MCAD Modeling Methods--3D Rapid Prototyping

New Models Expand Options for Materials and Resolution.

First it was "Go west, young man," then "Plastics are the future," now, at least for me, it's "Rapid prototyping!" For those of you not familiar with rapid prototyping, I'll briefly go over the basics. After you create or otherwise acquire a 3D model of whatever it is you want, you create an STL file, a format originally designed for stereolithography machines. It's a faceted representation of the model so it's much simpler than an engineering model (figure 1). You used to have to move your model so it occupied all positive space (positive x, positiv e y, positive z), but in today's programs that usually doesn't matter. Most modern 3D modeling programs provide some way to generate this kind of file. (If a vendor makes you pay for a translator for this basic file format, you should really question why you're using its software.) Some programs export models, some Save As. Once you have an STL file, the real fun begins.

Figure 1. An STL file is a simplified representation of your model made of myriad tiny triangles. Because rapid prototyping apparatus limits, the STL file doesn't have to be quite as exact as your engineering model.

Resolution's the Thing

Some programs can import STL files for rough space checking, but you usually can't measure to them very accurately. When you create an STL file, pay close attention to the resolution you specify, as it will affect your build time. Most rapid prototyping machines top out at a resolution of 0.0050 but are more comfortable at a less exact 0.0070 or even 0.010. Depending on the rapid prototyping machine, the details may be somewhat different, but the gist is the same. You open or import the STL file into the control program for the rapid prototyping machine. The program likely will position the faceted model on a virtual base. You then tell the program how you want the model oriented. What direction you face your model directly affects your rapid prototype. The program breaks your model down into very thin layers and sends them one at a time to the rapid prototyping machine. Each layer is traced out in the modeling material by the process the machine uses. Some machines use lasers to cure a polymer. Others extrude actual engineering materials. Some even cut thin sheets of various materials and adhere them together. Whatever the construction method, the model is built up one layer at a time until your prototype is finished. It's an amazing process!

Earlier I said orientation is important—now I'll explain why. If you build a snap feature vertically, it will have weld lines between the layers stacking horizontally up the feature. When you flex the feature, the weld lines aren't going to hold up very well. Snap! There goes your model. But if you build the model so the snap feature runs horizontally, the weld lines go up and down the length of the feature and don't separate when flexed (figure 2). It's the same as your basic metallurgy. Forged tools are always tougher than cast because of the structure induced in the metal by pounding it into shape.

Figure 2. Whenever possible, try to orient the model so that the layers strengthen the part instead of weakening it. When the layers follow the shape of the feature, they reinforce each other and make the model stronger.

The Garden of Eden

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At the high end of rapid prototyping machines is the brand-new Objet Eden 350/350V. The Eden 350/350V lays down its layers in 16-micrometer thicknesses. Put into the same units as other machines, that's 0.0006299210 resolution, an order of magnitude smaller than what we are used to. This resolution means that stair-stepping will be much less noticeable (figure 3). Again, orientation is all-important because it's only in the z-axis that the Eden achieves that resolution. The x and y axes are still closer in resolution to what competitors offer.

Figure 3. The Eden 350/350V has a resolution of 16 micrometers, so it lays down such fine layers that the stair-stepping that plagues other systems is barely noticeable.

The material used in the Eden 350/350V is acrylic, available in several colors. That may not sound appealing, but Objet does some interesting stuff with that material. Objet offers a flexible acrylic material in 65 and 75 durometers. Not only can you model rubber-like parts, but you can produce fully functional parts made of actual engineering materials. How's that, you ask? You can rapid prototype a rubber mold and cast your parts. The Objet Eden 350/350V is available in the United States exclusively through Stratasys.

Testing Your Metal

Plastic parts are all well and good, but what if you need metal? You used to have to make special porous models and infiltrate them with metal, usually a copper-filled alloy. Well, no more. Arcam, a Swedish company, has come out with the Arcam EBM S12 system. This rapid prototyping machine uses actual metal (titanium!) to build its models (figure 4). The machine uses layers of powdered metal melted by an electron beam. Parts are built in a vacuum chamber to minimize impurities and to give the electron beam a clear path. The vacuum also provides better stability and thermal balance. You can go directly into manufacturing with the resulting models—they are real metal and very solid. You might have to do a little final finishing work to clean up any stair-steps the process may leave on the model, but think of the lead-time gains! You could even run prototype molds and shoot plastic parts. Stratasys is selling the Arcam EBM S12 system in the United States.

Figure 4. Metal parts can be made on the Arcam EBM S12machine, so you can test your prototypes just as you would real parts. In fact, you can even go right into production with them.

Visions of Grandeur

Perhaps you don't need all these bells and whistles, or you're just not sure what you need. Maybe you're just starting out with rapid prototyping.

The InVision LD 3D printer from 3D Systems is a desktop modeler (figure 5). It sits on your desk, right there in your office. At $14,900, approximately one-third the price of larger systems, it's a great place to start. It's relatively quiet (though I can't say I've ever heard a terribly noisy rapid prototying machine), doesn't smell bad and needs no additional equipment for postfinishing models. You can take them right out of the machine and put them to use. Interestingly, the InVision LD 3D printer uses sheet lamination to build its models, much like the old LOMs (laminated object modelers). It uses a plastic sheet, though, not paper. What comes out looks a lot like a stereolithography part.

Figure 5. Having a great rapid prototyping system right on your desk helps you refine designs quickly and efficiently.

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Also in the entry-level range is Z Corp.'s latest model, the ZPrinter 310 Plus, which adopts features first implemented in the higher-end Spectrum Z 510 printer. These additions include a heated build chamber for higher printing speeds.

Resources to get you started

Take Part

It's never been a better time to get into rapid prototyping. It's a powerful tool that can help you to produce better designs and sell them to upper management or clients. With new machines pushing the frontiers of capability, it's only going to get better!

Mike Hudspeth, IDSA, is an industrial designer, artist and author based in St. Louis, Missouri.