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1.Rat tails, veins and buckles:
Rat tails, or veins, appear as an irregular line or crack on the casting, when the surface of the molding sand buckles up.
Rat tails usually occur on the surface of the mold bottom, an area covered with molten material.
Buckles are a more severe form of rat tails.
Causes and prevention of rat tails and buckles
Rat tails and buckles occur when excessive heat of the metal causes the sand to expand. This may be caused by:
Poor expansion properties of the sand: Add combustible additives to sand.
A hot pouring temperature: Reduce pouring temperature of metal.
Poor mold design: Large and flat sections are more prone to rat tails. The mold also should not be too hard, as it must allow for proper expansion.
Metal penetration occurs when liquid metal penetrates gaps in the molding sand. The penetration is visible to the naked eye as a rough and uneven surface finish of the casting.
Causes and prevention of metal penetration
Metal penetration is due to:
Use of sand with low strength and high permeability
Use of large or coarse sand grain: the coarser the sand grains, the more severe the metal penetration
Lack of mold wash
Soft ramming of sand
Prevent metal penetration by fixing these areas. Use high strength, small grain size, low permeability and hard ramming of sand.
Ensure a protective barrier against metal penetration by coating the surface of molds with a mold wash.
You can typically remove metal penetration by grinding down the rough surface of the casting.