CO2 sand casting or Hobby Aluminum casting is suitable for urgent and heavy orders with high accuracy and good surface finish on castings required. Hobby mould casters used this method because in this method elimination of core drying machine, but it is not an economical process. The only expense hobby mould caster encounters are the cost of the CO2 cylinder, regulator, and hoses and hand held applicator gun or nozzle. The operation is fast and the possibility of rejections is less. CO2 is a sand molding technique and it uses grain sand which is mixed with a solution of sodium silicate that acts as binding agent. CO2 gas is used to harden the mass of sand after the mould has been made. The chemical reaction takes place is explained below:
In this process, CO2 gas is passed through sand mix (0.25% moisture content Sodium Silicate), then the sodium silicate sand becomes stiff gel which gives enough strength for the mould. Additives used are Coal powder, Wood Flour, Sea coal, Dextrin, Kaolin clay, Aluminium oxide and Invert Sugar.
Properties of Additives:
1. Kaolin clay is used for mould stability
2. Aluminium oxide is used for hot strength of bonded sand
3. Invert Sugar is used after pouring to reduce the retained strength of the mould or core which removes from the mould with less shaking.
Steps involved
1. Mould material is pure Silica, 3-5% of liquid base binder of sodium silicate, mulled for 3 to 4 minutes
2. Now rammed around pattern in moulding box
3. CO2 gas forced into mould at about 1.4 to 1.5 kg/cm2 for a predetermined time (15 to 30 seconds)
4. Apply refractory coating, system ready for pouring
5. Use either wood or metal pattern
6. Suitable pressure reducing valves are used to reduce pressure of gas
7. Pattern rubbed with graphite before ramming to overcome removal of pattern from mould.





Advantages:
1. Low raw material cost
2. Fast production rate
3. No core or mould baking equipments
4. Semi skilled workers enough
5. Hollow moulds can be easily made
6. Easily mechanized
Disadvantages:
1. Difficult to reclaim the used sand
2. Poor collapsibility due to inorganic nature of bond
3. Moisture susceptibility
4. Less suitable for non ferrous casting
Applications:
1. For larger cores preparation
2. Heavy or thick walled castings
3. Producing cores of Fe, Al, Cu base alloys

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