Latest from Issues and Ideas

Investment Casting + Sand Casting

July 5, 2012
Hybrid cores promise the best aspects of both processes

Investment casting continues to be the “cool” sort of metalcasting, involving high-tech materials and proprietary techniques, and favored for producing high-precision parts for cutting edge industries, like aerospace and medical equipment. Investment casting also earns a lot of investment capital.

Now, sand casters may be able to snatch a bit of the magic. The Southwest Research Institute, an independent research center in San Antonio, working with the British foundry Grainger and Worrall Ltd., has developed a technology for producing heavily cored iron and steel castings in sand molds, but with cores that combine sand mixtures and ceramic structures. According to Marc Megel, asst. director of the Design Development Dept. of SwRI’s Engine, Emissions and Vehicle Research Division, “The Hybrid Ceramic-Sand Core Casting Technology will enable casting of narrow, complex passageways in a way not previously achievable with conventional iron casting techniques.”

Megel is listed as the principal developer of the technology with Keith Denholm of Grainger and Worrall. Their three-year, multi-phase research and development program set out to establish a technology for casting heavy-duty diesel cylinder heads that would achieve a higher-peak cylinder pressure than current state-of-the-art engines.

Typically, cylinder heads are manufactured as sand castings; it’s an affordable way to execute the high-volume production programs, and still suitable for most of the geometrically complex internal passages needed with such product designs.

However, the geometries developed for higher-peak cylinder pressure and high cooling velocity and efficiency involve internal passages that are too small to produce with reliability using standard sand casting.

“We needed to come up with a new way to create these very small passages,” Megel explained. “Ceramic cores, such as those used in the aerospace industry to cast cooling passages in turbine blades, do not break down in the presence of molten metal, even at very small sizes.

“Ceramic core casting is unusual in the automotive industry because it is expensive,” he observed. “In the new hybrid ceramic-sand core product, the ceramic section is used where coolant passages between the engine’s gas exchange port walls and fuel injector or spark plug are formed.”

Megel pointed out that the hybrid ceramic-sand core method would allow foundries to produce high-combustion, low-emission engines without compromising on performance, size, or weight. Foundries will be able to use conventional sand casting for most of the design, and adopt ceramic core inserts for the smaller passages.

Engine designers and manufacturers will be able to adopt technologies that rely on high cylinder pressure — which is critical to high power density and to many ultra-low emission, high-efficiency, low-CO2 emission diesel, natural gas, and gasoline combustion technologies — without requiring extraordinary process changes or system modifications from their metalcasting suppliers.

The technique is sufficiently novel and impressive to have earned a 2012 R&D 100 Award. It’s the 37th such award for the Southwest Research Institute, and will be presented later this year.