Investigating The Re-Usability Characteristics And Limits Of Polypropylene Powder In Laser Sinter Additive Manufacturing

Abstract

The aim of this research was to investigate the re-usability characteristics and limits of polypropylene (PP) powder in laser sinter additive manufacturing (AM). Initially, the objective was to analyse Laser PP CP 60 from Diamond Plastics GmbH, and preliminary work was carried out on it, but the focus eventually turned to Laser PP CP 75 after processing difficulties were experienced with the former material. Polymer laser sintering (LS) subjects feed powders to high temperatures, leading to degradation of their thermal, rheological, and physical properties, impeding their recyclability. Therefore, it is imperative to examine the degree of deterioration or aging of polymers before re-using the materials. The recyclability of polymers is measured based on powder particle size, morphology, density, rheological properties, and thermal characteristics. Attempts were made to determine suitable processing parameters of Laser PP CP 60 polypropylene powder grade from Diamond Plastics in the first part of the analysis. This was followed by experiments to characterize the powder after a single print cycle to quantify degradation at this stage using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and melt flow index testing (MFI). The focus was then turned to the recyclability of Laser PP CP 75 per print cycle by printing a set of test coupons at pre-determined positions in the build volume of an EOSINT P 380 LS machine. The powder remaining in the machine and the cake powder surrounding the coupons after each print cycle was thoroughly mixed using a concrete mixer for about 30 minutes. A sample of the mixed powder was re-introduced into the P 380 machine to print another set of test specimens. In this study, the re-usability of the PP powder was determined by characterizing and comparing the used powder after each cycle with powder from previous cycles and fresh material. It was established from the preliminary testing of Laser PP CP 60 that slightly different process parameter settings were required to those provided by the supplier to achieve the highest ultimate tensile strength, smoothest surface, and best dimensional accuracy of printed parts. The three characterized batches of Laser PP CP 60 powder (virgin, aged, virgin-aged mixture) exhibited poor (not 100% spherical) but acceptable morphology and particle size distribution. It was also found that the sintering window of virgin Laser PP CP 60 increased significantly (by 28%) after a single cycle of printing, from a value of 21.04 ℃ for the virgin powder to 26.95 ℃ for the recycled powder. The degree of crystallinity of virgin Laser PP CP 60 decreased from 13.92% to 12.12% after a single printing cycle and then increased to 12.48% after the addition of 50% virgin material. All the three batches, namely virgin powder, used powder, and a 50% virgin:50% used mixture, showed high degradation temperatures of 457.30 ℃, 456.05 ℃, and 455.95 ℃, respectively. Lastly, the three batches of powder showed low MFI values of 6.1, 6.5, and 6.4 g/10 min in the same order. Turning to Laser PP CP 75, it was concluded that the powder could be re-used for four re-use cycles without having to mix with virgin material because it does not form an “orange peel,” as is the case with polyamide-12 (PA 12) (used here as reference material). Besides, the MFI trend indicates that the viscosity of the material is not significantly degraded, which promotes recyclability. The DSC assessment established that the sintering window of Laser PP CP 75 increased with each re-use cycle. The results indicate that the shrinkage rate and curling of the material might reduce with the number of re-use cycles because a wide and sufficient sintering window prevents crystallization of the polymers during processing. From the TGA test, the breakdown temperature of Laser PP CP 75 was seen to increase slightly with the number of re-use cycles, from 455.53 ℃ (virgin material) to 457.53 ℃ after the 4th re-use cycle. Thus, the material does not break down at the temperatures prevailing during printing, making it suitable for LS processing and re-use. The SEM analysis revealed that the average size of Laser PP CP 75 powder particles is not significantly influenced by re-use cycles, making the powder recyclable. Furthermore, the powder did not exhibit signs of agglomeration for the 1st, 2nd, and 3rd re-use cycles. However, this phenomenon was observed in the 4th re-use cycle of printing. Lastly, tensile testing revealed that the material had the highest ultimate tensile strength after the 3rd printing cycle (7.4 MPa), after which the value decreased with recycling. Overall, it can be concluded that PP powder has superior recyclability properties to those of PA 12, which requires mixing with virgin material after every print cycle. Besides, the MFI, particle size and thermal properties are insignificantly altered, which does not hamper the re-use of the material for subsequent printing cycles. Parts printed with virgin Laser PP CP 75 had an average dimensional error of 3.02% and 4.06% after the 4th re-use cycle. Therefore, Laser PP CP 75 might not be commercially suitable because of dimensional accuracy and observed processing difficulties.