Characteristics and remedy of under casting defects in sand casting

Defect characteristics and detection methods

It shows that the sharp angle, edge angle, thin section and narrow gap of the casting are not filled by the alloy liquid, and the shape of the casting is incomplete. It can be found in the sand cleaning or the appearance inspection after sand cleaning.

Causes of formation
  1. The chemical composition of the alloy deviates and the fluidity of the alloy liquid is poor
  2. The refining of alloy liquid is not sufficient, and there are many inclusions of gas and oxide, which makes the viscosity of alloy liquid high
  3. The design structure of casting and pouring system is unreasonable, which makes the thin-wall part cool fast and the thick part slow, or prevents the smooth filling of alloy liquid
  4. The rough surface of the mold and mud core makes the flow resistance of the alloy liquid large, and the exhaust capacity of the mold is poor, which increases the gas pressure and prevents the alloy liquid from filling the sharp corners, slits and edges
  5. The large heat capacity or thermal conductivity of the mold material makes the thin wall and slit of the casting cool too fast, and the alloy liquid can not flow to the sharp corner, thin wall, thin rib and other parts.
Prevention methods and remedial measures
  1. Strictly control the chemical composition of the alloy liquid to meet the standards to ensure the fluidity of the alloy liquid
  2. Select the refining agent with good refining effect to fully refine the alloy liquid, minimize its gas content and oxide inclusion content, and improve the purity and fluidity of the alloy liquid
  3. If possible, improve the design structure of the casting, such as increasing the wall thickness of the thick and thin section, increasing the arc r at the joint of the thick and thin section, etc
  4. When the bottom pouring method is used for open pouring of alloy liquid, the continuous and uniform liquid flow should be used to fill the mold quickly
  5. Properly increase the pouring temperature of alloy liquid
  6. Select molding sand and core sand materials with good permeability and formula to control the compactness and evenness of mold and core
  7. Place gas risers or vent holes at the parts with under casting to eliminate the back pressure of gas in the mold cavity
  8. The mold cavity at the under cast and cold isolated parts is easy to be produced by applying heat insulation coating, which reduces the thermal conductivity of the mold here, so that the alloy liquid can be filled here