Shell mould casting method makes use of moulds made of relatively thin shells about 6 mm thick for casting.
The shells are made from clay free silica sand 60-140 AFS fineness mixed with 3 to 10 percent by weight of phenolic thermosetting resins like phenol formaldehyde.
The resin is used in powder form in the mixture and then dried on the sand grains. The mixture ought to be dry and free flowing.
In dry mixtures 112percent kerosene is added and mulled to avoid dusting and loss of binder throughout operation.
The sand resin mixture is invested on the pattern heated to a temperature of 200 to 400°C for a period of 15 to 45 seconds. During this period the sand mix around the pattern partially sets to a thickness of 6 to 15 mm depending on the dwell period and the type of mixture.
The unset sand is removed and also the partially set shell sticking onto the pattern is cured at a temperature of 250° to 350°C for a period of 1 to 3 minutes depending on the pattern intricacy and shell thickness.
The cured shell is ejected from the pattern plate using ejector pins on the pattern. To aid in shell stripping a suitable releasing agent like silicon solution is applied on the heated pattern before investing the mould mixture
The ejected mould elements are assembled with cores and clamped together for pouring. The assembled mould could be backed up by sand or shots if required.
The investment of the mould mixture is finished either by blowing onto the pattern or by dumping the mixture on the pattern.
Blowing is more often used in the manufacture of shell cores and the dumping method in the manufacture of shell moulds.
In the dumping method the heated pattern plate is placed inverted on a box filled partially with the mould mixture.
The entire assembly of the pattern plate and the box is inverted to dump the mixture on the pattern plate.
After the dwell period the box is brought back to the upright position to dump the unset mixture back into the box.
The pattern plate along with the sticking partially set shell is sent in for curing.
Modern shell moulding machines have been designed to complete a cycle of investment, dumping back of unset excess sand, curing and ejection automatically.
ADVANTAGES OF SHELL MOULDING :
Good surface finish of the order of 3 microns RMS and close dimensional tolerances of the order of ± 0.003 mm per mm.
Dimensions across parting surfaces can be held to within 0.1 mm per mm.
The resin binder in the mould and core completely burns due to the heat of the poured metal leaving only loose sand with the casting which is easily cleaned.
The process reduces the tendency for section variation in castings compared to green sand moulds.
DISADVANTAGES OF SHELL MOULDING :
Limitation of the size of the castings.
High initial cost
High cost of the binder
APPLICATION :
Used for manufacture of cams, pistons, piston rings, small pulleys, motor housings, fan blades.
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