1.Porosity
Features
There are smooth holes of different sizes on the inner or surface of the casting.
Main causes
① The charge is not dry or contains many oxides and impurities;
② Pouring tools or additives before furnace are not dried;
③ Too much water in molding sand or too much water brushing during mold lifting and repairing;
④ The core drying is not enough or the core vent is blocked;
⑤ Spring sand is too tight, molding sand permeability is poor;
⑥ Pouring temperature is too low or pouring speed is too fast.
2.Shrinkage and porosity
Features
The shrinkage cavity is mainly distributed in the thick section of the casting, with irregular shape and rough hole.
Main causes
① The structure design of the casting is unreasonable, for example, the difference of the wall thickness is too large, and there is no riser or cold iron at the thick wall;
② The position of pouring system and riser is wrong;
③ Too high pouring temperature;
④ The chemical composition of the alloy is unqualified, the shrinkage is too large, and the riser is too small or too few.
3.Sand holes
Features
There are sand filled holes inside or on the surface of the casting.
Main causes
① The strength of molding sand is too low or the compactness of sand mold and core is not enough, so the molding sand is flushed into the mold cavity by metal liquid;
② Local damage of sand mold when closing the box;
③ The pouring system is unreasonable, the direction of the inner gate is not right, and the sand mold is damaged by the molten metal;
④ The loose sand in the mold cavity or gate is not cleaned when the box is closed
4.Bonded sand
Features
The surface of the casting is rough with a layer of sand.
Main causes
① The original sand has low fire resistance or too large particle size;
② The clay content of molding sand is too high, and the fire resistance decreases;
③ Too high pouring temperature;
④ The content of pulverized coal in green sand is too small;
⑤ When dry casting, the mold is not brushed obliquely or the coating is too thin.
5.Sand inclusion
Features
A layer of molding sand is sandwiched between the sheet metal protrusion and the casting.
Main causes
① The hot wet tensile strength of molding sand is low, and the surface of mold cavity expands and cracks due to heating and baking;
② The local compactness of the sand mold is too high, the moisture is too much, and the surface of the mold cavity cracks after the moisture drying;
③ If the pouring position is not selected properly, the surface of the mold cavity will expand and crack due to the baking of high temperature molten iron for a long time;
④ The pouring temperature is too high and the pouring speed is too slow.
6.Wrong type
Features
There is relative displacement of casting along parting surface.
Main causes
① The upper mold and the lower mold of the pattern are not aligned;
② When the box is closed, the upper and lower sand boxes are misplaced;
③ The upper and lower sand boxes are not clamped or the upper box is not added with enough iron, so the wrong box occurs during pouring.
7.Cold separation
Features
There are incomplete fusion cracks or pits on the casting, and the joint is smooth.
Main causes
① The fluidity of the alloy is poor when the pouring temperature is too low;
② The pouring speed is too slow or the flow is broken during pouring;
③ Improper placement of the pouring system or too small cross-sectional area of the internal sprue;
④ The casting wall is too thin;
⑤ The height of sprue (including sprue cup) is not enough;
⑥ When pouring, the amount of metal is not enough and the cavity is not full.
8.Insufficient casting
Features
The casting is not filled.
Main causes
① The fluidity of the alloy is poor when the pouring temperature is too low;
② The pouring speed is too slow or the flow is broken during pouring;
③ Improper placement of the pouring system or too small cross-sectional area of the internal sprue;
④ The casting wall is too thin;
⑤ The height of sprue (including sprue cup) is not enough;
⑥ When pouring, the amount of metal is not enough and the cavity is not full.
9.Crack
Features
The casting is cracked, and there is oxide film on the metal surface at the crack.
Main causes
① The casting structure design is unreasonable, the difference of wall thickness is too large, and the cooling is not uniform;
② The yield of sand mold and core is poor, or spring sand is too tight;
③ Early sand falling;
④ Improper gate position causes uneven shrinkage of each part of the casting.