Selective Laser Sintering (SLS) is an Additive Manufacturing (AM) process that uses laser power to sinter powdered material, typically plastics, into a solid 3D model. Sintering—using atomic diffusion to create objects from powdered material—has been around for thousands of years. However, it was first developed as an AM technique in the 1980s at the University of Texas at Austin and has been used as a foundation to develop similar processes for metals, glass, ceramics, and some composites. Although desktop and hobby 3D printers are widely available, there are very few SLS versions due to the complexity of the process.

The main benefit of SLS over other AM processes (such as Stereolithography (SLA), Fused Deposition Modelling (FDM), or Fused Filament Fabrication (FFF)) is the unfused powder acting as a self- supporting material, eliminating the need for including support structures in the model. In addition to reduced print times, this results in the creation of intricate and complex geometries, including interlocking parts, moving parts, and complex lattice structures that maximize the part’s strength in desired locations, while also minimizing weight. SLS is a popular technique in the aerospace and medical device industries due to its ability to quickly produce low quantities at mass production accuracy levels, as well as the variety of materials that can be utilized.

The theory behind SLS is similar to other AM processes, in that source material is heated and built up layer by layer to complete a 3D structure imported as a STL file from Computer-Aided Design (CAD) software. The laser pulses on to the build platform, mapping a cross-section of the model on the loose powder, heating the powder to slightly below the melting point which fuses the particles together. While the layer height is dependent on the machine and the required accuracy, it is generally in the region of 100 microns. Once all the layers have been constructed, the object is left to cool before being removed for post-processing.

In addition to minimizing material wastage, post-processing can be less involved than other AM processes due to the absence of solid support structures. Unlike many other AM processes generally used for prototyping, SLS parts can be used in production, which would be the reason for the majority of post-processing. Due to the nature of the process, SLS parts have a grainy surface finish. Polishing can be included in post-processing for a smoother surface finish, including a coating with a watertight material to counteract the porosity of the finished part. Depending on the intended use of the part, metal plating can also be added. In comparison to other AM techniques, the grainy surface finish and internal porosity would be considered as negative aspects, but the main downside to SLS printing is its susceptibility to warping, which affects the accuracy of large flat surfaces and small holes.

Selective Laser Sintering is a relatively new technique that has allowed the manufacturing world the opportunity to reconsider the design process, design opportunity costs, and how parts and assemblies are produced, even within the sphere of AM.

About the author:
Chris Brennan is a Manufacturing Engineer and the founder/owner of Thirteen Design Consultancy based out of County Louth, Ireland.

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