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Articles > Rapid Prototypes

The term rapid prototyping (RP) refers to a class of technologies that can automatically construct physical models from Computer-Aided Design (CAD) data. At Ti Squared Technologies, our preferred method is"three dimensional printing" to allow direct casting of a titanium components from a prototype wax pattern.  This way we can quickly create prototypes of your design, for real world evaluation within a few short days. Prototypes have numerous uses. They make excellent visual aids for communicating ideas with co-workers or customers and they can be used for form, fit, function analysis. All of this can be done without incurring the expense of hard tooling.

For small pre-production runs, RP techniques can also be used to make tooling (referred to as rapid tooling). For complicated objects, rapid tooling is often the best manufacturing process available. Dramatic time savings allow manufacturers to bring products to market faster and more cheaply. From your solid geometry, a software package "slices" the CAD model into a number of thin (~0.1 mm) layers, which are then built up one atop another. Rapid prototyping is an "additive" process, combining layers of wax to create a solid object. This is in contrast to most machining processes (milling, drilling, grinding, etc.) which are "subtractive" because they remove material from a solid block. RP's additive nature allows it to create objects with complicated internal features that cannot be manufactured by other means.

Of course, rapid prototyping is not perfect. Part size is generally limited to 10 cubic inches or less and surface quality will be inferior to that of a casting made from a wax injected pattern.

The Basic Process

Although several rapid prototyping techniques exist, all employ the same basic five-step process. The steps are:

  1. Create a CAD model of the design
  2. Convert the CAD model to STL format
  3. Slice the STL file into thin cross-sectional layers
  4. Construct the model one layer atop another
  5. Clean and finish the model
  1. CAD Model Creation: First, the object to be built is modeled using a Computer-Aided Design (CAD) software package. Solid modelers, such as Pro/ENGINEER or Solid Works, tend to represent 3-D objects more accurately than wire-frame modelers such as AutoCAD, and will therefore yield better results. The designer can use a pre-existing CAD file or may wish to create one expressly for prototyping purposes. This process is identical for all of the RP build techniques.
     
  2. Conversion to STL Format: The various CAD packages use a number of different algorithms to represent solid objects. To establish consistency, the STL (stereolithography, the first RP technique) format has been adopted as the standard of the rapid prototyping industry. The second step, therefore, is to convert the CAD file into STL format. This format represents a three-dimensional surface as an assembly of planar triangles, "like the facets of a cut jewel." The file contains the coordinates of the vertices and the direction of the outward normal of each triangle. Because STL files use planar elements, they cannot represent curved surfaces exactly. Increasing the number of triangles improves the approximation, but at the cost of bigger file size. Large, complicated files require more time to pre-process and build and our solid object printer has limited resolution so it isn't necessary to produce a detailed STL file. Since the stl format is universal, this process is identical for all of the RP build techniques.

  3. Slice the STL File: In the third step, a pre-processing program prepares the STL file to be built. Build orientation is important for several reasons. First, properties of rapid prototypes vary from one coordinate direction to another. For example, prototypes are usually weaker and less accurate in the z (vertical) direction than in the x-y plane. In addition, part orientation partially determines the amount of time required to build the model. Placing the shortest dimension in the z direction reduces the number of layers, thereby shortening build time. The pre-processing software slices the STL model into a number of layers from 0.01 mm to 0.7 mm thick, depending on the build technique. The program may also generate an auxiliary structure to support the model during the build. Supports are useful for delicate features such as overhangs, internal cavities, and thin-walled sections. This build material will create a rough surface and requires special techniques for cleaning.  Each PR machine manufacturer supplies their own proprietary pre-processing software.
     
  4. Layer by Layer Construction: The fourth step is the actual construction of the part. Our solid object printer build one layer at a time from wax polymersl. The machine is fairly autonomous, needing little human intervention.
     
  5. Clean and Finish: The final step is post-processing. This involves removing the prototype from the machine and detaching any supports. Sanding, sealing, and/or painting the model will improve its appearance and durability.

Rapid Prototyping Techniques

Most commercially available rapid prototyping machines use one of six techniques. The two most common techniques us at Ti Squared are discussed below: Stereolithography Patented in 1986, stereolithography started the rapid prototyping revolution. The technique builds three-dimensional models from liquid photosensitive polymers that solidify when exposed to ultraviolet light. As shown in the figure below, the model is built upon a platform situated just below the surface in a vat of liquid epoxy or acrylate resin. A low-power highly focused UV laser traces out the first layer, solidifying the model's cross section while leaving excess areas liquid.

Figure 1: Schematic diagram of stereolithography. 7

Next, an elevator incrementally lowers the platform into the liquid polymer. A sweeper re-coats the solidified layer with liquid, and the laser traces the second layer atop the first. This process is repeated until the prototype is complete. Afterwards, the solid part is removed from the vat and rinsed clean of excess liquid. Supports are broken off and the model is then placed in an ultraviolet oven for complete curing.  The RP model can then be used to make rapid tooling as described below.

Ink-Jet Printing

Ink-Jet Printing refers to an entire class of machines that employ ink-jet technology. The first was 3D Printing (3DP), developed at MIT.  3D Systems version of the ink-jet based system is called the Thermo-Jet or Multi-Jet Printer. It uses a linear array of print heads to rapidly produce thermoplastic models (Figure 2d). If the part is narrow enough, the print head can deposit an entire layer in one pass. Otherwise, the head makes several passes.

Sanders Prototype of Wilton, NH uses a different ink-jet technique in its Model Maker line of concept modelers. The machines use two ink-jets (see Figure 2c). One dispenses low-melt thermoplastic to make the model, while the other prints wax to form supports. After each layer, a cutting tool mills the top surface to uniform height. This yields extremely good accuracy, allowing the machines to be used in the jewelry industry.

Ballistic particle manufacturing, depicted in Figure 2b, was developed by BPM Inc., which has since gone out of business.


Figure 2: Schematic diagrams of ink-jet techniques. 12

Rapid Tooling

Most rapid tooling today is indirect: RP parts are used as patterns for making molds and dies. RP models can be indirectly used in a number of manufacturing processes:

In the simplest and oldest rapid tooling technique, a RP positive pattern is suspended in a vat of liquid silicone or room temperature vulcanizing (RTV) rubber. When the rubber hardens, it is cut into two halves and the RP pattern is removed. The resulting rubber mold can be used to multiple wax patterns of the original RP pattern. These RP prototypes can be used as investment casting patterns.

How to create Stereolithography .stl files

The Rapid Prototyping Process uses .stl or Stereolithography files to build physical 3D Cad Models. Stl files are created using a mesh made from triangles to represent the physical part. Most CAD programs allow an easy export of .stl files.
Tips on creating perfect .stl files
Faceting, or flat spots on your part file are relative to the resolution of the .stl file. A good .stl file size is between .5 meg for a simple file to 10 megs for a large complicated part. Generally if your part is outside of these parameters it should be resized.
Checking your .stl file
Open the .stl file in your cad application and look at the faceting to ensure the part appears as designed checking for excessive flat spots on curves and contours. Also double check if your file is in inches or millimeters, some programs such as SolidWorks export in millimeters even if the drawing is in inches.

Resolution Too Low
When the faceting is too coarse you can see flat spots on curved surfaces. The flat spots in the .stl file will show up when the part is produced.
Resolution Too High
while the parts will print when the resolution is too high, it can cause delays in processing parts because of the large size. Increasing the resolution excessively does not improve the quality of the produced part. Please ensure your .stl files are under 10 megabytes.
Perfect Resolution
Good .stl files have faceting similar to the files pictured on the left, and are manageable to work with and produce excellent prototypes.

Here are instructions to create .stl files from Popular CAD applications.
Alibre
1. File
2. Export
3. Save As > STL
4. Enter File Name
5. Save
Ashlar-Vellum
1. File > Export...
2. Select STL Export Type
3. Set Export Options to Binary > OK
4. Enter Filename
5. Save
AutoCAD
Your design must be a three-dimensional solid object to output an STL file.
1. Make sure the model is in positive space
2. At the command prompt type "FACETRES"
3. Set FACETRES BETWEEN 1 &10. (1 Being low resolution and 10 high resolution for STL Triangles).
4. Next, at the command prompt type "STLOUT"
5. Select Objects
6.Choose "Y" for Binary
7. Choose Filename
Autodesk Inventor
1. Save Copy As
2. Select STL
3. Choose Options > Set to High
4. Enter Filename
5. Save
CADKey
1. Choose Stereolithography from Export options
2. Enter Filename
3. Click OK
I-DEAS
1. File > Export > Rapid Prototype File > OK
2. Select the Part to be Prototyped
3. Select Prototype Device > SLA500.dat > OK
4. Set absolute facet deviation to 0.000395
5. Select Binary > OK
IronCAD
1. Right Click on the part
2. Part Properties > Rendering
3. Set Facet Surface Smoothing to 150
4. File > Export
5. Choose .STL
Mechanical Desktop
1. Use the AMSTLOUT command to export your STL file.
2. The following command line options affect the quality of the STL and should be adjusted to produce an acceptable file.
Angular Tolerance - This command limits the angle between the normals of adjacent triangles. The default setting is 15 degrees. Reducing the angle will increase the resolution of the STL file. 
      A. setting of 1 would mean the height of a facet is no greater than its width. The default setting is 0,  ignored. 
      B. Aspect Ratio - This setting controls the Height/Width ratio of the facets. 
      C. Surface Tolerance - This setting controls the greatest distance between the edge of a facet and 
           the actual geometry. A setting of 0.0000 causes this option to be ignored. 
      D. Vertex Spacing - This option controls the length of the edge of a facet. The default setting is 0.0000, ignored.
ProE
1. File > Export > Model (or File > Save a Copy)
2. Set type to STL
3. Set chord height to 0. The field will be replaced by minimum acceptable value.
4. Set Angle Control to 1
5. Choose File Name
6. OK
ProE Wildfire
1. File > Save a Copy > Model
2. Change type to STL (*.stl)
3. Set Chord Height to 0. The field will be replaced by minimum acceptable value.
4. Set Angle Control to 1 5. OK

Rhino
1. File > Save As Select File Type > STL
3. Enter a name for the STL file.
4. Save
5. Select Binary STL Files SolidDesigner
1. File > Save
2. Select File Type > STL
3. Select Data
4. Click OK
ScanIP
1. File > Export > STL...
2. Select parts for export and export settings
3. Export
4. Enter Filename
5. Save
SolidEdge
1. File > Save As
2. Set Save As Type to STL
3. Options
4. Set Conversion Tolerance to 0.001in or 0.0254mm
5. Set Surface Plane Angle to 45.00
6. Save
SolidWorks
1. File > Save As
2. Set Save As Type to STL
3. Options > Resolution > Fine > OK
4. Save
Think3
1. File > Save As
2. Set Save As Type to STL
3. Save
Unigraphics
1. File > Export > Rapid Prototyping
2. Set Output type to Binary
3. Set Triangle Tolerance to 0.0025
4. Set Adjacency Tolerance to 0.12
5. Set Auto Normal Gen to On
6. Set Normal Display to Off
7. Set Triangle Display to On