Investing with rapid prototyping: with the ability to produce near-net shape engineered cast components in days, the combination of rapid prototyping and investment casting is a key tool for both metalcasters and design engineers.

It's Tuesday 4:30 p.m., and a particular engine component needs to be designed, cast and shipped by Friday afternoon. How? The time constraints are too tight, you say. What if the CAD design needs multiple refinements for manufacturability? It is impossible to overcome such a task. But then you discover a viable option that you believe would allow for completion of the component--using rapid prototyping to create a pattern for investment casting an engineered metal component.

Suddenly, the weight on your shoulders is lifted, as you realize rapid prototyping and investment casting can go hand-in-hand. Rapid prototype the pattern, coat the pattern in the refractory, invest the pattern in the ceramic mold, burn the pattern out and cast the component.

When time is of the essence for shape or production of cast metal components, metalcasters are discovering that rapid prototyping for investment casting can eliminate errors in production tooling or eliminate tooling in general, which in turn saves money and improves leadtimes. More importantly, rapid prototyping provides opportunities to correct and eliminate design errors and machining operations.

Although this procedure is not ubiquitous among all investment casting facilities, innovations have driven rapid prototyping's popularity since the first machine was commercialized in the 1980s. The advantages that rapid prototyping can provide the design-to-cast component process are becoming more readily discovered and benefiting those involved.

Why Rapid Prototype?

Rapid prototyping can be accomplished easily in only a few steps. It begins with converting a 3D CAD model to an .STL file, which is the standard digital file format for the prototyping machine to read. From there, the file is then "printed" three dimensionally using either photopolymer, thermopolymer, polystyrene, or other materials, depending on the method. (Note: not all rapid prototyped materials are suitable for investment casting, thus, its best to discuss with your vendor which ones have low ash content.) If they meet dimensional standards, the prototyped models can then be attached to a tree for standard investment casting.

Contrarily, if the models don't meet the dimensional or function/design standards or their castability seems futile, they may be modified in the .STL file and rapid prototyped again. And again and again. This augments the design process before any irreversible adjustments are made and alleviates the onus that design engineers may otherwise face.

Ray Sander, lead modeler for the Product Development Solutions group at the Battelle Memorial Institute, Columbus, Ohio, said running concept parts during the design phase by using a concept modeler can validate the design. This is a fast, economical process that catches problems early and possibly saves time and costs.

"If you wait to long in your design phase to try to get your final model, changes, if necessary, will be more complicated by several hundred entities that may be associated with each other," said Sander, who creates rapid prototyped models for the medical and defense markets and also is a former patternmaker at a metalcasting facility. "If you can address those problems early and get a quick attempt at evaluating your design and effectiveness of it, you're going to save a lot of time. You can also take this early model and bring in others for improved product feedback.

"We want to answer questions, we want to improve on the design, and we want to do this in a real timely manner. That's why we prototype."

Sander described how design engineers often begin the tooling process without ensuring that the design is fundamentally correct. Once the tooling is complete, if an error is discovered, the product would have to be re-tooled, which costs thousands of dollars and hours to fix, something that is not ideal when time is an issue. Unfortunately, some firms are forced into the situation where they have imperfect models, yet there is no time or money left to correct the errors. They then face the issue of asking if they am presenting a quality product to their customers.

"It's in the early stage that the (CAD) drawing's pretty simple," he said. "If you dimension a model from a place that you decide to cut away later, all of the sudden, that dimension is not tied to the model. Instead of going back to fix one feature, you'll have to fix two features, or four or five. So there's where your time comes in."

Design engineers who utilize rapid prototyping have acknowledged how much reduced time a project will allow if rapid prototyping is incorporated. Signicast Corp., Hartford, Wis., an investment casting facility that has used rapid prototyped parts for 10 years, appreciates the flexibility rapid prototyping provides under time and cost restraints. "It is an excellent choice with complex shapes, where tooling cost is high, and the required leadtime is short," say Todd McDonald, sales manager, and Mark Seiler, senior project manager, for Signicast.

"Rapid prototype patterns can be produced in days versus weeks for aluminum tooling. Today's market demands shortened product-to-market cycles. Rapid prototyping allows for producing and testing a product far quicker than conventional methods. It also is the path of choice for low-volume cast components--when tooling cost far exceeds the cost of one or two patterns."

McDonald and Seller discussed a situation when Signicast, "under an extremely compressed timeline," rapid prototyped multiple thermopolymer patterns to help create 20 castings, each measuring 2 x 2.5 x 7-in. By doing so, the company saved four weeks in production and $24,000 in tooling costs.

Know All Capabilities

The Signicast component easily fit in the common prototype model range of under a cubic foot. However, if a part exceeds this range, it can still go through the rapid prototyping process by dividing the model into multiple parts. Through this, each part may be prototyped and then bonded together to form the sample component. This can be done with glue or the modeling material itself.

"We saw there was a great need from the engineers' standpoint and designers' stand point to not only have a part that you get a prototyped plastic for form and fit, but when someone has a part that they need the integrity for a solid, well-gated steel and/or aluminum and/or magnesium part to put into use, this process is not just form and fit; it's form, fit and function," stated Larry Blum, chief engineer at Aristo Cast Inc.

"And that's what the engineers were looking for."

In 2003, Aristo Cast, an investment casting facility located in Almont, Mich., received an order for a military component to be used on an un-manned reconnaissance vehicle. The aluminum cast part would have a finished casting weight of 9 lbs., and measure 28-in. long, 7-in. tall and 8 in. wide with wall thicknesses from 0.08-0.125 in. Such dimensions exceeded the facility's rapid prototyping machine's build platform of 8 1/2 x 8 in. However, Aristo Cast, which has practiced rapid prototyping methods since 1999, managed to accomplish the task. The engineers sectioned the math data into several pieces that did not exceed the build size parameter. Six individual models were then rapid prototyped and re-assembled into one whole polymer wax pattern forming the original design. But, this too presented another quandary.

"We found on this job that re-assembling the wax pattern was going to be extremely difficult because

no surfaces tangent to our sections were straight and flat," Blum said. "We needed some holding fixtures to align each segment while being assembled, but the time frame we had would not allow us to make holding fixtures by conventional machining methods. We had our engineers retrieve the math data and build ABS plastic fixtures creating the outer profile of the part using our FDM (fused deposition modeling) machines. This reduced our fixture build from days to literally hours."

With the rapid prototyped fixtures, the firm was able to set the six individual models on surfaces fit specifically for them. The models were then bonded into one pattern, cast and shipped. "We produced 8 of the military components in 22 working days from receipt of the order," Blum explained. "Now that's rapid!"

Aristo Cast has found that their prototype machines are not used only to make a sellable product, but also used as a tool to produce components to facilitate their production casting manufacture.

Blum described how "alloy rapid prototypes at Aristo Cast can be produced in as short a period as 1-2 working days."

"We often send a full size or scaled clown cast or ABS part made front the furnished math data, when we submit our quotation for rapid prototypes or production parts," he said. "The engineers and designers really get excited when they see the actual part that up until now was only a computer model. A rapid prototype produced to validate a design gives the customer and engineers the luxury of changing dimensions or material to better suit form, fit or function, without incurring the high cost of tooling changes.

"To aid toolmakers in designing and quoting of how they're going to construct this tool or deciding how many inserts will be made are things that are a tremendous aid to people other than the customers."

Additionally, rapid prototyping can accelerate production due to the machines' abilities to create multiple patterns at one time. This allows the process engineer to save time in the production of rapid prototyped patterns. "The machine goes through its process whether there is one pattern or many patterns being grown," said Reg Gustafson, the project manager for Clinkenbeard & Associates, which is a metalcasting tooling supplier and a turnkey source lot machined castings. "Growing multiple parts in batches reduces the cycle time for each part significantly. (Metalcasters) can just dip the models and cast them without too much messing around."

Take Your Pick

Amid the numerous advantages that can he obtained through rapid prototyping, they all might not be equal when it comes to the actual machines themselves. For investment casting, there are a number of methods to choose from, but according to Gustafson, the two ideal ones are the SLA QuickCast and the SLS Cast Form.

The Stereolithography (SLA) QuickCast method creates hollow models made from fused liquid photopolymer that can be used in place of solid wax models and greatly assist investment casters due to their collapsibility advantages. Gustafson said these models are essentially support structures enclosed by a skin that maintain the integrity of the patterns. Because of the internal cavities, when the mold is flash-fired to remove the model before the component is cast and the metal is poured, the model will enclose on itself and fall out without breaking the mold shell. The shell can then be cleaned easily before casting, without using further tooling to correct any mold errors.

The Selective Laser Sintering (SLS) CastForm involves a polystyrene material that is created one layer at a time.

Once this model is created, it is infiltrated with wax to provide a better surface finish for the shell molds. Although these models are not hollow patterns, they have low densities and low ash contents that allow" for easy removal from the molds.

Other modeling processes that also are found throughout the industry are ThermoJet methods, which also create solid models. The biggest draw with ThermoJet is its speed; using a thermopolymer material, the machines can produce models up to three times faster than other rapid prototyping machines. However, this method is ideal for quick modeling to aid in the design evaluation process, whereas QuickCast is more common with the actual functional testing of models. Nevertheless, it is a popular item for investment casting facilities to own.

Gustafson stated there are several innovations in the future that will surpass the current rapid prototyping capabilities. One machine in development will build supports solely out of wax as opposed to the polymer material that is currently used. Because current supports are made from the same material as the prototyped models, they need to be carved off, and the model design is harmed. This new process would decrease finishing costs and, thus, reduce the cost of making the prototyped models.

"(Machines) have become and will continue to be more accurate with the printing compared with the old traditional systems as far as the printing mechanism goes," Gustafson said. "I don't think the part accuracy is quite there yet, but I think within the next five or 10 years, it's definitely going to be there."

For More Information

Visit the Rapid Prototyping Home Page at www.cc.utah.edu/~asn8200/rapid.html.

Visit the Rapid Prototyping Forum at www.rapidprototyping.net.

"Improve Casting Quality, Reduce Lead Time via Rapid Prototyping," R. Gustafson, Engineered Casting Solutions, Fall 1999, p. 37-40.

Kevin O'Shaughnessy, Assistant Editor