Shell moulding casting and it's process, advantages and disadvantages.

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.

Principle

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:

•        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

Steps involved:

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

Advantages of Shell Mould Casting:

·       Good casting detail and dimensional accuracy are possible

·       Moulds are lightweight and may be stored for extended periods of time

·       Has better flexibility in design than die-casting

·       Is less expensive than investment casting

·       Capital plant costs are lower than for mechanized green sand moulding

·       Metal yields are relatively high

·       Sand: metal ratios are relatively low

·       Gives superior surface finish and higher dimensional accuracy, and incurs lower fettling costs than conventional sand castings

Disadvantages of Shell Mould Casting:

·       Higher cost of match plate

·       Size of casting is limited

·       Serious dust and fume problems

·       Carbon pickup in case of steels

APPLICATIONS:

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.