Since its commercial introduction in 1988, Rapid Prototyping (RP) has progressed from a novelty into a standard tool for new product development, and is now poised to be an important technique for manufacturing the advanced products of the 21st century. Rapid Prototyping machines create solid objects from computer data files in an automated manner. Although numerically controlled milling machines have done this for decades, RP offers an important twist: as opposed to machining material away from a larger block, RP builds the part up as a series of very thin layers. This concept of additive, as opposed to subtractive, manufacturing offers some unique possibilities that are just beginning to be explored.
Rapid Prototyping Methods
There are currently more than 20 commercial suppliers of RP equipment worldwide, and a large number of universities performing research on their own systems. According to Wohler's Associates, an industry analyst, a total of 3,289 RP systems have been sold to industrial, academic, and government sites internationally, to date. Although the materials and processes differ among various machines, the concept is the same. The basic operation of any RP system consists of slicing a 3D CAD file into a series of thin cross sections, translating the result into 2D position information, and using this data to control the placement of solid material. This process is repeated for each cross section and the object is built from the bottom up, one layer at a time.
Stereolithography, manufactured by 3-D Systems Inc. of Valencia, CA, is the original and still most widely used system. A stereolithography machine builds objects by shining ultraviolet light from a laser onto the surface of a vat of photosensitive liquid epoxy. Wherever the light strikes the surface, the epoxy is solidified; a computer controls the laser to trace the desired pattern, thus creating a cross-section of the final object. When one layer is complete, the part is lowered into the vat, a thin layer of new liquid is spread over the surface, and the process is repeated. Each layer is as thin as 0.002 in., resulting in an object with very fine details.
Selective Laser Sintering, manufactured by DTM Corp. of Austin, TX, uses similar equipment, but builds objects from a bed of fine powder rather than from a vat of liquid. Selective Laser Sintering machines use a higher-power CO2 laser to selectively fuse a thin surface layer of plastic, or plastic-coated metal or ceramic particles.
Laminated Object Manufacturing, made by Helisys Inc. of Torrance, CA, also uses a CO2 laser, but cuts patterns in sheets of paper, each of which has been bonded to the previous layer. The resulting object has the look and feel of a block of carved wood.
Fused Deposition Modeling, manufactured by Stratasys Inc. of Eden Prairie, MN, is basically a high-tech hot glue gun mounted to a plotter. This system "prints" each cross section using a thin bead of melted plastic that solidifies upon cooling.
From Visual Models to Functional Prototypes
The initial use of Rapid Prototyping was to obtain quick visual models of a new product or design concept. The ability to physically interact with a model allows design teams to accurately communicate ideas; perform form-and-fit checks of complex mechanisms such as engine components; and show potential customers and suppliers a design before actual production begins. By automating a traditionally craftsman-oriented process, RP allowed a larger number of models to be created at a lower cost and in less time, and was immediately adopted by companies in the aerospace, automobile, and consumer products industries. Today, building prototype models is still the largest single application of RP; with more machines, better quality parts, and lower cost, this technology is available to an increasing audience of smaller companies. Companies who do not own RP equipment can send their designs via the Internet to a service bureau, which will "print" and ship a custom model within a day or two.
The single biggest limitation of RP in the early days was weak materials. Materials development has moved the capabilities of RP beyond the original brittle polymers into consumer-grade plastics, composites, and even metals. (Shown above are functional prototypes for testing before mass production.) This has opened up the use of RP equipment to produce functional prototypes, whereby a designer can create one-of-a-kind objects for actual testing before large-scale manufacturing. A functional prototype may be as simple as the ergonomic testing of a hand tool or as complex as studying the fluid flow in a new design for a pump impeller. Functional prototypes of consumer products allow for multiple design iterations before the production tooling is finalized. This often results in dramatic quality increase and saving in cost and time. In some cases, RP now serves as the final production method for a limited number of specialty products.
Interestingly, a significant portion of the RP industry has come full circle through the relatively new niche of Concept Modelers. Concept Modelers are machines designed to operate in an office environment with minimal training. They are priced at around one-third of the cost of an entry-level RP machine, and operate at up to 10 times faster. Although the resolution and capabilities of the materials used with these machines are extremely limited compared to other RP equipment, Concept Modelers are ideal for use in early-stage product development (hence the name). Concept Modelers are the "pin printers" of the RP world -- after printing multiple rough drafts of a product design on a Concept Modeler, the final draft is sent to the "laser printer" -- a high-end RP system.
Agile Tooling
Much of the focus of research today is directed toward using Rapid Prototyping equipment to build the tools that will be used to mass-produce plastic parts. This "Rapid Tooling" is an area of much confusion, as RP techniques can create tools that span the spectrum from prototype use, limited to less than 20 parts, to production-quality tools that compete with machined steel. A preferred term that describes an important need is Agile Tooling. An agile tool has the characteristics of being rapidly produced, inexpensive, robust, easily modified or recycled, and has the capability to cost-effectively support both prototype and low- through intermediate-volume production runs. This can potentially be achieved through RP technology, but currently is limited by existing RP materials and standard approaches to creating tools that do not take full advantage of the additive nature of RP processes.
For many products utilizing injection-molded components, the lead time and cost of tooling are major obstacles to successful product introduction. RP research in this area has focused on two approaches to reduce the cost and lead time of production-quality, high-volume (100,000+ units) tooling. The first approach is to create the tools directly on RP equipment. DTM's RapidSteel process is an example. During the RapidSteel process, steel powder coated with a plastic is fused by a Selective Laser Sintering machine into the tool form; only the plastic coating is actually fused, serving as a binder for the steel powder. The tool is then subjected to multiple furnace cycles in which the plastic binder is burned out and bronze is infiltrated into the form. The result is a production-quality tool with advanced wear and thermal conductivity characteristics.
The second approach uses RP equipment to create the geometry for the tooling; a secondary process turns this form into a material suitable for high-volume tooling. Examples of this approach are 3-D Systems' KelTool and the PHAST technology recently donated to Milwaukee School of Engineering (MSOE) by Procter & Gamble. Both of these processes begin with master patterns, generally created from stereolithography. KelTool patterns are typically of the tool cavity and core; PHAST patterns are of the part geometry itself. A transfer mold is created from the master pattern, and is used to create a powdered-metal "green part." This green part is subjected to a number of further process steps to produce a fully dense, production-quality insert that is a mixture of materials. An injection mold insert produced by the KelTool process, for example, is comprised of 70% A6 tool steel and tungsten carbide, and 30% copper. Lead times for both KelTool and PHAST are approximately eight days, compared to 8-10 weeks for traditional tooling. Cost is generally lower, and can be dramatically lower for complex tool geometries or features that would require significant EDM work if built using traditional toolmaking techniques.
Because RP methods are additive and compatible with extremely complex geometries, tools produced using RP have the potential to actually outperform their traditional counterparts. One important technology currently under development at MSOE and elsewhere is Conformal Cooling. Channels for cooling fluid that conform to the shape of the tool surface can be created immediately beneath the surface of an insert as it is being built. The proximity of these channels to the hot mold surface maximizes thermal transfer during use. This should ultimately reduce injection molding cycle times by up to 50%.
Technology Risks and Rewards
It is clear that Rapid Prototyping offers some significant advantages over "the old way" in the area of product development. A typical example is a Milwaukee-based manufacturer who reports that in 1990, a staff of 22 model makers produced 2,000 prototypes to support new products; by 2000, eight model makers produced more than 12,000 prototypes at a lower total cost. For all of the success stories such as this, however, there is still a lack of widespread adoption of RP technology. One often-quoted figure is that only 2% of the companies that should be using Rapid Prototyping actually are. The 2% of the companies who have adopted RP are becoming quite sophisticated regarding its use, while other companies run the risk of being left behind.
There are a few important barriers to a company's entry into RP. An obvious one is the initial cost of the equipment, which can run in the $500,000 range for the machine itself. An equally important barrier is the learning curve associated with the "smart" use of RP. RP is not a panacea, nor is it a replacement for machining or other traditional manufacturing and product development techniques. Because of some fairly high costs associated with the technology, using RP efficiently becomes critical. The rewards are great for those who are willing to learn, as are the resources for industry education. Companies interested in the technology can join machine-specific users' groups and attend RP seminars such as those offered at MSOE, trade shows, and university-run Rapid Prototyping Consortia.
Milwaukee School of Engineering's Rapid Prototyping Consortium
MSOE's Rapid Prototyping Consortium was established in 1991 to address the above mentioned risks of Rapid Prototyping and develop the use of these techniques in a manufacturing environment. There are currently 27 industry members in this highly successful industry/academic partnership, including Bombardier, Snap-On, Harley-Davidson, Kohler, Master Lock, and S.C. Johnson. Firms that are Consortium members represent a broad array of defense and commercial products, and span the spectrum of RP use from minimal employment of concept models through heavy use of leading-edge RP applications such as functional prototypes and direct manufacturing tooling.
A key feature of the Consortium is that membership is restricted to noncompeting companies. This allows for open exchange of manufacturing, product development, and management lessons learned and a sharing of successful innovations. Monthly meetings are the forum for exchange of this information, allowing members to keep up to date on the latest technology. Focused committees allow members with common interests to explore specific topics. Committees include CAD, RP Part Finishing, Tooling, and New Product Development.
Members in the Rapid Prototyping Consortium pay a yearly fee and receive a fixed number of machine hour credits. Part design is performed at individual companies, with files transmitted to the equipment at MSOE through a secure website. Status of individual part builds is provided through video snapshots and text updates, which are accessible by member companies through the website. All parts are built and finished by undergraduate and graduate students, which serves as an important component of the "hands on" engineering curriculum at MSOE.
The Consortium provides both a source of research needs and a testbed for research results. An advisory committee allocates funds for Consortium research, which is generally led by MSOE with coordination of specialty personnel and equipment resources from various Consortium member companies. The primary focus of Consortium research has shifted over the years, as RP materials and technology have developed. Earlier efforts were devoted to part accuracy, finish, and secondary processes for creating visual models and functional prototypes. Current work has advanced toward improving the efficiency of prototype and intermediate-volume manufacturing through the use of agile tooling produced by Rapid Prototyping.
Student Researchers on the Frontier
Undergraduate students at MSOE are pushing the limits of RP technology in a wide range of applications. Working with NASA, a student team is developing RP as an on-orbit system for producing new and replacement components for future human missions to Mars. The author and scientists at NASA Johnson Space Center and NASA Marshall Space Flight Center have modified a Fused Deposition Modeling system for operation in zero gravity. Recent experiments on this system have been performed aboard the NASA KC-135, a specially modified aircraft that flies a series of parabolas to achieve 0g for 25 seconds at a time. This experiment will be followed by a Space Shuttle payload devoted to RP in orbit, to be flown in late 2000.
Students have used RP techniques to produce extremely complex, multi-chambered hollow models of human vertebrae. CT images of various bone densities are turned into models built using RP. After these models are filled with appropriate composite mixtures in a separate step, the resulting objects replicate the nonhomogeneous mechanical properties of bone. These vertebrae can be used in applications such as high-fidelity crash test dummies and surgical implant certification. In addition, accurate 3D molecular models of proteins, DNA, and other structures are being built using RP for educational and research purposes.
Manufacturing in the 21st Century
Rapid Prototyping is continuing to evolve, serving not only the current needs of companies involved in product development, but also as a manufacturing technique in its own right for advanced materials and products. If one extends current manufacturing processes such as overmolding 20 years into the future, it is possible to imagine a cell phone that is produced not as an assembly, but rather as a single, extremely complex composite. Rapid Prototyping can be a key technology in enabling this vision. By building objects one layer at a time, it is possible to embed electronic components, making them an integral part of the resulting device. Piezoelectric and optical sensors, for example, have been embedded into composite sheets such as those used on the skin of high-performance jet aircraft. These integrated sensors can continually monitor stress and chemical changes within the skin of the aircraft, detecting damage and sending an appropriate signal. Custom-shaped "smart" devices such as wearable computers are also under development. These devices are built up in layers of polyurethane, with electronic components and interconnects embedded within the nonconductive housing package during the additive manufacturing. The end result is a waterproof, rugged item customized to the needs of the individual user.
This review has just touched on some of the exciting developments in this rapidly growing field. While the concept of a Star Trek-type replicator is far in the future, it is clear that automated production of extremely complex products will be required to keep up with developments in other technology areas. As we enter the new century, our ideas of products, materials, and manufacturing are changing dramatically; Rapid Prototyping will likely be an integral part of this paradigm shift.
For Further InformationMilwaukee School of Engineering Rapid Prototyping CenterDr. Robert Crockett(414) 277-7275www.msoe.edu/rpc
Indexes to Further Topics:www.wohlersassociates.com/Carnegie Mellon Univ.www.cc.utah.edu/~asn8200/rapid.html
RP Equipment Suppliers:www.3dsystems.comwww.stratasys.comhelysis.comwww.dtm-corp.com
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