CO2 Molds (Carbon dioxide Moulding) is the process of hardening the moulds and cores by using carbon dioxide and sodium silicate liquid base binder is one of the most widely used process in foundry. In this process clean dry silica is thoroughly mixed with 3.5 to 5% sodium silicate liquid base binder in a mixer. The sand is then backed in the flask or the core box by standard moulding machines, core blowers or by hand. Sometimes other ingredients such as coal dust, pitch, etc. are added to improve collapsibility and such other properties.
When the mould is packed properly CO2 gas is forced in the mould or the core at a pressure of about 15 N/cm2. The chemical reaction that takes place is
Na2OSiO3l.H2O+CO2 → Na2Co3 +SiO2.H2O
Sodium Carbon Sodium Silica gel
Silicate dioxide carbonate
The silica gel is formed and this harden the sand and acts as a cement to bind the sand grains together.
As shown in figure shows various steps followed in preparing the mould. The mould is prepared in the usual manner and CO2 gas is passed through the mould. The gassing is carried out by a probe having a number of holes at the bottom.
The success of this process depends upon the method of introducing the gas which should be simple, rapid and uniform throughout the body of the sand.
Following are the advantages of this process :
(i) The operation is rapid and expensive core baking equipment is not required.
(ii) The process can be mechanized and adopted for mass production.
Semiskilled labor can be employed.
(iii) Cores can be stored for a long period.
(iv) Same sand can be used for both moulds and cores.
(v) Handling of moulds is reduced.
(vi) Good surface finish and accuracy can be obtained.
This process can be used for both ferrous and non-ferrous castings. The sand used in this process cannot be reclaimed. If sand with a silica base binder is kept for a long time it hardens in air. This is the major disadvantage of the process.