Sand casting is an economical way to produce parts in a variety of metals. Shell mold casting can be an alternative process to consider given specific part design needs. This blog post introduces the shell mold casting process and compares it with sand casting so readers may understand their respective strengths and limitations.
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All casting processes need a pattern for the shape of the part to be cast, and shell molding is no exception. As with sand casting, investment casting and other casting processes, the pattern is used to create the cavity into which liquid metal is poured.
In shell molding a metal pattern tool is coated with a layer of sand. This contains a resin that melts when heated and cools to form a thin shell, (typically 1/4 &#; 3/8&#; thick,) around the pattern. Melting is achieved by heating the pattern before coating in sand, then heating the shell to fully cure the resin.
The shell is made in two halves so it can be released from the pattern. After each half shell, (sometimes called a &#;biscuit&#;), is lifted away from the pattern the two shells are bonded together, forming the cavity in the space between them. The assembled shell is then placed into a flask where it&#;s supported with granular material like sand or shot.
The shell is filled with liquid metal which is allowed to cool. Once solid, the shell is taken out of the flask and broken apart to release the cast metal part. A limitation is that while it&#;s not impossible to add cores to form hollow regions, it is difficult to do.
Shell mold casting uses a different sand to that used in green sand casting, and this is what gives the process its advantages. In sand casting the &#;green&#; sand, (green meaning not dried and not the color,) is held together by a binder of clay, usually Bentonite.
The silica sand used for shell molding is finer than that used for sand casting. The American Foundry Society uses a &#;fineness number&#; to characterize average grain size where a higher number indicates smaller grains. Green sand is typically 60 &#; 80 on this scale while shell molding sand is 100 or higher. In addition, rather than using a clay binder, the sand is given a thin coating of phenolic resin which is what melts to form the shell.
The fine grained sand packs more tightly than green sand, which reduces gas permeability. Sand casting requires a degree of permeability to let moisture, in the form of water vapor, out. This isn&#;t a problem in shell molding though because the shell is dried and heated before metal pouring.
The major benefits of finer sand are that it conforms more closely to find surface features on the pattern and provides a smoother surface for the metal.
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For even better replication of surface detail, and a smoother finish, a foundry may use zircon sand. This can have a fineness number as high as 150. It also has a lower coefficient of thermal expansion which allows application of tighter tolerances.
Superior surface finish and reproduction of fine detail are two of the major strengths of this process. In addition, shell molding requires minimal draft angles to release the pattern from the shell.
Whereas sand casting typically needs 2° &#; 3° of draft, the shell molding process gets away with 1°, and on short vertical walls it may be possible to have zero. This allows application of smaller machining allowances, which saves time and money in finishing operations.
Sand casting:
In comparison, shell molding:
In terms of capabilities and costs, shell mold casting falls between sand and investment casting. It&#;s more complex than sand casting and can produce more complex parts, but without the sophistication and complexity of a wax pattern process.
Impro makes aluminum aerospace parts by shell mold casting when it makes sense. If you&#;d like to discover whether the process is right for your parts, contact us. Our casting specialists will be happy to help.
Shell mold casting is a casting method to produce castings with thin shell mold, also it&#;s ideal for medium-to-high-volume production. Similar to the sand casting, in that molten metal, a dispensable mold is poured. Shell casting was invented by German J. Cronin in . It was first used in Germany in and began to be used in other countries after .
A kind of heat hardening molding sand (phenolic resin coated sand) is used to cover the metal template heated to 180-280 &#; to harden it into a thin shell (the thickness of the shell is generally 6-12mm), and then the shell is heated and solidified to achieve sufficient strength and rigidity. Therefore, the upper and lower shell can be clamped with clamps or bonded with resin, and the mold can be formed without sand box, The heating temperature of casting metal template is about 300 &#;, and the molding sand used is resin sand, that is, resin sand with phenolic resin as binder. Similarly, the core can be made into thin shell by the above method, and the tipping bucket method is commonly used to make the thin shell mold. The blowing method is commonly used in the manufacture of thin shell core.
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