History
SLS (Selective Laser Sintering) is one of the earliest 3D printing technologies. Today, the most widely used powder bed fusion systems are plastic-based and are referred to as SLS.
Principle
The SLS 3D printer first heats the print chamber, which is filled with nitrogen, keeping the temperature below the melting point of the powder. Next, the container holding the powder is lowered, and a powder-coating roller spreads a thin layer of powder over the build platform. A laser is then directed according to the cross-sectional contour of the 3D model, sintering the powder at the laser’s focal point. This process forms a solid layer at each cross-section. Once the first layer is sintered, the build platform lowers by the height of one layer, and the roller spreads a new layer of powder. This cycle repeats until the printing is complete.
After printing, the model needs to cool in the build chamber before it can be removed for post-processing.
Materials
The most commonly used material for SLS is Nylon, a thermoplastic that is lightweight, durable, and flexible. Nylon’s resistance to shock, chemicals, heat, UV light, water, and dirt makes it an excellent choice for rapid prototyping and production.
The most common Nylon variants used in SLS are Nylon 11 and Nylon 12, also known as PA 11 and PA 12. PA 11 offers slightly better flexibility and impact resistance, while PA 12 is stronger, more abrasion-resistant, and biocompatible.
Characteristics of SLS Models
A typical SLS-printed model has a porosity of about 30%, resulting in a unique granular surface. This characteristic also means that SLS models can absorb water, allowing them to be dyed in various colors in hot water. However, special care is needed when using these models in wet environments.
SLS models are prone to shrinkage and warping. As the newly sintered layer cools, its size decreases and internal stress increases, which may cause the lower layers to shift upward. Shrinkage is usually around 3%-3.5%, so machine operators should consider this when preparing designs and adjust the model size accordingly.
Oversintering occurs when radiant heat fuses unsintered powder around a feature, potentially causing the loss of small features like slots and holes. The likelihood of oversintering depends on the size and wall thickness of the features. For example, a 0.5mm wide slot or a 1mm diameter hole can be successfully printed on a 2mm thick wall, but these features may disappear on walls thicker than 4mm. As a general guideline, slots larger than 0.8mm wide and holes larger than 2mm in diameter can be printed effectively in SLS.
Since SLS doesn’t require support structures, parts with hollow sections can be printed with ease and accuracy. These hollow sections also reduce both weight and cost.
Unsintered powder must be removed from inside the model through designated holes, so it is recommended to include at least two holes with a minimum diameter of 5mm when designing your model.
If high strength is required, the model must be printed as a solid part. Alternatively, a hollow design without holes can be used, leaving tightly packed powder inside the model. This can increase the model’s mass and provide additional mechanical resistance. An internal honeycomb structure, similar to fill patterns used in FDM, can also be incorporated to further enhance the strength and reduce warping.
Post-Processing
SLS-printed models have a granular surface that is prone to smudging. However, through various post-processing techniques such as polishing, staining, and painting, the appearance of SLS models can be greatly improved. Additional functionalities, such as waterproofing or metal plating, can also be applied to enhance the model’s durability and performance.
In addition to rapid prototyping, SLS 3D printing is suitable for small batch production. One of its key advantages is that it doesn’t require support structures, as the print powder itself supports the model during printing. However, it does have some drawbacks, including the need for a specialized lab environment due to its industrial-grade requirements, and the high maintenance costs associated with the equipment. Additionally, the printed models may exhibit graininess and visible laser marks on the surface.
Compared to FDM and DLP, SLS technology is not as widely used due to its higher costs, complex post-processing, and large printer sizes. Nevertheless, with the ongoing development of new materials and advancements in SLS technology, it is expected that SLS will become as popular as FDM and other technologies in the future.
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