The road from rapid prototyping to rapid manufacturing seems more like a highway nowadays. More and more companies have started to produce metal tooling components and even some finished parts on solid imaging systems once consigned to prototyping duties. But the widespread adoption of "digital manufacturing" technologies still depends on the development of materials that can match—or even outperform—traditional machined metals. 3D Systems (www.3dsystems.com) has been doing its part lately with a collection of metals for its selective laser sintering (SLS) machines.
Working directly from CAD files, these machines create parts from layers of sintered metal or thermoplastic powders. On the metals side, the company has recently come out with a new material that can serve as a stand-in for P-20 tool steel. Called LaserForm ST-200, this new material starts out as a 420-stainless steel powder. After sintering on the SLS machine, ST-200 parts go through an oven processing step which infiltrates the steel with bronze, creating fully dense parts. Mark Kosek, 3D's business development manager for tooling and casting, likens it to a "stainless steel sponge filled with bronze." The resulting parts consist of 56% stainless steel and 44% bronze for a final density around 6.73 g/cm³. This steel-and-bronze matrix material exhibits a tensile strength of 435 MPa, a tensile yield strength of 250 MPa, a compressive yield strength of 277 MPa, and elongation of 6%.
The main property differences with P-20 relates to hardness and thermal conductivity. ST-200 measures 79 on the Rockwell B scale, significantly softer than the Rockwell C hardnesses of ordinary P-20 steels. "But the availability of softer steel can be an advantage for those interested in more fast finishing," notes Mervyn Rudgley, 3D's senior director of product management. And Kosek notes that, softer or not, ST-200 has already proved itself in real tools—in part, he says, because the matrix material has good impact properties. He cites a recent customer trial, for example, in which ST-200 inserts, slides, and rails for an injection mold showed no visible signs of wear after 40,000 shots of a 30% glass-filled nylon.
The thermal conductivity of ST-200, at 39 W/m8C, exceeds that of ordinary P-20—a characteristic that Kosek says can ultimately improve the molding productivity by cutting the cooling component of cycle times.
Easy UseST-200 isn't the first tool steel 3D Systems has offered. In fact, at first glance, this new steel seems to fall short of its predecessor, LaserForm ST-100, which actually offers higher mechanical properties and faster build times. But ST-200 has a couple of characteristics that should give it an edge over its forerunner: For one, ST-200 produces parts with roughly 50% more strength coming off the machine. For another, the material development went hand in hand with hardware and software enhancements that improve the overall resolution of the SLS system.
Together these changes make it easier to tackle parts with difficult geometries—such as delicate protruding features, sharp edges, or long thin sections. ST-100 has handled its share of challenging geometry in the past, but it did so with lower yields on the toughest jobs. Or as Kosek puts it, ST-200 makes "difficult geometry more buildable and more survivable." The extra green strength decreases the chance of breakage as the parts make their way through the oven and also allows the parts to withstand more aggressive removal of powder residue with a blast of air. "You can now be quite vigorous with the compressed air," Rudgley says. This last advantage is hardly trivial. Kosek notes that the core and cavity sets for advanced injection molds can have interior cooling passageways that easily trap powder residue. This ability to remove that powder can ultimately increase tooling productivity by enabling conformal cooling passages—ones that closely follow the contours of the molded part to increase heat transfer.
For all the material's promise in tooling applications, ST-200 also supports the move into metal part production. "More than 50% of our ST-100 customers already make as many finished parts as tooling components," Rudgley reports. And he points out that these finished metal components tend to have the very sort of delicate features that ST-200 targets. "Some users shied away from jobs that pushed the envelope on geometry," he says. "Now, they'll be able to tackle these jobs."
More Metals Coming
3D Systems has set its sights on producing other useful metals for tooling and finished components. The first one, due out early this year, closely approximates A6 tool steel. "This material gets us well into the Rockwell C range," Rudgley reports. It's intended for those applications in which maximum longevity and durability outweigh any benefit from faster finishing. And 3D Systems also has its first aluminum slated for introduction later this year. "That one will be really big," Rudgley predicts.