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Laser Sintering

A Brief Tutorial
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Laser Sintering was originally developed at the University of Texas. While the equipment generally remains very expensive and large, the ability to produce parts in a range of real engineering plastics and metals has enabled it to compete with less expensive technologies. It's often the method of choice for additively manufactured parts with critical material properties in fields such as aerospace and medicine.

SLS Schematic The same basic procedure is used to make either plastic or metal parts. The powder delivery system uses a piston in a cylinder. The piston moves upward incrementally to supply a measured quantity of powder for each layer. Metal or thermoplastic powder is then spread by a roller over the surface of the build cylinder. The piston in the build cylinder is moved down one object layer thickness to accommodate the new layer of powder.

A laser beam is then traced over the surface of this tightly compacted powder in the build cylinder to selectively sinter and bond it to form a layer of the object. The entire 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 cycle is repeated until the entire part is fabricated.

After the part 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. That's not the complete story, though. It may take a considerable length of cool-down time before the part can be removed from the machine. Large parts with thin sections may require as much as two days of cooling time.

Metals are handled in a similar way. They can be directly sintered using a much higher power laser than that used with plastics. Alternatively, the metal powder particles can be coated with a thin layer of plastic which enables the use of a lower power laser to bond them into a so-called green part. A green part is very fragile but it can be made into a high strength part by carefully heating it to a high temperature in an oven and infiltrating it with a second metal, often bronze in the case of steel parts.

Laser sintering 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.

Since the objects are sintered they are porous. As mentioned, it may be necessary to infiltrate the part, especially metals, with another material to improve mechanical characteristics. Variations on the process have become available over the years from several vendors which make that unnecessary, however. Selective laser melting (SLM) uses exactly the same procedure but higher-powered lasers to make fully-dense parts. Electron beam melting (EBM) from Arcam AB (Sweden) replaces the laser with a scanned electron beam and also creates fully-dense parts in a variety of alloys. The technology has found success in medical applications such as implants.

A word about terminology: The term Selective Laser Sintering is claimed by 3D Systems as a tradename, although it and the acronym SLS are quite often used generically. Laser sintering is the preferred generic term.

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Stereolithography (SLA)
  & Photopolymer-based Systems.
Fused Deposition Modeling (FDM).
  & Thermoplastic Extrusion Systems.
Inkjet-based Systems.
Three Dimensional Printing (3DP).
(Selective) Laser Sintering (SLS/LS).
Laminated Object Manufacturing (LOM).
Laser Powder Forming (LPF).

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REV 5 - - - 8/17/12