Rapid prototyping rocks on

Rapid prototyping may be something of a misnomer when it comes to those systems that turn out metal, rather than plastic, parts.

Solidica's Form-ation rapid prototyping system combines ultrasonic bonding and on-the-fly milling to build metal prototypes.

These direct-metal processes are now poised to make the leap from the world of prototyping into full-blown rapid manufacturing. Direct-metal systems have already turned out tooling components used for production runs. Even more promising, these systems could soon create new design possibilities—by turning out finished parts whose low production volumes or complex shapes don't lend themselves to traditional manufacturing processes.

The recent SME Rapid Prototyping Exhibition highlighted these emerging direct-metal capabilities. Among the developments, Solidica Inc. (Ann Arbor, MI) unveiled a new system that build parts from layers of ultrasonically bonded and machined metal. And EOS GmbH (Munich, Germany) showed its latest high-resolution systems, capable of bringing new levels of detail to finished parts. Here's a closer look at both systems:

The sound of metal. Like other rapid prototyping systems, Solidica's new Form-ation works from CAD data to build up parts, layer by layer. But the similarities stop there. Rather than sintering powdered metal, the Solidica machine makes parts from layers of ultrasonically joined metal sheets, which serve as sort of a rough cut of the prototype's net shape. As the layers come together, the machine also uses an integrated, three-axis machining center to clean up the part surfaces and add detailed features on the fly. "A true net shape part emerges from the machine," says Dawn White, Solidica's president and the technology's inventor.

According to White, Solidica's patented ultrasonic consolidation (UC) technology offers a couple of key advantages over rapid prototyping methods that must melt metal powders: Ultrasonic energy creates a solid-state bond between adjoining layers, so the process dodges the consequences of liquid-solid phase transformations. "We avoid the residual stresses, dimensional changes, and metallurgical incompatibilities," she says. And this notion of avoiding metallurgical mismatches ties into potentially avoiding some design limitations related to metals. Right now, Solidica supports only a handful of aluminum alloys, but White says that the underlying UC technology works with a much wider variety of metals. She reels off a list that includes mild and stainless steels, magnesium, copper, nickel alloys, and titanium. UC can join these metals not just to themselves but also in various combinations, making possible the creation of parts whose properties vary through their cross sections.

Another capability of the process that could bolster rapid manufacturing efforts comes from an ability to encapsulate electronic and thermal-management components. White says the Solidica process has successfully encapsulated tungsten films, stainless steel mesh, various sensors, and optical fibers—all in aluminum. "We think would could encapsulate other things as well," she says citing structural fibers and MEMS devices as two possibilities. Short of full encapsulation, the system's on-the-fly milling operations can also cut pockets for circuit boards and other electronic components.

For all the high-tech potential of the system, it will initially target somewhat more down-to-earth tooling applications. The reason is that the Solidica machine can turn out nearly complete injection mold inserts in a single step, cutting as many as three steps from conventional rapid tooling methods. All the inserts need once they come off the machine is polishing or texturing. White reports that a 4- × 6-inch core and cavity set takes about nine hours to make. In addition to injection molds, the process has tackled thermoforming and blow molding tooling, too.

As for process capabilities, the Form-ation machine has a 24- × 36- × 10-inch work envelope and build speeds up to 200 ipm. The first versions of the machine, five of which are now in final testing, offer an accuracy of ±0.008 inch over the entire work envelope. White notes, however, that the commercial Form-ation machine features design improvements that bring that accuracy within ±0.002 inch.

Finer powders. In powders, EOS has bumped up the level of accuracy and fine detail obtainable with its Direct Metal Laser Sintering (DMLS) system. This system produces solid metal parts layer by layer from metal powder mixtures whose constituents have different melting points. Energy from the EOS machine's CO₂ laser melts the mixture's low-melting point components, which then bind together the unmelted particles of the high-melting-point components into a strong matrix. "This aspect of our process is related to conventional sintering," notes Michael Shellabear, a mechanical engineer who oversees the DMLS product line.

Yet the differences from conventionally sintered parts are what make DMLS well-suited to the manufacture of precision parts. For one thing, DMLS uses no binders, eliminating the debinding step and the shrinkage associated with it. "Our parts have their final size and mechanical properties right out of the machine," Shellabear says. For another, EOS has picked its metals and its process to create an expansion reaction during the liquid-phase sintering. "New alloy elements are created that have a higher volume than the initial components," Shellabear explains. And this volume increase compensates for the sintering shrinkage that would otherwise hurt dimensional accuracy.

Keeping shrinkage at bay represents just one element of the system's accuracy. EOS, like other makers of rapid prototyping machinery, has over the years refined the hardware and software features that contribute to accuracy. The company's most recent strides, however, have come in the form of materials enhancements. EOS now offers bronze- and steel-based alloys that make parts in 20-micron layers, down from 50 microns in the past. "The 20-micron materials are selected for applications that require high detail resolution or surface quality," Shellabear says. The new materials also make higher densities possible—because smaller powders pack together tighter. EOS can adjust final part densities according to application requirements, but Shellabear cites maximum densities of nearly 100% for the 20-micron powders, up from 95% with the earlier materials. "And higher densities mean higher mechanical properties," he says. "As a rule of thumb, our bronze powders have similar mechanical properties to aluminum and the steels are broadly similar to cast iron." Temperature limits, meanwhile, are roughly 650C for the steel and 450C for the bronze.

These new capabilities have been put to good use in tooling applications needing good surface quality, fine detail, or both. "Inserts for injection molds are still the largest use," Shellabear says. But EOS systems have produced die-cast, rubber, sheet metal stamping, and blow molding tools, as well. The 20-micron DMLS powders can produce parts with surface finishes as smooth as Ra0.25 microns when used in conjunction with a shot peening finishing step. "Further polishing is no problem, even up to a mirror-like surface," Shellabear says. As for the high-resolution detail, Shellabear shows off parts with narrow slots, thin walls, and small holes down to 0.001 inch.

The DMLS process also allows tooling design twists that would be tough to machine. A recent tool for a golf ball, for example, had conformal cooling lines built into the inserts and left some regions of the cavity intentionally porous, in order to create a venting system. Some of the molds created with the EOS powders have also proven durable enough to cross the line from prototype to production tooling. As an example, Shellabear points to a glass-filled PC/ABS modem base that has run on a DMLS tool for more than 100,000 shots. Another tool, this one for a die-cast aluminum clutch housing, has lasted for more than 500 shots. Then there's speed. Shellabear says it typically takes less than a week to go from CAD data to injection molded parts—and only a fraction of that time is the CAD system's build time.