Beyond Appearance Models

Rapid Prototyping Shifts to Production of Functional Parts and Tooling

James D. Destefani
Senior Editor

Time to market is a key to survival in the global manufacturing marketplace. If you don't make it quickly and inexpensively someone else will.

One potential answer to the need for speed in product development is rapid prototyping (RP). Over the past ten years, RP technology has evolved from a laboratory curiosity to a place among the standard array of product development tools available to design and manufacturing engineers.

RP now encompasses a wide variety of processes and an ever-expanding list of applicable materials, and growth has been impressive. In 1996, the market for RP systems and services totaled $421 million, according to consultants Wohlers Associates Inc. (Ft. Collins, CO). That's a 42.6 percent increase from 1995. And Wohlers expects growth to continue, with the market topping $1.1 billion by 1999.

The main use of RP has been to produce "appearance models"--nonfunctional prototypes that can help product developers visualize their designs and make necessary adjustments before committing to production. Recently, however, attention has focused on rapid manufacturing of tooling durable enough for limited production and on fast production of functional prototypes using novel manufacturing processes. These new application areas for rapid prototyping have resulted in increased usage of a new term: rapid prototyping and manufacturing (RP&M).

Fast Tooling
One of the promises of RP&M has been to circumvent the relatively costly and time-consuming conventional mold-making process. "Rapid tooling" processes either use an RP part as a master from which molds are produced, or use an RP model as a sacrificial pattern for investment casting. In either case, the way the master is produced is less important than the next step: development of tooling durable enough to make at least a few hundred parts from the end-use material.

An example of a rapid tooling process beginning to emerge from the laboratory is a spray-forming technique developed at the Idaho National Engineering and Environmental Laboratory (INEEL), Idaho Falls, ID. The lab's rapid solidification processing (RSP) tooling technology has the potential to provide a rapid, low-cost alternative to conventional fabrication of plastic injection molds and metal-forming dies, according to spray forming group leader Kevin M. McHugh.

"Conventionally produced tooling is very expensive," he says. "A simple plastic injection mold may start at about $10,000. Large ones, for automotive bumpers and fascias, for example, can cost $200,000-$300,000. Stamping dies for automotive hoods, fenders, and similar parts can exceed $1 million, and turnaround times might range from three to six months up to a year. The bottom line is, only the most conservative ideas ever make it to the showroom or marketplace, because mistakes are very costly."

RSP replaces machining of a solid with direct deposition of sprayed metal droplets onto a pattern that contains the part geometry. Molten tool steel or another alloy is pressure-fed into a nozzle, where it contacts a high-velocity inert gas that atomizes it into droplets about 50 m in diameter. The gas flow then deposits the droplets onto a pattern--wax, clay, plastic, or ceramic, depending on the metal being sprayed--that is manipulated in the spray jet. The deposit builds up to replicate the shape and surface texture of the pattern, which can be generated using a "conventional" RP process such as stereolithography, selective laser sintering, or laminated object manufacturing.

The pattern material used depends on the alloy sprayed, but INEEL uses ceramic patterns for tool steels. "The ceramics we use are readily available, castable materials," McHugh says. "Alumina and fused silica are the materials of choice, and we've also used zirconia. All the materials we work with give good replication of details on the masters and are relatively inexpensive." A mold release agent helps in removal of the steel deposit from the ceramic pattern, he adds.

The process can spray-deposit any tool steel, including P20, H13, and D2 grades. McHugh says it also works with particulate-reinforced metal-matrix composites, and has the potential to co-deposit systems of two or more alloys to create unique mold and die materials.

INEEL is currently working with a benchtop system that can fabricate tooling inserts about 4" on a side. But deposition rates even using the small, single-nozzle system can exceed 500 lb/hr, McHugh reports.

RSP can produce as-sprayed surface finishes of 15 in. rms for tool steels--more than adequate for most applications, according to McHugh. Accuracy of about 0.001-0.002" per foot has been achieved, he adds.

The spray--a combination of solid, semi-solid, and liquid particles-is about 70 percent solid when deposited, McHugh says. The resulting tooling has properties very similar to those of forged, heat-treated tool steel, but the rapid cooling of the metal particles results in a unique metallurgical benefit, he adds. "Because of rapid solidification, alloying elements in the material are in a supersaturated state," he says. "This allows us to tailor hardness, toughness, thermal fatigue, and other properties by artificial aging at relatively low temperature. Hardness in the 61-64 Rc range can be developed for peak-aged H13 tool steel.

"The benefit of artificial aging is, there's no distortion involved. When you heat treat tool steels, the phases change. The different phases have different specific volumes, and that's what gives rise to distortion." The materials also can be conventionally heat treated if desired.

It all sounds good, but how does RSP stack up to conventional processes in terms of turnaround time and cost? "We have about a one-week turnaround time, and cost is anywhere from two to ten times less than conventional mold-making, depending on complexity," McHugh cites the example of one company that sent a room temperature vulcanizing (RTV) rubber mold master for processing. "We received the mold master on Thursday, cast our ceramic molds on Friday, and fired them over the weekend. On Monday, we spray-formed the mold. On Tuesday, we cleaned the mold up and sent it out. The company said it took about eight hours for them to square up the sides of the P20 tool steel insert and start making parts."

The next step is to scale up and commercialize the technology, something McHugh hopes will happen with the help of a consortium currently being formed under the auspices of the National Center for Manufacturing Sciences (NCMS), Ann Arbor, MI. Tools produced using RSP need to be put into production and run, and we need to scale up both melt capacity and the number of nozzles to produce larger tools," McHugh says. Companies exploring the consortium approach include the Big Three auto-makers, Eastman Kodak, General Electric, Lockhead Martin, Pitney Bowes, Toledo Molding & Die, and United Technologies.

Parts Without Patterns
Tooling is one thing, but what if you need functional prototypes of a complex metal part in days?

One answer to that question might be direct shell production casting (DSPC), a rapid manufacturing process developed by Soligen Technologies Inc. (Northridge, CA). Based on a 3D printing technique developed at the Massachusetts institute of Technology (MIT), the process involves rapid production of ceramic shell molds for investment casting directly from customers' CAD files.

DSPC machine in action DSPC works by building up the ceramic mold layer by layer. After deposition of each layer of ceramic powder, an inkjet printer head selectively deposits a liquid binder according to the mold's cross section, including complex integral cores. The rest of the process resembles conventional investment casting.

DSPC-produced parts have better accuracy than conventional sand castings, according to Soligen. As cast part finishes include "stair steps" of 0.005-0.007" (0.13-0.18 mm); parts can he hand finished to 125-150 in. rms. The company also performs machining, heat treating, and other operations to supply customers with finished, ready-to-use components.

An early user of the process was Caterpillar Inc., which received complex oil manifolds in one week. "Caterpillar sent us their design file by modem on Wednesday evening," Soligen president and CEO Yehoram Uziel says. "By Friday, we completed two casting shells. On Monday, we cast the A356 aluminum parts. They were heat treated on Tuesday, finish-machined on Wednesday, and shipped overnight the same day for installation on an engine." The cost of the DSPC parts was $10,000; Uziel estimates that sand casting would have required six weeks and cost $25,000.

Other DSPC-produced parts have included:

DSPC also has been used to manufacture production tooling. Engineers at Chicago Pneumatic Tool Co. Inc., first used the process to concurrently evaluate multiple design options for components of a new power tool. When Chicago Pneumatic finalized the design, Soligen personnel used the CAD file to make aluminum tooling for full production of the component.

Originally published in Manufacturing Engineering - October 1997