Cores' Role in Casting Design
One of metalcasting’s strongest selling points is its ability to encompass several parts, often in the form of a welded assembly, into one component. This is possible because the nature of the metalcasting process lends itself to complex geometries. At the heart of many of these complex geometries is a core or core assembly.
A core is a shaped body, usually made of sand, which forms the interior part of the casting, like the cavity the pit makes in the flesh of a peach. In metalcasting, the mold provides a space for the molten metal to go, while the core keeps the metal from filling the entire space.
Cores allow you to incorporate holes in your design, but these holes don’t have to be limited to the see-through kind you’d find in a length of pipe. Cores can take on a variety of angles and shapes, and more than one can be used per casting. Sometimes, an assembly of cores is constructed to create a web of internal passageways and chambers. For many seemingly impossible parts, imagination and cores can turn a floundering design into a winning engineered component.
“Go after the geometry you want and let the metalcaster figure out how to accomplish it with the best balance of economics and properties,” said Mike Gwyn, vice president of metals technology at Advanced Technology Institute Corp., Mt. Pleasant, S.C.
How a Core Works
Most cores are made of sand, although they also can be made of ceramic or metal. The core acts as a negative, displacing molten metal as it is poured into the casting mold. Following the solidification of the metal, the sand core is shaken out, revealing the void. In investment casting, a cored hole is formed by the ceramic shell mold and then knocked out after solidification. In permanent mold casting, metal cores are used, and semi-permanent molding makes use of sand cores. Although cores usually are used to form interior passageways in a casting, they also can be used to shape an external part of a more intricate casting. If a section of a casting is undercut, for example, a core can be used to help the pattern be withdrawn from the mold without distorting it. Additionally, sometimes cores are used to strengthen or improve an inner or outer surface of the mold.
In order for a core to successfully produce a quality casting, it must have several different types of strength. It must be strong enough to maintain its form while it is prepared and handled, withstand the hot metal poured into a mold, and exhibit minimal expansion or contraction in order to maintain good dimensional accuracy. These necessary strengths should be considered during casting design in order to ensure an accurate component can be cast in an efficient and productive manner.
“You have to keep in mind the mechanical and physical limitations of the core itself,” said Larry Stahl, General Motors. “Despite one’s best efforts, you can’t overcome physics.”
Sand cores can be made in a handful of ways, and most metalcasting facilities use more than one type of core in their production. The metalcaster chooses the type of core based on what would fulfill the application’s physical and dimensional requirements in the most economical way. While a core’s strength is important, care also must be made to ensure it will easily disintegrate after the metal solidifies in order to be removed during shakeout.
As a customer, it’s beneficial to have a clear understanding of the properties that are truly required for the part. This can save you money and time.
“You don’t want to pay dearly for perfection if you don’t need it,” said Wayne Rossbacher, president and owner of Foundry Consulting, Sugar Grove, Ill. “For example, you want to use the coarsest grain possible to give an acceptable finish.”
Stahl suggests including core design in your initial discussions with the metalcaster to avoid potential setbacks in the coreroom. One of the primary core design issues is the location of the cavities needed in the component.
“How are you going to support those cores? How are you going to keep the cores in place when the metal comes?” Stahl asked. “What are the production requirements that have to be met? These all become questions that have to be answered.
“One of the most critical steps is when the casting engineer meets with the customer. This is when you find the balance between what the designer thinks he needs and what he can actually make.”
Cores need patterns just as molds need patterns. These patterns are used in conjunction with coreboxes to produce a core. Core production can be performed manually or through automation, but the logistics remain the same. In low volume operations, core sand is fed into a corebox from a mixer. Sand also may be blown in when the cores are smaller, with high production rates. In the case of a coldbox core, time is then given for the core mix to cure. In the case of shell, hotbox or warmbox cores, the sand mixture in the box is heated to set the core. Once it is set, the core is removed from the corebox.
Some factors of core production should be considered in core design. Because coreboxes are made in two halves that are clamped together, they will have a parting line, particularly if the sand is blown in. If this parting line is not sealed when the corebox is closed, coresand may blow out of the box to create fins on the cores. This will require more time in production to file the fins off the cores.
Sand is mixed with high pressure air when it is blown into the corebox for compaction. Sometimes, the air already in the corebox does not have enough time to escape, resulting in poor sand distribution and poor compaction. To prevent this problem, patternmakers use blow tubes and vents to keep air from being trapped in the corebox. However, the tubes and vents will leave marks on the surface of the core, and these will transfer onto the surface of the corehole or passageway in the metal casting. Metalcasters can dip or paint the core with a coating to remove the marks if they will cause a problem in the final application of the component.
Cores in Action
The use of cores will add to the final cost of your casting, and a casting design that fails to accommodate core design will be felt in the project’s budget. But the advantages of cores to casting design are major money savers, lending design freedom to manufacturing a component. A well-thought-out core package can create intricate passageways that either save money, reduce weight or improve functionality. For instance, a hydraulic valve body gray iron casting made at the green sand facility of Grede Foundries Inc. – Iron Mountain Div., Kingsford, Mich., incorporated two components into one to reduce weight by 11.3%.
The component integration was made possible with seven individual shell cores that were built together to form a one-piece assembly. The assembly process required that proper tolerances were held to meet near-net shape requirements. The final core assembly was stacked on two previously-assembled cores and needed to hold a height tolerance in order to kiss against the mold surface tight enough to ensure the printout of the cored holes did not distort.
This complex use of cores helped reduce the customer’s inventory, avoid extra machining costs and eliminate five hydraulic connections, which improved safety and quality.
In another example, casting supplier Denison Industries, Denison, Texas, overcame the challenge of a crossover configuration of an intake manifold while still using a one-piece core rather than an assembly. The passageway core was made via the coldbox process and the main body core was blown. This casting also made use of an external core produced with three chunk cores.
“Typically, customers don’t know how the cores are being used in a part,” Gwyn said. “Be aware that cores exist. Once you start thinking about the geometry of the part, start talking with the metalcasting facility’s engineer about the capabilities of cores.” Metalcastingdesign.com