The causes of six common defects in castings and ways to prevent them

A. Stomata (bubbles, choke holes, air nests)

Features: air holes are holes that exist on the surface or inside the casting, round, oval or irregularly shaped, sometimes multiple air holes form an air mass, and are generally pear-shaped under the skin. Choke holes are irregularly shaped and have a rough surface, while air nests are a piece of the casting surface recessed into the surface, with a smoother surface. Bright hole appearance inspection can be found, subcutaneous pore by mechanical processing to be found.

Causes of formation.

1, mould preheating temperature is too low, liquid metal through the pouring system when cooling too fast.

2, mould exhaust design is not good, gas can not be dischargedsmoothly.

3, the coating is not good, the exhaust itself is not good, or even itself volatile or decomposition of gas.

4、The surface of the mould cavity has holes and pits, after the liquid metal is injected into the holes and pits, the gas rapidly expands and compresses the liquid metal, forming choking holes.

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

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

7, the deoxidizer is not good, or the amount is not enough or improper operation, etc.

Prevention methods.

1、The mould should be fully preheated, the particle size of the coating (graphite) should not be too fine, and the permeability should be good.

2、Use inclined pouring method to pour.

3、The raw material should be stored in a ventilated and dry place and preheated when used.

4、Select deoxidiser (magnesium) with good deoxidising effect.

5, pouring temperature should not be too high.

Second, shrinkage (shrinkage loose)

Features: shrinkage is a kind of surface rough hole on the surface or inside the casting, slight shrinkage is many scattered small shrinkage, namely shrinkage, shrinkage or shrinkage at the grain is coarse. Often occur in the casting near the sprue, the root of the riser, the thick part, the wall of the thick and thin transition and has a large plane of the thick and thin.

Formation causes.

1, mould working temperature control does not meet the requirements of directional solidification.

2, improper choice of paint, different parts of the paint layer thickness control is not good.

3、The position of the casting in the mould is not designed properly.

4, pouring mouth design failed to achieve the role of full complementary shrinkage.

5, pouring temperature is too low or too high.

Prevention and cure methods.

1、Increase the temperature of the grinding tool.

2, adjust the thickness of the coating layer, paint spraying to be uniform, paint off and replenish the coating can not form a local paint pile phenomenon.

3、Heat the mould locally or use adiabatic material for local insulation.

4、Set copper block at the hot section, for local excitation and cooling.

5、Design a heat sink on the mould, or accelerate the cooling speed of the local area through water, or spray water or spray outside the mould.

6、Use detachable cooling blocks and place them in the cavity in turn to avoid inadequate cooling of the cooling block itself during continuous production.

7、Design a pressurising device on the mouth of the mould.

8、The design of the pouring system should be accurate, choose the appropriate pouring temperature.

Third, the slag hole (melt slag or metal oxide slag)

Features: slag hole is the casting on the bright hole or dark hole, hole in all or part of the slag filled, irregular shape, small dots of flux slag is not easy to find, the slag removed, showing a smooth hole, generally distributed in the lower part of the pouring position, near the inner sprue or castings at the dead end, oxide slag more than a mesh distribution near the casting surface of the inner sprue, sometimes in the form of flakes, or irregular clouds with wrinkles, or The formation of lamellar interlayer, or to group flocculation exists inside the casting, fracture often from the interlayer, oxide in which, is one of the roots of the formation of cracks in the casting.

Formation causes: slag hole is mainly due to the alloy melting process and pouring process caused by (including the incorrect design of the pouring system), the mould itself does not cause slag hole, and metal mould is one of the effective ways to avoid slag hole.

Prevention and cure methods.

1. Set up the pouring system correctly or use a casting fibre filter.

2、Use inclined pouring method.

3、Select the melt and strictly control the quality.

IV. Cracking (hot crack, cold crack)

Characteristics: The appearance of the crack is straight or irregular curve, the surface of the hot crack fracture is strongly oxidised dark grey or black, no metallic lustre, the surface of the cold crack fracture is clean and has metallic lustre. Generally the outer cracks in castings can be seen directly, while the inner cracks can only be seen with the help of other methods. Cracks are often associated with shrinkage, slag and other defects, mostly occurring in the inner corner of the casting, the intersection of thick and thin sections, pouring mouth and casting connected to the hot section area.

Formation reasons: metal mould casting is prone to crack defects, because the metal mould itself does not give way, cooling speed, easy to cause increased stress in the casting, open too early or too late, pouring angle is too small or too large, the coating layer is too thin, etc. are easy to cause the casting to crack, mold cavity itself when there are cracks are also easy to lead to cracking.

Prevention and cure methods.

1, should pay attention to the structure of the casting process, so that the uneven wall thickness of the casting parts of the uniform transition, the use of appropriate rounded size.

2, adjust the thickness of the coating, as far as possible to make all parts of the casting to achieve the required cooling rate, to avoid the formation of too much internal stress.

3, should pay attention to the working temperature of the metal mould, adjust the mould slope, as well as the appropriate time to draw the core cracking, take out the casting slow cooling.

Five, cold partition (poor integration)

Characteristics: Cold partition is a kind of seam or a round edge of the surface seam, the middle is separated by oxide, not completely fused into one, cold partition is serious when it becomes "under-cast". Cold partition is often found on the top wall of the casting, thin horizontal or vertical surfaces, thick and thin wall joints or on thin helper plates.

Reasons for formation.

1, metal mould exhaust design is not reasonable.

2、The working temperature is too low.

3, Bad quality of paint (artificial, material).

4、Improper location of sprue opening.

5, pouring speed is too slow, etc.

Prevention and cure methods.

1, correct design of sprue and exhaust system.

2, large area of thin-walled castings, the coating should not be too thin, appropriate thickening of the coating layer is conducive to molding.

3、Appropriately increase the working temperature of the mould.

4、Adopt inclined pouring method.

5、Adopt mechanical vibration metal mould pouring.

Six, sand eye (sand hole)

Characteristics: Relatively regular holes are formed on the surface or inside the casting, the shape of which is consistent with the shape of the sand grains, and the sand grains inlaid on the surface of the casting are visible when the mould is just released, from which the sand grains can be pulled out, and when multiple sand eyes exist at the same time, the surface of the casting is orange peel-like.

Formation reason: due to the sand core surface falling sand grains are wrapped by copper liquid exist with the casting surface and the formation of holes.

1, sand core surface strength is not good, burnt or not completely cured.

2, the size of the sand core does not match the outer mould, crushing the sand core when closing the mould.

3、The mould is dipped in graphite water with sand contamination.

4、The sand dropped by the friction of the sand core at the pouring bag and the sprue is washed into the cavity with the copper water.

Prevention and cure methods.

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

2, sand core and the size of the outer mold match.

3, is the ink to clean up in time.

4、Avoid friction between pouring bag and sand core.

5、Blow clean the sand in the mould cavity when putting down the sand core.