Casting defects
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The document discusses various casting defects that can occur during sand casting including gas defects, shrinkage cavities, molding material defects, pouring metal defects, and mold shift. Gas defects like blowholes and porosity are caused by entrapped gases in the molten metal or gases released from the mold during pouring. Shrinkage cavities result from liquid shrinkage during solidification which can be prevented with proper feeding and risers. Pouring metal defects include mis-runs from insufficient metal flow and cold shuts where two streams of molten metal do not fuse together properly.
Casting defects
- 2. CASTING DEFECTS The following are the major defects, which are likely to occur in sand castings 1. Gas defects 2. Shrinkage cavities 3. Molding material defects 4. Pouring metal defects 5. Mold shift
- 3. GAS DEFECTS Trapping of gas in the molten metal or by mold gases evolved during the pouring of the casting Blowholes, Porosity, Pinholes Blowholes are spherical or elongated cavities present in the casting on the surface or inside the casting Pinhole, porosity occurs due to the dissolution of hydrogen gas, which gets entrapped during heating of molten metal.
- 4. CAUSES OF GAS DEFECTS The lower gas-passing tendency of the mold, which may be due to lower venting, lower permeability of the mold or improper design of the casting The lower permeability is caused by finer grain size of the sand, high percentage of clay in mold mixture, and excessive moisture present in the mold. Metal contains gas Mold is too hot Poor mold burnout
- 5. SHRINKAGE CAVITIES caused by liquid shrinkage occurring during the solidification of the casting. To compensate proper feeding of liquid metal is required & risers are placed at the appropriate places in the mold. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick sprues to avoid shrinkage cavities
- 6. METAL PENETRATION molten metal enters into the gaps between sand grains. rough casting surface. the sand is coarse or no mold wash was applied on the surface of the mold. The coarser the sand grains more the metal penetration.
- 7. FUSION Fusion of the sand grains with the molten metal. Brittle, glassy appearance on the casting surface. Clay or the sand particles are of lower refractoriness or that the pouring temperature is too high.
- 8. Swell • metallostatic forces, move the mold wall back causing a swell in the dimension of the casting. •A proper ramming of the mold will correct this defect. Inclusions •Particles of slag, refractory materials, sand or deoxidation products are trapped in the casting during pouring &solidification. •The provision of choke in the gating system and the pouring basin at the top of the mold can prevent this defect.
- 9. POURING METAL DEFECTS Mis-runs Cold shuts.
- 10. MIS-RUNS when the metal is unable to fill the mold cavity completely and thus leaves unfilled cavities. A mis-run results when the metal is too cold to flow to the extremities of the mold cavity before freezing. Long, thin sections are subject to this defect and should be avoided in casting design.
- 11. COLD SHUT When two streams of molten metal while meeting in the mold cavity, do not fuse together properly thus forming a discontinuity in the casting. When the molten metal is poured into the mold cavity through more-than-one gate, multiple liquid fronts will have to flow together and become one solid. If the flowing metal fronts are too cool, they may not flow together, but will leave a seam in the part. Such a seam is called a cold shut, and can be prevented by assuring sufficient superheat in the poured metal and thick enough walls in the casting design.
- 12. MOLD SHIFT The mold shift defect occurs when cope and drag or molding boxes have not been properly aligned