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Option 2 : Metals penetration
The following are the major defects, which are likely to occur in sand castings
- Gas defects: A condition existing in a casting caused by the trapping of gas in the molten metal or by mould gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity.
- Shrinkage cavities: These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required.
- Moulding material defects: The defects in this category are cuts and washes, metal penetration, fusion, and swell.
- Pouring metal defects: The likely defects in this category are Mis-runs and cold shuts, Slag inclusions
- Mould shift: The mould shift defect occurs when cope and drag or moulding boxes have not been properly aligned.
Cut and washes: These appear as rough spots and areas of excess metal and are caused by erosion of moulding sand by the flowing metal. This is caused by the moulding-sand not having enough strength and the molten metal flowing at high velocity.
Metal penetration: When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mould wash was applied on the surface of the mould. The coarser the sand grains more the metal penetration.
Fusion: This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high.
Rat Tails: This is caused by compression failure of the skin of the mould cavity. Due to excessive heat, sand expands resulting in a small line on the casting surface.