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Space Systems Academic Group

Rapid Prototyping / Additive Manufacturing (3D Printer).

The NPS Space Systems Academic Group procured in 2008 a Fortus 400mc (link to Fortus web site where product and materials information can be found) rapid prototyping machine through school funds.  The machine employs fused deposition modeling (FDM) for additive manufacturing of three-dimensional parts from computer-aided design (CAD) geometry.  This Wiki is intended to provide information for potential users of the machine.  The 3D printer is open to the NPS community on a cost-reimbursable basis for appropriate activities, e.g. instructional parts, research, and thesis projects.  Interested in producing parts?  Contact Dan Sakoda and provide the following information.

How does it work?

CAD models are imported in the stereo lithography (.STL) file format to the printer software.  After orienting the part for the build, the software then slices horizontal build planes and creates toolpaths for each slice of the part based on the configuration of the machine and user-selectable parameters.  Once print jobs are generated, another software application is used to arrange them on the build area and send them to the machine. In this way, time can be saved by printing a number of parts, as long as they can fit within the envelope of the machine and the build footprint.

The tool path is defined by contour lines that outline the part, and raster, or fill lines.  Parts can be made as completely solid, or to save on material and time as a sparse build, where the internal volume is similar to a honeycomb structure.  By default, each alternating layer is raster-filled in alternating directions for higher strength.  This figure shows the build parameters of a simple block part.

What Can Be Printed?

Parts which would be difficult (or impossible) to machine can be readily printed as a single part.  Some post-printing work may be necessary to remove support material.  Since the part is built in the vertical direction, one slice at a time, it may be necessary to have a sacrificial support structure on which to build.  Any cantilevered feature would need this, for example.  The following figures show the CAD model and the 3D part of a female head printed for MOVES as a part of their studies on tangible 3D virtual humans (Dr. Amela Sadagic);  an MAE Spacecraft Robotics platform (Dr. Marcello Romano) using a number of 3D-printed parts for the structure, and a Physics Robotics (Dick Harkins) project that utilizes a number of 3D parts for housings, and working parts such as the 'whegs' that propel the platform over terrain and obstacles.  Sometimes, a simple bracket is needed to hold an instrument or sensor, such as the figure below with an inertial measurement unit mounted by an RP bracket (1 hour, 11 minutes to build) on a pendulum for a thesis project (Xiaoping Yun, advisor).

  
  

 NPSAT1 Half-Scale Model Assembly.

An interesting innovative use of 3D-printed parts was done in building laser reflectors to do large-scale modal testing.  Here, the large bi-focal mirror in the basement of Halligan Hall is outfitted with a number of reflectors printed on from the Fortus 400mc.  Some post-printing work was done to yield a good reflective surface. 

  


STL File Requirements

If you're using one of the NPS-licensed CAD programs, it's very likely you can export your part geometry into the .STL file format.  Here are some settings for the more popular CAD programs.  It's best to have your model in English Inch units, but millimeters will work, also.  The basic settings for export deal with Angle Tolerance (how much the normals of the surface triangle can deviate from one another), and Deviation (how much the mesh is allowed to deviate from the CAD part).

SolidWorks:

  • File > Save As
  • STL > Options
  • Change the resolution to Custom
  • Change the deviation to 0.0005in (0.004 mm)
  • Change the angle to 4.75

NX I-DEAS

  • File > Export > Rapid Prototype File > OK
  • Select the part to be prototyped
  • Select prototype device > SLA500.dat > OK
  • Set absolute facet deviation to 0.00039
  • Select Binary > OK

NX 7.5 (and later)

  • File > Export > STL...
  • Output Type:  (Binary)
    • Triangular Tolerance:  0.0003 (inch)
    • Adjacency Tolerance:  0.0003 (inch)
    • Auto Normal Gen:  ON
    • Normal Display:  OFF
    • Triangle Display:  OFF
    • OK
  • Save to File:  (navigate to folder and give file name)
  • File Header Info.:  (leave blank)
  • Select Object(s):  
    • Pick part to export
    • OK
  • 2. Discontinue
  • 2. No

It's always a good idea to try importing your STL file to see that it is what you expect.  If your CAD program is a solid modeler, it's good to check that the volume of the CAD part matches the volume of the imported STL file.

Stratasys "Best Practices" PDF document describing some other CAD programs and how to export STL files can be found here.

What Does It Cost?

Approximate cost (part material alone) is about $4.50 per cubic inch.  Canisters of material and support come in 92 cubic inch canisters and cost between about $400 and $450 each, depending on the type of material.  Note that some waste is involved with calibration parts and in the normal operation of the machine.

Does it outgas in a vacuum?

A concern for any material in space or for research is whether they outgas -- will they be a source of contamination.  NASA has performed testing to determine the amount of outgassing of various (read many, many) materials in a vacuum.  The numbers provided are in TML (percent of total mass loss), CVCM (percent of collected volatile condensable materials) and WVR (percent of water vapor regained).  For more information on the NASA outgassing tests, see http://outgassing.nasa.gov.  The Stratasys polycarbonate (PC) material was tested and results are favorable for use in space, of course, verification should be done to ensure the specific application is consistent with the NASA test results.  Below is the output from the NASA on-line outgassing report (when doing a search on "Stratasys"):

STRATASYS POLYCARBONATE PC10 - RAPID PROTOTYPE MATERIAL
% TML:  0.17
% CVCM: 0.00
% WVR:  0.14
STRATASYS POLYCARBONATE PC10 - SUPPORT MATERIAL
% TML:  0.10
% CVCM: 0.00
% WVR:  0.07
STRATASYS POLYCARBONATE PC10 MODEL MATERIAL
% TML:  0.14
% CVCM: 0.00
% WVR:  0.12

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