By Audrey Doyle
It's been said that if a picture is worth a thousand words, a rap id-prototype model is worth a thousand pictures. And in the world of computer-aided product design and manufacturing, this adage couldn't be truer.
Nothing beats being able to hold in your hands a physical model of a product or part before committing time and money to have it manufactured. By being able to interact with a model at such an early stage, rather than relying on a representation of it on a computer screen, designers and engineers can get a better sense of whether the product or part will function properly, and can then change the CAD file from which it originated if it doesn't. They also can show these prototypes to potential customers for feedback before production begins, saving valuable time and money in the manufacturing process.
|One of the latest uses for rapid prototyping technology is in the pre-operative planning stage of complex surgical procedures. (Image courtesy of Medical Modeling Corporation)|
Because of such benefits, designers and engineers have relied on rapid prototyping (RP)-which quickly builds physical models and prototype parts layer by layer from liquid, powder, or sheet materials-to generate prototypes of their product or part designs ever since the technology came on the market in the late 1980s. Today, almost 5500 RP machines are installed in 53 countries worldwide, according to Terry Wohlers, president of Wohlers Associates consulting (Fort Collins, CO).
A vast majority of these users come from the aerospace, automotive, and consumer product industries, and the models they output are used strictly as prototypes. After the prototypes are approved, a cast is made based on the prototype, and from that cast final parts and products are manufactured.
But as the following success stories indicate, RP's benefits are spreading to a far more diverse range of applications, including medicine, art, and education. Furthermore, the technology is increasingly being used to output actual finished pieces, eliminating the casting process.
Medical Modeling Corp. of Golden, Colorado, specializes in creating life-size anatomical models of patients' bodies from Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) data. Surgeons worldwide are using these models to enhance pre-operative planning for complex reconstructive surgeries, particularly for operations to correct facial and skull abnormalities resulting from car accidents, congenital deformities, and tumors. To output the models, the company uses RP machines from 3D Systems and Z Corp.
"Very few companies are similar to ours, where our work involves making models of people's bone structures for surgeons to use in reconstruction cases, in which analyzing the model prior to surgery is beneficial," says Medical Modeling Corp. general manager Andy Christensen. "I think medical rapid prototyping has been slow to take off, because people in the medical field generally think of this as a technology used in product design and manufacturing, not as a surgical aid." This is unfortunate, he adds, because surgeons frequently rely on models of patients' anatomical structures before performing complex surgeries, and RP is a quick way to accurately build these models.
|An RP model of the skull of a patient injured in a car accident, created from CT or MRI scans, is a valuable tool for surgeons preparing for reconstructive surgery.|
For instance, say a person's skull has been crushed in a car accident. A few weeks after initial surgery, the patient would undergo the first in a series of operations during which surgeons would reconstruct the skull as best they could. "Before performing those operations, surgeons might have a model of the patient's skull built, so that they can better understand the trauma and know more clearly what they're getting into," Christensen explains.
Historically, to obtain such a model, surgeons take scans of the patient's skull and forward the data to a lab that would use a computer numerically controlled milling machine to mill a 3D model of the skull from foam or polyurethane. However, because CNC milling is a subtractive process, in that the machine is carving a shape from a solid material, the resulting model is solid. Therefore, the skull model in this case can't accurately show the trauma because surgeons can see only the surface of the skull, not what the trauma looks like from inside the skull.
Conversely, Christensen says, "because rapid prototyping is an additive process, where you're building the model in layers based on spatially correct scan data, you end up with a faithful reproduction of the part being modeled, cavities and all."
The process of creating an anatomical model begins with a call from a surgeon in need of Medical Modeling Corp.'s services. "Say it's a person with a skull deformity. The hospital sends us CT scans of the skull in a format proprietary to whatever CT scanner they use," Christensen says.
After importing the data into a high-end Windows NT workstation, the Medical Modeling Corp. team uses Mimics, a 3D image-processing package developed for the medical field by Materialise (Leuven, Belgium; Ann Arbor, MI), to build a 3D model of the patient's skull. "Mimics interfaces with all common scanning formats," Christensen says. "So pretty much whatever data we get from a surgeon or hospital, we know the format will be compatible with Mimics."
Next, the team converts the resulting file into STL format, the standard used for RP machines. "At that point, we're ready to build the model," Christensen says, and adds that an onsite specialist analyzes the model each step of the way to ensure accuracy. After the model is built, it's sent to the surgeon. The entire process, from the time they receive the data to the time the model is finished, takes a week on average, but has occurred in as little as a day.
According to Christensen, Medical Modeling Corp. uses two RP technologies: 3D printing and stereolithography. The 3D printer, Z Corp.'s Z402-which is based on core technology invented at and patented by MIT and licensed to Z Corp. in 1994-works by building models layer by layer, using a powder composed primarily of corn starch and sugar. The stereolithography machines-the SLA 250 and 350 from 3D Systems-use a computer to control the movement of a laser that shines ultraviolet light onto a vat of photosensitive liquid epoxy or acrylate; wherever the light strikes the surface, the epoxy or acrylate solidifies. The model is lowered into the vat, a new layer of the material is spread over the surface, and the process is repeated until the model is complete.
|In 1989, artist Stewart Dickson created his first stereolithograph based on a mathematical equation. After creating "Trefoil Um bilical Torus," in the Grass computer system, he sent the model to a 3D Systems SLA machine for output. |
Deciding which technology to use depends on how quickly the surgeon needs the model. "In the SLA machine, a typical model can take 24 hours to build. In the 3D printer, the average time is 6 to 8 hours, and nothing has taken us longer than 10 hours," Christensen says. "So we use the 3D printer in cases where surgeons need the model right away."
A second factor has to do with cost. "Stereolithography is still an expensive technique, and an average model created from stereolithography costs from $2000 to $2500," he says. "3D printing is much less expensive, so we can sell models for an average of $800."
A third has to do with whether the surgeon wants to bring the model into the operating room. "The acrylate used in stereolithography can be sterilized," Christensen explains, "so the surgeon can use the model as a reference during the actual surgery."
Regardless of which technology is used, Christensen says the resulting model provides valuable benefits to the surgical team. With an RP model, surgeons can not only visualize the trauma and communicate ideas with colleagues, they also can rehearse the surgery on the model using the tools they'd use during the operation, greatly reducing the risk of surprises while making room for experiments. "It's also been shown that using models in surgery reduces by 20 percent the amount of time a person is under anesthesia," Christensen says. "A lot of these surgeries can take 10 to 15 hours, so that reduction can be significant."
Stewart Dickson might hold the job of senior technical director at Walt Disney Feature Animation (Burbank, CA) during working hours, but in his spare time he creates sculptures based on complex mathematical equations, most of which he comes up with himself.
A 1981 graduate of the University of Delaware with a bachelor's degree in electrical engineering, Dickson began sculpting in the late '70s-first by hand, and later designing his creations in Wavefront software and outputting them on a CNC milling machine. In 1989, when he first had access to an RP system, a 3D Systems SLA machine, he used it to build his first stereolithograph, which he designed in the Grass interactive computer graphics system developed by computer graphics luminary Tom DeFanti. "It's called 'Trefoil Umbilical Torus,' but I like to think of it as a green snake tied in a knot," Dickson chuckles. Then, in 1991, he turned to Wolfram Research Inc.'s Mathematica, an integrated technical computing system that combines interactive calculation and visualization tools with a complete programming environment.
He still uses Mathematica today for the first step of the design process, running the software on a Power Mac G4 and an old SGI 4D20G Personal Iris. "For the most part, I've been working on the mathematical ideas I create these objects from since the '80s," explains Dickson. "I also get ideas by going to mathematical conferences. Then I create my own formulas and use Mathematica to see if they work as art." If he feels a particular equation will look great as a piece of sculpture, he imports the file to autodessys' formZ, where he uses the software's parallel surface function to transform it into a buildable object.
"A mathematical graphing system like Mathematica is concerned with an infinitesimally thin theoretical surface that you need to be able to extrude normal to itself to make it buildable," he explains. With formZ's parallel surface function, Dick son can turn the 2D parametric surface created in Mathematica-which has a U and V dimension but no thickness-into a 3D computational solid geometry (CSG) object. He also sometimes uses formZ's Boolean functions to perform CSG operations on the computer model, such as slicing the object to expose parts of its internal structure.
|Dickson's model, "A Three-Dimensional Zoetrope" (below), was output (left) using the Stratasys Genisys 3D Printer.|
When he's finished with his design, he exports the file in STL format to whatever RP device he plans to use for that piece. In the past he has received grants from various organizations, including Siggraph, to build stereolithographs using 3D Systems machines. More recently, he worked out a deal with Z Corp. whereby he created some mathematics-based objects for the company to use during demos of its 3D printer, and in return the company output some of his models as sculptures. Thanks to a similar agreement with Helisys, he recently output some of his designs via the company's Laminated Object Manufacturing (LOM) machine. LOM uses a laser to cut patterns in sheets of paper, each bonded to the previous layer, resulting in a sculpture that looks and feels like a block of carved wood.
Since creating that first stereolithograph in 1989, Dickson has built nearly three dozen sculptures designed in Mathematica and formZ and output via RP. A recent sculpture is called "Annotated Hyperbolic Paraboloid." This piece, created in 1998 and output as a stereolithograph, is based on a mathematical equation that shows the synthesis of a parabola along a hyperbolic path. According to Dickson, the idea be hind the sculpture is twofold: "It's mainly to give blind students access to math and science," he says, "but it's also to test this wacky theory of mine about the integration of multidimensional information in three-space through the physical interface of touch." To do that, Dickson printed the mathematical equation, in Braille, onto self-adhesive plastic sheets using the Tiger 1000 personal tactile graphics embosser. He then covered the sculpture with the sheets.
Another recent sculpture, called "A Three-Dimensional Zoetrope," is based on the "Genus One, Three-Ended Minimal Surface of the Costa-Hoffman-Meeks family." This infinitely large class of minimal surfaces was proposed by a graduate student named Celsoe Costa in Rio de Janeiro and discovered by David Hoffman, William Meeks III, and James Hoffman at the University of Massachusetts, Amherst, between 1983 and 1985. Dickson's sculpture, which was exhibited in the Siggraph 2000 Art Show, was output using the Stratasys Genisys fused deposition modeler, which prints cross sections of a model using a thin bead of melted plastic that solidifies upon cooling.
|Toymaker Hasbro collaborated with the Smithsonian Museum to build a model from scans of the museum's deteriorating triceratops skeleton, using a stereolithography machine for output.|
According to Dickson, although his sculptures are based on complex mathematical equations, he believes they work well on their own as unique works of art. "I enjoy seeing if these formulas work as sculpture," he says. "Most of the time, researchers leave their ideas in the computer. I think it's important to bring them out into our world."
Hasbro (Cincinnati) uses RP machines to output prototypes of the toys it manufactures, which include everything from Batman to Poo-Chi to Pokemon. According to Bill Smith, vice president of engineering at Hasbro, the company began using the technology five years ago to enhance its product design cycle.
"Traditionally, engineers and designers would make a working model out of a variety of material-polystyrene boards, rubber, Styrofoam, metal, plastic-in order to build a working prototype of the toy," Smith says. Sales and marketing would bring the prototype back to the customer for input, and any necessary changes were implemented into the design. The final product was built based on the approved prototype. According to Smith, the whole process, from concept to final, took between 12 and 14 months on average.
In an effort to enhance its design capabilities as well as bring its toys to market more quickly, Hasbro incorporated CAD and CNC milling into the design cycle more than 15 years ago. Then, five years ago, it added stereolithography machines to the mix. Today, the company designs new toys using a variety of CAD packages running on Hewlett-Packard Kayak and SGI Indigo2 workstations. Working prototypes are created from the designs using DTM's Sin terstation 2500plus selective laser sintering machine, which uses a CO2 laser to build objects from a bed of fine powder. To create the finished master from which the final toy will be built, Hasbro uses 3D Systems' SLA 7000. They often use the CNC milling mach ines to mill the final products.
According to Smith, Hasbro's current design and manufacturing process has dramatically cut the cost and time it used to take to bring products to market. Such savings have proven crucial a number of times-most notably in 1998, when Has bro unleashed the wildly popular Furby electronic toy. "We had just acquired Tiger Electronics early in the year," Smith recalls. "And our CEO took one look at Tiger's Furby designs and became determined to see that Furby was on store shelves that Christmas.
"A Furby has about 70 or 80 gears, mechanisms, and other working parts," Smith says. Hasbro met the deadline, but probably wouldn't have done so in that timeframe without CAD and rapid prototyping. "It would have taken at least a year," concludes Smith.
Although Hasbro has reaped numerous benefits from its RP machines, late last year the toy manufacturer used its SLA system for an application far beyond its traditional use: to create dinosaur models for the Smithsonian Museum.
|Computer-designed parts output by a DTM Sinterstation at Rocketdyne go directly into actual rocket engines. |
The problem began when the Museum discovered that its triceratops skeleton, which had been on display since 1905, was deteriorating. As part of a preservation project, Scansite 3D Services (Mountain View, CA) was hired to take a 3D scan of the dinosaur skeleton. As the company scanned each of the 250 bones in the original skeleton, it turned the files over to Hasbro, which built two separate one-sixth-scale triceratops models from the digital data. "We didn't design the skeleton in CAD; the scan files were used directly as output to the stereolithography machine," says Smith. In total, it took about seven months to create the two models.
According to Smith, one of the RP replicas will be on display in the Smithsonian. Hasbro hopes to donate the other to the Cincinnati Museum of Natural History, so visitors can learn not just about the triceratops, but also about RP technology. "Although this isn't what we normally use our system for, we volunteered to do it because we thought it represented a unique application for the technology," Smith says. "We also hope to be able to give something back to the people in our community."
Rocketdyne (Canoga Park, CA) has been building rocket engines in support of national defense and US involvement in space since the mid-1950s. Among other projects, the company's current responsibilities include designing and building the liquid rocket engines for the Space Shuttle's main engine and the architecture for the space station's end-to-end electric power system. In addition, Rocketdyne is developing the linear aerospike engine for the subscale X-33 test vehicle designed by Lockheed Martin to demonstrate key technologies and lower operating costs that are needed for the next generation of reusable launch vehicles.
As part of its manufacturing process, Rocketdyne has relied on RP technology since the early 1990s. "We started designing parts and manufacturing models in IBM's Catia in the late 1980s," recounts Roger Spielman, lead engineer in Rocketdyne's Accelerated Digital Design and Manufacturing (ADDM) Center. "At that time, we were milling models of our parts using high-speed machining, and then they'd be investment-cast or injection-molded." The resulting part, called an MTD, for manufacturing test and development, was used only for test purposes. If the part broke or cracked during the testing phase, it had to be built all over again.
According to Spielman, this process was not only time-consuming, but expensive. "A typical part might be scheduled for a 63-week build cycle and cost over $100,000 to build. And that's just the test part. That's not anything close to the final part," he states.
In 1994, Rocketdyne replaced its CNC milling machines with a DTM Sinterstation 2000. "We chose the Sinterstation because we liked the fact that we could build metal parts and parts made from other materials, like thermoplastic," he says. A year later, the company began building end-use rocket engine parts directly from the Sinterstation, eliminating the milling and investment-casting/injection-molding processes. "Now, a typical part can be built using the DTM system in six weeks for $10,000," Spielman says.
More important, however, is the fact that Rocketdyne is going directly from computer design to final part, and that the final part comes directly from an RP machine. "That's the real innovation here. We're actually building end-use flight parts that come right out of the Sinterstation and go onto the aircraft," Spielman continues. "The material that the Sinterstation uses, and the process it uses to create the parts, have been qualified for this application."
According to Spielman, the design process today begins in Catia or in Pro/Engineer running on IBM RS6000 workstations and Dell Windows NT-based PCs. After the part is designed, it is sent from the Rocketdyne design department via network to the ADDM Center, where Spielman and his colleagues use the surface modification and analysis tools in SDRC's Imageware Surfacer software running on 600mhz PCs to prepare the part to be built. "We use Surfacer to orient the part, repair the file if necessary, scale the part, things like that," he says.
Next, the part is converted to STL format and is sent to the Sinterstation 2000 or to one of the Center's two Sin terstation 2500plus machines for output. Parts that normally would be injection-molded, such as ducts, capacitor boxes, and electrical fittings, are output in DTM's DuraForm Polyamide or Glass-Filled thermoplastics material. For metal parts, such as brackets and high-speed impellers, Rocketdyne uses a metal material the company developed in-house.
To ensure quality, the ADDM Center builds production control samples at the beginning of each build cycle and then tests the samples to qualify them. If a part needs to be tweaked, it goes back to the designers. If not, the final build continues.
According to Spielman, Rocketdyne is a trailblazer in terms of its use of RP technology. "People's lives are riding on these parts. That's our number-one concern," he states. "Because of that, we abide by a strict set of rules when building them." In addition to documenting the material's full pedigree back to its raw state, for instance, Rocketdyne also records and documents every step of the RP process as well as conducts ancillary tests, such as burn and fungal tests, directly on the parts themselves. "The fact that both the RP process and the materials pass such strict guidelines is very exciting," he concludes. "In a very real sense, it shows just how far this technology can go."
Freelance writer Audrey Doyle is a Computer Graphics World contributing editor. She can be reached at firstname.lastname@example.org.
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