Guangdong machining factory shares with you = common defects and prevention methods of six castings
1、 Air hole (bubble, choke hole, air socket)
Characteristics: porosity is a hole on the surface or inside of the casting, which is round, oval or irregular. Sometimes, multiple pores form a gas mass, and the subcutaneous cavity is usually pear shaped. The choke hole is irregular in shape and rough in surface. The air cavity is a concave part of the casting surface, and the surface is smooth. The appearance of open hole can be found, and the subcutaneous pores can be found only after machining. Guangdong Machinery Factory is really good, it is worth paying attention to!
Reasons for formation:
1. The preheating temperature of the mould is too low, and the liquid metal cools too fast when it passes through the pouring system.
2. The exhaust design of the mould is poor, and the gas can not be discharged smoothly.
3. The paint is not good, its exhaust performance is not good, even volatile or decomposed into gas.
4. There are holes and pits on the surface of the mold cavity. After the liquid metal is injected, the gas in the cavity expands rapidly and compresses the liquid metal, forming choke hole.
5. The surface of mold cavity is rusted and not cleaned.
6. Raw materials (sand cores) are not stored properly and are not preheated before use.
7. The deoxidizer is not good, or the dosage is not enough or the operation is improper.
Prevention method:
1. The mold should be preheated sufficiently, the particle size of coating (graphite) should not be too fine and the air permeability is better. 2. Pour by tilting. 3. Raw materials shall be stored in a ventilated and dry place, and preheating shall be carried out during use. 4. The deoxidizer (magnesium) with better deoxidation effect was selected. 5. The pouring temperature should not be too high. Guangdong Machinery Factory is really good, it is worth paying attention to!
2、 Shrinkage (shrinkage)
Features: shrinkage is a kind of rough surface hole on the surface or inside of casting. Slight shrinkage is a lot of scattered small shrinkage holes, that is, shrinkage porosity, and coarse grain at shrinkage or shrinkage. It often occurs near the casting inner runner, the riser root, the thick and large part, the thick and thin junction of the wall and the thick and thin parts with large plane.
Reasons for formation:
1. The working temperature control of the die did not meet the requirements of directional solidification. 2. The coating thickness of different parts is not well controlled due to improper selection of coating. 3. The position of castings in the mold is not designed properly. 4. The design of the riser failed to fully supplement the shrinkage. 5. The pouring temperature is too low or too high.
Prevention and treatment methods:
1. Increase the grinding tool temperature. 2. Adjust the thickness of coating layer, spray the coating evenly, and the coating will not accumulate when it falls off and the coating is repaired. 3. Heat the mold locally or keep it with insulating materials. 4. The hot spot is set with copper block to quench the local part. 5, design heat sink on the mold, or accelerate the cooling rate of the local area through water, or spray water outside the mold, spray. 6. The cold block can be disassembled and placed in the mold cavity in turn to avoid insufficient cooling of the quench block itself during continuous production. 7. The pressure device is designed on the riser of the die. 8. The design of the pouring system should be accurate and the suitable pouring temperature should be selected. Guangdong Machinery Factory is really good, it is worth paying attention to!
3、 Slag hole (slag inclusion or metal oxide slag inclusion)
Features: slag hole is an open or dark hole in the casting. All or part of the hole is filled with slag, and the shape is irregular. It is difficult to find the slag inclusion with small point flux. After removing the slag, it presents smooth holes, which are generally distributed at the lower part of the pouring position, near the inner casting channel or the dead corner of the casting. The oxide slag inclusion is distributed on the surface of the casting near the inner casting channel in a mesh, sometimes in the form of thin sheet, The irregular cloud with wrinkles, or the formation of lamellar interlayer, or the presence of the inside of the casting in the form of flocculent, often breaks from the interlayer when it breaks. Among them, oxide is one of the root causes of cracks.
Reasons for formation:
Slag hole is mainly caused by alloy smelting process and pouring process (including incorrect design of casting system), and mold itself will not cause slag hole, and metal mold is one of the effective methods to avoid slag hole.
Prevention and treatment methods:
1. The casting system is set correctly or cast fiber filter is used. 2. Inclined pouring method is adopted. 3. Select flux and strictly control quality.
4、 Cracks (hot cracks, cold cracks)
Features: the appearance of cracks is straight or irregular curve. The surface of hot crack fracture is strongly oxidized to dark gray or black, and has no metal luster. The surface of cold crack fracture is clean and has metal luster. The external crack of the casting can be seen directly, while the internal crack can be seen by other methods. Cracks are often related to shrinkage, slag inclusion and other defects, which are mostly found in the inner corner of casting, the junction of thick and thin section, and the hot joint area between riser and casting.
Reasons for formation:
The crack defects are easy to occur in metal mold casting, because the metal mold itself has no relevancy, the cooling speed is fast, which easily causes the internal stress of the casting to increase, the opening mode is too early or too late, the pouring angle is too small or too large, the coating layer is too thin, etc. it is easy to cause the crack of the casting, and the crack can also be caused when the mold cavity itself has cracks.
Prevention and treatment methods:
1. Attention should be paid to the structural and technological characteristics of the casting, so that the uneven parts of the casting wall thickness are evenly transitional, and appropriate fillet size shall be adopted.
2. Adjust the coating thickness to make the parts of the casting reach the required cooling speed as far as possible, so as to avoid forming too much internal stress.
3. Attention should be paid to the working temperature of metal mould, adjust the inclination of the mold, and timely pull out the core to crack, and take out the casting for slow cooling. Guangdong Machinery Factory is really good, it is worth paying attention to!
5、 Cold insulation (poor fusion)
Features: cold insulation is a kind of surface crack with through seam or round edge, separated by oxide skin in the middle, not completely integrated, and becomes "under cast" when the cold insulation is serious. Cold insulation often occurs on the top wall of casting, thin horizontal or vertical plane, thick thin-walled connection or on thin auxiliary plate.
Reasons for formation:
1. The exhaust design of metal mould is unreasonable. 2. The operating temperature is too low. 3. The quality of the coating is not good (man-made, material). 4. The position of the sprue is not correct. 5. Pouring speed is too slow.
Prevention and treatment methods:
1. The sprue and exhaust system are designed correctly. 2. Large area Thin-walled Casting, coating should not be too thin, appropriate thickness coating is conducive to forming. 3. The working temperature of the mould should be increased properly. 4. The inclined pouring method is adopted. 5. The metal mould is used for pouring by mechanical vibration. Guangdong Machinery Factory is really good, it is worth paying attention to!
6、 Sand hole
Features: relatively regular holes are formed on the surface or inside of the casting, and the shape is consistent with the shape of sand particles. When the mold is just released, the embedded sand particles can be seen on the surface of the casting. Sand grains can be taken out from the sand grains. When multiple sand holes exist at the same time, the surface of the casting is orange peel.
Reasons for formation:
The sand particles falling on the core surface are wrapped by copper solution and form holes with the surface of casting.
1. The surface strength of sand core is not good, scorched or not completely solidified.
2. The size of sand core is not in accordance with the outer mold, and the core is crushed when the mold is closed.
3. The mold was dipped in graphite water contaminated with sand.
4. The sand falling from the friction between the pouring bag and the sand core at the runner is pushed into the cavity with copper water.
Prevention and treatment methods:
1. The sand core shall be manufactured strictly according to the process and the quality shall be checked. 2. The sand core is in accordance with the size of the outer mold. 3. It is the ink that needs to be cleaned up in time. 4. Avoid friction between pouring bag and sand core. 5. When the sand core is lowered, the sand in the mold cavity shall be cleaned.