casting 101 C
by heat to produce sturdy, rigid molds. It provides a number of component advantages, including excellent dimen- sional precision and accuracy, a high de- gree of part-to-part reproducibility and better near-net-shape capability than other sand casting methods. However, patterns must be made of metal and the process is energy-intensive, which increases costs compared to green sand. Shell casting is ideal for thin-
S
walled parts with close tolerances, cast parts requiring fi ne detail or superior surface fi nish or parts requiring mini- mal draft. Often, casting parts in shell molds can reduce weight reduction and/or machining stock. T e specialized base material for
producing shell molds is known as shell sand. As with all sand casting processes, shell mold making begins with a pattern. Shell patterns typically consist of two halves, cope (top) and drag (bottom). T ey are often made of gray iron due to its heat transfer characteristics, but the metal can make tooling expensive. T e features on the pattern are referred to as cavity inserts. In addition, shell molds have locators, which are cured sand alignment pins on the mold face that facilitate joining the cope and drag halves. T e mold is created
during the “invest” cycle. Free fl owing shell sand is dumped onto a hot pattern (usually between 450-600F). As heat is transferred from the pat- tern to the sand, the resin that coats the individual sand grains begins to fl ow and bridge between sand grains. When the mold has absorbed suffi cient heat from the pattern, a thermosetting
Understanding the Shell Molding Process AMERICAN FOUNDRY SOCIETY TECHNICAL DEPARTMENT
hell molding is a common sand casting process that also is used to make cores. In this process, fi ne, resin coated sand is activated
reaction occurs. After the mold reaches a predetermined thickness in the invest cycle, excess sand is removed. T e sand mold that remains resembles a shell of sand over the pattern. Shell molds are made in halves and
are glued or clamped together before pouring. Shell cores can be made whole or as multiple pieces glued together. Once the mold or core has been com- pletely assembled and glued, molten metal may then be immediately poured into the mold or the mold or core may be stored indefi nitely. T e ability to store molds means that component runs can be produced with shorter lead times. T e cooling rate of shell castings can
be controlled and sped up with appro- priate backing media around the shell mold. After pouring and solidifi cation has taken place, the castings and sand are separated in the shakeout process. Clean- ing typically involves little more than shot blasting to remove residual sand. Castings produced by shell molds tend to have a low defect rates, mostly because of the robust nature of the mold. For example, sand defects due to mold erosion during pouring com- monly can be prevented. Also, the frequency of gas-related defects tends to be less because thin shell molds are naturally well-vented.
One of the most notable benefi ts of the shell process is the dimensional precision and accuracy with which parts may be produced. Manufactur- ing processes with similar dimensional capabilities tend to be substantially more expensive and less conducive to high-volume production. Because of the dimensional preci-
sion and accuracy of shell casting, less machining may be required to produce the fi nal shape. Good stock fi nish allowances on shell molded castings are in the range of 0.04-0.06 in. T is is possible due largely to the improved strength and integrity of shell molds at metalcasting temperatures. Also, machining stock can be minimized be- cause shell molds typically require less draft on casting cavities. Where other molding processes may require draft angles greater than 1 degree, features on shell molds often can be produced with draft angles of 0.25-1 degree. Dimensional tolerances are good as
well. Relative to the parting line, cast- ings may be produced with a tolerance of ±0.02 in. parallel to the parting line or ±0.025 in. across the parting line. In addition, some properly engineered features with dimensional tolerances of ±0.012 in. are possible. Because shell sand is dry, fl owable and typically fi ner than sand in other molding processes, casting cavity surfaces tend to be densely packed and less friable than in other types of sand molds. Unmachined cast- ings produced from shell molds may have a surface fi nish of 150-350 micro- inches or less. Heat-cured, resin-
Castings produced via shell molds can produce thin sections with minimal draft and machine stock allowances.
48 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2015
bonded shell sand has a relatively high tensile strength, which improves the ability to form a pat- tern with sand projections intact. As a result, deep draws sometimes can be cast without a separate core and/or with less draft. ■
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