Differences Between Permanent Mold & Sand Casting
Permanent mold casting techniques help create the thousands of simple metal parts that are identical copies needed to replace worn out components in machines around the world. Sand mold castings help make complex and intricate parts, and often very large parts that are
impossible to make using other techniques like die casting or permanent mold casting. While the molding process is similar, these two techniques do have important differences.
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Components
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Sand casting requires an oversized pattern, cores to funnel the metal into the pattern, two halves of a mold (a box that is split into two parts, which can be secured into one unit), a lubricant and sand. Permanent mold casting consists of two halves of a metal mold, a sand or metal core, and a ceramic coating. Each require metal to melt down and a furnace to melt the metal.
Sand Mold Casting Process
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The first step in sand mold casting is to create the mold. Sand is packed tightly in the two halves of mold around the pattern and cores. After the pattern and cores are removed from the sand, a lubricant is applied to the surface of the sand to help separate it from the metal. The cores are positioned in their respective places and the mold is clamped shut. The molten metal is poured into the mold until it is completely full and it is allowed to cool. Afterward, the mold is broken and the metal is shot blasted to remove any sand that is stuck to the surface. Excess material is trimmed from the final part.
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Permanent Mold Casting Process
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The first step in permanent mold casting is to heat up the metal mold between 300 and 500 degrees Fahrenheit. The metal is covered with a ceramic coating to help remove the part. Metal or sand cores are added to the mold, and the mold is clamped together. The metal is poured into the cavity and allowed to cool. After the metal has cooled, the mold is opened and the part is removed. Excess material is cut away from the final part.
Shrinkage
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Sand mold casting requires the artisan to account for shrinkage depending on the type of metal used. For example, if a part is being made using nickel alloys, the artisan needs to make the pattern larger by a factor of 1.05. The metal does not shrink as significantly in the permanent molding process, due to the fact that the mold expands with the molten metal.
Cost
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Sand mold casting is expensive, in the sense that the mold must be prepared every time a part is needed, which translates to fewer units per production run. Permanent molds are reused up to several thousand times before a new mold needs to be prepared.
Uses
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Permanent mold casting is ideal for repetitive, high production parts, like in the automotive industry. Small, uncomplicated aluminum, copper or magnesium alloys used in gears or gear housings are common. Other uses include pistons, wheels and impellers. Sand mold casting is ideal for unique parts, artistic metal pieces and large metal works because the sand mold is destroyed after each single use.
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