Unlocking Powder Recycling in Polymer Additive Manufacturing

by Jenny Burt, Tuesday, 19th January 2021

The Future is Additive

Additive Manufacturing (AM) is currently revolutionizing the manufacturing world. Using a 3D model, materials are selectively connected, often layer by layer, to produce a part. This means these parts can be much more complex than those produced by traditional subtractive methods. Plus, the AM process usually generates less waste.

Below we show a typical Powder Bed Fusion (PBF) process in action. If you are familiar with AM, you have probably heard of Selective Laser Sintering (SLS), which is a type of PBF process. A thin layer of powder is formed, then a thermal source such as a laser is used to selectively fuse the powder particles together. This is repeated for each layer and the finished part is then extracted from the powder cake. The unused powder can then be recycled for the next part, usually by blending it with a certain amount of fresh powder. Two material types commonly used in PBF processes are metals and thermoplastic polymers, usually nylon 12.

Figure 1. Diagram showing a typical Powder Bed Fusion (PBF) process

Recycle It!

Did you know that in a typical SLS process using nylon 12 as much as 85-95% can end up not being sintered?^[1] The ability to recycle unused powder gives PBF processes the potential to be very resource-efficient. However, this efficiency depends strongly on the condition of the recycled powder, as any change risks a drop in final part quality.

The Malvern Panalytical AM toolkit enables us to investigate if and how our material is changing as we recycle it more and more times. Our latest application note, From Polymer to Powder: Investigating the Additive Manufacturing Recycling Process, discusses how the Morphologi 4 automated imaging system can be used to investigate particle size and shape differences of nylon 12 powders through recycling. With it, we can classify and compare new and recycled samples based on their shape, as shown below. Particle size and particle shape are critical as they affect powder flow and packing, which in turn will affect layer formation in the PBF process.

Figure 2. Classifying particles as elongated, potato-shaped or very irregular/agglomerates based on shape parameters

Probing Polymer Properties

What about the molecular characteristics of the nylon 12 material itself? We also discuss how size exclusion chromatography (SEC) using the OMNISEC system can be used to investigate changes to the polymer’s molecular weight and structure. Thanks to multi-detection SEC with RI, UV-Vis, RALS/LALS and viscometer detectors the OMNISEC is able to identify a very low concentration of high molecular weight species present in the recycled nylon 12, as shown below. Studies have shown that the presence of high molecular weight species impacts the quality of the final parts.^[2]

Figure 3. Overlay of the RI and RALS chromatograms for duplicate injections of the new nylon 12 (**red** and **purple**) and the recycled nylon 12 (**green** and **black**). The RALS chromatogram shows the presence of high molecular weight species in the recycled nylon 12 sample

The Morphologi 4 and OMNISEC analyses can help understanding how many times a powder can be recycled, as well as the optimum powder refresh rate (used/new powder ratio) to use. If you would like to find out more, read our Application Note here.

Want to learn more about the importance of powder quality and recycling in powder bed fusion AM processes? The upcoming webinar ‘Characterizing the particle size and shape of polymer powders for additive manufacturing’ is now open for registration.

References

L. Feng, Y. Wang and Q. Wei, PA12 Powder Recycled from SLS for FDM, Polymers, 2019, 11, 727.
S. Dadbakhsh, L. Verbelen, O. Verkinderen, D. Strobbe, P. Van Puyvelde, J-P. Kruth, Effect of PA12 powder reuse on coalescence behaviour and microstructure of SLS parts, European Polymers Journal, 2017, 92, 250-262.