What is Shell Molding?
Shell Molding (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.
The pattern is made up of metal and it can be used to form multiple shell molds.
And because of these reusable patterns, higher production can be achieved with less cost.
6 Steps of Shell Molding Process
Shell molding process is done in 6 steps as mentioned below.
- Heating of pattern and blowing a sand-resin mixture on it.
- Removing the excess loose particles.
- Hardening the mold in oven.
- Separating the hardened mold from the pattern.
- Assembly of molds.
- Pouring of molten metal, Cooling of metal and Removal of casting.
Let’s discuss these steps of shell molding process one by one.
Step 1: Heating of pattern and blowing a sand-resin mixture on it
- The very first step involved in the shell molding process is the manufacturing of shell mold.
- The sand which is used in shell molding has a very smaller grain size as compared to that of the green sand.
- This small grained sand is mixed with a thermosetting resin binder.
- The metal pattern used in this process is coated with a parting agent which will help in separating the pattern from the shell.
- The metal pattern is heated at the temperature between 175 °C to 370 °C.
- As shown in the image, the sand mixture is blown over this hot metallic pattern.
- As the sand mixture contains a thermosetting resin binder, its reaction takes place with the hot pattern and thus the thin shell is formed on the surface of pattern.
- The thickness of the sand layer is kept more if heavy casting is required to be manufactured.
- Generally, for shell molding casting process, the thickness of mold is kept around 7.5 mm.
- The thickness of the mold will be more if the sand mixture remains in contact with the pattern for a longer time, and vice versa.
Step 2: Removing the excess loose particles
- As shown in the above image, the loose particles of sand are removed and it is left with the sand-resin mixture.
Step 3: Hardening the mold in oven
- Now, in order to harden the sand-resin mixture on the pattern, the shell and pattern are kept in the oven for a few minutes (see above image).
Step 4: Separating the hardened mold from the pattern
- The pattern is then separated from the shell by using ejector pins as shown in the above image.
- Besides the shell manufacturing, the shell molding process can also be used for manufacturing of sand cores with higher accuracy and smooth surface finish.
Step 5: Assembly of molds
- Now, as shown above, the hardened shells are assembled with each other by using clamps or gluing material.
Step 6: Pouring of molten metal, Cooling of metal and Removal of casting
- After this process, the shell mold is completely ready for the casting. As shown in the above diagram, the shell is now ready and molten metal can be poured for further casting procedure.
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.
Having said that, let’s discuss the advantages and disadvantages of Shell molding process.
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.
- Shell mold casting is not suitable for manufacturing complex shapes components.
Applications of Shell Molding
The applications of shell mold casting are mentioned below.
- By using the shell molding process, parts like gears, connecting rods, valve bodies, cylinder heads, bushings, etc can be manufactured.
- Shell mold casting is suitable for casting ferrous as well as non ferrous metals. These include cast iron, stainless steel, alloy steel, aluminum alloys and copper alloys.