Characteristics and remedy of burning black defect in sand casting

Defect characteristics and detection methods

There are gray white powder like spots or scabs on the thick parts of the casting, near or opposite to the internal sprue, at the root of riser, at the sharp corners of the core sand forming the cavity, or the fracture surface is black spots. On the X-ray film, it is similar to loose, but the outline is clearer than loose

Causes of formation
  1. Insufficient protection in sand mold and core material or burnt during baking
  2. The protective agent in the mold where the alloy liquid flows most is burnt and loses its protective effect
  3. Combustible sundries such as coal powder, grass root, sawdust and sulfur are mixed in the molding materials
  4. Too high pouring temperature of alloy liquid
  5. There is too much water in the sand mold or there is oil or water drop on the surface of the mold cavity
  6. Poor air permeability of mold
  7. The air flow is too fast during pouring, so-called “cross wind” blows through
Prevention methods and remedial measures
  1. Add necessary amount of protective agent to sand mold and sand core
  2. Before sand mixing, check whether there are combustible sundries such as coal powder, grass root, sawdust, sulfur mixed in, to avoid oil and water dripping into the mold. If it is found that it has been dripping, bake it before pouring
  3. It is forbidden to sprinkle or touch water during the modification, so as not to reduce the protective effect of fluorine additives
  4. The sand core shall be dried thoroughly, but the binder and protective agent shall not be invalid due to overburning
  5. Spray boric acid water solution (boric acid 5% ~ 8% + talc powder 1% ~ 2% + water 94% ~ 90%) or fluorine additive water solution (fluorine additive 20%, water 80%) on the surface of the dried sand core, and bake for 45min ~ 1H at 140 ~ 160 ℃
  6. Strictly control the humidity and compactness of the sand mold, make each part uniform, ensure that the sand mold and sand core have enough air permeability, and at the same time, some air holes can be pierced
  7. Place cold iron on the thick part of the casting. Cold iron should be dried to prevent moisture
  8. When pouring high magnesium aluminum alloy, spray 50% sulfur and boric acid mixed protective agent on the surface of the sand mold cavity of the pouring system and the thick part of the casting, but pay attention not to spread too thick. Or add 0.02% beryllium into the alloy liquid to prevent severe oxidation and burning loss of the alloy liquid
  9. During pouring, the pouring height (i.e. the distance between the ladle nozzle and the sprue cup) shall be shortened as far as possible to reduce the impact force of the alloy liquid on the cavity wall. If necessary, the mixture of sulfur and boric acid protectant can be sprayed on the surface of the alloy liquid. Prevent the pouring site from blowing too much wind
  10. When the sand mold forming the cavity is very thin, pay special attention to make the core without sharp angle
  11. In the design of the internal sprue, try to make the alloy liquid enter the mold cavity from more internal sprues, so as to avoid the heat concentration on less (1-2) internal sprues
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