What is sand casting? The quality of the sand and core sand directly affects the quality of the casting.
The quality of the sand and core sand directly affects the quality of the casting. The poor quality of the sand can cause defects such as pores, sand holes, sand sticks and sand inclusions. Good molding sand should have the following properties:
1) Breathable type
After the high-temperature metal liquid is poured into the mold, the mold is filled with a large amount of gas, and these gases must be smoothly discharged from the mold. The sand-forming property of the gas is called gas permeability. Otherwise, the casting will have defects such as pores and insufficient pouring. The gas permeability of the mold is affected by factors such as particle size, clay content, moisture content and sand compactness. The finer the grain size of the sand, the higher the clay and moisture content, the higher the sand compaction, the worse the gas permeability.
The ability of sand to resist external damage is called strength. The molding sand must have sufficient strength to prevent collapse during molding, handling, and boxing, and will not damage the mold surface during casting. The strength of the sand should not be too high, otherwise the castings will be defective due to the decrease in gas permeability and concession.
3) Fire resistance
The high-temperature metal liquid is poured into the mold to have a strong thermal effect on the mold, so the molding sand should have the ability to resist high-temperature heat, that is, fire resistance. If the fire resistance of the modeling material is poor, the casting is prone to stick sand. The more SiO2 content in the molding sand, the larger the molding sand particles and the better the fire resistance.
The finger sand deforms under the action of external force, and the ability to completely maintain the existing shape after removing the external force. The modeling material has good plasticity, convenient shape operation, and the shape of the sand shape is accurate and the outline is clear.
When the casting is condensed, the volume shrinks and the molding sand should have a certain ability to be compressed, called concession. The sand has a poor concession, and the casting is prone to internal stress or cracking. The tighter the sand, the worse the concession. Adding wood chips and the like to the molding sand can improve the repellent property.