Prediction of absorptivity in Multi-Jet Fusion manufactured polypropylene structures through laser flash and corrected porosity method

Abstract

For the first time in the literature, this study validates the absorption phenomena in Multi-Jet Fusion (MJF) printed polypropylene (PP) structures through Laser Flash (LFA) and Corrected Porosity (CP) methods. The influence of process parameters such as build height and build orientation was investigated on tensile properties, crystallinity, porosity and thermophysical attributes in MJF printed PP coupons. Results showed that both crystallinity and tensile performance did not significantly vary with either location or build orientation. Interestingly, samples printed in the Z orientation showed a 35% decrease in strain, indicating that Z-oriented MJF coupons were more brittle than the flat samples (XY). Samples printed in Z orientation also possessed higher porosity and relatively lower crystallinity than the XY orientation. However, large deviations within porosity values were an obstacle to determining a suitable build chamber location for manufacturing dense samples. Therefore, a detailed investigation on porosity of printed samples using micro-CT scans and CT image analysis was necessary. Initially, poor contrast was obvious when MJF printed samples were positioned vertically in the micro-CT chamber which was mainly due to high value of horizontal intensity profile (HIP ~ 70%). Contrast in MJF samples improved significantly in the horizontal orientation (HIP ~ 40%). In parallel, the half-time and heat loss were measured in LFA to understand changes in absorption phenomena with height and orientation of the build. A direct correlation was found between LFA half-time and porosity only when the porosity correction method was implemented. Corrected porosity value was found to be inversely proportional to the heat loss of printed PP samples which indicated higher absorption for samples printed in the bottom of build chamber, XY12, whereas lower absorption was observed for less dense Z samples. Finally, heat loss phenomenon was verified using dense reference Pyroceram samples as they possess high diffusivity and low half-time and porosity compared to MJF printed samples. There is a science behind understanding the absorptivity of the MJF process which is related to the complexity of the process and is challenging to address in MJF PP samples when mixed with carbon black. The study showed that accurately determining the level of porosity is the key to validate absorption phenomena within MJF printed coupons. The contributions of this work are the investigation of the light absorption phenomena in MJF printed PP structures, and the establishment of the absorption-porosity correlation. These contributions help to predict the mechanical properties and subsequently the overall quality of the produced parts which can save cost and time in effectively utilising the MJF process.