Shell Molding

Shell mold casting is recent invention in molding techniques for mass production and smooth finish. Shell molding method was invented in Germany during the Second World War. It is also known as Carning or C process which is generally used for mass production of accurate thin castings with close tolerance of +_ 0.02 mm and with smooth surface finish. It consists of making a mould that has two or more thin lines shells (shell line parts, which are moderately hard and smooth. Molding sand is prepared using thermosetting plastic dry powder and find sand are uniformly mixed in a muller in the ratio 1: 20. In this process the pattern is placed on a metal plate and silicon grease is then sprayed on it. The pattern is then heated to 205°C to 230°C and covered with resin bonded sand. After 30 second a hard layer of sand is formed over the pattern. Pattern and shell are then heated and treated in an oven at 315°C for 60 sec. Then, the shell so formed as the shape of the pattern is ready to strip from the pattern. The shell can be made in two or more pieces as per the shape of pattern. Similarly core can be made by this process. Finally shells are joined together to form the mold cavity. Then the mold is ready for pouring the molten metal to get a casting. The shell so formed has the shape of pattern formed of cavity or projection in the shell. In case of unsymmetrical shapes, two patterns are prepared so that two shell are produced which are joined to form proper cavity. Internal cavity can be formed by placing a core. Hot pattern and box is containing a mixture of sand and resin. Pattern and box inverted and kept in this position for some time. Now box and pattern are brought to original position. A shell of resin-bonded sand sticks to the pattern and the rest falls. Shell separates from the pattern with the help of ejector pins. It is a suitable process for casting thin walled articles. The cast shapes are uniform and their dimensions are within close limit of tolerance ± 0.002 mm and it is suitable for precise duplication of exact parts.

The shells formed by this process are 0.3 to 0.6 mm thick and can be handled and stored. Shell moulds are made so that machining parts fit together-easily, held clamps or adhesive and metal is poured either in a vertical or horizontal position. They are supported in rocks or mass of bulky permeable material such as sand steel shot or gravel. Thermosetting plastics, dry powder and sand are mixed ultimately in a muller. The process of shell molding possesses various advantages and disadvantages. Some of the main advantages and disadvantages of this process are given as under.

Advantages

The main advantages of shell molding are:

(i) High suitable for thin sections like petrol engine cylinder.

(ii) Excellent surface finish.

(iii) Good dimensional accuracy of order of 0.002 to 0.003 mm.

(iv) Negligible machining and cleaning cost. (v) Occupies less floor space.

(vi) Skill-ness required is less.

(vii) Moulds formed by this process can be stored until required.

(viii) Better quality of casting assured.

(ix) Mass production.

(x) It allows for greater detail and less draft.

(xi) Unskilled labor can be employed.

(xii) Future of shell molding process is very bright.

Disadvantages

The main disadvantages of shell molding are:

1. Higher pattern cost.

2. Higher resin cost.

3. Not economical for small runs.

4. Dust-extraction problem.

5. Complicated jobs and jobs of various sizes cannot be easily shell molded.

6. Specialized equipment is required.

7. Resin binder is an expensive material.

8. Limited for small size.