Selective laser sintering (SLS)

Selective Laser Sintering is a layer manufacturing process that enables the manufacturing of complex 3D parts through the successive addition of powder layers.
Selective laser sintering/melting is a powder- and laser-based additive manufacturing technology that can process many types of materials such as polymer, metal, ceramic, and composite. Its industrial applications include the production of components for the automotive, aerospace, tooling, biomedical, and architecture
SLS is a type of Powder Bed Fusion (PBF) technology wherein a bed of powder polymer, resin or metal is partially (sintering) or fully (melting) targeted by a powerful directional heating source such as a laser that results in a solidified layer of fused powder.
Such a process requires the use of a high-energy laser beam to fuse particle granules directly into complex, net-shaped 3D components. Parts are produced in a layer-by-layer way by repeating a scanning of the laser beam over a single layer of the powder granules, thereby consolidating them via full or partial melting. CAD and CAM software establish the process scheme. These manufacturing conditions lead to advantages such as high geometrical design freedom, high flexibility, and near net shape production. Those advantages have also enabled specialists of the field to develop the technology further.
SLS is often seen as the sister of metal SLM technologies. Like SLM technologies, the plastic printed part is produced layer-by-layer with SLS technology.
However, both technologies do not require the same sintering temperature. A polyamide material (e.g., Nylon, PA12) for instance, must be sintered at 160–200°C, using a high-wattage laser whereas metal laser 3D printing technologies withstand a much higher temperature.

Basic selective laser sintering

During the manufacturing process, the SLS 3D printer preheats the bulk powder material in the powder bed below its melting point, allowing the laser to raise the temperature of the selected regions to the melting point. A roller applies a layer of fresh polymer powder. Once the sintered powder is obtained, the build plate lowers before applying a new layer of powder on the printing area. The process is repeated until the desired part is created.

Once the printing process is over, the powder bed and the manufactured parts (often referred to as a «partial cake») must cool down before being removed from the build chamber. Depending on the type of polymer used, it is possible to reuse the non-sintered powder. The manufactured parts are ready to use or can be refined according to the requirements.

Several process parameters of a 3D printer can be modified. They include laser parameters (e. g. laser power and scanning speed) and construction parameters (e. g. layer thickness). Settings that are usually modified, are described as below:

  • Laser power – Applied power of the laser as it scans the surface of each layer
  • Scanning speed – The speed at which the laser beam moves when it passes through a scanning vector
  • Scanning spacing – Separation between two consecutive laser beams.
  • Number of scans – Number of times the laser beam passes through a scanning vector per layer
  • Laser scanning strategy – Any scan pattern or exposure method that is used to influence a dependent variable during the SLS process
  • Layer thickness – A measure of the layer height of each successive addition of material in the additive manufacturing or 3D printing process in which layers are stacked.
  • Build plate temperature – Heated chamber or bed temperature during the printing process.

Operators can produce several parts at the same time during the printing process. In this case, each powder layer can contain cross-sectional layers of several parts.