Moulding Sand in Shell Moulding Process

Moulding Sand in Shell Moulding Process
The moulding sand is a mixture of fine grained quartz sand and powdered bakelite. There are two methods of coating the sand grains with bakelite. First method is Cold coating method and another one is the hot method of coating.
In the method of cold coating, quartz sand is poured into the mixer and then the solution of powdered bakelite in acetone and ethyl aldehyde are added. The typical mixture is 92% quartz sand, 5% bakelite, 3% ethyl aldehyde. During mixing of the ingredients, the resin envelops the sand grains and the solvent evaporates, leaving a thin film that uniformly coats the surface of sand grains, thereby imparting fluidity to the sand mixtures.
In the method of hot coating, the mixture is heated to 150-180 o C prior to loading the sand. In the course of sand mixing, the soluble phenol formaldehyde resin is added. The mixer is allowed to cool up to 80 – 90o C. This method gives better properties to the mixtures than cold method.
Adv: dimensionally accurate
Smoother surface
Lowered draft angle
Thin section
No gas inclusion
Small amount of sand needed
Simple processing
Limitation: patterns are expensive
Size of casting – limited
Complicated shapes
Sophisticated equipments needed.