Thermal and Oxidative Aging Effects of Polyamide-11 Powder Used in Multi-Jet Fusion

The transition of additive manufacturing (AM) from a technique for rapid prototyping to one for manufacturing of near net or net components has been led by the development of methods that can repeatedly fabricate quality parts. High-speed laser sintering and the recently developed multi-jet fusion (MJF) processes have seen quick adoption from industry due to their ability to produce high-quality components relatively quickly. However, the recommended refresh ratios of new powder led to notable amounts of used powder being discarded. In this research, polyamide-11 powder, typically used in AM, was thermally aged to investigate its properties at extreme levels of reuse. The powder was exposed to 180 °C in air for up to 168 h and its chemical, morphological, thermal, rheological, and mechanical properties were examined. To decouple the thermo-oxidative aging phenomena from AM process related effects, such as porosity, rheological and mechanical properties characterisation was performed on compression-moulded specimens. It was found that exposure notably affected the properties of both the powder and the derived compression-moulded specimens within the first 24 h of exposure; however, consecutive exposure did not have a significant effect.

Experimental Characterisation and Finite Element Modelling of Polyamide-12 Fabricated via Multi Jet Fusion

The HP Multi Jet Fusion (MJF) technology is a relatively recent addition to powder bed fusion additive manufacturing (AM) techniques. It differentiates itself from selective laser sintering (SLS) technology through the use of fusing and detailing agents to control part geometry, and the use of a planar infrared radiation (IR) source that sweeps over the powder bed to initiate the sintering process. Depending on the printing methodology, AM processes can introduce mechanical property anisotropy that is dependent on print orientation. In the case of MJF-fabricated parts, there is a general disagreement over the influence of print orientation on tensile mechanical properties in the literature. In this work, MJF-fabricated PA12 (AM PA12) is printed at various orientations and characterised in terms of tensile and compressive mechanical properties. The orientations have been selected to take into account the alignment of the IR source sweep direction to the test load. We observe that orientating parts towards the vertical direction for printing tends to favour enhanced tensile mechanical properties. The anisotropy in mechanical properties is attributed to more complete polymer powder fusion as a result of the increased number of IR source sweeps when parts are orientated towards the vertical direction. Both tensile and compressive stress-strain data were used as experimental data input for calibrating the Elastic-Plastic with combined hardening (EPC) material model in the commercial finite element analysis (FEA) package-Abaqus. We demonstrate that the EPC material is a suitable material model for the FEA of AM PA12.

Creep and Recovery Behaviour of Polyolefin-Rubber Nanocomposites Developed for Additive Manufacturing

Nanocomposite application in automotive engineering materials is subject to continual stress fields together with recovery periods, under extremes of temperature variations. The aim is to prepare and characterize polyolefin-rubber nanocomposites developed for additive manufacturing in terms of their time-dependent deformation behaviour as revealed in creep-recovery experiments. The composites consisted of linear low density polyethylene and functionalized rubber particles. Maleic anhydride compatibilizer grafted to polyethylene was used to enhance adhesion between the polyethylene and rubber; and multi-walled carbon nanotubes were introduced to impart electrical conductivity. Various compositions of nanocomposites were tested under constant stress in creep and recovery. A four-element mechanistic Burger model was employed to model the creep phase of the composites, while a Weibull distribution function was employed to model the recovery phase of the composites. Finite element analysis using Abaqus enabled numerical modelling of the creep phase of the composites. Both analytical and numerical solutions were found to be consistent with the experimental results. Creep and recovery were dependent on: (i) composite composition; (ii) compatibilizers content; (iii) carbon nanotubes that formed a percolation network.