On the Reuse of SLS Polyamide 12 Powder

Abstract

In the Selective Laser Sintering (SLS) technique, the great majority of the powder involved is not included in the final printed parts, being just used as a support material. However, the quality of this powder is negatively affected during the process since it is subjected to high temperatures (close to its melting temperature) during a long time, i.e., the printing cycle time, especially in the neighborhood of the printed part contour. This type of powder is relatively expensive and large amounts of used powder result after each printing cycle. The present paper focuses on the reuse of Polyamide 12 (PA 12) powder. For this sake, the same PA 12 powder was used in consecutive printing cycles. After each cycle, the remaining non-used powder was milled and filtered before subsequent use. Properties of the powder and corresponding prints were characterized in each cycle, using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), computed tomography (CT), and tensile tests. It was concluded that subjecting the same powder to multiple SLS printing cycles affects the properties of the printed parts essentially regarding their morphology (voids content), mechanical properties reproducibility, and aesthetical aspect. However, post-processing treatment of the powder enabled to maintain the mechanical performance of the prints during the first six printing cycles without the need to add virgin powder.

Keywords: Polyamide 12; Selective Laser Sintering; mechanical properties; milling and filtering; morphology; powder reuse.

Figure 1

Schematic view of the SLS equipment used.

Figure 2

Sequence and main stages involved in the methodology used. Note: ‘n’ is the number of processing/printing cycles and is used to name powder and printed parts.

Figure 3

Parts printed in each (re)processing cycle: (a) tensile test sample and (b) CT sample for porosity characterization (dimensions in mm).

Figure 4

Computed tomography analysis: (a) 3D view; (b) one of the three longitudinal cuts made in each sample, showing the four sections where porosity values were taken; and (c) one of the three transversal cuts made for sample shape checking.

Figure 5

DSC heat flow curves obtained for the virgin powder (0 cycles, P0 in the graph) and for reused powder collected after some of the 1 to 12 cycles (samples P1 to P12 in the graph).

Figure 6

Powder DSC results: (a) melting temperature and (b) melting enthalpy.

Figure 7

SE/SEM images for: (a) virgin powder and (b) powder after 12 printing cycles.

Figure 8

CT results: (a) porosity level for different printing cycles, where statistically homogenous groups are indicated by the same letters (a, b, and c); transversal cuts done through CT at 10 mm from the base of samples for different printing cycles: (b) 1; (c) 4; (d) 8 and (e) 12.

Figure 9

Commercial parts printed in parallel with the test samples used in this study for (a) printing cycle 1 and (b) printing cycle 12.

Figure 10

Representative stress–strain curves corresponding to the first printing cycle of tensile samples printed in horizontal (H) and vertical (V) orientations.

Figure 11

Tensile properties of samples printed in horizontal (H) orientation: (a) Young’s modulus, (b) yield strength and (c) tensile strength.