The versatility in pattern material
Cores made in one sand process can be used with molds made in another process in order to optimize the strength of both. Here, a chemically-bonded core is placed into a green sand mold.
choice means more opportunities in cost savings, as patterns and core- boxes can be tailored to production and metal. For example, if only a few castings are required, a good grade of pattern lumber can be sufficient. Larger casting quantities and higher accuracy requirements will dictate an aluminum pattern. Nobake and green sand casting methods can incorporate cores made in another sand process. For instance, a nobake core can be placed in a green sand mold to provide tight tolerances and smoother surface fi nish for interior passageways and holes. Cores add cost, but they can provide a balance between less expensive molding and complex geometrical requirements. Core design and coremaking details discussed with a metal casting supplier will reveal the cost benefi ts of pairing cores with ei- ther green sand or nobake molds based on a specifi c part’s requirements. Several of the benefits of both nobake and green sand molding processes overlap, but a few general questions should help in ultimately
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deciding: • Is it a high volume part? • Is it larger than 20,000 lbs.? • Is it a relatively simple part with no cores?
• Does it have a surface fi nish requirement?
• Does the part have multiple inte- rior passageways, holes, mounting pads or geometries?
• Will the part benefi t from added features that would reduce machining?
• How thin are the part’s walls? • What are the tolerance requirements?
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• Is it a short-run or one-off part for which tooling costs should be kept to a minimum?
Anything that can be cast in a green sand mold can be cast in a nobake mold, but the reverse isn’t true. If a designer fi nds the metal casting sup- plier must add several costly steps to the green sand process through coring, coating, machining, pattern adjust- ments and trial runs to make a success- ful casting, the nobake process most likely will be the better choice.
36 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2014
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