Processes - Additive Manufacturing
Selective Laser Sintering (SLS)
Selective Laser Sintering (SLS) was developed at the University of Texas in Austin, by Carl Deckard and colleagues. The technology was patented in 1989 and was originally sold by DTM Corporation. DTM was acquired by 3D Systems in 2001. The basic concept of SLS is similar to that of SLA. It uses a moving laser beam to trace and selectively sinter powdered polymer and/or metal composite materials into successive cross-sections of a three-dimensional part. As in all rapid prototyping processes, the parts are built upon a platform that adjusts in height equal to the thickness of the layer being built.
Additional powder is deposited on top of each solidified layer and sintered. This powder is rolled onto the platform from a bin before building the layer. The powder is maintained at an elevated temperature so that it fuses easily upon exposure to the laser. Unlike SLA, special support structures are not required because the excess powder in each layer acts as a support to the part being built. With the metal composite material, the SLS process solidifies a polymer binder material around steel powder (100 micron diameter) one slice at a time, forming the part.
The part is then placed in a furnace, at temperatures in excess of 900 °C, where the polymer binder is burned off and the part is infiltrated with bronze to improve its density. The burn-off and infiltration procedures typically take about one day, after which secondary machining and finishing is performed. Recent improvements in accuracy and resolution, and reduction in stair-stepping, have minimized the need for secondary machining and finishing. SLS allows for a wide range of materials, including nylon, glass-filled nylon, SOMOS (rubber-like), Truform (investment casting), and the previously discussed metal composite.
Advantages
Broad Material Compatibility
SLS supports a wide range of material options, including various polymers, ceramics, and even glass powders. This versatility makes it a widely adopted 3D printing method across multiple industries.
No Need for Support Structures
Unlike many other additive manufacturing processes, SLS does not require dedicated support materials. The unsintered powder surrounding the printed part provides natural support during printing. This significantly reduces post-processing time, minimizes material waste, and enables the creation of complex geometries that would otherwise be difficult or labor-intensive to produce.
High Productivity
SLS is among the fastest additive manufacturing technologies due to the rapid scanning capability of the laser and the short exposure time needed to fuse each layer. Moreover, parts can be densely nested within the build chamber with minimal spacing, maximizing build volume and throughput for batch production.
Excellent Mechanical Properties
The SLS process creates strong interlayer bonding, resulting in parts with near-isotropic mechanical properties. This means the tensile strength, hardness, and elongation at break are relatively uniform across the X, Y, and Z axes, making SLS parts suitable for functional testing and end-use applications.
Disadvantages
Surface Finish and Porosity
SLS parts typically have a grainy, slightly rough surface finish due to the powdered feedstock. Additionally, they are porous, which makes them unsuitable for applications requiring watertightness or high impact resistance without further post-processing.
Extended Build Time
Although printing itself is relatively fast, the overall production cycle can be longer due to necessary preheating and controlled cooling phases. However, modern SLS systems often feature removable build platforms, allowing these thermal cycles to occur offline and thereby improving overall machine utilization.
Post-Processing and Cleaning
Post-processing involves a meticulous cleaning stage in which parts must be carefully extracted from the surrounding powder bed—a process known as unpacking—and then cleaned with compressed air. This step can be time-consuming and requires care to avoid damaging intricate features.
Capabilities
Disclaimer: All process specifications reflect the approximate range of a process's capabilities and should be viewed only as a guide. Actual capabilities are dependent upon the manufacturer, equipment, material, and part requirements.