Selective Laser Sintering

• What it is
• Process
• Materials
• Advantages and Disadvantage
• Cost
• Size and Shape Limitations of Parts
  
  

Selective laser sintering is a process that employs a powdered material approach to rapid prototyping. It was initially created and patented in 1989 at the University of Texas, and commercialized by DTM Corporation through company operated Service Bureaus. "SLS technology is a free form fabrication process that allows for the creation of engineered components by the precise thermal fusing of powdered materials. Complex part geometries are built in successive layers that define cross sections of the component features" (Mr. Kent L. Nutt, "The SLSTM Selective Laser Sintering Process," DTM Corporation USA).

Process:

The SLS process begins with the atmosphere preparation in the process chamber, which is heated to the operating temperature and filled with nitrogen. The 3-D CAD data in the .STL file format is input into the SLS system. The system slices the part into its cross-sectional data. Typical slice thickness is 0.005 to 0.006 in. One powder feed piston rises to distribute a layer of material. At the same time, the part building cylinder lowers to the desired layer thickness. The other powder feed piston also lowers to accommodate any surplus material, which the leveling roller transfers across the build area. The deposited powder is heated to a temperature just below its melting point. Using a raster scanning pattern, the laser draws one cross section of the desired part to sinter the powder particles. Unsintered powder remains to support the next layer, which is then distributed, leveled, and sintered. This process continues until the part is complete. The part building cylinder then raises to allow the part to be removed for cooling. Excess powder is cleaned off the part by brushing or air blowing. With the exception of the use of ceramic materials, no post- production curing is required with this system.

Materials:

• Poly carbonate is a hard plastic that can be run on machines with only a 10-20 W laser. It is good for visualizing parts and making working prototypes that do not carry heavy loads.
• Nylon is a more flexible plastic than poly carbonate. It also has low power requirements and parts made with it can have fine features since there is little shrinkage while the part is in the powder bed. It is good for prototyping because it is very tough.
• Ceramic is a polymer coated silicon carbide (SiC) powder that provides exceptional dimensional accuracy because only 20% (by volume) of the powder is sintered. The part (once created with an SLS machine) is then fired in a kiln and only the ceramic part survives.
• Calcium hydroxyapatite is a material very similar to what makes up your bones. It is being investigated as a possible material for medical applications.
• Direct metal powders are just that: powders of metals. These require a powerful laser to sinter (50-100 W!) Hopefully, these will be developed so that working prototypes can be made directly using SLS, rather than the traditional technique of making a wax mold and casting the part. Current research is focusing on how to densify parts made from metal powders. Right now, we are working on Hot Isostatic Pressing (HIP).

Advantages and Disadvantages:

Advantages:
• Ability to provide support supported building. The unsintered powder surrounding the part in the build cylinder acts as a natural support for the next layer. No elaborate supports need to be built such as in some photo polymer systems.
• Excess powder material can be returned to the powder feed cartidges for reuse.
• Different materials can be sintered for different applications using this process.
• Accuracy as good as + / - 0.05 inches across the part cylinder in the X, Y, and Z coordinate.
• Process can be cheaper and faster because no mold is needed and part can be produced directly from a CAD drawing.
• With the exception of ceramic materials, there is no post production curing needed.
Disadvantages:
• Part needs to be sent away to be manufactured.
• Large material quantity is required.
• High initial capital cost relative to some alternative molding processes.
• Size limitation based on size of chamber of machine.
• High temperature leads to high energy costs.
• 20-40% shrinkage in ceramics.