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 Powder-based RP methods are showing a lot of promise as future parts production technologies for custom or tool-less manufacture. Selective laser sintering (SLS) and three dimensional printing (3DP) continue to mature and evolve, but variations on these themes are chipping away at their limitations. One of the more interesting recent developments is Selective Inhibition Sintering (SIS) which is being worked on by a group headed by Behrokh Khoshnevis at the University of Southern California. Parts made by SIS are surprisingly pretty considering that the technology is still at an early stage of development.
SIS has similarities to both selective laser sintering and three dimensional printing. A layer of powder is formed in a build cylinder as with those methods (A), but instead of inkjet printing a binder onto the layer as in 3DP, a material which selectively inhibits sintering is deposited by inkjet (B). The entire layer is then sintered with a blast of thermal radiation from an infrared heater (D). A radiation-minimizing frame (C) is positioned before sintering to prevent areas of the powder layer which lie outside the part envelope from sintering. After all the layers are fabricated, the final part can be removed from the powder bed (E) as in SLS or 3DP (F). The result is a fast cycle time and the potential for low machine cost since there are no expensive lasers or environmental control systems involved.
  There are also some potential advantages that aren’t quite as obvious: Since much less liquid material is required to inhibit sintering than to bond particles as is the case with 3DP, there is the potential for very high resolution if fine powders are used. Since no binders are necessary for metals or ceramics as in some forms of SLS, shrinkage is potentially greatly reduced for those materials. Resolution and definition of parts is potentially better than with competing methods that use an electrostatically-generated infrared mask as does similar technology being developed in Sweden by Speed Part AB. That’s because the inhibiting material is deposited directly on the powder layer whereas a mask must always be spaced at minimum a short distance from it which introduces at least some diffusion. The initial parts shown here are all made from polystyrene powder and some of them have been colored by inkjetting inhibitor containing dyes. The alpha test bed is said to be mechanically inaccurate, accounting for the rough finishes, and mainly designed to study the inhibition process and material parameters. Work is proceeding to further study the chemistry and to improve the radiation-minimizing frame technology. Bulk sintering is also being investigated which has the potential to simplify machine architecture quite a bit, and might be a good solution for small production run applications.
 
Recent patent applications show that very similar work is going on at the University of Texas and MIT is producing fine parts using 3DP from slurries in substantial numbers. Two companies are said to be close to licensing SIS technology, so one way or another, powder-based methods will almost certainly be a vital part of RP’s future.For more info contact:
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