Shell mould casting is a metal casting process similar to sand casting, in that molten metal is poured into an expendable sand-based mould. However, in shell mould casting, the mould is a thin-walled shell created by 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 moulds. A reusable pattern allows for higher production rates, while the disposable moulds enable complex geometries to be cast.
Process Steps of Shell Mould Casting
Pattern Creation
A metal pattern (usually made of aluminum or steel) is created, which is the exact replica of the desired part.
The pattern is heated to around 175°C – 370°C.
Coating with Resin Sand
Sand mixed with a thermosetting resin (like phenolic resin) is poured onto the hot pattern.
The heat melts the resin, causing the sand to bond and form a hard shell layer on the pattern.
Curing the Shell
The coated sand is held on the heated pattern for a few minutes to allow the resin to harden.
The excess loose sand is removed, leaving a thin, solid shell.
Shell Removal
The hardened shell is stripped from the pattern in two halves.
The two shell halves are joined together to form the final mould cavity.
Assembly
Cores (if required) are placed inside the shell to create hollow sections.
The shell halves are clamped together and supported by backing materials.
Pouring
Molten metal (steel, aluminum, cast iron, etc.) is poured into the mould cavity through the gating system.
Cooling & Solidification
The metal is allowed to cool and solidify inside the shell mould.
Shell Removal
Once the casting is cooled, the shell is broken off using vibration or mechanical means.
Finishing
Final cleaning, shot blasting, fettling, and machining (if necessary) are done to remove excess metal and improve surface quality.
Compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labour requirements. It is often used for small to medium parts that require high precision.
Shell mould casting is 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 using this process are small-to-medium in size and require high accuracy, such as gear housings, cylinder heads, connecting rods, and lever arms.
Shell Mould Casting Process Feature
- 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 with sand casting because a finer sand is used. The resin also assists in forming a very smooth surface.
Advantages of Shell Mould Casting
- High dimensional accuracy (±0.5 mm)
- Smooth surface finish
- Complex shapes with fine details
- Suitable for mass production
- Thin wall sections can be cast
Applications of Shell Mould Casting
- Automotive parts (cylinder heads, camshafts)
- Pump components
- Gear housings
- Valve bodies
- Turbine blades
Conclusion
Shell mould casting (or shell molding) is a precision casting process that uses a resin-covered sand to form the mould. This method is widely used for small to medium-sized metal parts that require high dimensional accuracy, smooth surface finish, and intricate shapes.