This article discusses the rapid solidification process (RSP) which is being developed by Idaho National Engineering & Environmental Laboratory, a US Department of Energy research lab in Idaho Falls. This process drastically lowers the costs and lead times of production tools. In an approach that is radically different from conventional tool making, in which a mold core and cavity are machined from a block of steel, RSP equipment creates a shape by spraying molten metal onto a pattern, faithfully reproducing the pattern's shape, details, and texture. Tools can be completed in as little as three days, and are suitable for both prototyping and production runs. RSP differs significantly from other commercial spray metal techniques, broadly known as thermal spray. RSP tooling can undergo either conventional heat treatment or low-temperature heat treatment known as artificial aging. Artificial aging allows the tailoring of steel properties, such as hardness, toughness, and thermal heat resistance, without the risk of tool distortion.
Many mass-produced items, ranging from cell phones to automotive parts, are formed from molds or dies that require specialized machining and long lead times to produce. Manufacturers, for their part, are always on the lookout for ways to cut tooling costs and get products to market sooner.
A new technique, which is called the rapid solidification process, or RSP, is being developed by Idaho National Engineering & Environmental Laboratory, a U.S. Department of Energy research lab in Idaho Falls, to drastically lower the costs and lead times of production tools.
Those costs can be significant, according to Kevin M. McHugh, advisory scientist at the lab and a researcher on RSP technology. Plastic injection molds, for example, can cost upward of $300,000 and require three to six months to produce, followed by another three months for tool checking and part qualification. Large casting dies and sheet metal stamping dies can cost even more— $1 million—requiring lead times of 40 weeks.
In an approach that’s radically different from conventional toolmaking, in which a mold core and cavity are machined from a block of steel, RSP equipment creates a shape by spraying molten metal onto a pattern, faithfully reproducing the pattern’s shape, details, and texture. Tools can be completed in as little as three days, and are suitable for both prototyping and production runs. The process can create tools of virtually any common tooling alloy, including P20, H13, and D2 tool steels, copper, aluminum, gray iron, Invar, and Kirksite. RSP can be used to manufacture tools for a range of processes, including injection molding, blow molding, die casting, stamping, and forging.
The lab is completing an agreement with Global Metal Technologies Inc., a die caster headquartered in Chicago, which plans to sell RSP production equipment to tooling companies and operate service bureaus that are capable of making tools.
Ed Buzanoski, a rapid prototyping consultant and president of BKP Intech Inc. in Dearborn, Mich., believes that RSP will be used initially to make prototype tools, but eventually will find its way into production tooling as users become more familiar with the process. Buzanoski has studied the process and has used the initial RSP equipment at the lab in Idaho to produce tools in benchmark tests, successfully producing molds of 60 Rockwell hardness in one week.
RSP tooling begins with a mold design described by a CAD file, which is converted to a tooling master by a rapid prototyping technology such as stereolithography, or SLA, McHugh said. Metal spray is deposited on a pattern, which can be made of various materials, depending on the tooling alloy that is being sprayed.
SLA to RSP
Because the temperature of molten tool steel is too high for most plastics, a castable ceramic pattern is made from the rapid prototyping master—a process that takes intermediate steps. Ceramic casting techniques are extremely accurate, and are often used in the tooling industry. Lower melting point alloys, such as Kirksite, may be sprayed directly on an SLA part.
RSP itself is relatively straightforward. Molten metal is sprayed against the ceramic pattern, replicating the pattern’s contours, surface texture, and details. It’s done one time for the core and once for the cavity.
After spraying, the molten tool steel is cooled at room temperature and separated from the pattern. The irregular periphery of the freshly sprayed insert is squared off, either by machining or, in the case of harder tool steels such as H13, by wire EDM. The squared-off insert is then fixed to a mold base. Injection molds or die-cast molds require two inserts, one for each mold half.
Starting with the master, turnaround can be completed in as little as three days, said McHugh. The most time-consuming portion of the process is preparing the pattern for spraying, he noted. The actual spraying of the molten tool alloy onto the pattern takes as little as four minutes. “It’s a rapid buildup,” he said.
McHugh estimates that, after designing an insert, it takes one day to produce an SLA model, one day to cast the ceramic pattern, five minutes to spray the molten metal on the pattern, and one to three days to mount the insert in a mold base.
The process starts with nearly any form of the metal— cast ingot, forged metal, powder, or scrap—that has the desired chemistry of the tool alloy. The metal is loaded into a crucible, where it is heated to about 100°C above its melting point. The molten metal is injected into a nozzle, where it is exposed to a flowing gas stream. From that point, the device operates much like a conventional paint sprayer. The high-velocity gas jet breaks the molten metal into tiny droplets, on the order of 50 microns each, which are carried by the gas stream and deposited on the pattern surface. Transit time from the nozzle to the substrate of the pattern is about 10 milliseconds.
RSP equipment is a self-contained unit. The equipment is configured so that the nozzle remains stationary while a robotic arm manipulates the pattern. The metal is maintained in a liquid state while it is inside the nozzle. Upon exiting, the gas stream flowing through the nozzle entrains gas equal to about eight times its own volume. Gas at room temperature is drawn into the jet, cooling the droplets and causing them to solidify rapidly. The spray of droplets results in a huge increase—by 10 orders of magnitude—in the surface area of the metal, explained McHugh. The large increase in surface area causes heat to be extracted rapidly from the particles as they travel from the nozzle to the substrate. About 70 percent of the droplets are solid by the time they hit the pattern; the remaining liquid fraction is enough to “weld” the metal together on the pattern surface.
As an extreme example of rapid solidification, McHugh spray coated a thin layer of tin alloy on an inflated party balloon. The tin alloy, which had a very low liquid fraction, would not be a suitable tooling material, but it does provide a striking example of the heat dissipation possible with the process. The alloy was heated to 300°C, yet dissipated enough heat in the spray so that it did not pop the balloon on contact.
Because the metal solidifies very rapidly, it forms a continuous buildup (rather than layering) on the pattern. “We prefer to bathe the entire pattern in a uniform mass and heat flux of the spray to ensure that the properties are good and we don’t see any warpage in the spray deposit,” McHugh said.
At this point in its development, RSP is limited to creating tools with a maximum aspect ratio of 3:1. McHugh estimated that the process could be used to create about 70 percent of the tools used in industry.
RSP differs significantly from other commercial spray metal techniques, broadly known as thermal spray, according to McHugh. One of these, arc spraying, uses the feedstock metal in the form of two wires that are kept on a spool. The wires are fed into a spray gun and an arc is struck, producing enough energy to melt the wire tips. Gas is blown over the tips to form droplets that are deposited on a pattern.
In arc spraying, the metal must be ductile enough to be wound into a wire, which may eliminate using materials such as tool steels, McHugh said. Arc spraying systems typically use materials such as zinc-based alloys, which are quite ductile, and are limited to outputs of approximately 15 pounds per hour.
RSP, by contrast, can be applied to a broad range of tooling alloys and can handle high outputs. A bench-scale system now at the laboratory is capable of deposition rates of 500 pounds per hour. Plastic injection molds typically have core and cavity thickness of 1 or 2 inches; die-cast mold inserts may be thicker, from 4 to 6 inches thick. In both types of tools, spray buildup of the tool steel is rapid, taking only a matter of minutes. McHugh added that the two processes also have differences in the thermal and physical properties of the droplets, resulting in more subtle differences in the microstructures of the deposited materials.
RSP tooling can undergo either conventional heat treatment or low-temperature heat treatment known as artificial aging. Artificial aging allows the tailoring of steel properties, such as hardness, toughness, and thermal heat resistance, without the risk of tool distortion.
Conventionally processed tool steel that is melted and cast at the steel mill contains segregates, resulting in nonuniform material properties that make high-temperature heat treatment necessary. Segregation occurs when molten metal solidifies slowly. Typical tool steels, which are purchased in a soft form so they can be machined, must undergo heat treatment, typically at about 1,000°C, followed by quenching and tempering, to increase their strength, hardness, and other properties.
With RSP, tool steel is melted and mixed up in the crucible, then solidifies rapidly during spraying, to prevent a chance for segregation to take place. RSP spray forming results in tool steel with a more uniform carbide distribution, allowing it to be artificially aged at a lower temperature. In tests, H13 tool steel that has been artificially aged compares well with commercial forged and heat-treated steel, said McHugh. “Strength is somewhat higher, and it seems to be maintained better at higher temperatures compared to the commercially heat-treated tool steel,” he said.
RSP also may enable the use of unique tooling alloys that would be prohibitively expensive with standard tooling, according to James R. Knirsch, RSP tooling development manager at Global Metal Technologies’ Solon, Ohio, plant, which is licensing the process. Premium grade tool steels such as H13 are made in huge lots with carefully controlled alloy additions. Carbide-forming additions that take advantage of the inherent rapid solidification in the RSP tooling process can be added to the melt, he said. Knirsch added that RSP makes it possible to experiment with exotic alloys, such as spraying very hard materials that are difficult to machine, including titanium nitride.
RSP tools offer the possibility for more efficient cooling. For example, it makes conformal cooling—in which cooling lines closely follow the contours of a mold cavity—easier to incorporate into molds, according to McHugh. Conformal cooling normally isn’t possible with conventionally machined molds and dies. With RSP, “You can spray metal onto a pattern, stop the spray, attach a cooling line, and spray metal to encapsulate the cooling line,” he explained. The material used to back up the steel can be nearly any metal, including good thermal conductors such as aluminum or copper, he added. During production runs, conformal cooling reduces the part cycle time and improves productivity.
The RSP technique can be used to manufacture either prototype or production tools. But advocates of the process say the real advantage is that production tools can be produced in prototype tool lead times. RSP also enables the fabrication of true prototype parts—in other words, with the right dimensions and surface finish—to be incorporated directly into the production run. This can be a big advantage to automotive companies, which normally require two sets of tooling, one to produce parts for destructive testing and another for production, McHugh said. RSP technology may make it possible to qualify a part and go directly into production with one set of tools.
In die casting, for example, prototype parts are not made on production tools, and frequently react differently when the job goes into production, said Knirsch. “In aluminum die casting, prototypes are generally made out of a different type of aluminum and on a different process, with different accuracy. You test them, and everything is fine. Then you run them in a die-cast process, and things are different.” True prototypes can save die casters as much as a month in developing a production process, since this production enhancement can now be done during the production stage, he said. He estimated that RSP technology could reduce die-cast tooling lead times by 50 percent.
Knirsch also expects RSP to help die casters offset replacement tooling costs. In die casting, a portion of the total costs is earmarked for replacement tooling. Tool life varies according to the size and complexity of the parts being produced. Knirsch estimated that die-cast molds typically last for 120,000 to 150,000 parts before needing replacement. This estimate is based on midsize aluminum die-cast parts molded on die-cast machines from 1,000- to 1,500-ton clamp force. Some industry estimates claim higher numbers for small to midsize parts. For an automotive program that may need 3 million pieces, a die may have to be replaced 20 times over the life of the part. To provide for that, tool replacement costs are incorporated into the piece price.
Die casters are often faced with rising tool costs cutting into profits over the life of a part, said Knirsch. Saving replacement costs can be a boon to both die casters and customers, in his view.
“If we can reduce the replacement costs, we can lower the piece price, meet obligations, and continue to stay in business,” he said. In tests that Global Metal Technologies has run comparing RSP die-cast molds with standard inserts, the RSP inserts last 20 percent longer. Longer life is apparently due to better resistance to wear and heat checking, in which small cracks form in the surface, he said.
RSP is very accurate in reproducing the surface of the pattern the steel is sprayed on—so accurate, in fact, that it will even reproduce fingerprints on the pattern, according to McHugh.
However, it can take several steps to go from the original mold design to the ceramic casting, and every step introduces the chance to lose accuracy, Knirsch noted. One area of development is to reduce the number of steps to produce a pattern. Currently, converting from SLA to sprayed-on tool steel requires three steps. Global Metal Technologies has succeeded in going from SLA to freeze-cast ceramics, to eliminate one step in creating a ceramic pattern, he said. He sees the next step as one of two possibilities: making the ceramic using a rapid prototyping technique; or using a machinable ceramic that would provide good accuracy and finish and could offer some time savings. He said the second alternative is now possible, but expensive, although he believes that new materials could make the process more affordable.
In addition, improvements in rapid prototyping equipment are producing SLA parts that are more accurate, with better layering and surface accuracy, providing better parts to work from, he added.
Getting to Market
The laboratory and Global Metal Technologies are working to commercialize RSP. A lab version of RSP equipment at Idaho Falls can produce samples approximately 4 inches in diameter and 2 inches thick. A beta version, expected by this summer, will be able to produce mold inserts 6 inches in diameter and 6 inches thick. Production units are slated to be available in January 2001.
Plans exist to produce machines capable of making inserts or cavities 14 inches in diameter by 10 inches thick, followed by machines that can produce inserts 48 inches square and 12 inches thick.
Global Metal Technologies and the laboratory are working with three other partners: Air Products and Chemicals of Allentown, Pa., which is producing components to handle the gas stream; Inductotherm of Rancocas, N.J., a supplier of induction heating equipment; and Belcan Engineering of Solon, Ohio, which is supplying overall system engineering. They have also teamed up with The Technology House, located in Solon, to form a service bureau to use the first production machine.
Machines will be capable of producing inserts at a rate of one an hour, or 4,000 inserts per year on a two-shift operation. The current accuracy of inserts is within ±0.003 inch, but this is expected to improve as rapid prototyping machine accuracy improves. The price of these machines is expected to be about $750,000, although Knirsch cautions that is a very preliminary estimate.
Global Metal Technologies plans to use the machines in-house, as well as making them available for the open market. Knirsch said the company plans to sell the equipment to large tooling companies and service bureaus, and also plans to start service bureaus of its own.