MCAD Modeling Methods-Real-life Replicas1 Sep, 2005 By: IDSA ,Mike Hudspeth
3D printing and rapid prototyping revolutionize design process
RP (Rapid Prototyping) has been around for some time now, but its downstream applications and derivations such as 3D printing, RP and rapid tooling offer more exciting potential than ever before. All you do is build a 3D computer model of the part you want and send it to a machine. Presto! Using a variety of techniques, the machine produces the part for you, usually in just a few hours.
The first RP device I ever saw was a 3D Systems SLA (stereolithography) machine. This type of machine is still in widespread use today. It builds a model one layer at a time. Software breaks a 3D computer model into a series of layers, each only 0.005" thick, and sends them to a laser, which begins to draw on the surface of a vat of photocuring polymer. Wherever the laser touches, the resin hardens. The vat has an elevator that raises and lowers a platform in 0.005" increments. When the laser finishes a layer, the platform lowers another 0.005" and the next layer is cured. Layer by layer, the model is built up into a true 3D object. This process can make parts that would be difficult or very nearly impossible to build traditionally.
Materials in the old days were terribly fragile and expensive. I remember an executive who picked up one of our models and shook it for emphasis. It slipped from his hand and shattered on the table—$800 down the drain. Nowadays, the materials are tougher and less prone to distortion. Moisture in the air still has an effect (swelling and warping), but not what it used to. Today's resins come closer than ever to actual engineering materials: They range from nearly opaque to water-clear, and there's even a rubber-like, flexible material available in different durometers (hardness ratings). You can perform limited functional testing on some.
The resolution of SLA machines usually peaks at about 0.003", but 0.010" to 0.005" is more the norm. That means most surfaces that are not horizontal display a visible stair-step effect. With some sanding and painting, you can produce a model that looks amazingly like a production part. You can even make a functional part for testing, but it will break very easily, so be careful. Also be careful with the supports that the machine builds to support the structure of the model. These need to be removed, usually by hand (figure 1). A stereolithography machine can cost anywhere from $75,000 to $800,000.
Figure 1. Regardless of the process used to build them, stair-stepping is visible on most rapid prototypes, even at very high resolutions. Note the support structures throughout and beneath this model. These must be removed in a secondary operation. This model was created using a 3D Systems SLA system.
Other RP processes are available. All use the same computer file format, STL, which most 3D modelers today can output. The most common are SLS (selective laser sintering), FDM (fused-deposition modeling) and LOM (laminated object manufacturing). If I didn't mention your favorite, remember that this is a column of limited length.
SLS machines use a laser to melt together grains of plastic powder. You choose materials from a selection of engineering plastics, although theoretically anything that melts can be used. The laser works basically the same way as with the SLA. These machines are accurate and fairly speedy. No supports are necessary with SLS because the powder supports the weight of the model. But you will need a way to remove the unfused powder from any internal cavities. The surface finish is all right, but does have a grainy texture. An SLS machine runs around $300,000.
FDM machines use long, thin rods of plastic that feed through a heated print head. They extrude the melted plastic and draw with it. Most machines require two material cartridges to function: the modeling material and the support material. At roughly $250 a cartridge, the models aren't free. FDM machines are usually not the most accurate, but they are great for touchy/feely presentations. Because they use actual engineering materials, you can test with them. The models can be sanded and painted with very good results. The density of a model is not the same as that of an injection-molded part, which means the model is more fragile. Also, because accuracy is relatively limited, don't expect to make small parts or parts with small features. FDM machines cost between $25,000 to $300,000 and don't need any special environment.
You don't see many LOM machines any more. The original developer, Helisys, went out of business a few years ago. The technology languished until Cubic Technologies gave it a new home. LOM machines use large rolls of paper and sharp cutters to build models. A sheet of paper is adhered to a previous layer, and the cutter traces the pattern. Scraps are removed and the next layer is glued down and cut out. The finished model has much the same density and texture as wood. The accuracy is as good as the thickness of the paper used. You can create pretty large parts, but they are susceptible to environmental changes so don't leave them in the sun or out in the rain. A LOM machine costs anywhere between $120,000 and $240,000, but its paper material is the cheapest of all the RP processes.
As exciting as RP is, a fairly new branch of its family is set to put the industry on its ear. In-house RP capability is not an option for every company because of costs. But what if you could get a machine for around $25,000 to $30,000? 3D printers provide most of the benefits of RP. The resolution isn't usually as high, and the size of model you can build is limited, but they are fast! Most work very much like a common inkjet printer. The material is sprayed through a printhead onto the print platform. The platform lowers, and the next layer is applied. The materials tend to be somewhat waxy, but are improving every year. One great thing about 3D printers is that they need no special environment. You can use one on your desk.
One company, Z Corp., has a 3D printer that prints in full color. It enables you to print a model of your product with different color grips or a display that has images right on it. You can even create a model with labels already applied.
The best thing about 3D printers is that they are so fast you can print several models in a day. Iterative design is what these printers are all about. You can meet in the morning to go over preliminary designs. model the changes before lunch, and then run another model in time for a review that afternoon. Try that with an outside RP vendor. The parts are pretty fragile, though, so you won't be able to do much testing with them.
Some RP machine vendors are coming out with lower cost, easier-to-use devices to compete with the 3D printers. Some of these new machines even use the same kinds of materials as their more expensive brethren. I'd suggest you consider one of these lower-cost RP machines if you can take advantage of the properties of the materials they use.
We are beginning to see applications of RP technology that go well beyond prototyping. Doctors are using RP to create body parts to practice on before surgery. Rapid tooling uses RP machines to build the cavities of a mold rather than the part itself (figure 2). You can use a sintering process that includes waxy particles that melt out, leaving a very porous part. Then infiltrate it with metal, and you have a mold you can shoot actual parts from. The mold might not hold up to production runs, but for limited runs it's cheaper than cutting traditional tools.
Figure 2. With rapid tooling, you model the tool instead of the part. This approach lets you shoot actual parts for limited runs. The example above was created using a system from Z Corp.
Rapid manufacturing is the next step in this ongoing evolution. Even now, many companies make actual production parts with RP technology. For items within a certain range of operational properties, RP can produce on a level untouchable by any other technology.
Recommendations and Considerations
Try the process you are interested in before you go out and plunk down any cash. Material prices will be an ongoing issue, so be sure to price those as well. A factor frequently overlooked is the machine environment. Some machines use some pretty noxious supplies and need isolated and controlled environments, which can be expensive. You'll have to weigh the plusses and minuses of each type of system for yourself. This is exciting technology with many far-ranging capabilities. With these machines, you can save time and expense while improving reliability.
RP system vendors
Mike Hudspeth, IDSA, is an independent designer, artist and author based in St. Louis, Missouri.
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