Advantages and disadvantages of shell molding casting

Shell molding casting is a main sand casting production process. The castings have good surface smoothness, less surface defects, and good dimensional accuracy. Our foundry has used this process for many years.

The followings are the advantages of shell molding castings:

1. Good surface quality

Because shell molding uses phenolic resin as the sand binder, so the smooth and hard surfaces of sand molds make the castings have good surface smoothness. The following photo could be taken as sample for the surface quality.

Moreover, this process have less sand residue during production, so could reduce some iron casting defects, such as sand inclusion, sand holes and air holes.

2. High rough casting dimensional accuracy

This molding material is a type of hard mold, so there will be less swell of sand molds, so the dimensional tolerance will be smaller. This advantage will be very useful for producing high accuracy rough castings, and reduce machining cost.

3. Thin wall thickness and complex castings

Less than 5mm wall thickness will be taken as very thin as for sand castings. Only shell molding process could produce these cast products.

In addition, hot shell and core molds are made by molding machines, so it could produce the castings with complex structures, especially complex inside structures.

4. Less manpower and molding skill requirements

Since the main works have been completed by the molding machines, so this process could be operated by women workers, and there is no special skill required. This is very different with green sand casting process.

The followings are the disadvantages of this process.

1. High production costs and casting prices

The phenolic resin sand is more expensive than green sand and furan resin sand, and it could not be recyclable. Therefore, shell molding castings will have higher prices.

2. High pattern costs

This process needs to use metal patterns (iron patterns), which will be more costly. So, it is not suitable for producing small quantity castings and small orders.

3. Size and weight limitation

The shells and cores of castings are produced by shell molding machines. These machines have dimensional limitation. So, most of shell molding castings will be less than 400mm length, and less than 20kg weight. Too long or too heavy can not be produced by this process.

Although shell and core molding process has these disadvantages, but its advantages are also very important. So, more and more iron foundries in China are using it to produce small and middle iron castings. As we know, in other countries, many metal foundries are using it to produce steel castings to replace lost wax casting process.

Shell Mold Casting

Shell mold casting is a metal casting process similar to sand casting, in that molten metal is poured into an expendable mold. However, in shell mold casting, the mold is a thin-walled shell created from applying a sand-resin mixture around a pattern. The pattern, a metal piece in the shape of the desired part, is reused to form multiple shell molds. A reusable pattern allows for higher production rates, while the disposable molds enable complex geometries to be cast. Shell mold casting requires the use of a metal pattern, oven, sand-resin mixture, dump box, and molten metal.

Shell mold casting allows the use of both ferrous and non-ferrous metals, most commonly using cast iron, carbon steel, alloy steel, stainless steel, aluminum alloys, and copper alloys. Typical parts are small-to-medium in size and require high accuracy, such as gear housings, cylinder heads, connecting rods, and lever arms.

The shell mold casting process consists of the following steps:

1. Pattern creation - A two-piece metal pattern is created in the shape of the desired part, typically from iron or steel. Other materials are sometimes used, such as aluminum for low volume production or graphite for casting reactive materials.
2. Mold creation - First, each pattern half is heated to 175-370°C (350-700°F) and coated with a lubricant to facilitate removal. Next, the heated pattern is clamped to a dump box, which contains a mixture of sand and a resin binder. The dump box is inverted, allowing this sand-resin mixture to coat the pattern. The heated pattern partially cures the mixture, which now forms a shell around the pattern. Each pattern half and surrounding shell is cured to completion in an oven and then the shell is ejected from the pattern.
3. Mold assembly - The two shell halves are joined together and securely clamped to form the complete shell mold. If any cores are required, they are inserted prior to closing the mold. The shell mold is then placed into a flask and supported by a backing material.
4. Pouring - The mold is securely clamped together while the molten metal is poured from a ladle into the gating system and fills the mold cavity.
5. Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting.
6. Casting removal - After the molten metal has cooled, the mold can be broken and the casting removed. Trimming and cleaning processes are required to remove any excess metal from the feed system and any sand from the mold.

Shell Mold Casting

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