Casting is a primary source of manufacturing. It offers minimum material loss. It is able to manufacture complex geometrical objects. But during casting or in particular sand casting, many defects arises which leads to rejections. Some defects can be ignored while some needs attention. There are various types of defects.
1. Porosity: This defect arises due to gases. During casting process molten metal is poured in the mould at a very high temperature. There are various reactions occurring in the mould. Various types of gases evolve in this process. But sometimes gases do not find way to come out from the mould. They get entrapped inside and causes defect in casting. Blow holes, pinholes, open holes are various types of defects arising due to gases. Their shape may be round or oval. This defect is found in cope part of molding boxes. Increased moisture content, less permeability, very fine sand grains, hard ramming poor venting are the various causes of porosity.
2. Shrinkage: It is contraction in the volume of molten metal. Due to uncontrolled solidification of molten metal a cavity is formed. This defect is referred as shrinkage. Shrinkage defect can be minimized by unidirectional solidification. Temperature of pouring metal should not be high. Chills can help in proper cooling.
3. Defects of molding material: the different defect occurring in this category are swell, wash, metal penetration, cut, rat-tail , drop etc.
Swell: This is expansion of mould cavity due to pressure of molten metal. This result in
oversize of casting. Poor ramming is the main cause of swell. Also ramming is not even.
Drop: Sometimes sand of mold falls inside the mold cavity. This refers to drop defect. Main reason of this defect is soft ramming. Strength of sand should be improved to overcome this defect.
Wash: It is a projection on lower surface of casting. Mostly it is seen in drag part. This occurs due to poor hot strength of molding sand.
Rat-tail: It is a long angular depression on flat surface. Reason is poor hot strength of sand.
Penetration: It occurs due to fusing of molten metal with molding sand. Reason is poor refractoriness of molding sand. This looks like projections. However it can be removed by chipping.
4. Defects arising due to defective pouring: Defective pouring results in different types of defects such as misrun, coldshut etc.
COLD-SHUT: This defect causes a line marking on casting surface. When molten metal from different gates enter into mould, they do not fuse properly and solidifies. It appears as crack with round edge.
MISRUN: Sometimes due to fall in temperature molten metal solidifies before reaching the last point of mold cavity. This results in vacant space or unfilled mould cavity. This defect is referred as Misrun.
5. Defects of metallurgy: It includes hot tear, hot spots etc.
HOT TEAR- It is crack developed on surface of casting. It is due to tensile forces developed. There are residual stresses which acts during cooling and finally develops cracks.
HOT SPOTS: Sometimes some region of casting cools rapidly compared to surrounding areas. Due to this these regions become hard. This defect can be eliminated by proper cooling practice or proper material composition.