Powder Surface Roughness as Proxy for Bed Density in Powder Bed Fusion of Polymers

Abstract

Powder bed fusion of polymers is becoming increasingly adopted by a variety of industries to tailor the strength, weight and functionality of end-use products. To meet the high standards of the modern manufacturing industry, parts built with powder bed fusion require consistent properties and to be free of defects, which is intrinsically connected to the quality of the powder bed prior to melting. The hypothesis of this work is that the roughness of the top surface of an unmelted powder bed can serve as a proxy for the powder bed density, which is known to correlate with final part density. In this study, a laser line scan profilometer is integrated onto the recoater arm of a custom powder test bench, which is able to automatically create layers of powder. A diverse group of polymers was investigated including polyamide 12 (PA12), polyamide 11 (PA11), polypropylene (PP), and a thermoplastic elastomer (TPU) under different recoating speed in order to increase the variance of the dataset. Data analytics were employed to compare roughness to measured powder bed density and a statically significant correlation was established between them.

In-Situ Monitoring of Powder Bed Fusion of Polymers Using Laser Profilometry

“…When considering the respective 𝜌 𝑚 of the two polymers, the average 𝜌 𝑝 values for Duraform PA12 and iCoPP are 0.37 ± 0.03 g cm −3 and 0.35 ± 0.05 g cm −3 . These numbers are higher than previously reported [9]: 0.26 ± 0.02 g cm −3 for Duraform PA12 and 0.29 ± 0.02 g cm −3 F. Sillani et al for iCoPP. The experimental conditions, and even the measurement procedure, are quite different for these two sets of 𝜌 𝑝 : in fact, [9] used a blade instead of a roller, and the powder was recoated in a metal cavity with nominal depth approximately equal to 150 μm at room temperature.…”

“…These numbers are higher than previously reported [9]: 0.26 ± 0.02 g cm −3 for Duraform PA12 and 0.29 ± 0.02 g cm −3 F. Sillani et al for iCoPP. The experimental conditions, and even the measurement procedure, are quite different for these two sets of 𝜌 𝑝 : in fact, [9] used a blade instead of a roller, and the powder was recoated in a metal cavity with nominal depth approximately equal to 150 μm at room temperature. Given the use of the roller and higher temperatures, the spreading conditions are different in the present work.…”

“…Therefore, the difference between the 𝐸𝐿𝑇 calculated in this work and the ''real'' 𝐸𝐿𝑇 is negligible, under the assumption of constant interlayer shrinkage. So, this methodology allows to ascertain with good precision the absolute packing density of the powder during production, an important metric associated to part quality in powder bed fusion of polymers [8,9].…”

“…13b and c, respectively. So increasing temperature to the processing region has a smoothing effect on the surface roughness 𝑆 𝑞 and thus the resulting powder layer density would increase [9]. On the other hand, increasing the recoating speed has a negative effect on the surface smoothness, and thus decreases 𝜌 𝑝 .…”

“…Therefore, an higher recoating speed is desirable for productivity, but has a detrimental effect on the packing density, which is a predictor for the final part density [8]. [9] experimentally measured a decrease of powder layer density (𝜌 𝑝 ) with increasing recoating speed for several materials that are commercially available for PBF-LB/P.…”