Carbon Dioxide Process for Mould Hardening | Casting | Metallurgy

Formerly this process was widely used in Europe for hardening of moulds and cores made of green sand, but now- a-days, it has been adopted in several countries because of rapid hardening of sand. It consists of thorough mixing of silica sand (clean, dry and free from clay) with 3.5 to 5% by weight of sodium silicate liquid base binder in a muller.

Sometimes coal-dust, pitch, graphite and wood flour are also added so as to improve the collapsibility property. The mixture is then put into core-boxes by anyone of the conventional methods. After packing, CO₂ is forced into the mould at a pressure of about 1.4 kg/cm². The sodium silicate present in the mould reacts with CO₂ and gives a hard substance called the silica gel.

Na₂SiO₂xH₂O + CO₂ ——— > Na₂CO₃ + (SiO₂xH₂O)

The silica gel is a hard substance like cement and hence helps in binding of sand grains. The method of introducing the gas must be simple, rapid and uniform throughout the sand body.

Normally time taken to harden a medium size body is about 20 to 30 seconds. Fig. 3.72 gives the steps for preparing a casting by CO₂ hardening process. The method of introducing the CO₂ gas must be simple, rapid and uniform throughout the sand mass.

The time to harden a small or medium size body of sand ranges from 15 to 30 seconds. Over gassing is wasteful and results in deterioration of sand.

Pattern used for this process is either of metal or wood. If wood is used, it is given a coating of alkaline resistant solution of that varnish which does not have any chemical reaction with mould material. This process can be applied for both ferrous and non-ferrous castings.

The primary application of the CO₂ process for moulding is as a substitute for dry sand moulding, the elimination of stoving being the principal attraction. This process is used for the production of large moulds from wooden patterns, and for small repetition castings to a high standard of surface finish and close dimensional tolerance thus acting as a substitute for shell moulding.

Advantages of Carbon Dioxide Process:

The advantages of this process are as follows:

(i) Operation is speedy. Pattern withdrawal is easier.

(ii) Castings obtained have close tolerances and good surface finish.

(iii) Baking operations for hardening the cores and moulds are avoided.

(iv) Cores and moulds can be stored for a long time. Cores can often be made hollow, with a consequent saving in binder.

(v) Permeability and flow ability of sand are improved.

(vi) The process being simple, requires semi-skilled labour.

(vii) Evolution of gases can be reduced to one-third of the gases given by green sand and dry sand moulds.

(viii) A uniform sand may be used for both moulds and cores production.

(ix) Less possibility of distortion of mould and core during the process of hardening.

(x) While pouring, the sand does not flow with the metal.

(xi) Accurate castings with sharp corners can be made.

Limitations of Carbon Dioxide Process:

(a) There is always a tendency for sand with silica base binder to air hardens, if kept for a long time.

(b) Sand mixture is costly due to use of binders and additives.

(c) Due to hardening of mould and core, the collapsibility property sometimes gives trouble.

(d) Sand with a silica-base binder has tendency to air harden to some extent, if kept for a long time.

(e) Lack of any cheap method for recovery of the spent sand.