The light has been turned on at some firms. A few manufacturers are finally seeing the opportunities that rapid prototyping (RP) technology can bring to custom-made production parts, a goal often termed mass customization. The most recent consumer example is the widely advertised Invisalign teeth-straightening process by Align Technology Inc. (www.invisalign.com) that is widely advertised on television and in print media. An orthodontist obtains an impression of the patient's teeth, which is then scanned to create a digital image. This CAD file is then modified to show the position of the teeth in as many as 30 progressive steps as the teeth move from malocclusion to perfection. Stereolithography, usually abbreviated as SLA, is used to create molds or patterns from this CAD file over which flexible polycarbonate "aligners" are formed. The aligners are then dispensed to the patient over time as the treatment progresses and the teeth move to the intended position. Use of similar technology for the manufacture of custom, high-cost parts will become more common, particularly if Invisalign is a commercial success.

Within the Rapid Prototyping Consortium at the Milwaukee School of Engineering (MSOE) we are also seeing a greater realization that RP can be a real time- and cost-saving technique if it is used earlier in the design process. For example, the use of concept modelers, such as the Z-402 by Z Corporation (www.zcorp.com), allows the inexpensive production of six different iterations of a handheld prototype for a tool manufacturer like Snap-on Incorporated. These concept models are not as accurate or as durable as more typical RP parts but are good for preliminary evaluation, even before the basic design is finalized. This not only accelerates the creative side of the design process but does so at a very low cost. Of the six alternative ideas, only one or two are retained for further evaluation -- and the rest are discarded.

The RPC is also seeing migration of its industrial membership to the need for and greater use of functional prototypes. In the past, prototypes produced by any of the typical RP processes were most often used for evaluations of form, fit, and finish. Now, the pressure is on RP equipment manufacturers to produce materials that allow functional testing of a design concept. The adjacent photo shows an intake manifold for a dirt-track race car that was fabricated using the selective laser sintering (SLS) process (www.dtm-corp.com) and a glass-filled nylon material called Duraform GF. The strength and heat resistance of this material allowed this manifold to be used directly on the engine for track testing rather than just as a pattern for a casting or a machined metal component.

Porsche had a similar need to assess the cooling problems of a high-performance V6 racing engine. Using SLA equipment (www.3dsystems.com), Porsche built a transparent flow model of a cross-flow water jacket for this engine, including all the crucial sections of the crankcase and cylinder head. Since the SLA builds the model directly from the CAD data, accurate representation was assured from the start. Over 60 sensors in the water jacket helped determine local flow temperature and pressure conditions. The transparency of the SLA model was key to the success of the cooling test as tiny air bubbles were injected into the coolant fluid and their motion recorded with a high-speed video camera. A careful frame-by-frame analysis of these images exposed stagnation zones, revealing insufficiently cooled sections within the water jacket. It is this type of functional prototyping that is becoming more common and essential in a globally competitive economy.

In some quarters the term rapid prototyping has also been expanded beyond the basic layer-by-layer fabrication process that is common to all commonly available RP processes to include high-speed machining (HSM). Advances in tool-pathing software and the allowable feeds and speeds of cutting tools have made HSM a viable choice for today's production of rapid prototype components and tooling. Many readers with traditional manufacturing experience may see the use of machine tools as little more than a new twist on old technology.

A closer look suggests that with recent improvements in machine control software and cutting-tool technology, HSM has become a viable RP process need that can also be extended to the rapid tooling. For example, a blow mold for plastic soft-drink bottles can be finish machined at 10,000 rpm and a feed rate of 150 inches per minute (ipm) using a coated 8 mm ballnose end mill. In this case, the material of choice is H-13 steel with a hardness of 50 Rc. At this speed, the entire milling operation can be completed in less than one hour, clearly demonstrating HSM as another means of getting to a functional prototype or production tooling quickly. As the typical RP materials are still limited by mechanical properties that may differ significantly from the desired material in the finished part, designers and manufacturers alike should expect to see a wider application of HSM in the future.