the production process of investment castings, some defects of investment castings will inevitably occur due to some wrong operations. Here are some common investment casting defects and how to prevent them:
Blowholes are pores that exist on the surface or interior area of a casting. It can be round, oval, or irregular in shape, and is sometimes an air mass composed of multiple pores. Internal pores are usually pear-shaped. The choke hole is irregular in shape and has a rough surface. Air pockets are cast indentations in the surface, which are much smoother. Surface pores can be detected by visual inspection, but subsurface pores can only be detected after machining.
The preheating temperature is too low, and the liquid metal cools too quickly through the gating system.
The exhaust design of the mold is not good, and the gas cannot flow out.
Poor coating material, poor exhaust, and even volatilization or decomposition of the gas itself.
There are holes and pits on the surface of the cavity. When the liquid metal is poured, the gas in the holes and pits expands rapidly, compressing the gas liquid metal, thereby forming a suffocation hole.
The surface of the cavity is rusted and not cleaned.
The raw materials (sand cores) were stored improperly and were not preheated before use.
Poor deoxidizer, improper dosage or improper operation, etc.
The mold should be fully preheated, the particle size of the coating (graphite) should not be too fine, and the air permeability should be good.
When pouring, the inclined pouring method is adopted.
Raw materials should be stored in a ventilated and dry place, and should be preheated when used.
Choose a deoxidizer (magnesium) with better deoxidation effect.
The pouring temperature should not be too high.
Investment casting shrinkage cavities are rough surface pores on the surface or inside of the casting, and microscopic cavities are many scattered small shrinkage cavities, that is, shrinkage cavities. The shrinkage pore particles are large and coarse. Often appear in casting sprue, riser root, thick area, thin wall and wall thickness connection and other parts.
Mold working temperature control does not meet the requirements of directional solidification. Improper coating
When choosing, the thickness of the coating in different areas is not well controlled.
The location of the investment casting in the mold is not designed properly.
The pouring riser design does not fully achieve the effect of feeding.
Increase mold temperature.
Adjust the coating thickness to make the paint spray evenly and avoid local coating accumulation when the coating paint peels off or fills up.
The mold is heated locally, or thermal insulation materials are used for local insulation.
Design cooling fins in the mold, or the cooling rate is accelerated by water to localize the cooling rate, or water is sprayed outside the mold.
Detachable handling cooling blocks are placed in sequence in the cavity to avoid sufficient cooling of the blocks during continuous production.
Design a pressure device at the die riser.
Accurately design the pouring system and select the appropriate pouring temperature.
3. Slag hole (flux slag or metal oxide slag)
Slag holes are obvious or unclear holes in castings. Fully or partially filled with slag and irregular in shape. Small-shaped slag is not easy to be found, and smooth holes will appear after slag removal, which is generally distributed in the bottom area of the pouring position, around the cross section of the runner, or near the corners. Oxidized slag is distributed in a net shape on the surface of the casting near the gate, sometimes in the form of flakes, or irregular cloud, wrinkled, or forms a flake interlayer, or exists inside the casting in the form of floc.
Slag holes are mainly caused by alloy smelting process and pouring process (including improper design of pouring system). The mold itself will not cause slag holes, and metal molds are one of the effective methods to prevent slag inclusion.
Correct gating system design, or use cast fiber filters.
Adopt inclined pouring method.
Choose flux and strictly control the quality.
4. Cracks (hot cracks, cold cracks)
Investment casting cracks The appearance of cracks is straight line or irregular curve. The surface of the hot crack is dark gray or black due to strong oxidation, and has no metallic luster. Cold cracks have clean fractures and metallic luster. Generally, casting cracks can be seen directly, but internal cracks can only be seen by other methods. Cracks are often related to defects such as shrinkage cavities and slag inclusions. It mostly occurs in the inner side of the pouring corner, the thickness of the joint, and the hot section between the pouring riser and the casting.
Metal mold casting is prone to crack defects because the metal mold itself does not have the ability to deform. The fast cooling rate is easy to increase the internal stress of the casting. The inclination angle is too large or too small, the coating thickness is too thin, etc. may cause casting cracks. In addition to the cracks in the mold itself, it is also easy to cause cracks.
Attention should be paid to the casting structure so that the non-uniform wall thickness transitions evenly. And use the appropriate rounded corner size.
Adjust the thickness of the coating layer, try to make each casting area reach the required cooling rate, and avoid the formation of excessive internal stress.
Attention should be paid to the working temperature of the metal mold, and the angle of the mold should be adjusted.
5. Cold off
A cold seal is a groove or surface crack with rounded edges, separated by scales in the middle, so it is not fully bonded. When the cold shot is serious, it will become "undercast". Cold segregation often occurs on the top wall of castings, thin horizontal or vertical planes, or thin-walled-thick joints.
The exhaust design of the metal mold is unreasonable.
The operating temperature is too low.
Poor coating quality or improper handling.
Improperly designed riser position
Pouring speed is too slow.
Properly design runners and exhaust systems.
Appropriately thicken the coating for thin-walled and thick investment castings.
Increase the mold working temperature.
Adopt inclined pouring method.
Casting is carried out using mechanically vibrating metal molds.
6. Sand hole
Sand holes are formed on the surface or inside of the casting and are relatively regular. The shape is the same as the shape of the sand. When the investment casting is taken out of the mold, we can see that the sand grains are embedded in the surface of the casting.
Sand core surface strength is not good, charred or not fully cured.
The size of the sand core does not match the outer mold, or it is crushed when the mold is closed.
The mold is contaminated with graphite water and sand.
Sand is flushed into the cavity.
Sand cores are made by process division.
Keep the core size consistent with the shape.
Clean the graphite water in time.
When placing the sand core, clean the sand in the cavity.
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