Shell Moulding (or) Croning Shell Process
Shell moulding is a process for producing simple or complex near net shape castings maintaining tight tolerances and a high degree of dimensional stability. Shell moulding is method for making high quality castings.
The process is based on the principle of capability of a thermosetting resin and sand mixture to assume the shape of a preheated metal pattern to form a dense, quickly hardened shell mould.
Process parameters of shell moulding Process
Sand coated with a thermosetting plastic resin is dropped onto a heated metal pattern, which cures the resin.
The shell segments are stripped from the pattern and assembled. When the poured metal solidifies, the shell is broken away from the finished casting.
Advantages: Faster production rate than sand moulding high dimensional accuracy with smooth surfaces.
Limitations: Requires expensive metal patterns. Plastic resin adds to cost; part size is limited.
Common metals: Cast irons and casting alloys of aluminium and copper.
Size limits: 30 g minimum usually less than 10kg; mould area usually less than 0.3 m2
Typical tolerances: Approximately 0.005 cm
Draft allowance: 1/4 to 1/2 degree
Surface finish: 1/3 – 4.0 microns
There are different stages in shell mould processing that include:
1. Initially preparing a metal-matched plate
2. Mixing resin and sand
3. Heating pattern.
4. Inverting the pattern (the sand is at one end of a box and the pattern at the other, and the box is inverted for a time determined by the desired thickness of the mill).
5. Curing the shell and baking it
6. Removing investment
7. Inserting cores
8. Repeating for the other half
9. Assembling the mould
10. Pouring the mould
11. Removing casting
12. Cleaning and Trimming.
The shell mould casting process consists of the following steps.
a) 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.
b) Mould 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.
c) Mould assembly:
The two shell halves are joined together and securely clamped to form the complete shell mould. If any cores are required, they are inserted prior to closing the mould. The shell mould is then placed into a flask and supported by a backing material.
The mould is securely clamped together while the molten metal is poured from a ladle into the gating system and fills the mould cavity.
After the mould has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting.
f) Casting removal:
After the molten metal has cooled, the mould 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 mould.
Advantages of Shell Moulding Casting
1. Good casting detail and dimensional accuracy are possible.
2. Moulds are lightweight and may be stored for extended periods of time.
3. Has better flexibility in design than die-casting.
4. Is less expensive than investment casting.
5. Capital plant costs are lower than for mechanized green sand moulding.
6. Metal yields are relatively high.
7. Sand: metal ratios are relatively low.
8. Gives superior surface finish and higher dimensional accuracy, and incurs lower fettling costs than conventional sand castings.
i) Higher cost of match plate
ii) Size of casting is limited
iii) Serious dust and fume problems
iv) Carbon pickup in case of steels.
Cylinders and cylinder heads for air cooled IC engines, automobile transmission parts, cast tooth bevel gears, brake beam, hubs, and track rollers for crawler tractors, steel eyes, gear blanks, chain seat brackets, refrigerator valve plate, and small crank shafts.