ing conditions. Binders that generate more gas need increased venting—in total number and/or size—to allow for proper gassing of the mold cavity. Vents should be placed to ease cleaning of core gas residue. Engineers also must determine if the mold requires active or passive gas venting, that is, if the mold design and filling pattern will push the gas out naturally or a more aggressive vacuum assist is necessary. Gas evolu- tion also can correlate with condensa- tion in the mold cavity.
Core Handling and Placement
Handling sand cores and placing them into a hot metal mold can be sources of trouble. Some practical sug- gestions include: Core Positioning: A cold core may
fit differently into a cold mold versus a hot one. Te core should be positioned in the mold so it’s fully supported during pouring. Core prints should
Fig. 2: A stripper plate can be used to remove core fins and parting lines in higher volume applications.
locate the core without constraining it in a way that could lead to fracture. Prints should be easy to clean and
discourage residue buildup, which could lead to misalignment. Core prints also must allow for the thick- ness of the mold core coating and ensure adequate core print clearances. It is important to focus on both halves of the mold and include proper core clearances, at least 0.005–0.015 in. (0.127–0.381 mm) per side. Some metalcasting facilities provide additional close-over clearances of 0.005–0.01 in. (0.127–0.254 mm). Because prints wear excessively, they should be easy to access and repair. Also, since closing the mold will likely shave some sand from the core, pre- pare for residue in the mold. Te core print may be an ideal spot for venting. Because there is no liquid metal sur- rounding the core print, it provides an easier path for venting core gasses that won’t mark the casting surface. Core Handling: Handling is a con- cern with cores that are heavy, fragile,
September 2013 MODERN CASTING | 37
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