Cutting die casting leadtimes via prototyping.

Inside This Story:

* Prototyping with hard tooling can lead to excessive time and costs for customers.

* Rapid prototyping methods can help smooth the preliminary production of diecast components and reduce overall leadtimes.

Imagine the following--you're a diecaster, and one of your customers is in the quoting process of an innovative product. This customer asks your firm to create 10 prototype components to examine the casting design as well as test for form, fit and function. If this customer receives the project, your diecasting facility likely will receive the bid to produce the high-volume component. However, the customer's time demands are too narrow to allow you to make production tooling.

But with rapid prototyping techniques, customers can obtain preliminary castings as precursors to diecast components. These castings will have properties similar to diecast parts while being produced in weeks or days while the tooling is made. Several investment casting and robber plaster mold (RPM) casting facilities conduct such prototyping procedures, and referring your customer to them likely will prove positive for all personnel in the production cycle.

Two metalcasting facilities and their customers already have benefited from similar relationships.

Investment Cast Die Castings 

The investment casting process has always been an ideal choice to produce rapid prototype parts because wax models (and other materials) can be created and turned into castings in as little as one day.

The process begins with the creation of ABS wax models from a digital design file (.STL). These models then are attached to an investment casting tree, coated with ceramic slurry and burnt out, and poured molten metal takes the shape of the hollow ceramic shell to obtain a prototype cast component. After the parts are cast, they can be machined and shipped as normal. This process from design to delivery of prototypes can take from days to a few weeks depending upon volume.

The engineers for Trico Products, Rochester Hills, Mich., have come to understand this.

As a manufacturer of windshield wiper components for automotive companies, Trico endured a quandary with a diecasting supplier in 2003. Trico was in the advanced design phase for a new windshield wiper motor (which included six cast parts) for a major automotive manufacturer and had decided to outsource its prototype models to a foreign diecaster. However, after three months of conflicting discussions about the parts' designs, such as tolerances and draft angles, the diecaster still had not begun not begun to cut a tool.

Trico was pressed by constraints and needed to have something in hand to even consider showing the new concept motor to the automobile manufacturer.

At the suggestion of one of his businesses partners, Trico's Manager of Research and Development, Dave Peck, went to Aristo Cast Inc., Almont, Mich., to discuss the metalcasting firm's capabilities with rapid prototyping.

Peck presented Aristo Cast the same part design that the diecaster found incompatible and asked the firm to create a rapid prototype casting. Within a week, Aristo Cast had a one-off part completed. "Aristo Cast was able to simulate in the investment casting process die cast palls," said Peck. "Basically (investment casting) gets into draft angles, tolerances, dimensions and wall thickness. So, it's a different methodology of what you pour into, but other than slightly rougher surface texture, the parts would be identical dimensionally. In the key areas where you have to machine the part, there's no difference."

After the first tests were completed, Trico and Aristo Cast examined several rapid prototype design iterations of one of the six motor parts to improve the parts' rigidity. Each time, Aristo Cast made, machined and shipped five magnesium parts from wax models in less than two weeks, and Trico assembled them to create a preliminary testable motor.

Once a design was finalized, Trico submitted a digital file to Aristo Cast for semi-production tooling. Aristo Cast then modified one of its investment casting tools to inject wax molds and several weeks later, cast in magnesium alloy AZ91D more than 50 rapid prototype components of each of the six parts and machined them. Trico then performed mechanical and system development tests and presented the parts to its own management and potential customers.

As a result, Trico is the only firm that has presented a viable 4 x 4-in. (10 x 10-cm) wiper motor to the automotive firm. This has led to the development contract, which is currently being finalized. Through investment casting, the component achieved the required wall thicknesses of as little as 0.04 in. (0.1 cm), and mechanical properties were similar to die castings for yield and ultimate tensile strengths. Trico saved more than 20% in prototype expenses than if it had kept the order with the foreign diecaster, and the whole process with Aristo Cast took less than three months.

"Typically, when we would have things rapid prototyped in the past, we would have to have parts made from aluminum tools to try them out in the diecasting process and then go to steel tools later on for production," Peck noted. "But that would take months before you could ever get one part. When we used rapid prototyping, that not only gave us a blueprint to put on the table, it gave us a functional part to test."

Aristo Cast Chief Engineer Larry Blum pointed out that the maximum cost of tooling for an investment casting facility can be the same price as the cheapest diecasting tooling block prior to cutting. Thus, diecasting customers are looking for alternatives.

Aristo Cast currently is investigating more about the diecasting market and how to produce 100 pieces for semi-production. Through this, as Blum said, "all the bugs and design problems are worked out to a degree before engineers get to the final part when they start cutting diecast tooling."

A method commonly used to avoid tool cutting is creating stereolithography (SLA) models, which are made from sintered photopolymer material. SLA components may display the correct size and shape, but they also tend to be fragile and have the potential to break when not handled carefully. "(The investment castings) are actual metal parts; you can touch and feel them," Peck said.

He also noted that SLA models do not provide for functionality, whereas with investment cast components, Trico's staff could assemble the motor, turn it on and watch the gears work.

"If you actually have a real part that resembles what the part will look like with the materials in it, and if you're getting it for a relatively inexpensive price quickly, it helps you sell advanced concepts to your own management and certain customers," Peck said.

RPM Die Casting

Using another process as a precursor to diecasting is not limited to investment casting. In the RPM casting process, a master cope and drag is made from epoxy cast against a reverse pattern machined from aluminum. An inverse-patterned rubber mold then is cast against the epoxy mold, and this rubber mold is used to cast plaster into a new mold. Once the plaster solidifies, the plaster mold is cut out and ready to cast with metal.

The RPM process allows the firm to mimic diecast tooling almost to the exact scale as production diecasting molds. Further, the rubber mold flexibility and heat insulation helps eliminate the need for draft and uniform wall thicknesses, and this minimizes the tooling process for the RPM cast parts. "(RPM castings) don't need the features die castings need, so once we have the part in a diecast design, we can do it," said Terry Carlson, vice president--sales for A.L. Johnson Co., Camarillo, Calif., an RPM metalcasting firm. "It's a simple thing for us. Changes in design are easily and usually inexpensively done with tooling (at a fast pace)."

RPM casting utilizes low-cost tooling that can be completed in less than a month, and when tooling changes are needed, they can be corrected in only a couple days. Further, the rubber molds are reusable and can be duplicated as many times as needed. As Sure Power Industries, Tualatin, Ore., has discovered, these characteristics make the RPM process ideal for prototyping.

Sure Power, which manufactures electrical devices, such as battery equalizers and isolators for heavy trucks, busses and marine vehicles, began to examine producing parts through diecasting, which would provide for lighter weight parts and be less time-consuming than the company's extrusion processes.

Matt Clark, a mechanical engineer for Sure Power, described how the firm was investigating its first diecast enclosure for a voltage alternator for an all-terrain vehicle. Sure Power was trying to determine how it would meet prototype requirements based on customer demands for time, so Clark contacted A.L. Johnson about its RPM capabilities to obtain the prototypes and deliver them to the customer.

Soon after that, Sure Power submitted a design to A.L. Johnson to produce more than 50 RPM cast parts. Although few augmentations were made to the original design, A.L. Johnson helped machine small mounting holes and other fragments into all the parts. After only five weeks, Sure Power sent 40 of the 7.5 x 4.5-in. (19.05-11.4 cm) prototype regulators to its customer where they were placed as part of a test assembly. Within nine weeks from when A.L. Johnson first received the design, Sure Power sent the order to a diecasting firm to begin high-volume production.

Similar to Peck and investment casting, Clark discussed how having an engineered cast component in hand through RPM casting can be more valuable than an SLA model. Because Sure Power handles electronic parts, many of its pieces can dissipate a lot of heat, thus, a part's thermal properties cannot be overlooked.

"With rubber plaster molding, we go through (mechanical testing processes) before releasing the design to hard tools as well as thermal testing that you can't do with an SLA model because it is plastic," Clark stated. "If we mount our electronics (in an SLA part), it wouldn't represent what kind of heat dissipation we can expect because it would lack the same thermal characteristics as a diecast part."

Save More

In addition to "production-grade" castings, Trico and Sure Power noticed other advantages to rapid prototyping. These methods are less labor-intensive not only for avoiding cutting die tools prematurely, but also staying clear from fabrications.

Peck and Clark noted how fabricating a part from an aluminum billet does not provide for an internal composition that casting processes allow, which would likely lead to contrary evaluations on internal structural properties. Also, casting alloy properties differ from those of billet metals, and machining additional draft into a billet (something that can be achieved easily through a casting process) adds higher costs and additional labor.

Although labor factors into the equation, both Peck and Clark agree that a critical factor saved with rapid prototyping is time.

"Time kills you," Peck said. "Rapid prototyping allowed me to get something in my hand. so I could evaluate a problem right away."

This resulted in Trico cutting its prototyping timeline in half by working with Aristo Cast and made design refinements in one-tenth of the time that it would take for diecast prototypes. With investment casting, time also can be saved with short-run production castings, not just prototypes. While tooling is being made, these short-run parts may still be cast through rapid prototyping methods.

The time equation also is necessary for companies to keep pace in the market. "If you don't seize the opportunity, that one project goes away, and next year there could be a couple more coming up," he said. "But every time you lose one of those windows of opportunity-, they don't always open up again. (With the wiper motor project), had I procrastinated, I would've lost this 500,000-product window.

"So, time is bigger than the cost."

For More Information

"Investing with Rapid Prototyping," K. O'Shaughnessy, MODERN CASTING, Fall 2004, p. 23-26.