Rapid Prototyping Applications

Toolmaking through rapid prototyping

Aronson, Robert B

A "sometimes" answer

Rapid prototyping (RP) is evolving from just a means of making prototypes to a technique for making production tooling, chiefly dies for plastic parts.

Initially the prototypes were quite fragile and the various techniques produced products that could only show form. As materials improved and prototypes became stronger, the products could be tried for fit and measured. Today some systems can produce parts strong enough to run briefly in a machine and do low-volume production of parts.

It didn't take RP manufacturers long to realize there's more profit in making a usable mold than making one prototype, so many system makers are concentrating on making molds for injection-molding operations.

This industry moves cautiously. The good news is that there are a wide variety of rapid tooling (RT) systems; the bad news is no black and white answers exist on how to match a project with a system. And a few companies in the mix just might overstate their capabilities.

A number of techniques still in the laboratory stage still need debugging. A lot of work is needed to convert them into practical systems that are able to operate at a competitive cost.

Milling May Be Better. The initial "rapid prototyping" system was-and still is for the most partthe conventional machine tool. In many cases users find it simpler to dash off a part the usual way. However RP systems do not require any elaborate front-end processing or software. Once you have created a solid model and output an STL file, you're ready to begin the build. Rapid tooling has an edge when you need many parts and, in particular, when that part is very complex. Cost, speed, and precision govern the decision on whether to use rapid prototyping or conventional machining.

"It's taking the developers of RT technology longer to commercialize and make a business out of what they have to offer," according to Terry Wohlers, Wohlers Associates, Inc. (Fort Collins, CO). "They're finding it is difficult to match the precision, flatness, and surface finish offered by CNC machining."

Three Ways to Go. There are three levels of rapid tooling. Some users need only a few prototypes. Called soft tooling, these parts can usually be made from room temperature vulcanizing (RTV) materials. The next, "bridge tooling," includes those production situations that require up to several hundred items to cover the time between early prototype and full production. The third type, hard tooling, employs tools for actual production.

Most of the examples following are used for bridge or hard tooling.

Government Job. A group working specifically on the Rapid Solidification Process and consisting of a consortium of 10 major industries, has been organized through the National Center for Manufacturing Sciences (Ann Arbor, MI). The process reportedly produces steel molds and dies.

Detail transfer between the mold and inserts is described as exceptional, with an EDM-like finish. The process completes projects that normally take a week in a day. This spray-forming process uses hot inert gas to atomize molten metal alloy. Currently it can produce 3" (76 mm) parts, but the goal is 12" (300 mm).

Extrude Hone (Irwin, PA). This company uses an MIT-developed system based on an ink-jet printer. It deposits liquid-binder material. Sintered and then infiltrated with a second metal, the resulting form is +/- 0.125 mm plus +/- 0.05 mm per 25 mm. The process can use almost any metal powders or mix of metals and powders but stainless steel is usually preferred.

3D Systems (Valencia, CA). Company uses the Direct AIM process for bridge-type production, this system is based on a stereolithography machine. First an insert is made using a CAD design, then the core and cavity are formed by stereolithography. The process is used for rather small parts.

3D Keltool is the company's process for volume production and typically starts with a CAD design. Then the operator forms a core and cavity using stereolithography. Next, cast silicon rubber creates a mold into which a mixture of metal power (tool steel and tungsten carbide) and binder is poured. Sintering burns away the binder and fuses the metal. Finally copper infiltration gives the material the properties of tool steel. Typical 3D Keltool accuracies range from 0.1% to 0.2%.

Epoxy SteeL Dynamic Tooling (Fresno, CA) starts with an RP pattern then forms the tool with a blend of 90% steel and 10% epoxy. The process can produce molds suitable for glass-filled nylon, ABS, and wax. It is possible to produce more than a thousand parts with an accuracy of +/- 0.025 mm without secondary machining. The molds are 400% stronger than aluminum and kirksite (a zinc-based alloy) and shrinkage is said to be negligible, with dimensional repeatability of +/- 0.013 mm.

In a rather surprising turn, GM is favoring this newcomer to the industry. The company has built three molds for working parts that measure about 1 ft^sup 2^ (930 cm^sup 2^). In one case, 18,000 parts were reportedly made from a single mold. They are working with the manufacturer to produce a 2 f^sup 2^ (1858 cm^sup 2^) part.

GM likes the process because the molds produced provide good wear characteristics, can mold abrasive material, and endure high pressure and temperature.

A GM spokesman points out that it is not always high accuracy or ability to work with exotic materials that gets the nod, but features important to a specific industry or process, such as speed and cost/part.

Electroforming Process. The operation of this technique from Express Tool (Warwick, RI) begins with a CNC-machined mandrel which is coated with a 1-2 mmthick layer of nickel that has a hardness of Rc 20-40. Cooling lines are then formed with a 2-4 mm layer of copper that reduces hot spots and cuts cycle time. Then the form is backed with a composite-filled material and the mandrel removed. Production versions have reportedly lasted for 200,000 shots.

DTM Corp. (Austin, TX). The selective sintering process (SLS) is another fabrication technique for tool making. The user first creates digital models of the core and cavity, as in similar techniques. The CAD system is then used to form a tool in a powder consisting of stainless steel with a polymer coating.

In the SLS process, a CO^sub 2^, laser fuses the powdered steel together in the shape described by the digital drawing. After processing, the core and cavity are placed in a reducing atmosphere furnace to burn out the polymer and partially sinter the steel. After another trip to the furnace, the part is infiltrated with bronze to form a steel/bronze tool. The tools are fitted with ejector pins before being use in a molding machine.

Another system, using a copperpolyamide casting material, is intended for short-run tooling applications. The material is a metal-plastic composite and with SLS processing a tool can be turned around in a day. The mold must be finished and fitted into a mold base. Copper polyamide has been used to mold plastics such as polyethylene, propylene, ABS, and nylon. Because of its heat resistance and thermal conductivity, the system can mold parts with a very short cycle time, like cycle times achieved on parts made using conventional tools.

Metal-Matrix Composites Co. (Chesterfield Twp., MI) This group works with a metal-filled vinyl ester compound called Vestalloy. It's claimed to exhibit high heat properties and better heat conduction. Reportedly, the material works well in some reaction-injection molding and thermoforming operations in the 500 deg F (260 deg C) plus range. According to the manufacturer, the material has 20% greater wear resistance (and double the heat conductivity) of conventional epoxies