Shell Moulding : It is the casting tecqnic for mass production and smooth finish. It was invented in Germany during second world war. ( By Mr. J. Corning, hence also called as Corning process or C process.

The process consists of a mould with 2 or more thin shells or shell like parts which are moderately hard and smooth, with the texture similar to a well made sand core, consisting of thermosetting resin bonded sand.The sheels are 0.3/0.6 mm thick, and can be easily handled and stored. Shell moulds are made so that machined parts fit together easily,held with clamps or adhesive and metal is poured either in vertical or horizontal position.They are supported in racks or mass of bulky permeable materilal such as sand stell shot or gravel.

Patterns for shell moulding ; The patterns for making shell moulding are made of metal as they are heated to 540deg C during the cycle. The patterns should have provision of ejector pins for removal of shell from the surface of pattern.

Materials for making shell: The sand used is fine dry silica sand, preferably round grained free from organic impurities and with not more than 1% Clay. In addition to it some additives have also been proposed, which are plumbago, iron oxide, sulphur and various minerals.

Zircon sand is also used for sheel moulding. Although this is costly, it gives stronger shell than silica sand for the same % of resin binder. It produces better surface finish because of its small round grains and a dense mould of high thermal conductivity, which rapidly and evenly cools the castings. The binders used are

thermosetting synthetic resin (3 to 10% by weight), phenol formaldehyde, and urea formaldehyde.

The proportions in which they are mixed are as follows:

Dry silica sand 100 parts by wtPowdered phenol 2 to 8 parts by weight Formaldehyde Resin binder

Wetting agent 0 – ¼ parts by weight

Process:1. Thermosetting plastics, dry powder and sand are mixed in a

Muller.2. Metal pattern is sprayed with separating silicon grease,

heated to 205 to 230 deg C 3. Covered quickly with the resin bonded sand. 4. After 30 sec a hard layer of sand is formed over the pattern.5. Pattern and shell are then heat treated in an oven at 315 deg

C for 60 secs. 6. Now sheel is ready to strip from the pattern. 7. A complete mould is made in 2 or more pieces and similarly

core is made by the same technique. 8. The sheels are clamped securely and usually embedded in a

gravel, coarse sand or metal shot.

9. Then the mould is ready to use.

Merits: 1. Very suitable for casting thin section of high definition ex

: petrol engine cylinder.2. Surface finish obtained by this process is excellent and

much superior than that possible with green sand mouding. Finer details can be reproduced.

3. The dimensional accuracy obtained is of the order of 2- 3 microns per mm and in large castings it may be reduced to as low as 1 microns in the line of the shell

4. Machining and cleaning cost is negligible. 5. It occupies less floor space and matl handling equipment.6. The total sand used is only 5 – 10 % that of green sand

mould. This reduces sand handling difficulties7. As the resin does not absorb moisture, it offers more

flexible scheduling8. Capital layout for shell moulding is considerably low for

mass productions compared to other foundry techniques. 9. Process being semi mechanicsed cycle time involved in

producing a shell mould is considerably less compared to sand mould.

10. Less skilled labor is required11. The moulds can be stored until required12. The thin shell does not have as great a chilling effect as a

sand mould and the gasses easily escape through the walls. Therefore a better quality of casting is assured.

Demerits : 1. Initial cost of patterns and sand is high and hence

unsuitable for small production2. Specialized equipments are to be used..3. Resin binder is an expensive materials.4. This process is limited to small sizes.5. Max thick of the section that can be cast is 4 mm

6. Certain casting shapes are unsuitable when proper partings and gatings are not available.

7. This process is more suited for large qty’s castings prdn, so that initial tooling cost can be off set.