Castings of common defects, the formation of causes and preventive measures

Air Holes (Bubbles. Choke holes. Air pockets)

Characteristics: 

Air holes are holes that exist on the surface or inside the casting, which are round. Oval or irregularly shaped, sometimes more than one air hole to form an air mass, the skin is generally pear-shaped. Choke holes are irregular in shape and have a rough surface, air pockets are a piece of the casting surface concave in, with a smoother surface. Ming hole appearance inspection can be found, subcutaneous air holes can only be found after machining.

Formation causes:

1. The preheating temperature of the mold is too low, and the liquid metal cools too quickly when passing through the pouring system.

2. Poor mold exhaust design, the gas can not be discharged smoothly.

3. The paint is not good, the exhaust itself is not good, or even volatile or decomposed gas itself.

4. There are holes on the surface of the mold cavity. Pit, liquid metal injection after the hole. The gas in the pit expands rapidly and compresses the liquid metal, forming a choking hole.

5. The surface of the mold cavity is corroded and not cleaned up.

6. Improper storage of raw materials (sand core), without preheating before use.

7. Poor deoxidizer, or insufficient dosage or improper operation.

Prevention methods:

1. The mold should be fully preheated, the grain size of the paint (graphite) should not be too fine, and the air permeability should be good.

2. Use inclined pouring method to pour.

3. Raw materials should be stored in ventilated and dry place and preheated when used.

5. The pouring temperature should not be too high.

Shrinkage (Shrinkage)

Characteristics: 

Shrinkage is a rough surface hole on the surface or inside the casting, minor shrinkage is a number of scattered small shrinkage holes, that is, shrinkage, shrinkage or shrinkage at the grain is coarse. Often occurs in the casting near the inner sprue. Root of the riser. Thick parts, wall of the thickness of the transfer and have a large plane of the thickness of the place.

Formation reasons:

1. Mold temperature control does not meet the requirements of directional solidification.

2. Improper selection of paint, different parts of the paint layer thickness control is not good.

3. Castings in the mold in the position of the improper design.

4. The design of pouring risers fails to achieve the role of full complementary shrinkage.

5. Pouring temperature is too low or too high.

Prevention and control methods:

1. Increase the temperature of the grinding tool.

2. Adjust the thickness of paint layer, paint spraying should be uniform, paint shedding and make-up coating can not form a local paint accumulation phenomenon.

3. Local heating of the mold or local insulation with adiabatic materials.

4. Set copper block at the hot section, and carry out localized cooling.

5. Design heat sink on the mold, or accelerate the local cooling speed through water, or spray water outside the mold, spray.

6. Use removable cooling blocks, placed in the cavity in turn, to avoid insufficient cooling of the cooling block itself during continuous production.

7. Design the pressurizing device on the riser of the mold.

8. The pouring system should be designed accurately and the appropriate pouring temperature should be selected.

Slag hole (Molten slag or metal oxide slag)

Characteristics: 

Slag hole is a casting on the open or dark holes, holes in all or partially filled with slag, irregular shape, small dots of flux slag is not easy to find, the slag will be removed, presenting a smooth hole, generally distributed in the lower part of the pouring position, near the inner sprue or casting dead center, oxide slag is mostly distributed in a net shape on the surface of the casting near the inner sprue, sometimes in the form of a flake, or with wrinkles in the form of an irregular cloud, or The formation of lamellar interlayer, or to the existence of a group of flocculent inside the casting, fracture is often broken from the interlayer, the oxide in which is one of the root causes of the formation of cracks in the casting.

Formation causes: 

Slag hole is mainly due to the alloy melting process and pouring process (including the design of the pouring system is incorrect), the mold itself will not cause slag hole, and metal mold is one of the effective ways to avoid slag hole.

Prevention and control methods:

1. Pouring system set correctly or use casting fiber filter.

2. Use tilted pouring method.

3. Choose the melt and strictly control the quality.

Cracks (hot cracks. Cold crack)

Characteristics: 

The appearance of crack is straight line or irregular curve, the surface of the fracture of hot crack is strongly oxidized as dark gray or black, without metal luster, the surface of the fracture of cold crack is clean and has metal luster. Generally, the external cracks of castings can be seen directly, while the internal cracks can be seen only with the help of other methods. Cracks are often associated with shrinkage. Cracks are often associated with shrinkage, slag and other defects, mostly occurring in the casting of sharp corners of the inner, thick and thin section of the intersection, pouring and riser and casting connected to the hot section area.

Formation causes: 

Metal mold casting is prone to crack defects, because the metal mold itself does not have concessions, the cooling rate is fast, easy to cause the casting of internal stress increases, open too early or too late, pouring angle is too small or too large, the coating layer is too thin, etc. are prone to cause the casting of cracks, mold cavities themselves are prone to cracks when the cracks also lead to cracks.

Prevention and control methods:

1. Attention should be paid to the casting structure craftsmanship, so that the casting wall thickness of uneven parts of the uniform transition, the use of appropriate rounded corner size.

2. Adjust the coating thickness, as far as possible, so that the casting parts to achieve the required cooling rate, to avoid the formation of too much internal stress.

3. Attention should be paid to the working temperature of the metal mold, adjust the mold tilt, as well as the appropriate time to extract the core cracking, remove the casting slow cooling.

Cold segregation (poor fusion)

Characteristics: 

Cold segregation is a kind of through the seam or have a rounded edge of the surface of the seam, the middle is separated by the oxide skin, not fully integrated, cold segregation is serious when the “undercasting”. Cold segregation is often found on the top wall of the casting, thin horizontal or vertical surfaces, thick and thin wall joints, or on thin auxiliary plates.

Formation reasons:

1. Metal mold exhaust design is unreasonable.

2. Working temperature is too low.

3. Poor quality of paint (man-made . Material).

4. Improper location of sprue opening.

5. Pouring speed is too slow, etc..

Preventive methods:

1. Correct design of sprue and exhaust system.

2. Large thin-walled castings, the paint should not be too thin, properly thickened paint layer is conducive to molding.

3. Increase the working temperature of the mold appropriately.

4. Adopt tilting pouring method.

5. Use mechanical vibration metal mold pouring.

Sand eye (sand hole)

Characteristics: 

The formation of relatively regular holes on the surface or inside the casting, its shape and the shape of the sand particles, just out of the mold can be seen on the surface of the casting inlaid with sand particles, from which you can pull out the sand particles, a number of sand holes at the same time, the surface of the casting was the shape of orange peel.

Reason: 

The sand particles dropped from the surface of the sand core are wrapped by copper liquid and form holes on the surface of the casting.

1. The surface strength of the sand core is not good, burnt or not fully cured.

2. The size of the sand core does not match with the outer mold, and the sand core is crushed when the mold is closed.

3. The mold is dipped in graphite water contaminated with sand.

4. The friction of the sand core between the sprue and the sprue is washed into the cavity with the copper water.

Prevention and control methods:

1. Sand core production in strict accordance with the production process, check the quality.

2. The size of the sand core and the outer mold is consistent.

3. Ink should be cleaned up in time.

4. Avoid friction between casting package and sand core.

5. Blow clean the sand in the mold cavity when the sand core is under.