To date, some rapid tooling techniques have been successfully deployed in the automotive industry, such as the process developed by the Manufacturing Systems department of Ford Motor Co.'s (Dearborn, MI) Scientific Research Laboratory (see the article "Rapid Tooling Technology From Ford Country" in the November 2001 issue of Manufacturing Engineering.) Ford has been licensing the technology to companies for producing dies, punches, and other tools by a thermal-spray process it developed by which a ceramic master of the working die surface is spray coated with molten metal, then backfilled with epoxy to create a working die.

Traditional rapid prototyping approaches employ several additive prototyping processes, such as stereolithography (SLA), selective laser sintering (SLS), fused deposition modeling (FDM), laminated object manufacturing (LOM), and 3-D printing, that all typically produce rapid prototypes created from various types of plastics, thermoplastics, photopolymers, and liquid resins. Most additive technologies used in rapid prototyping, rapid manufacturing, or rapid tooling are layer-based processes that create plastic or metal prototypes and small-run production parts directly from 3-D CAD models. They offer an alternative to subtractive processes for parts production with metal-removal methods using CNC metalcutting.

Direct metal technologies that generally use powdered metals for rapid tooling and manufacturing have recently gained favor among manufacturers. In August, several metal-based rapid technologies were outlined in papers presented at the Direct Metal Systems in Manufacturing technical forum sponsored by the Rapid Prototyping Association of SME (RPA/SME). The program detailed several metal-based rapid technology efforts either currently offered or under development at companies including ArCam AB (Mölndal, Sweden), EOS GmbH (Munich, Germany), POM Group Inc. (Auburn Hills, MI), the ProMetal division of Extrude Hone (Irwin, PA), Solidica Inc. (Ann Arbor, MI), 3D Systems (Valencia, CA), and the US Army's Tank-automotive and Armaments Command (TACOM, Warren, MI).

In contrast to the plastics, thermoplastics, and resins used in rapid prototyping processes, most of the metal-based rapid processes involve powdered metals and other metallic feedstock, including binder-coated powder, wire, flat wire, ribbon, foil, sheetmetal, and atomized spray, according to Vito Gervasi, Milwaukee School of Engineering, who presented at the Direct Metal Systems forum. Gervasi notes that the list of creative direct metal processes continues to advance, with a growing list of players and machines in service. Capable of producing metal parts to more than one meter in size, Gervasi says that a number of the processes are no longer lab curiosities, and are in use by industry.

With tougher thermoplastics, rapid prototypes like this automotive differential made with the fused deposition modeling process from Stratasys can be used within working systems to test form, fit, and function.

Among these technologies, the POM Group's patented Direct Metal Deposition (DMD) process features a laser-based additive metal process in which POM claims to produce fully dense metal parts of 99.8% density, which is slightly better than the 99.6% density of 'handbook grade' metals from castings manufacturers, according to Dwight Morgan, president of The POM Group Inc. (Auburn Hills, MI). Originally developed at the University of Michigan (Ann Arbor, MI) by POM co-founder and CEO Jyoti Mazumder, the DMD process isn't considered rapid manufacturing by Morgan, who instead refers to the process as "direct manufacturing," because DMD makes parts directly from powdered metal.

"If we were to take 10 of the most prominent names in rapid prototyping and ask us all to write down the definition of rapid manufacturing, I'm not sure that you'd get a consensus, but I'm sure if you asked what rapid prototyping is, you would get a consensus," says Morgan. "That has to do with the maturity of that particular industry, and the fact that people know what rapid prototyping is.

"My definition of rapid manufacturing is using a rapid prototyping technology as the precursor to a pattern used to build something in the future," Morgan adds. "In other words, an SLA, SLS, or LOM model is used to create a seed part, which you would then use to make tools. Using that definition, I would definitely concur that, on a commercial perspective, no one is out there using rapid manufacturing, because it's not yet an economically viable process. If you think about rapid prototyping as a process that is quite cost-effective when used to make a rapid prototyping part, and to then convert that into the pattern, you need to put a lot of time into it to perfect it, and then you begin the manufacturing process. The time and cost penalty is one of the drawbacks involved in using rapid manufacturing.

"Having said that, POM views itself as a direct manufacturing company, not a rapid manufacturing company, in that we bypass the reliance on a rapid prototyping technology as the precursor to ours, and we work directly from the native data file," Morgan says. "We call it 'From CAD to Steel Directly,' and that's what we're trying to get at--that whole concept of direct manufacturing."

Similar to RP systems, POM's DMD takes 3-D solid models from any CAD system and inputs the data directly into its machines. "We take that solid geometry and the machine slices it mathematically into layers, processes it like a rapid prototyping process--except that it's building it in the material from which you want to make the part," Morgan notes. "It's a computer-controlled, layer-based microcasting process, where we're actually recasting fully dense material, layer by layer.

Porous metal parts can cause problems if the parts produced are more brittle than desired, and some direct metal approaches have had difficulty with higher levels of porosity that affect rapid metal parts' longevity. For instance, ProMetal says it has improved the porosity percentages of parts made through its metal-based parts' 3-D printing process, but other companies say this continues to pose problems for manufacturing reliable parts. ProMetal's process has made strides in improving porosity on the prototypes in achieving less than 1.5% porosity, and later this year the company will announce a fully sintered stainless steel.

"We are the only process that works in what we call homogeneous metals," notes Morgan. "It does not require any binders, which create problems. Some other processes use the binder as an adhesive that binds the actual powdered metal together. It holds it in place as their machine jets it onto the form shape, so they actually ink-jet it basically, it's metal printing. Instead of printing out ink, it prints with metal."

Military and aerospace applications with lower-volume parts production have made for a good fit with POM's DMD process, he adds. POM recently has been working with the Russian government on refurbishment of older military aircraft engine turbine blades for use in power-generating steam turbines. "Obviously, you're not going to build a million pieces this way, because it's just like asking someone to use something like SLA to make a million plastic parts. There is a much better way to do that--it's called plastic injection molding. The same thing is true with making metal parts. We look at companies that need to make fully functional prototypes. That can be really any manufacturing company that needs to make a one-off metal prototype which replicates the microstructure and fully replicates the physical properties of the part they would manufacture by die-casting, for example."

Aside from niche applications in the general industrial repair and overhaul market, POM is focused on refurbishing tooling. Automotive suppliers, appliance makers, lawn tractor manufacturers all have bought one-off prototypes in full metal, Morgan says, and the POM process can manufacture high-strength alloys, including titanium and Inconel. "They don't want to rely on a process that's limited on which metals they can use, or that relies on a binder, because they cannot rely on the physical properties of the metal. Also, the part wouldn't replicate the aluminum or steel part that they were going to use in production. Our process relies on a commercial powder-metal market that existed before POM ever started. Any powdered metal that you can buy can be used in our process. For aerospace, we make a lot of Inconel direct-metal prototypes."

Direct metal casting with Z Cast powdered metal in Z Corp.'s 3-D printers allows manufacturers to quickly build molds for metal parts.

The US Army TACOM also has tested direct metal approaches to parts production with its initial prototype application of its Mobile Parts Hospital (MPH) using Optomec Inc.'s (Albuquerque, NM) Laser Engineered Net Shaping (LENS) technology. TACOM plans to debut a second rapid manufacturing system prototype of the mobile parts hospital, which aims to rapidly produce metal replacement parts for military vehicles in the field, using POM's direct metal manufacturing process. (For more information, see the MPH Web site at www.mobilepartshospital.com.)

To meet TACOM's requirements, POM is converting its system to a direct diode laser, instead of its system's previous CO² or Nd:YAG lasers, in order to reduce the amount of electrical power needed to run the MPH rapid manufacturing system. "They want to take CAD geometry for military vehicle parts and build them in the field, in the actual arena of war," Morgan adds. "We're converting our process to a diode laser, which is a high-powered laser but very efficient electrically. It can be driven off a Caterpillar generator. We are building the next-generation machine that will go in their Phase 2 MPH, Mobile Parts Hospital, that'll be out by this December."

Building functional rapid prototypes for manufacturers out of newer, tougher thermoplastics capable of withstanding heat up to 400ºF allows traditional rapid prototyping companies, including Stratasys Inc. (Eden Prairie, MN), to supply RP systems that are effectively used in rapid manufacturing. "We don't sell them as rapid manufacturing systems, although people tend to use them for that," says Stratasys' Joe Hiemenz.

"If you look at materials like PPSF [polyphenylsulfone] the strength and the heat deflection that come into play allow manufacturers and designers to build products and prototypes with properties that are very close to what they're already going to be in the final product," notes Patrick Robb, Stratasys product manager. "It means we can work in a high-temperature environment, and customers can build not only parts for prototype, checking form and fit, but function. One of our customers strapped rocket boosters on these plastic models and shot them off to test capabilities and functionality."

"A trend that has emerged over the last few years is that people want more and more functional models," Hiemenz adds. "The days of just looking at them are fast ending, and that's one of the things that have been propelling us, because that's our forte. With ABS, PPPC, and the PPSF, we're taking off in tooling, so that's right into this new rapid manufacturing realm, because these are really tough materials."

Rapid metal's successes could eventually result in dramatic changes for the traditional rapid prototyping market, according to Marina Hatsopoulos, co-founder and CEO of Z Corp. (Burlington, MA), a longtime supplier of lower-cost 3-D printing rapid prototyping machines. In the past year, Z Corp. introduced its new Z Cast powdered metal, which is a proprietary mixture of powdered metal, plaster and ceramic composite designed for casting aluminum and other nonferrous metals, aimed at high-speed, low-cost moldmaking for direct metal parts production. With Z Cast, manufacturers can use powdered metal in Z Corp.'s existing 3-D printers to fabricate molds for molten metal.

"Our 3-D printers that use the other materials are more useful for early-stage concept development, but the [Z Cast metal] casting really allows us to get into low-volume production or functional testing," Hatsopoulos says. "If you think about the traditional casting process, you would normally have to go through a machining process to make a tool that could then be used to make the molds, and our approach circumvents that entire step. It enables you to go directly to making a mold of any complexity. That only makes sense in lower volumes--if you're getting into very high-volume production, it wouldn't make economic sense."

Custom manufacturing in lower part volumes has made more rapid manufacturing approaches of metal parts feasible, she adds. Small production runs of 10 - 20 parts can allow using the Z Cast material. "There's been a push to custom manufacturing," says Hatsopoulos. "It could be that there are parts that are slightly different, so that would require making a completely unique tool for each one.

"The way we see the market diverging is that the rapid prototyping market of the last decade will really go away," Hatsopoulos states. "Thinking about stereolithography, what we believe is that ultimately it's going to diverge in two directions: one direction is going to be concept modeling, 3-D printing, which is the bulk of what we sell, and that has to be extremely fast, office-compatible, very easy to use, and very economical. It has to be the equivalent of a printer--you want to get the workpiece fast and cheap in your hand, full color. And then the other direction will be meeting the needs of the manufacturing community by having parts in their materials. We think that the Z Cast product goes right to that, because it gets you a full-metal part, a real metal part. If you think about some of the other RP technologies, they do neither one nor the other. They're not giving you a part in your final material--they're slow, they're expensive--and we think that that middle ground of RP will ultimately go away."

POM Group Inc Extrude Hone Corporaton Stratasys Inc Z Corporation