Defect characteristics and detection methods

It is often found after sand cleaning or sand blasting of castings that the layered oxide scar on the outer surface of each part of castings or star like scar on the inner surface of castings will peel off from the surface of castings in the form of strip or scale when hammered

Causes of formation

1. The air cushion pressure in the mold cavity is greater than the static pressure of pouring alloy liquid
2. There is gas in the mold cavity
3. Poor permeability of the mold

As a result of the above reasons, the molten alloy will boil and oxidize strongly and inhale

Prevention methods and remedial measures

1. Improve the permeability of sand mold and core ① Use molding sand and core sand with good permeability. ② Various special vent holes are made in the core. ③ Control the humidity, compactness and evenness of sand mold
2. Adopt molding sand, core sand and binder materials with low gas output
3. Adopt molding sand with low humidity or dry core and sand mold
4. Use suitable and carefully treated cold iron
5. Properly reduce the pouring temperature of alloy liquid and shorten the pouring time as much as possible
6. When the adhesive material with hygroscopicity is used as the binder of sand core, if the holding time of the core exceeds one day and night, the core shall be baked again before assembling the sand mold (mold closing or box closing)
7. Add a gas outlet and an exhaust channel of the cavity