The shell mould casting is categorized into a metal casting process, the same as sand casting. Mainly, molten metal is poured into an expendable sand-based mould in this procedure. Actually, it’s a thin-walled shell made using a mixture of sand-resin around a pattern. This process involves the resin-covered sand for moulding.
A metal piece in the shape of the expected part is reused to form many shell moulds. Because of this reusable pattern, it’s easy to reach for a higher production rate. Apart from that, the disposable moulds warrant complex geometries to be cast.
6 steps of the shell moulding process
Though it seems complex, the shell moulding process is comparably easy, containing just six steps.
1) Making the Pattern
The shell moulding process requires which pattern is they are starting with. This is the first and most important process of mould casting. Making the pattern is not a complex thing.
Typically, these patterns used in this moulding process have two parts of metal, like iron or steel, in a custom shape aimed for the finished casting. The shell moulding process requires covering all the steps in a quality way.
2) Creating the Mould
After making the pattern, a mould has been made. Within this step, both of the pattern parts are warmed and then treated with something like a lubricant.
After that, you can insert the pattern parts into a huge dump box container with resin-covered sand. When it inserts into the dump box, the pattern parts themselves will become covered in the sand.
3) Merging the pattern parts
Here, mainly focus on merging the pattern parts to make the final product. At this step, the top and bottom pieces of the pattern are merged. According to the type of pattern, it will show a locking mechanism that keeps both parts together.
We can insert that merged two parts into a flask when it is fully secured. In this case, they’ll be exposed to molten metal, which is the starting step after the pattern parts have been gathered.
4) Submission to Molten Metal
The next step of the shell moulding process is pouring molten metal into the mold’s cavity. In this process, two different types of metals and alloys can be used in this shell moulding process: steel and iron. The metal or alloy is warmed until it turns into a liquid form. Next, the molten metal is poured into the mold’s cavity to make a hardened shell.
5) Keep to Cool
Before making a hardened shell, the molten metal should keep cool. After that, it starts to do the casting process. Typically, the pattern parts cool at room temperature. When the pattern parts cool, the molten metal will be roughened to make a shell.
6) Ejecting the Casting
The last step of the shell mould casting process is emitting the cast from the mold’s cavity. Since this casting procedure uses a two parts pattern, castings are easily sent out by “opening” the pattern. The top and bottom pattern parts are detached from each other, at which point the newly made casting can be protectively ejected.
Shell moulding applications
The shell moulding process is essential for making industrial products like gearbox housing, connecting rods, small-sized boats, truck hoods, cylindrical heads, camshaft, valve body, etc. Average tolerances are just 0.005 mm/mm, and the cast surface completion is 0.3–4.0 micrometers (50–150 μin). Apart from that, the resin also assists in forming a very smooth surface. There is no moisture present in the shell. An only a tiny amount of gases will be made. Those gases would easily escape through the thin shell walls since the porous sand.
The shell moulding process has a higher productivity rate and lower labor needs than sand casting. We use it for small to medium parts that need high precision most of the time. Shell mould casting is vital for both ferrous and non-ferrous metals. The most commonly used materials are cast iron, carbon steel, stainless steel, aluminum alloys, and copper. This shell moulding process lets various shapes in a vast range of sizes with compatible surface finish to be cast. Little scrap is created, and what there is can be recycled. We can recycle the sand-resin mix by burning the resin at elevated temperatures. Tooling prices are low and short lead times are possible.
Pros and cons of Shell Moulding Process
What are the shell moulding advantages and disadvantages?
Advantages:
- Having the ability to cast a thin and complex section.
- Elevated dimension accuracy.
- Good surface completion.
- It can be automated easily.
- Has the ability to operate using a semi-skilled operator.
- No need for other machining methods.
- Lowers labor/machining cost compared to the sand casting
- Facilitates for mass production
Disadvantages:
- There should have a special metal pattern that makes it costliest for large casting.
- That is not necessary for little batch production.
- Shell moulds are low permeable than green sand mould.
- Quantity and limitation of weight.