What is Shell Molding Casting?
Shell Molding Casting (also known as Shell Casting or Shell Mold Casting) is a casting process in which the molten metal is poured into an expandable mould. In Shell Mold Casting, the mould used is the resin covered sand.
Shell molding has more advantages as compared to sand casting because it gives better surface finishing, better dimensional accuracy and higher productivity rate.
Shell Molding Process (6 Steps)
Shell molding process is done in 6 steps as mentioned below.
- Creating a pattern
- Creating a mold
- Assembly of mold
- Pouring of molten metal
- Cooling of metal
- Removal of casting
The molding sand used in shell molding is made from a mixture of dry powder of thermosetting plastic and fine sand in the ratio of 1:20.
In shell molding process, the pattern is placed on the metal plate and silicon grease is sprayed on it.
After that, the pattern is heated at the temperature range between 205 °C to 230 °C and it is covered with resin bonded sand. (See above image).
Because of this heating, the hard layer of sand is formed on the pattern (step 3 in image).
The resin bonded sand remains stick to the pattern surface and rest of the sand falls down.
The pattern is then separated from the shell by using ejector pins as shown in the above image (step 4).
If there is a requirement of a complicated shape, then the pattern can be made into two or more shapes.
Also the core is made inside the shell. Finally the parts of shell are joined to make the complete mold cavity.
The mold formed by this method has the same shape of the pattern. The mold formed is just a replica of the pattern.
After this, the molten metal is poured into the mold cavity and it is allowed to solidify. In this way, the casting is prepared using a shell molding process.
If the casting requires unsymmetrical shapes then two different shells are prepared and they are joined to form a mould cavity.
In some cases, cores are also placed inside the mold cavity as per requirements of casting.
By using shell molding process, accurate castings can be made which can have tolerance of ±0.02 mm and this type of molding is suitable for duplicating the exact parts.
Facts about Shell Molding
- Shell Molding method was invented in Germany during the time period of second world war.
- This method was invented by German engineer Johannes Croning. Hence this method is also known as Croning’s method.
- Shell mold casting is generally used for achieving mass production of the castings with good surface finish.
- Castings obtained from shell molding are accurate and have a tolerance of ±0.02 mm.
Advantages of Shell Molding
The advantages of Shell Mold Casting are mentioned below.
- Accurate surface finish is obtained using the Shell Molding Process.
- Dimensional accuracy of 0.002 mm to 0.003 mm can be obtained by using shell molding.
- It requires very less machining and cleaning.
- Shell molding uses very less floor space, so it consumes less space in industries.
- Less skilled workers can also use shell molding process for manufacturing the castings.
- Shell Mold Casting process is used for mass production work.
Disadvantages of Shell Molding
Disadvantages of shell molding process are mentioned below.
- In shell molding process, the pattern cost is higher.
- Resins used for mixing the sand also have a higher cost.
- This method is not economical if it is used for manufacturing a small number of castings.
- Shell mold Casting is used for small size castings only.