Process:

Dry and fine sand (90 to 140 GFN) which is completely generally free of the clay is utilized for preparing the shell moulding sand. The grain size to be selected depends on the surface finish wished on the casting. Too fine a grain size needs large amount of resin that makes the mould expensive.

The synthetic resins utilized in shell moulding are fundamentally thermosetting resins, which obtain hardened irreversibly through heat. The resins, most broadly used, are the phenyl formaldehyde resins. Joint with sand, they give very great strength and resistance to heat. The phenolic resins utilized in shell moulding usually are of the two stage kinds, that is, the resin has excess phenol and behave like a thermoplastic material. Throughout coating along the sand, the resin is combined along a catalyst hexa-methylene tetramine in a proportion of approximate 14 to 16% so as to build up the thermosetting characteristics. The curing temperature for these would be approximate 150^oC and the time needed would be 50 to 60 sec.

Additives might sometimes be added up into the sand mixture to develop the surface finish and ignore thermal cracking throughout pouring. Some of the additives utilized are calcium carbonate, coal dust, manganese dioxide, pulverized slag, and ammonium borofloride and magnesium silicoflouride. Some of the lubricants such like calcium stearate and zinc stearate might also be added up to the resin sand mixture to develop the flowability of the sand and allow easy release of the shell from the pattern.

The primary step in preparing the shell mould is the preparation of the sand mixture in such kind way that every sand grain is thoroughly coated along resin. In order to achieve this, primary the sand, hexa and additives, that are all dry, are mixed up inside a Muller for a period of 1 min. After that the liquid resin is added up and mixing is constant for another 3 minutes. To this warm or cold air is introduced into the Muller and the mixing is constant till all the liquid is eliminated from the mixture and the coating of the grains is attained to the desired degree.

As the sand resin mixture is to be cured at approximate 150^oC temperature, just metal patterns with linked gating are utilized. The metal utilized for preparing patterns is grey cast iron, majorly because of its easy availability and outstanding stability at temperatures involved in the procedure. But sometimes-additional risering provision is needed as the cooling in shell mouldings is slow.