Selective laser sintering (SLS) is an additive manufacturing technique that uses a laser to selectively sinter powdered material (typically nylon or polyamide) to create a solid 3-dimensional structure. SLS has the potential to produce serial parts with high quality and high mechanical properties. Although the technique has high potential and has already been adopted by many companies for producing final parts, there remain some challenges facing this approach.  

One of these challenges is cost. The main contributor to the high cost of building serial parts using SLS is the high cost of polyamide 12. This is compounded by the fact that less than 30 % of the powder employed in the process is used to build the part. Coupling this with the high cost of polyamide/nylon 12 (PA12) drives the need to reuse the material multiple times. On paper this makes sense but in practice it is not always that straightforward as the material is prone to aging during the build process which can limit its reuse. 

Material aging takes place when the powder is exposed to temperatures near the melting point of the polymer, changing the materials thermal and physical properties. To minimize these effects and to compensate for the material used in prior builds, the reused powder (part cake) is blended with virgin powder; however, achieving build consistency can be difficult if the properties of the powder blend vary between builds. In some cases, the aging process can prove beneficial, such as sintered parts having increased mechanical properties, but this is still not well understood.  

To better control or to deal with aging, the mechanism responsible for the material changing must be firstly understood, and this has been the focus of Prof. Katrin Wudy’s research during her time at Friedrich-Alexander-Universität (FAU) Erlangen-Nürnberg in Germany. Prof. Wudy was recently appointed as Professor for Laser-based Additive Manufacturing at the Technical University of Munich (TUM).  

Prof. Wudy has used a Malvern Panalytical multi-detection Gel Permeation Chromatography (GPC) system to investigate the influence of processing time and temperature on molecular changes and thermal properties of polyamide 12 in selective laser sintering. Multi-detection gel permeation chromatography (GPC) combines refractive index (RI), right and low angle light scattering detectors (RALS and LALS) and a differential viscometer, to give detailed information absolute molecular weight and molecular structure. This has proved crucial to better understand the aging of SLS PA12.

Average molecular weight and dispersity for different cumulative build times and SLS systems in Prof Wudy’s work.

Prof. Wudy was able to illustrate the major influence of cumulative build time on the aging in terms of molecular weight increase but also using it to produce Mark-Houwink plots and show the growth was linear, not crosslinking or branching. This work is summarised in our latest application note: Using triple-detection GPC to study the aging of polyamide/nylon 12 in Selective Laser Sintering.  

Detailed information on the measurement methodology, on the powder preparation and on the building process layout as well as further results can be found in [Wudy, K., & Drummer, D. (2019). Aging effects of polyamide 12 in selective laser sintering: Molecular weight distribution and thermal properties. Additive Manufacturing, 25, 1-9].  

For more information about how multi-detector GPC can be used to characterize polymers for additive manufacturing, you can also watch my recent webinar on the subject or download the application note on optimizing the macromolecular properties of natural polymers for 3D-bioprinting