peak melting temperature and last trace of crystallinity; where T$_M$$^0$ was equal to 188 and 205 ℃, respectively. The Avrami and Tobin model was used to explore the isothermal crystallisation kinetics. Of which, the Avrami and Tobin model were successful in describing the isothermal crystallisation kinetics of PA-12. In the Avrami model $n$ ranged from 2.73 to 3.63 and in the Tobin model $n$ ranged from 3.15 to 4.23 for the primary crystallisation process. The non-isothermal crystallisation kinetics were studied between 5 to 40 ℃/min. It was demonstrated that as the cooling rate increased, the time taken for crystallisation decreased. The Jeziorny-modified Avrami model was successful in describing the non-isothermal crystallisation kinetics of PA-12. In the Jeziorny-modified Avrami model $n$ ranged from 4.42 to 7.68. The influence of the FA, DA and their chemical constituents on the thermal behaviour of PA-12 was studied by creating PA-12 blends at varying wt. % compositions. The blends included PA-12/FA, PA-12/DA, PA-12/water PA-12/carbon black, PA-12/2-PYR and PA-12/2-PYR/water. Since the powder used in MJF often is refreshed, PA-12 was aged artificially in an oven at 170℃ for 7 days to represent aged powder. The thermal behaviour of aged PA-12 powder and aged PA-12 blends was studied. It was demonstrated that the 2-PYR had a significant impact on the thermal properties of PA-12 via its action as plasticiser: DSC revealed a depression in polymer glass transition temperature (T$_g$), of up to 17 ℃, and a depression in melting temperature, of up to 10 ℃, with increasing composition of 2-PYR. Moreover, the crystallisation behaviour was also affected by the 2-PYR component in that its presence decreased the supercooling required for the onset of crystallisation. FTIR analysis revealed a small peak at 1690 cm$^{-1}$ which was assigned to 2-PYR. This indicated that 2-PYR was retained in the PA-12 and interacted with the polymer via hydrogen bonding. Optical microscopy showed that the 2-PYR constituent did not affect the spherulite sizes, but in the case of the carbon black constituent, it was found to be rejected to the inter-spherulitic regions and this was coupled with a notable reduction in spherulite size. This showed that carbon black and 2-PYR were fundamental components within the FA and DA with respect to their impact on the thermal properties and microstructure of PA-12