Despite the advantage of speedy mold building, rapid-tooling technologies have gained only a toehold in the general moldmaking arena. But that could change as new developments improve dimensional accuracy, durability, and cooling efficiency to compete with traditional methods for making production tools.
After a decade of "gee-whiz" headlines, the bundle of technologies known as rapid tooling has made little headway in replacing traditional machining methods, especially for production tooling. These radically new techniques promise dramatic increases in speed of mold manufacture, but they have been plagued with questions about accuracy, durability, and mold finish. In the last three years, at least six suppliers have entered the market with new rapid-tooling approaches that address the former limitations. Those new entries, as well as enhancement of earlier technologies, have made rapid tooling better and even faster to produce, and have proved that it can compete in not just prototype but commercial production tooling. Now there are signs that rapid tooling (RT) at last may be turning the corner toward wider acceptance:
- Dynamic Tooling, a custom molder and RT supplier, has built 200 molds with its Polysteel technology since it was developed two years ago. Polysteel molds account for 70-80% of the firm's molding business. "We recently built three tools for polycarbonate parts in three weeks," says president Paul Vawter.
- Optomec, a user of the seven-year-old LENS rapid-tooling process, has built production molds for half a dozen clients and has four or five other customers ready to give it a try, including a major consumer-products firm. The LENS process reportedly can generate a small tool in a matter of days.
- A new RT process from the POM Group produced a tool for an automotive body part in 31 days, whereas the best a conventional toolmaker could offer was three months.
- Rapid Tooling Technology (RTT), the world's largest user of the 3D Keltool RT process, has a dozen Keltool machines and is looking to add more to keep pace with orders. RTT has refined the 3D Keltool process so that it can now turn out production-grade mold inserts in five days, down from nine days previously. Three years ago, the process took two to three weeks.
The need for speed
Although high-speed machining and other advances have accelerated the pace of conventional moldmaking, "rapid" tools have retained their speed advantage. "Molders have to respond more rapidly to changing consumer demand for automotive, telecommunication, medical, and consumer products, hence they need to tool up faster," says Vawter of Dynamic Tooling. "Not only do molders now need tools faster, but they have shorter product life cycles. And they are often designing tools that are more complex than in the past. These trends run counter to traditional methods of making tools on a longer delivery cycle and designing them to last for millions of cycles."
With delivery times that can range from four to more than 16 weeks (depending on size and complexity), traditional tool making remains one of the slowest and most expensive steps in the injection molding business, says rapid-prototyping/rapid-tooling consultant Terry Wohlers, president of Wohlers Associates. On the other hand, RT technologies today can create molds in two to three weeks and small tooling inserts in as little as 48 hr. The advantage is most pronounced in tooling that involves complex shapes and surfaces.
What makes this speed possible is a fundamental characteristic of RT methods: They are additive processes that build up tools by spraying or "ink-jet" deposition of metal droplets, laser sintering or compression forging of metal powders, electroforming (a plating process), or casting. Conventional moldmaking is a subtractive process that starts with a block of metal and removes material until what remains is the desired shape.
Most RT technologies start with a 3-D CAD model of a part or tool design. Some approaches require the CAD model to be split into "slices" and then the metal-deposition equipment is driven directly from the "sliced" CAD file. Other approaches use the sliced CAD data to drive a rapid-prototyping machine that generates a model or pattern from which the tool is formed.
The latter alternative has been used as a means of reverse engineering. "If we have a die-cast or injection molded part and no information on how it was built, we can cast the core and cavity off the part," says Brad Fox, president of RTT.
Not just for prototypes
Apart from the 3D Keltool process, RT methods have little long-term record of production molding use. RT found easier acceptance at first in prototype or short-run tooling. Nowadays, suppliers of RT technology promote their methods as applicable to full production tooling, as well. "Rapid tooling can serve a market for low-volume prototyping (a few hundred parts), mid-volume production (a few thousand cycles), and high volume (50,000-plus cycles)," says Drew Santin, president of Santin Engineering, a provider of consulting and industrial design and engineering services. Some RT vendors claim their molds can run over a million shots. Rapid-tooling technology can also provide an efficient means of repairing or rebuilding mold surfaces, even those that were made using traditional machining.
As production molding becomes a significant market for RT, suppliers are putting more emphasis on the idea that "rapid" tools are not just quick to build but also can make parts 35-40% faster than conventional molds. That's because additive moldmaking processes lend themselves to incorporation of "conformal cooling"—building in cooling lines that follow the contours of the core or cavity. Conventionally machined molds are limited to straight gun-drilled channels that are often less efficient. RT also allows selective use of thermally conductive metals.
According to Wohlers and Santin, weaknesses can still be found in virtually all RT technologies. These include limited ability for some tools to handle corrosive materials such as PVC or highly abrasive resins such as glass-filled polymers. Other issues include dimensional control when high shrinkage is encountered in powdered-metal sintering or casting methods. However, RT suppliers say they can achieve dimensional accuracies of ±0.005 in. or better, similar to conventional tooling, though secondary machining may be required.
Tools made by some RT processes may have limitations on hardness that could affect service life. But some of today's rapid tooling offers hardness from 25 to 70 Rockwell C, comparable to P-20 and H-13 tool steels, respectively. In addition, RT processes may produce rough initial surfaces, but they can be polished, just like conventional tools. Thus, RT reportedly can attain high-quality surface finish suitable for processing even clear materials.
Fully dense in one step
Among new RT technologies that emerged in the last few years, one is POM Group's DirecTool technique, which was introduced to a wider audience at last year's NPE show in Chicago. Developed at the Univ. of Michigan over 11 years and commercialized in late 1998, this Direct Metal Deposition (DMD) process builds cores and cavities using an industrial laser and powdered tool steel. The tool is built up on a metal plate, following a digital tool path generated from a CAD model of the part. First, a small amount of powdered metal is injected through a nozzle onto a 1-2 mm spot. Then a 2500-watt laser built into the nozzle melts the metal into a small pool, which is shaped by the laser. "As the laser moves away, the heated and shaped metal cools at about 1 million degrees per second," says POM president and COO Dwight Morgan. The process uses an inert-gas environment to prevent oxidation. DirecTool produces a fully dense, homogenous metal tool without sintering.
The mold is built up in 0.01-in.-thick layers at rates of 1-3 cu in./hr, a rate equal to the final finishing speed of a CNC machine, Morgan says. The process has been used with various tool steels, including D-2, F-7, 420 and 316 stainless, H-13, H-19, H-21, and P-20. It can also be used with beryllium-copper alloy. A combination of metals can be used for cost savings. "A low-carbon 1025 or 1020 steel can be used for the base, while the core and cavity are produced from tool steel," says Morgan. Compared with conventional machining, he claims, "The cost savings can be easily 35%, and the lead times have been documented up to as much as 70% shorter." DMD tools reportedly can reach 50 Rc surface hardness and can be highly polished. "The process was used to make a mold set for two optical mirrors for a NASA space application," Morgan notes. The mirrors were polished to 40 angstroms, which is a thousand times finer than the finish needed for an auto headlight tool. "High optical quality allows no tool porosity," he adds.
DirecTool can produce mold components up to 2 x 2 ft and is said to be accurate to ±0.005 in. POM claims 25 clients have used its tools to mold polycarbonate, TPOs, and even PP nanocomposites. The DMD method can also be used to repair existing molds in a process POM calls NuTool.
POM also uses DMD for its CoolMold technology, which provides conformal cooling, and/or a layer of highly conductive metals in the core and cavity with a tool-steel overlay. The process can be used to modify existing molds that have cooling, shrinkage, or warpage problems, and it reportedly can raise mold productivity by 60-65%. Morgan cites one example: "We did a project for a customer making an exterior automotive part. The initial mold cycle was 90 sec. CoolMold cut the cycle time to 65 sec."
No sintering needed
Rapid Solidification Process (RSP) is a new metal-spray deposition method that produces 99.5-99.8% dense cores and cavities. The process was developed at the Idaho National Engineering & Environmental Laboratory (INEEL) in a two-year consortium with 11 companies, including Chrysler Corp., Ford Motor Corp., Johnson Controls, Procter & Gamble, United Technologies, and Lockheed Martin Aeronautical Systems.
The process uses a ceramic pattern built by stereolithography, selective laser sintering, or other rapid-prototyping process. The dimensions of the pattern must be exact because the spray deposition process will mimic surface detail closely, says Marty Sorensen, manager of industry and material technologies at INEEL. The starting metal can be an ingot, forging, powder, or scrap before it is melted in a crucible. Molten metal is injected into a nozzle, where it encounters a channeled flow of inert gas that conveys droplets 1.5 to 2 ft to the part surface. The droplets cool rapidly in flight and solidify instantly when they encounter the pattern. A tin alloy heated to 300 C was sprayed onto the surface of a balloon to demonstrate the rapid cooling effect.
The nozzle can deposit up to 500 lb/hr of material in a bench-scale system. Mold components, depending on complexity, can be built in minutes, says Sorensen. "On a small sampling of selected parts, we hit dimensional tolerances better than ±0.002 in.," he adds. Tool hardnesses can reach Rockwell 60 C. The tooling can also be heat treated and tempered. Conformal cooling lines can be added to the tool by stopping the spray process, placing copper tubing, and then encapsulating it with more sprayed metal.
The RSP process reportedly can be used with any tool steel or softer materials. RSP is in the last stages of R&D, and INEEL is trying to scale up the technology. INEEL is close to licensing RSP to die caster Global Metal Technologies Inc. (GMTI) in Solon, Ohio. In tests, GMTI found that RSP created an H-13 steel tool with a 20% longer life than a conventional H-13 tool. Injection molder Hach Plastics in Loveland, Colo., was part of the consortium that helped develop the RSP process and has a machine in-house.
Another direct-metal tooling technology is the Laser-Engineered Net Shaping (LENS) technology, which was developed in 1993 by Optomec's founders while they were employed at Sandia National Laboratories. Optomec was started four years ago to commercialize the technology.
The three-year-old Directed Material Deposition technique uses a high-powered laser focused on a metal substrate to create a molten puddle on the substrate surface. Metal powder is injected into the melt pool, expanding the volume of the puddle. As the laser scans across the surface, lines of material are deposited in a solid bead of metal. Layers of metal are added until the tool geometry is complete. This process deposits up to 4 cu in./hr. Resulting molds are fully dense and non-porous and have a fine grain size that enhances strength and ductility. The process can use tool steels, stainless steels, aluminum, and alloys of copper, nickel, or titanium. Tool hardness runs 45 to 56 Rc.
LENS technology has been used for mold repair and rapid tooling, says Lisa Taute, director of marketing at Optomec. A year ago, the company developed the ability to build large structures of H-13 tool steel, making LENS applicable to plastics molds. The company also developed gradient material structures that join different materials for higher thermal conductivity or other purposes. Optomec is now awaiting results from an H13 tool that was built with conformal cooling channels—the first such application for this process.
Six commercial-scale LENS machines are at work in the field. Optomec both builds molds and offers LENS equipment capable of making parts up to 18 x 18 x 42 in.
'Print' powdered metal
MoldFusion is a new 3-D metal "printing" process that uses a variant of a rapid-prototyping technique to create molds from layers of powdered steel and a photo-cured polymeric binder (see process schematic). The "green" tool is sintered to remove the binder, and then is infiltrated with molten bronze to produce a fully dense mold. The resulting tool can last for hundreds of thousands of shots, says Larry Navarre, director of business development for D-M-E, which is marketing the process and related application-engineering services to plastics moldmakers.
After seven years of research at the Massachusetts Institute of Technology, MoldFusion was commercialized about six months ago by the ProMetal Div. of Extrude Hone Corp., which supplies the metal-deposition machinery. The ProMetal machine spreads a layer of powdered stainless steel onto the tool-building surface, and then uses an "ink-jet" type of spray to "print" a layer of liquid polymer droplets onto the surface of the metal-powder layer. The droplet pattern is derived from a 2D slice of the CAD model. An integral lamp quickly dries the binder on the steel powder. The tool-build piston lowers the model to accept another layer of powder and binder, and the process is repeated until the part is finished.
Then the intermediate part is placed in an oven where the binder is burned off and the metal particles are fused together. "We do not sinter the green part to full density. We sinter to create a structural skeleton that can withstand machining and, ultimately, the molding forces," Navarre says. This approach reduces the amount of shrinkage during sintering, which yields accuracy benefits. Infiltrating the porous metal part with molten bronze in a second furnace yields a 60%-steel/40%-bronze part that can be machined, ground, polished, coated or plated. "Moldfusion does not generate a net-shape tool. You cannot mold directly from this. So we are partnering with mold makers to supply them the technology, and they will apply the finishing touches," Navarre says. Total build time can take as little as a week.
He says the process can build tools with the strength, compressibility, and hardness (26-30 Rc) of P20 tool steel. Tool shrinkage is estimated at 1.5% ±0.2%. "We can get a base tolerance of ±0.005 in., and variance over the length of the part is 0.002 in./in.," Navarre says. Current build volume is limited to 12 x 12 x 10 in., but Extrude Hone plans to introduce a 20 x 40 x 10 in. unit this year.
Navarre notes that MoldFusion offers several opportunities for improved heat transfer and faster molding cycles. For one, the 40% bronze content improves thermal conductivity. Also, the process can accommodate conformal cooling, which reportedly can trim 25-30% off cooling cycles.
D-M-E is also developing an approach called Structural Mass Reduction (SMR), which involves incorporating honeycomb structures and trusses as a means of improving thermal isolation and reducing the mass of metal from which heat must be removed. D-M-E is also working on Dynamic Thermal Cycling, which incorporates SMR along with selective heating and cooling of the mold. D-M-E provides assistance to moldmakers in applying these cooling technologies to MoldFusion. Navarre says he receives two solid leads each week.
Zero shrinkage is a distinctive feature touted by Dynamic Tooling for its eight-month-old Polysteel II and Polysteel II+ processes for rapid tooling. These methods mold metal powder and a proprietary polymeric binder around a stereolithography pattern. Conformal-cooling channels can be molded into the tool. "Because I am molding the metal with zero shrink, I am getting zero stress in the mold," says president Vawter. The raw material is 90% A6 or other tool steel. The forming process creates a mold with isotropic properties, Vawter explains. In contrast, conventional machining leaves stresses in the mold steel. When heated, the mold can warp.
With Polysteel, Vawter says, "We can get dimensional accuracies down to ±0.001 in." Polysteel II can create molds with an expected tool life of 10,000 to 500,000 cycles, surface hardness of Rockwell 35 C, and surface finish of 2 RMS. Polysteel II + molds can survive 500,000 to 1,000,000 cycles with a Rockwell 70 C hardness and similar surface finish. "We can generate the mold and produce parts in one to three weeks," he says.
Dynamic Tooling also developed a powder-metal forging process that forms steel inserts against a ceramic pattern in roughly one-quarter of the time conventional machining takes. A test for Ford Motor Co. a few years ago produced mold inserts for a plastic wing nut in 18 hr, saving 52 hr and 80% of the labor of standard practices.
Laser sintering two ways
Selective Laser Sintering (SLS), a process used for rapid prototyping since 1989, has now been adapted to rapid tooling. DTM, which patented the SLS process, recently came out with LaserForm ST-100, a 420 stainless-steel powder with a polymer binder. This powder can be used with DTM's Sinterstation machines, which use a laser to fuse together the particles in order to build up a part layer by layer. The resulting "green" metal part is then placed in an oven, where the binder is removed and the steel is sintered and infiltrated with molten bronze—all in one 24-hr cycle.
Total production time for tooling inserts is three to four days, with little or no additional finishing or polishing, DTM says. Tooling parts with higher finishing/polishing requirements can take five to 10 days. DTM sources say participants in beta tests repeatedly found they could obtain complex tooling in half the time required for traditional methods. A recent test produced a small insert with LaserForm ST-100 in 53.9 hr, 26 hr of which involved sintering and infiltration. Traditional methods consumed three electrodes and 122 hr. DTM expects LaserForm ST-100 tools to offer the greatest benefit in small, complex tooling. These molds are expected to last for hundreds of thousands of cycles with most plastics.
A somewhat similar process has been developed by Electro Optical Systems (EOS) in Germany, said to be Europe's leading supplier of RT and rapid-prototyping systems. Its Direct Metal Laser Sintering (DMLS) system can make tool inserts for prototype to small production runs with lead times of one to two weeks. The system builds the tool up layer by layer. It differs from the DTM process in that it uses no polymer binder and requires no separate burn-out and sintering step. Laser sintering occurs as the metal powder is deposited. This RT process, called DirecTool, has been available since 1995, and more than 55 machines have been sold in Europe and Asia.
Complex geometric shapes and conformal cooling can be created in a single piece with the EOSint M series machine. Tools up to 250 x 250 x 185 mm size can be built on the most recent model, called M 250 X. Build speed ranges from 0.4 to 3.3 cu in./hr, depending on material. Tooling components can be infiltrated with liquid epoxy resin, if desired.
EOS offers a line of DirectMetal powders adapted for different applications, from quick production of prototype tools without fine details to finely detailed tools and heavy-duty injection molds. A new grade, DirectSteel 20-V1, will be released in a few months for making tools with even finer detail resolution and smoother surfaces. It builds layers only 0.02 mm thick, vs. 0.05 to 0.1 mm for other grades. EOS also offers an automatic surface treatment for tool inserts. The Micro Shot peening process produces Rz values of less than 20 microns, which additional polishing can reduce to an Rz of less than 1 micron.
According to DMLS product manager Mike Shellabear, the commercial success of this technology is due to the one-step process and its high accuracy: "The accuracy of our process is such that no post-machining is necessary. Many of our customers just shot-peen the tool surfaces and then use them to mold thousands of parts."
The original RT method
For two decades, injection mold inserts have been made by the Keltool metal-powder sintering process. As noted above, recent R&D by its biggest practitioner, RTT, has cut tooling production time by 44%. The process was originally developed by 3M Co. but today is offered for license by 3D Systems. It involves creating silicone rubber molds of a core and cavity from a stereolithography pattern, then filling the molds with powdered A6 tool steel and an epoxy binder. The rubber mold is removed after the mixture cures, and the cured part is sintered. The 70%-dense metal part is then infiltrated with copper.
Says RTT president Brad Fox, "These inserts can run millions of parts. Final dimensions are accurate to ±0.001 in. They have a surface finish of 20-25 microinches or an SPI grade 3 to 4." Their hardness of 32 Rockwell C can be heat treated to a 42-46 Rc. The 3D Keltool process today can produce parts up to 5 x 8.5 x 5 in.
Electroforming bows in
The first two commercial tooling applications of a nickel-shell electroforming process may soon be announced by ExpressTool. Its electroforming process is not really "rapid," in that it produces a tool in about the same time as conventional machining. However, its higher thermal conductivity reportedly yields 15-30% shorter molding cycles. The ExpressTool process uses a CNC-machined pattern made from a proprietary conductive material that accepts electroplating, says Tom O'Connor, president. The plating process deposits a 1-2 mm shell of nickel backed up by 3-4 mm of copper, and a backing support of a proprietary composite. Similar coefficients of thermal expansion keep the three layers from separating during use.
The nickel-copper tooling insert provides five to 10 times greater thermal conductivity than P-20 or H-13 tool steel, says O'Connor. The nickel provides surface hardness of 20-40 Rc. Electroformed tools reportedly can be produced to the same precision as molds made with EDM electrodes and can withstand injection pressures above 20,000 psi, melt temperatures up to 800 F, and mold temperatures of 350 F.
ExpressTool molds have been tested at up to 300,000 cycles, but there has not been a commercial project yet. One of the candidates for a first commercial use is a luggage tag. The other is for a tool to produce 750,000 to 800,000 shots. In the meantime, ExpressTool is also aiming to promote its technology for retrofit tooling.
POM Group's Direct Metal Deposition process can repair molds as well as build new ones. The NuTool process deposits metal powder on areas requiring a rebuild and a laser instantly melts the powder. No post-sintering is required. Cameras monitor the process.
New MoldFusion process from Extrude Hone and D-M-E is one of several rapid-tooling technologies that build molds from metal powder directly from a CAD model. As shown here, conformal cooling channels make these tools "rapid" in molding cycles as well as build time.
A new spray-metal deposition approach to rapid tooling called Rapid Solidification Process (RSP) was developed by the Idaho National Engineering & Environmental Laboratory. Molten metal droplets sprayed onto a pattern cool so rapidly that, as shown in this demonstration, a spray of molten tin does not destroy a child's balloon.
Conformal cooling channels that follow the contours of the mold core and cavity are said to speed injection molding cycles by 20-40%.
MoldFusion "prints" successive layers of powdered metal and polymeric binder. (1) A spreader takes metal powder from the supply basin and spreads a thin layer on the surface of the build table. (2) The print head, driven by CAD data, deposits droplets of binder on the metal powder. (3) The lamp quickly dries the binder. The build table then drops 0.005 to 0.007 in. to accept a new layer of metal powder. The completed mold is sintered and then infiltrated with molten bronze.
Polysteel, a new metal and polymer sintering process from Dynamic Tooling, saved two months in developing this mold.