Why Don’t We See More Polymers in SLS?
One of the most popular AM methods is Selective Laser Sintering (SLS) due to the complexed geometries, isotropic properties and detailed parts that are achievable in this process. That said, there are not too many thermoplastic powders commercially available. The most popular ones are based on Polyamide 12 and Polyamide 11, both can be filled or unfilled grades. Lately, PAEK grades (PEK & PEKK) are also available for more demanding applications.
The SLS process consists of melting or fusing thermoplastic powder to form a uniform bulk material by using a laser beam. Although printing time is relatively long, the time under direct laser is short. To receive proper coalescence of the particles and satisfying mechanical properties of the part, several properties of the powder must be fulfilled, Both intrinsic (e.g. thermal, optical and rheological) and extrinsic properties (e.g. particle shape and size) should be addressed.
The scheme below demonstrates the complexity in developing a thermoplastic powder for the SLS process.
Let’s have a look on the properties that were discussed also in the FDM articles, so we can understand why PA11, PA12 and potentially PEKK are dominant in this process.
These polymers are Semi-Crystalline (SC) but have slow crystallization rates. The slow crystallization rates (Lower Tc) enable better interparticle molecular diffusion, pretty similar to the FDM process. The difference between the melting temperature (Tm) and the crystallization temperature (Tc) is called the sintering window also known as the Super-cooling state.
The DSC (Differential Scanning Calorimeter) is an analytical method to detect transition temperatures. It is demonstrated in the DSC curve (Figure 2) that SLS grade PA12 has a wider sintering window (Turquoise colour) than the injection molding grade window (Orange colour). A similar trend can be found while comparing PEEK and PEKK.
What about amorphous materials?
Since amorphous polymers don’t crystallize, they would be perfect for SLS, right? Amorphous materials are less popular in SLS due to two main reasons. The first is viscosity related. The SLS process requires relatively low viscosity to enable the molecular diffusion and the particles coalescence. Amorphous polymers typically have high viscosity, which inhibits proper coalescence and usually ends with brittle and instable parts. The second reason is related to the fact that the crystalline regions in SC polymers restrict the flow of adjacent molecules during reheating. Whereas, with amorphous polymers, the material flows above Tg and can cause defects that reduce part details resolution.
So, tailoring thermal properties is challenging but as the chart above implies, there are many more properties to fulfill, such as the powder extrinsic properties.
Other critical parameters are the particles’ geometry (shape), size and particle size distribution (PSD). These parameters affect on powder flow properties which affect layer thickness and deposition rate.
The powder particles can be round similar to the PA2200 (Figure 3) or have more sharp edges and rough surface like the HP3 PEK (Figure 4). Both EOS materials, are benchmark for SLS powders but there are also new PEKK powders developed by ARKEMA and SOLVAY. The HP3 was reported to give smooth surface finish despite the more “edgy” morphology. The manufacturing process of the powder significantly affects the powder’s morphology.
Many of the required parameters for a high-quality powder for SLS were not discussed, but this article might partially answer the question “why can’t we see more polymers?” for this truly amazing AM process.