Characteristics and remedy of porosity in sand casting

Defect characteristics and detection methods

Small holes in the surface and inside of the casting. Its surface is generally smooth, clean and bright, and round. The subcutaneous pores are pear shaped, mostly distributed on the surface of the casting or near the core of the mold wall and under the gas outlet riser. Surface porosity was found after sand blowing, and internal porosity was found after X-ray fluoroscopy and machining.

Causes of formation
  1. Due to the incorrect design of the pouring system, gas is brought in during the pouring of alloy liquid
  2. Due to the poor air permeability of the mold, the air in the mold cavity cannot escape
  3. The sand mold or core material is mixed with organic matters or minerals (coal slag, coke, etc.) to generate gas
  4. There are pores and pits on the inner and outer cold iron surfaces and working surfaces
Prevention methods and remedial measures
  1. Design the pouring system correctly, adopt the bottom pouring or gap type open pouring system (sprue ratio is straight: horizontal: internal = 1:3:3), adopt the cone-shaped straight sprue, its diameter should be ≤ 20 mm, to prevent eddy current
  2. The joints of all parts of the pouring system shall be processed into smooth joints to avoid the impact of alloy liquid on the cavity wall and core. During pouring, continuous and stable liquid flow shall be injected to prevent air from entering.
  3. Molding sand and core sand with good air permeability shall be used to prevent organic matters or minerals from being mixed, and air holes shall be set at the end of the runner
  4. Mix molding sand and core sand sufficiently to make the binder evenly cover the surface of each sand particle
  5. The assembled sand mold and core shall be poured within 4H to prevent moisture absorption due to staying too long
  6. Reasonable setting of risers for feeding and exhausting
  7. Install filter screen and slag collecting buffer bag in the pouring system to keep the alloy liquid flowing stably
  8. Reasonable placement of cold iron
  9. For the plane without air hole or looseness, the machining allowance can be enlarged to remove the surface with air hole defect during machining, or the cold iron can be placed on this plane