New materials and better processes let rapid-prototyping machines
fabricate tooling that is good enough to produce parts.

Paul Dvorak
(Senior Editor)
MACHINE DESIGN

23 July 1998

The rapid tooling process from Vista Technologies LLC, White Bear Lake, Minn., starts with the construction of a shelled mold from an SLA machine. Wraps of copper tubing for cooling water surround the part's protusion.

Even in the early days of rapid prototyping, the long-term goal of practitioners was to quickly generate real parts rather than just concept molds. Today, that goal is being realized. The same techniques that generate fragile prototypes are increasingly being extended to provide tough production tooling. In many cases this tooling produces parts having tolerances and surface finishes acceptable for finished products.

After a 24-hr. cure, the mold (back of a core is shown) can be fitted to a base and mounted in an injection-molding machine to generate parts. The company can deliver 20 to 100 parts, depending on material, in less than two weeks.

The first examples of rapid tooling became available in 1993 when Soligen Technologies Inc. commercialized a process invented at M.I.T. for creating casting models more quickly without patterns. Three years ago, DTM Corp. in Austin developed a method called RapidTool for creating powder-metal green parts that are sintered and infiltrated with copper to produce production molds. More recently, a process called Kettool has also been used to quickly make tool-steel molds. Now owned by 3D Systems, Valencia, California, this process results in injection molds capable of withstanding a million shots when used with nonabrasive resins.

A finished mold produced by the same process makes golf equipment.

Recent developments aim at reducing the amount of postprocessing required for rapidly produced molds. Researchers are also experimenting with new materials such as thermoplastics, composites, and maybe even cement. Bolstering this work are improvements in underlying RP hardware, particularly lasers.

RT SPEEDS UP
It has long been a goal of rapid-prototyping companies to use powder metal as an RP material. The newly devised Laser Engineered Net Shaping or Lens process now uses powder metal to build parts with excellent material properties, say its developers. The process comes out of Sandia National Laboratories and works like most additive RP methods: a computer slices a solid model into many layers, and a machine builds parts layer-by-layer from metal powder or liquid resin. The machine uses a 700-W laser focused onto a substrate to create a molten puddle into which a mechanism injects metal powder. A computer moves the substrate relative to the laser beam depositing thin metallic lines until the part finishes.

Parts Now, Northridge, California, cast the full-sized upper intake manifold from 356-T6 aluminum in 15 business days for an auto manufacturer. The part has complex cores and thin walls, features that could only be cast. Parts Now is the production arm of Soligen Technologies, the company that developed the method of quickly producing ceramic molds without first building a pattern. The process is called direct shell production casting. Soligen’s most recent machine, the DSPC300G, produces larger molds in a 14 x 18 x 12 in. volume. It works 30% faster than the previous model. The company says part size is not a limitation for machines and that they have cast parts weighing hundreds of pounds.

However, engineers with developer Optomec Design Co., Albuquerque, stress that Lens can build a limited overhang, to about 30 degrees. And because it works slowly compared to other systems, at rates of 0.3 to 1.0 in.³/hr., quick production of tools rather than parts will be its forte. The firm hopes to eventually build models with internal sensors to more closely monitor pressure and temperatures. Molds devised in the Lens process could also feature built-in cooling channels that conform to the shape of the part. This optimizes cooling and will allow the shortest possible process times.

FASTEST STEREOLITHOGRAPHY MACHINE YET

Engineers with Aaroflex Inc., Fairfax, Virginia, say their Solid Imager RP system is the fastest yet thanks to an imaging device that accurately positions a laser beam within 0.0005 in. over 10 in. at speeds to 1,250 ips. The system builds parts in raster and vector modes. Vector scans (400 ips) work best for parts with little interior fill while raster scans are best for large areas. A 3-W, 70-kHz laser delivers up to 3,000 dpi. Parts are built in a 25 in. cube. Resins can come from DuPont or RPC AG in Switzerland.

The half-dozen materials tested so far include 304 and 316 stainless steel, iron-nickel alloys, and titanium. Surface finishes are also encouraging. A laser-remelt process improves the standard finish of about 400µin. to about 10µin.

The plastic design on the right is the SLA part engineers with ProtoCAM Inc., Northhampton, Pennsylvania, used to make a 3D Keltool mold. The darker insert in the tool-steel block is the finished mold, and the clear block shows the approximate geometry and dimensions of the mold. The core is not shown. “The Keltool process delivers an A6 tool-steel mold capable of thousands of parts,” says Ray Biery, managing partner. “And because the copper in the mold dissipates heat better than an all steel mold, shot times are shorter,” he adds. Building the mold takes about five to six weeks. That compares well to the 16 to 20 weeks required by a conventional tool shop.

Other developments in rapid tooling focus on processing new materials in exiting machines. For example, one experimental rapid-production method showing good results so far marries powder injection-metal feed stocks, from injection-metal molding methods, with an RP machine from Sanders Prototype Inc., Merrimack, N.H. Powder-injection metals contain a polymer that lubricates and suspends the powder as it’s deposited in the RP machine.

“A lot of what’s happening now is just the confluence of existing technology,” says Rand German, professor of powder metals at Pennsylvania State University, University Park, PA. German is working with the Sanders machine partly because it boasts one of the best accuracies available. This makes it a good candidate for producing short-run complex parts such as gears.

The seahorse is molded from a powder-metal material developed by Rand German’s group at Pennsylvania State University. The sculpture measures only 1 in. long, but shows the intricate details possible with the material which German says has a hardness and wear comparable to tool steel, a surface finish lower than 1-µm average roughness, and shrinkage from the original CAD-produced model of less than 0.1%. German says the technology promises to deliver a new generation of moldmaking materials.

“A group member recently benchmarked available powder metals for strength, wear, and dimensional change,” says German. “Then we set goals to top their characteristics and limit shrinkage to near zero,” he says. With a proprietary combination of off-the-shelf powders, German’s crew came up with a material having an R hardness range of 30 to 35, a wear resistance that is four times better than tool steel, and a surface finish of two micrometers as sintered.

SLA PART TOUGH ENOUGH TO PUMP HOT ANTIFREEZE To test the design of a new water-pump impeller, engineers with Airtex Product, Fairfield, Illinois, used a recently introduced SLA material that was fitted around a steel rod. Mounted in an aluminum housing, the device ran at speeds of up to 5,000 rpm until it broke. What’s notable is that the impeller was moving a relatively hot 225 degrees Fahrenheit water-glycol mixture at 48 gpm and about 17 psi. The material, SLA 5210, comes from 3D Systems, Valencia, California. “Because there were almost no step lines in the SLA prototype, we were able to more accurately gauge the performance of the impeller under working conditions,” says Kerry Austin, product designer for Airtex.” We might even be able to hit 7,000 rpm on future applications by adding strengthening features such as improved fillets or increasing the draft on the impeller.” Engineers with 3D Systems credit an advanced vinyl ether/acrylate formulation for the durability of the resin. They say parts exhibit almost no swelling or softening when exposed to extreme levels of moisture, and have good side-wall surface quality.

The pump impellar on the left, made of SLA 5210 resin, is similar to the one tested by Airtex's Austin. The other is a production part. The resin is the first in a series of niche product materials, says a spokesman for 3D Systems, which supports applications requiring a tolerance for high temperatures.

Other efforts in rapid tooling concern the making of production molds out of ordinary SLA resins. Though SLA epoxies would melt at typical injection-molding temperatures, judicious cooling can let molds last long enough to make a few dozen parts. The process works like this: an RP machine constructs a core and cavity with hollow backs. Copper tubing for cooling is looped about the protrusion of the part shape inside the mold. The shell is filled with an epoxy mixed with aluminum powder to improve heat transfer, and 24 hours later the mold can be mounted to a base ready to make parts. “We can get 20 to 100 parts out of a mold depending on the material,” says James Mishek, developer and president of Vista Technologies LLC, White Bear Lake, Minnesota. “Polycarbonate must be shot at high temperatures. The technique gives about 20 parts before the mold breaks down. But rubber or polypropylene is less abrasive and tends to give closer to 100 parts,” he adds. Accuracy has been to about +/-0.002 in. The advantage of the system is that it produces parts in the production material in less than two weeks. Mishek also says he expects SLA materials soon that tolerate temperatures beyond the 158 degrees Fahrenheit limit of the most rugged existing resins. Then the part count and surface quality from a mold should improve.

Inventor Khoshnevis (khoshnevis@usc.edu) is in his University of Southern California lab with his Contour Crafting system. The image below shows a servo-controlled trowel smoothing and shaping a cone. Hollow structures might also be filled with material for stability.

NEW MACHINES, PROCESSES
When Berok Khoshnevis was troweling plaster onto the wall of his home not long ago, it occurred to him that a small trowel on the nozzle tip of a rapid-prototyping machine might remove the layered appearance that characterizes these parts. He tested the idea and it worked. With funding from the National Science Foundation, the professor at the University of Southern California came up with a system that could work with a wide variety of materials. Theoretically, it can work with any substance that can be extruded. The system can use standard thermosets, thermoplastics, and photopolymers as well as those not associated with PR such as plaster, cement, clay, or concrete.

Loopers for high-speed sewing machines from Union Special Corp. are investment cast from patterns produced on a ModelMaker rapid prototyping machine from Sanders Prototype Inc., Merrimack, N.H.

So far Khoshnevis has built a limited-capacity machine and fabricated smooth-walled cones and cylinders as test shapes. The build method resembles fused deposition modeling, extruding UV curable or air-dry materials one layer at a time, which Khoshnevis terms as Contour Crafting. “Surface quality is improved by removing the obvious stair steps,” he says. One cylinder built by the machine has a surface of 24 microns and was made with 2-mm layers. The machine works faster than existing systems that use fused deposition. Preliminary studies show that even for layers 0.25-mm thick, the process would be about eight times faster and give a much smoother surface than current FDM. Use of thicker layers would speed the process even more without affecting surface quality.

In-process tasks

The mechanics involved in Contour Crafting are scalable. This opens the door to the possibility of RP machines that incorporate large gantry robots. These might produce not only large prototypes but also final products that could include boats and internal components for aircraft and automobiles. What’s more, structural properties of large parts could be improved through the introduction of resins with fillers such as fibers or glass.

So what next? Pennsylvania State University's Rand German half jokes that since most developments have come from shoe-string budgets, "with a little serious funding, we could be dangerous." He's suggesting there are many more ideas looking for funding and the rapid tooling we've seen so far is just the tip of the iceberg.

RP CUTS 33% FROM DEVELOPMENT SCHEDULE
An RP system produced intricate sewing machine parts that range from 12.7 to 152 mm high with an accuracy of 50 microns. Union Special Corp., Huntley, Ill., used a ModelMaker RP machine from Sanders Prototype Inc, Merrimack, N.H., to produce patterns for loopers. These are sewing-machine parts that work with needles to form an interlocking stitch. In doing so, the firm cut nine days from a developed cycle that once spanned 27.

Loopers have complex blends and compound angles that are easily defined in CAD software but difficult to show in a drawing or to a pattern maker. Such small parts are usually investment cast. Thus concept models fabricated on RP machines generally are scaled up. The RP machine chosen has a dimensional accuracy of 24 microns with 2-micron rms surface finish.
"We were able to produce benchmarks for hard tooling while eliminating nine days, and that saved $3,750 in labor costs per master pattern over the previous process, " says Walt Taylor, project engineer with Union Special. The chart above compares the time and cost of the old wire-EMD-and-machining process versus RP and investment casting.

ADVANCED MATERIALS FOR RP
“Just a year ago, we had three materials for three different machines,” says Mervyn Rudgley, director of product management for 3D Systems in Valencia, California. “Today we have 11 materials for our SLA machines and materials capable of 300 and 400 degrees Fahrenheit are not far off.” Other developers are making similar announcements. For example, DTM Corp. in Austin recently announced three additional materials for its equipment and DuPont Co., Wilmington, Delaware, introduced two for SLA machines. The following table highlights a few recent unveilings.