The material used to make sand mold is called molding material, the material used to make sand mold is generally called molding sand, and the molding material used to make sand core is called core sand. Generally, molding sand is composed of raw sand (mountain sand or river sand), clay and water in a certain proportion, of which clay is about 9%, water is about 6%, and the rest is raw sand. Sometimes a small amount of additives such as coal powder, vegetable oil and wood chips are added to improve the properties of molding sand and core sand.
The quality of molding sand and core sand directly affects the quality of castings. Poor molding sand quality will cause defects such as blowholes, sand holes, sand adhesion, and sand inclusion in castings. Good molding sand shall have the following properties:
1) Breathable type
After the high-temperature molten metal is poured into the mold, the mold is filled with a large amount of gas, which must be smoothly discharged from the mold. The property of molding sand that allows the gas to pass through is called permeability. Otherwise, the casting will have defects such as air hole and insufficient pouring. The permeability of mold is affected by the grain size of sand, clay content, moisture content and sand mold compactness. The finer the particle size of sand, the higher the clay and water content, and the higher the sand mold compactness, the worse the air permeability
2) Strength
The ability of molding sand to resist external force is called strength. The molding sand must have sufficient strength to avoid collapse during molding, handling and box closing, and to avoid damaging the mold surface during pouring. The strength of molding sand should not be too high, otherwise the casting will have defects due to the decline of permeability and yield.
3) Fire resistance
The high-temperature metal liquid will have a strong thermal effect on the mold after pouring, so the molding sand should have the ability to resist the high-temperature thermal effect, that is, fire resistance. For example, molding materials have poor fire resistance, and castings are prone to sand sticking. The more SiO2 content in molding sand, the larger the molding sand particles, and the better the fire resistance.
4) Plasticity
It refers to the ability of molding sand to deform under the action of external force and maintain the existing shape completely after removing the external force. The plasticity of the molding material is good, the modeling operation is convenient, and the shape of the sand mold made is accurate and the outline is clear.
5) Abdication
When the casting is condensed, its volume will shrink, and the molding sand should have a certain ability to be compressed, which is called the yielding property. The castings are easy to produce internal stress or crack due to poor castability of molding sand. The tighter the molding sand is, the worse the yielding property is. Adding wood chips and other materials to molding sand can improve the yield.