Sprint to the Finish

Rapid manufacturing methods streamline low volume production.

Denise Kapel, Senior Editor

(Click here to see the story as it appears in the Nov./Dec. issue of Metal Casting Design & Purchasing.)

Metalcasters and casting buyers have more options than ever before among the methods for making molds and cores.  This specialized area relies on skilled, knowledgeable staff as well as current technology. Casting molds are prepared using skills as diverse as hand woodworking or programming computer-operated machinery, and it can involve a combination of both. For customers requiring small quantities of castings, whatever the size, one of several rapid manufacturing methods often fits the bill.

“Each rapid manufacturing technique offers its own set of advantages and no one technique will obsolete the others,” according to Dave Hockemeyer, owner/engineer, Peridot Inc., a rapid prototyping, polymer molding and custom metalcasting operation in Hoagland, Ind.

As CNC machining capabilities develop, as well as additive manufacturing, users among metalcasters and independent patternmaking shops are evolving their methods in step with the technologies.

“[Companies] positioned to offer these latest techniques will use the various ‘tools’ to build more complex components with shorter lead times at lower costs, producing a win-win for the project,” said Hockemeyer.

A Variety of Choices

Among the rapid methods for metalcasting are several additive manufacturing (AM) technologies for “printing” plastic, wax, sand or metal tooling.

“Each of these typically offers advantages over traditional CNC machined foundry tooling in the way of less lead time and lower costs,” said Hockemeyer. “One way in which they excel is that they are not affected by part complexity. In fact, the more complex the part geometry, the more effective AM is at producing an efficient solution.

“For example, when a complex wax pattern made with conventional techniques requires a soluble core, a die with moving action or challenging draft requirements, a printed wax pattern can be made in one piece. The layering process eliminates those concerns and the need for the wax die altogether,” he said. “Similarly, when a complex sand mold pattern requires draft, cores, multiple glued cores, offset parting or loose pieces, a printed sand mold can avoid those concerns by building a complete mold ready to be poured with metal, without the need for a pattern. Printed plastic patterns offer advantages since they require less programming and setup time as compared to a machined pattern, and the AM machine itself can run 24/7 unattended.”  

Soft tooling produced using additive manufacturing methods can be ready quickly and maintain the metal casting supplier’s ability to make up to several hundred parts. “This commonly takes place when a plastic master model is printed, followed by a mold off the master model, and ends with a cast polymer pattern impression made from a material with good wear characteristics allowing for hundreds of sand molds to be made,” Hockemeyer explained.

Thomas Mueller, CEO of Express Prototyping in Keego Harbor, Mich., prints investment casting patterns in PPMA (Plexiglas). “3-D printing technology allows us to print every thinkable shape and form. No draft angle is needed,” he said. The firm specializes in plastic patterns sized up to 19.7 x 15.8 x 11.8 inches, which it can turn around in two days.

“I can build quite a lot of parts in a very short time,” Mueller said. “So it works pretty well for low volume. The breakeven point in cost determines whether it is worthwhile. Whether a job is easy or hard to cast, it requires tooling. What if it’s a thousand dollars for tooling, and [the cast component] costs 10 bucks? If I print a plastic pattern for 10 bucks, you can only use it once, but once you’ve passed the breakeven point, then it makes more sense.  If you don’t need many parts, it costs more to make the tool than it would cost only to order those parts to be produced using a 3-D printer.”

Often, tooling techniques are used in combination. One such application Hockemeyer described involved mounting a printed plastic pattern to a pattern plate and using a printed sand core. “This combination can result in a simplified pattern with a complex core and saves cost since the pattern is easy to make and there is no need for a corebox,” he said.

“Every kind of molding you can think of, whether green sand or airset, has had 3-D printed cores in it,” said consultant Steve Murray, Hoosier Pattern Inc., Decatur, Ind. “We’ve put together 3-D printed sand molds with ceramic cores because the customer wants that very smooth finish inside the pumps and turbochargers where you cannot machine.”

Sand molds also can be CNC machined subtractively from a prepared block to achieve toolless production on a short-run casting too large for 3-D sand printers or other rapid machines. However, Murray noted the value of having no need for patterns when preparing a one-off, very large casting.

“Essentially, if you’re doing a really big casting, you put your sand in the corebox and start building it, the same as we’re doing with 3-D printed sand molds. I like to say you’re building with Legos, because you’re building these interlocking features and you can put it together. This can be limited in size only by your imagination. You can use this in pit molding. You can use this in many different things.”

Hockemeyer added, “AM techniques bring capabilities that are simply not possible with conventional tooling. Wax patterns can be built with complex gating systems that could never be molded in a wax die. And sand molds can be printed with bottom-fed runner systems that are not feasible with standard pattern plates. The net result of these advantages is a more sound casting, because metallurgical concerns can be optimized regardless of parting lines and other traditional concerns.”  

Finding the Right Fit

“This whole subject of short-run production casting is striking a chord at this time in manufacturing,” said Murray. He shared a recent visit with a customer planning a short run of specialized heavy construction machines that raised questions beyond traditional casting concerns.

“We were thinking about using rapid prototyping technology to make their castings on the whole production run, because it was only going to be 210 to 300 units,” said Murray. “So, my question was, ‘Is that going to be 210 to 300 units in one year, over a five-year period or in seven to 10 years?’”

The customer asked why, and Murray replied, “If it’s one year, you might be better off making tooling. But if it’s going to be over a little bit of time, we all know that as soon as the engineer gets feedback on those first machines out in industry, he’s

going to make changes.” Looking ahead, that could amount to many changes over a five or seven-year span, leading to significant tooling costs.  “If we use rapid prototyping technology, you wouldn’t have to change the tooling. We would just print a new one, and off you go,” he said.

The customer’s engineers, designers and marketing staff are determining which method will be the most economical based on their long-term expectations for the project.

“Realistically speaking, if they go ahead and make permanent tooling for all the castings over a five-year build, if 20% of the parts make an engineering change, it’s cheaper to produce the job using 3-D sand printing.”

Another situation where planning ahead and employing rapid manufacturing technology can benefit the customer is on products that are made to order. “What happens if somebody’s customers want custom work? That suddenly changes the game,” he said.

For short run metal casting jobs, the criteria to determine the right method are the size and the alloy to be cast. “Then, it’s also going to come down to foundries that cast that alloy—what are they comfortable using? If it’s lost foam, maybe that’s what they want to do, make a one-off lost foam pattern and do it the traditional way. We would like to show them the benefit of having a 3-D printed sand pattern casting and do it that way.”

Reducing Front-End Costs

“With some of the work that we quote and build production tooling for, one thing we look at is where we can take advantage of 3-D printing specifically on low volume production,” said Steve Shade, project manager for Craft Pattern & Mold, Montrose, Minn.

“A great example would be a complex valve housing, which might have six, eight or 10 cores assembled,” he explained. “A variety of tooling is required to do that; there’s a lot of risk of variation and different things that come into play. So, some of the requirements of the tooling to do that just don’t make sense from an economic standpoint. When you can 3-D print those cores as a single package or a couple pieces and assemble, it can be very competitive, and in fact it can be very cost-effective, even for 25, 50 or 100-piece quantities. That can offset the cost of tooling that’s $100,000, so it’s beneficial.”

“With tooling, there are multiple ways we can do it, and the same with 3-D printing,” added Tony Cremers, president. “So, if it’s going to be repeat orders, we go from there.” He asks questions to determine the details on the project and sometimes gives customers a number of options to help guide their decision making.

In one case, a one-off machined aluminum impeller was quoted three ways: an SLS (selective laser sintered) investment cast aluminum with secondary machining; a 3-D printed sand cast with the same requirements; and a low production pattern tool with a cope and drag airset sand mold process.

“We didn’t know the surface quality yet, and if there are certain tolerances on the area, we might have to add machine stock to make sure we meet their requirements,” said Cremers. “For this one part, if it’s small enough, 3-D printing is ideal [from a cost perspective]. If they’re looking to make multiple orders and it’s going to be 20-25 pieces or more, then you’ll probably want to machine out a pattern tool, a soft tool or low production tool. It balances on the customer’s requirements or request.”

Cremers sets the cost effectiveness limit for 3-D printing around 12 or 16 inches. “Once it gets past that in size, there’s a bigger cost factor. The dollars start matching or going over the cost of building a pattern tool with the traditional CNC route,” he said.

Geometry is another area where rapid technologies like AM can provide significant benefit.

“Certain parts and especially aluminum parts typically are diecast intent, so we’re dealing with a lot of thin wall sections,” said Shade. “Some of the traditional casting technologies like sand casting might have difficulty with those thin walls. So we employ investment casting techniques, 3-D printing using SLS patterns to investment cast thin wall parts. But we can also build an aluminum wax tool to shoot the wax patterns for investment castings for low volume production. So that’s a great bridge to full production with diecasting.”

In industries where metal casting weight is of great concern, such as aerospace, rapid manufacturing technologies are showing promise in converting parts such as welded assemblies into single castings.

“Very complex castings can be produced to optimize performance, function and available real estate,” said Hockemeyer. “So, whether the project requires a one-off printed sand mold/core or wax pattern, a multi-impression soft pattern for low volume production, or several hundred sand cores or wax patterns, the influence of rapid manufacturing methods continues to play a vital role with a promising future in the metal casting arena.”     ■

Thanks to streamlined simulation, tooling, casting and machining capabilities, an intricate water passage went from purchase order to prototype in just 17 days.