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Shell Mold Casting

shell mold casting

What Is Shell Mold Casting?

Shell Mold Casting is a typical metal casting process which similar to sand casting. The shell casting mold is a thin-wall hardened shell created by applying a sand-resin mixed material around a pattern which is made from a metal piece in the shape of the desired geometry. The pattern is reused to form multiple shell moulds. A reusable pattern allows for higher production rates, while the disposable moulds enable complex geometries to be cast.

Shell mold casting can be used for both ferrous and non-ferrous metals. The most commonly used are cast iron, carbon steel, alloy steel, stainless steel, aluminium alloys, and copper alloys. Typical parts cast by this process are small-to-medium size and require high accuracy, such as gear housings, cylinder heads, connecting rods, and lever arms, etc.

Shell Mold Casting Process

In the course of the shell molding process, the first step is to create a pattern which is typically composed of two pieces of different metals, namely iron or steel in the shape as desired for the finished part or component. Apart from steal and iron, other materials are often used, such as aluminum for production in low volume or graphite to cast the reactive materials.

Once the pattern is ready, it will be heated to around 550 degrees F and covered in sand. The resin-coated sand grains will bind together and begin to solidify. After several minutes, there are enough sand bonding against the core box walls. After that you could pour out the rest sand, leaving a hollow core. Generally, the wall of a shell mold ranges from 7 to 8mm thick.

In the shell mold casting process, the shell mold is complete only after the two shell halves are joined totally by using glue or other fasteners and securely clamped. If there is a need to insert a core, it should be inserted preceding closing the mold. And then, the shell mold will be placed into a flask and propped by a piece of backing material.

The fourth step in the shell casting process is to pour molten metal into the shell mold’s cavity. In the shell mold casting process, no matter what kind of property and types is, the metals or alloys will be melted into liquid state, pouring into the shell mold’s cavity and forming a hardened shell.

The molten metals are allowed to cool and solidify into the shape of the final shell mold casting. The pattern parts should be chilled at the room temperature generally. Accompanying the cooling of pattern parts, the molten metal will gradually become a tough geometry as desired.

This is the last step in the shell casting process—removing shell molds. After the molten metal has chilled, the outer mold can be broken up and the castings taken out. The shell casting process involves the using of a two-piece pattern, the shell mold casting is easily ejected by “opening” the pattern. With the top and bottom pattern parts are separated from each other, the newly created shell casting can be simultaneously ejected.

Advantages of Shell Mold Casting

Shell mold casting delivers many benefits like following:

  • It can be completely automated which lowers labour costs and facilitates mass production.
  • Good and consistent surface finishes result in lower machining costs compared to sand casting.
  • As no moisture is present in the shell, very few gases are produced and those that are escape through the thin shell walls easily as the sand is permeable.
  • The burning of the resin binder on the surface of the shell helps to make removal from the cast component easy.
  • The process allows complex shapes in a large range of sizes with good surface finish to be cast.
  • Little scrap is produced and what there is can be recycled. The sand-resin mix can be recycled by burning off the resin at high temperatures.
  • Tooling costs are low and short lead times are possible.
  • A wide variety of metal alloys can be cast with this process.
  • Typical tolerances are just 0.005 mm/mm and the cast surface finish is 0.3–4.0 micrometers (50–150 μin) – this is better than sand casting because a finer sand is used. The resin also assists in forming a very smooth surface.
shell mold casting products

Disadvantages of Shell Mold Casting

There are some disadvantages to the shell mold casting process including:

  • The gating system must be part of the pattern because the entire mould is formed from the pattern, which can be expensive.
  • The phenolic resin is expensive, although not much is required because only a shell is being formed.
  • Equipment costs can be high.
  • Poor material strength is achieved compared to other casting processes.
  • Shrinkage can be a problem.
  • High porosity can be a problem.
  • Secondary machining often required even though the surface finish is better than that can be achieved with sand casting.
  • Labour costs can be high if the process isn’t automated.

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