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Thermoplastic powder is spread by a roller over the surface of a build cylinder. The piston in the cylinder moves down one
object layer thickness to accommodate the new layer of powder. The powder delivery system is similar in function to the
build cylinder. Here, a piston moves upward incrementally to supply a measured quantity of powder for each layer.
A laser beam is then traced over the surface of this tightly compacted powder to selectively melt and bond it to form a
layer of the object. The fabrication chamber is maintained at a temperature just below the melting point of the powder
so that heat from the laser need only elevate the temperature slightly to cause sintering. This greatly speeds up the
process. The process is repeated until the entire object is fabricated.
After the object is fully formed, the piston is raised to elevate it. Excess powder is simply brushed away and final
manual finishing may be carried out. No supports are required with this method since overhangs and undercuts are supported
by the solid powder bed.
SLS offers the key advantage of making functional parts in essentially final materials. However, the system is mechanically
more complex than stereolithography and most other technologies. A variety of thermoplastic materials such as nylon,
glass filled nylon, and polystyrene are available. Surface finishes and accuracy are not quite as good as with
stereolithography, but material properties can be quite close to those of the intrinsic materials. The method has also
been extended to provide direct fabrication of metal and ceramic objects and tools.
Since the objects are sintered they are porous. It may be necessary to infiltrate the part, especially metals, with
another material to improve mechanical characteristics.
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