In this article we will discuss about:- 1. Meaning of Shell Moulding 2. Patterns and Materials for Making Shell Mould 3. Process 4. Merits 5. Demerits.
Meaning of Shell Moulding:
Shell moulding is a recent invention in casting techniques for mass production and smooth finish. It was originated in Germany during Second World War. For some time, it was called by the name of Corning process (after the name of J. Corning who invented it), C-process, and finally shell process or shell moulding.
It consists of making a mould that has two or more thin shells or shell like parts which are moderately hard and smooth, with a texture similar to that of a neatly made sand core, consisting of thermo-setting resin bonded sands. The shells are 0.3 to 0.6 mm thick, and can be easily handled and stored.
Shell moulds are made up of a mixture of dried silica sand and phenolic resin, formed into thin, half mould shells, which are held with clamps or adhesive and metal is poured either in a vertical or horizontal position. They are supported in racks or mass of bulky permeable material such as sand, steel shot or gravel.
Patterns and Materials for Making Shell Mould:
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The patterns for making the shell mould are made of metal as they are heated to 540°C during the cycle. The patterns should have provision of ejector-pins for removal of shell from the surface of pattern.
Materials:
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 another raw material used for manufacture of shell.
Although zircon sand is costly but gives a stronger shell than silica sand for the same percentage of resin binder. Zircon sand produces better surface finish because of its small round grains, and a dense mould of high thermal conductivity which rapidly and evenly cools the casting. The binders used are thermosetting synthetic resin (3 to 10% by weight), phenol formaldehyde and urea formaldehyde.
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The proportions in which they are mixed are as follows:
Dry silica sand – 100 parts by weight
Powdered phenol-formaldehyde resin binder – 2 to 8 parts by weight
Wetting agent – 0 to 1/4 parts by weight.
Process of Shell Moulding:
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Thermo-setting-plastics, dry powder and sand are mixed intimately in a muller. Metal pattern is sprayed with a separating silicon grease, heated to 205 to 230°C and covered quickly with the resin-bonded sand. After 30 seconds, a hard layer of sand (3 to 4.5 mm thick) is formed over the pattern.
Pattern and shell are then heat-treated in an oven at 315°C for 60 seconds, and then the shell is ready to strip from the pattern. A complete mould is made in two or more pieces and similarly core is made by the same technique. The shells are clamped securely and usually embedded in gravel, coarse sand or metal shot. Then the mould is ready for use.
Following are the steps in shell moulding (Refer Fig. 3.73):
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(a) Hot pattern is clamped to the dump box containing a mixture of sand and thermo-setting plastic resin.
(b) Pattern and box are inverted and kept in this position for some time. Hot pattern melts resin in contact with it.
(c) Box and pattern are again inverted and brought original position. A thin shell of resin-bonded sand sticks to pattern and the rest falls.
(d) Pattern with shell in placed in oven and heated to cure resin bond.
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(e) Shell is stripped from the pattern with the help of ejector pins.
(f) Two shells are assembled, clamped and properly backed with sand in a suitable box. This forms a shell mould ready to receive the metal.
Merits of Shell Moulding:
(i) Shell moulding is very suitable for casting thin sections of high definition, for example, petrol engine cylinder.
(ii) Surface finish obtained by this process is excellent and much superior to that possible with green sand moulding. Finer details can be reproduced.
(iii) The dimensional accuracy obtained is of the order of 0.002 to 0.003 mm per mm and in large castings, it may be reduced to as low as 0.001 mm in the line of the shell.
(iv) Machining and cleaning cost is negligible.
(v) It occupies less floor space and material handling equipment, thus greater production from a given floor space.
(vi) The total sand used is only 5 to 10% that of green sand mould. This reduces sand handling difficulties.
(vii) As the resin does not absorb moisture, it offers more flexible scheduling.
(viii) Capital layout for shell moulding is considerably low for mass production as compared to other foundry techniques.
(ix) Process being semi-mechanised, cycle time involved in producing a shell-mould is considerably less compared to sand mould.
(x) Less-skilled labour is required.
(xi) The moulds can be stored until required.
(xii) The thin shell does not have as great a chilling effect as a sand mould and the gases easily escape through the walls, therefore, a better quality of casting is assured.
Demerits of Shell Moulding:
(a) Initial cost of patterns and sand is high and hence unsuitable for small production.
(b) Specialised equipment are to be used.
(c) Resin binder is an expensive material and little of sand can be economically reclaimed.
(d) This process is limited to small sizes.
(e) Minimum thickness of the section that can be cast is 4 mm.
(f) Certain casting shapes are unsuitable when proper partings and gatings are not available.
(g) This process is more suited to the production of castings in large quantities so that the initial tooling cost can be offset.
(h) Due to use of fine and dry silica sand, adequate dust extraction equipment is required to be installed.