Tech Briefs

Paper drawings and the associated delays in fabrication are eliminated.

Parts with complex three-dimensional shapes and with dimensions up to 8 by 8 by 10 in. (20.3 by 20.3 by 25.4 cm) can be made as unitary pieces of a room- temperature- curing polymer, with relatively little investment in time and money, by a process now in use at Johnson Space Center. The process is one of a growing number of processes and techniques that are known collectively as the art of rapid prototyping. The main advantages of this process over other rapid-prototyping processes are greater speed and lower cost: There is no need to make paper drawings and take them to a shop for fabrication, and thus no need for the attendant paperwork and organizational delays. Instead, molds for desired parts are made automatically on a machine that is guided by data from a computer-aided design (CAD) system and can reside in an engineering office.

Two-Piece Mold is used to make a complex three-dimensional part.
The process centers around the Actua 2100 (or equivalent) office-compatible rapid-prototyping machine. This machine is essentially a three dimensional printer that builds a part directly from a CAD data that specify a solid mathematical model, in the same manner as that of a rapid-prototyping machine of the stereolithographic or fused-deposition-modeling type. A CAD operator merely builds a plot file and submits it to the machine (this submission takes approximately one minute per part), then the machine builds the part. The time that it takes to build the part could be a few hours or as much as 30 hours, depending on the size of the part.

The machine builds parts with extremely fine detail but with two severe drawbacks. One of the drawbacks is that it makes parts of a wax that lacks toughness and strength. The other drawback is that any surfaces that are facing down with respect to the machine are covered with supports. These supports can easily be cleaned off by a light manual brushing, but the resulting surfaces are not smooth.

The present rapid-prototyping process overcomes these drawbacks. The steps of the process are the following:

  1. The CAD system is used to design the desired part.
  2. Taking advantage of a solid-modeling subtraction capability, the CAD system is used to design a mold that contains a cavity of the size and shape of the desired part.
  3. The CAD model of the mold is sliced into appropriate pieces to eliminate any downward-facing surfaces (to prevent the production of supports on surfaces of the molded part).
  4. Filling ports and vents are added to the CAD model to complete the mold design.
  5. The data from the CAD model of the mold pieces are submitted as a print job to the rapid-prototyping machine, then the machine builds the mold pieces.
  6. The mold pieces are taped together and filled with a room-temperature-curing polymer. The polymer used by the developer of this method is a durable polyurethane that becomes cured sufficiently for removal from the mold in about 1/2 hour.
  7. The mold is removed and, after removal of any minor flashing, the part is ready for use.

One advantage of using a wax as the mold material is that mold can be removed from the part by melting, if necessary (the melting temperature of the wax is less than that of the polyurethane). Of course, if the mold is melted and it is desired to produce more copies of the part, then more copies of the mold must be built from the CAD files.

This work was done by Scott A. Swan of Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at under the Manufacturing category. MSC-23035.

This Brief includes a Technical Support Package (TSP).

Relatively Inexpensive Rapid Prototyping of Small Parts (reference MSC-23035) is currently available for download from the TSP library.

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