3D Printing - SLS

Table of Contents

Selective Laser Sintering (SLS)

In an SLS printer, a layer's height of powder is spread on a heated bed by a roller. This bed keeps the powder just below its melting point, and a laser scans this layer to fuse the powder particles together. Once this layer is fused, the build platform moves down a layer's height and this process repeats until the model is complete. With this type of printing, the powder that surrounds the part supports it as it gets built, which eliminates the need for support structures [1].

SLS printed parts are high quality and have competitive functionality when compared to products manufactured by more traditional methods. These parts are durable and strong, and can be used for a variety of engineering applications such as functional prototyping and fixtures. They can be used as end-use products and printing times are much faster than SLA printing as the laser scanning process is quicker with powder [2]. Additionally, this technology can be used to create parts that impossible to machine traditionally or without splitting into pieces. Even with the capabilities of this print technology, there are still limitations that affect the quality of the piece. 

Designing parts for laser sintering is similar to injection molding with a few differences. Undercuts, negative draft, and interior features are no problem with LS technology. A minimum wall thickness of 1mm is usually necessary and holes in thicker walls will be smaller because of "hoop shrink", which can be minimized with a wall thickness of less than 3mm. The LS process naturally adds a small radius fillet (0.4 mm) to corners so break edges are not necessary to add in unless additional stress relief is required. Furthermore, LS can create 90 degree interior corners but it is recommended to have a stress relief fillet with a radius of 0.4 mm. 

Factors affecting print quality:

• Shrinkage and Warping

Since SLS materials are processed at a high temperature, it is important to control the cooling of the part to ensure shrinkage and warping does not occur. Large surfaces that are not supported by support structures are also at risk of warping [3].

• Tolerances and Part Features [4]

  • Dimensional tolerances for SLS printed parts are +/- 0.05mm/mm
  • Walls should maintain a minimum thickness of 0.7 to 2mm depending on the material
  • Features should maintain a minimum size of 0.8mm to prevent them from detaching
  • Holes should maintain a minimum diameter of 1.5mm
  • Hollow parts need escape holes, similar to SLA printing, for unfused powder to be removed

• Surface Finish and Post Processing [4]

  • The surface finish of SLS printed parts is high quality due to small layer heights however may appear grainy due to the use of powder
  • To improve this graininess, parts can be tumbled in media tumblers or vibro machines. These machines contain ceramic chips that gently abrade the part to create an even and smooth surface finish
  • Parts can be easily dyed due to their porosity
  • Painting, lacquering, and coating
  • Further reading on post processing for SLS printed parts [4]

• Materials [1]

The primary materials used in SLS printing are Nylon-based materials. There are various types used, as well as composites that combine other materials such as carbon and aluminum. Popular choices include:

• • Polyamide (PA) 12 ( Nylon 12)
  • PA 11
  • Aluminum-filled Nylon (Alumide)
  • Glass-filled Nylon (PA-GF)
  • Carbon-fiber filled nylon (PA-FR)
  • SLS Materials and their properties and applications

Direct Metal Laser Sintering (DMLS) and Selective Laser Melting (SLM)

DMLS and SLM are very similar to SLS except instead of the printing with polymers, these processes fuse powdered metal. DMLS fuses powdered metal alloys, and SLM is better suited for pure metals. DMLS also only fuses the exterior of the powder with other particles rather than fusing whole particles together. SLM and SLS completely fuse powder particles together which significantly increases print time and energy [5].

DMLS and SLM still have a much quicker turn around time than traditional machining because it does not require any tooling and can create high complexity parts. There are many different metals that can be used in this process including and not limited to: Aluminum, Stainless Steel, Titanium, and Copper. The parts produced using these materials have good strength and can be used in production. However, because of the powder usage, the finish produced is not smooth like a machined part. Polishing can be done but it is time consuming and makes it difficult to maintain tolerancing.

References

[1] A. B. Varotsis, “Introduction to SLS 3D printing,” 3D Hubs. [Online]. Available: https://www.3dhubs.com/knowledge-base/introduction-sls-3d-printing/

[2] “Guide to Selective Laser Sintering (SLS) 3D Printing,” Formlabs. [Online]. Available: https://formlabs.com/blog/what-is-selective-laser-sintering/

[3] S. Oceanz, “How to design parts for SLS 3D printing,” 3D Hubs. [Online]. Available: https://www.3dhubs.com/knowledge-base/how-design-parts-sls-3d-printing/

[4] B. Redwood, “Post processing for SLS printed parts,” 3D Hubs. [Online]. Available: https://www.3dhubs.com/knowledge-base/post-processing-sls-printed-parts/

[5] G. Jones, “Direct Metal Laser Sintering (DMLS) – Simply Explained,” All3DP, 13-Apr-2019. [Online]. Available: https://all3dp.com/2/direct-metal-laser-sintering-dmls-simply-explained/