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 combi- nation 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 be- cause the customer wants that very smooth finish inside the pumps and turbochargers where you can- not 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 prepar- ing 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 do- ing with 3-D printed sand molds.
Additive manufacturing technology often is used for prototyping as well as short runs. Nov/Dec 2014 | METAL CASTING DESIGN & PURCHASING | 21
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Soft tooling produced using additive manufacturing methods can be ready quickly and maintain the metalcaster’s ability to make up to several hundred parts.”
I like to say you’re building with Legos, because you’re building these interlocking features and you can put it together. Tis 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 tech-
niques bring capabilities that are simply not possible with conven- tional tooling. Wax patterns can be built with complex gating sys- tems 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 cast- ing, because metallurgical con- cerns can be optimized regardless of parting lines and other tradi- tional concerns.”
Finding the Right Fit “Tis 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 ma- chines 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?’” Te 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
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