The Shell Mold Process

Shell molding is a process for producing simple or complex near net shape castings, maintaining tight tolerances and a high degree of dimensional stability.

The process begins with a precision-machined pattern, which is heated and then covered with a fine resin sand mix. As the mix polymerizes, excess sand is removed, leaving a hard, thin "shell" in the contour of the pattern. The shell is ejected while still hot, and split into a cope and drag. The two halves are pasted together to form a mold. If required, cores and filters can be inserted before the halves are joined. Mold locators built into the patterns minimize the probability of mismatch, or parting line shift.

The resulting mold is then poured to your material specifications in a non-conventional manner that helps assure:
Precise temperature control
Cleaner metal with less oxide potential
Reduced Metal Static Pressure

While much of our process is automated to maximize productivity, exceptional quality requires special techniques. The molds are brought to the metal via conveyor, rather than transporting metal to the molds. The two main advantages of this method are that metal temperatures are maintained with extreme accuracy and metal turbulence is minimized. Experienced hands ladle the metal directly from the melting furnace into the molds with speed and artful precision. Hand pouring also minimizes the distance from the ladle to the sprue, reducing the potential for oxides to enter the mold.

After cooling, the castings are separated during the "shake out" process, and then cleaned. The results are near net shape, semi-precision castings characterized by uniform dimensional control and excellent surface finish, ready for the next step in the manufacturing process.

Design Advantages of the Process
Because of the rigidity of our molds and the positive cope and drag locators, we can provide a number of advantages in the design of your product:

Tolerance Control: dimensional control on one half of the mold will typically be equal to, or in some cases superior to, the investment casting process. Tolerances across the parting line are superior to sand cast capabilities.

Concentricity and Location of Core to Mold Configurations: the consistency of our castings is ideal for the tighter tolerances required by the CNC Machining Centers. We specialize in multi-hole applications, such as manifolds, where the concentricity requirements are beyond sand cast capabilities.

Finish Requirements: casting accuracy allows for a reduction of up to 50% in finish required on machine surfaces. In some cases, this eliminates roughing cuts and reduces both casting weight and metal cost.

Draft Requirements: drafts (both I.D. and O.D.) of 1/2 degree are common in our pattern design. Cores are often eliminated through our ability to "draw" deep holes. The bushings illustrated here are made coreless.

Permanent Tooling: since our process is subject to heat, all of our tooling is fully machined to exacting tolerances with high wear material such as continuous cast iron or conductivity copper. High production parts are generally hard coated. Tooling has produced hundreds of thousands of parts from a single pattern.

Competitive Pricing: our process is the most productive method of manufacturing near net shape castings, competing favorably with other processes and generally providing a superior product.

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info@southwestcustom.com
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