Causes and prevention methods of five common casting defects
There are many kinds of casting defects, and the reasons for the defects are very complicated. It is not only related to the casting process, but also related to a series of factors such as the properties of the casting alloy, the melting of the alloy, and the performance of the molding material. Therefore, when analyzing the causes of casting defects, it is necessary to proceed from the specific situation, conduct a comprehensive analysis according to the characteristics, location, process and sand used of the defects, and then take corresponding technical measures to prevent and eliminate defects.
can not pour
1. Features
Parts of the casting are incomplete, often in the thin-walled part, the part farthest from the runner or the upper part of the casting. The incomplete corners are smooth and shiny without sticking sand.
2. Causes
(1) The pouring temperature is low, the pouring speed is too slow or the pouring is intermittent;
(2) The cross-sectional area of the runner and inner runner is small;
(3) The content of carbon and silicon in the molten iron is too low;
(4) Too much moisture and pulverized coal content in the molding sand, large gas generation, or too high mud content, poor air permeability;
(5) The height of the upper sand mold is not enough, and the pressure of the molten iron is insufficient;
3. Prevention methods
(1) Increase the pouring temperature, speed up the pouring speed, and prevent intermittent pouring;
(2) Increase the cross-sectional area of the runner and inner runner;
(3) Adjust the ingredients after the furnace, and appropriately increase the carbon and silicon content;
(4) Strengthen the exhaust in the casting mold, reduce the amount of coal powder and organic matter added in the molding sand;
(5) Increase the height of the upper sand box;
Not filled
1. Features
The upper part of the casting is incomplete, the level of the molten iron in the sprue is the same as the level of the molten iron of the casting, and the edge is slightly rounded.
2. Causes
(1) The amount of molten iron in the ladle is not enough;
(2) The runner is narrow and the pouring speed is too fast. When the molten iron overflows from the pouring cup, the operator mistakenly thinks that the mold is full and stops pouring too early.
3. Prevention methods
(1) Correctly estimate the amount of molten iron in the ladle;
(2) For the mold with narrow runner, slow down the pouring speed appropriately to ensure that the mold is full.
Damage
1. Features
The casting is damaged and broken.
2. Causes
(1) The casting sand is too violent, or the casting is damaged by collision during the handling process;
(2) When the drum is cleaned, the castings are improperly loaded, and the weak parts of the castings are broken during rolling;
(3) The cross-sectional size of the riser and the riser neck is too large; the riser neck has no knock section (groove). Or the method of knocking out the pouring riser is incorrect, so that the casting body is damaged and lacks meat.
3. Prevention methods
(1) When the casting is cleaned and transported by falling sand, pay attention to avoid various forms of excessive collision and vibration, and avoid unreasonable throwing;
(2) When the drum is cleaned, it should be operated in strict accordance with the technical regulations and requirements;
(3) Modify the dimensions of the riser and the riser neck, make a knock section of the riser neck, and correctly grasp the direction of the pouring riser.
Sticky sand and rough surface
1. Features
Sticky sand is a surface defect of castings, which is characterized by the adhesion of sand particles that are difficult to remove on the surface of the castings; for example, after the sand particles are removed, the castings have uneven and uneven surfaces, which are called rough surfaces.
2. Causes
(1) The sand grains are too coarse and the sand mold compactness is not enough;
(2) The moisture in the molding sand is too high, so that the molding sand is not easy to be compacted;
(3) The pouring speed is too fast, the pressure is too high, and the temperature is too high;
(4) Too little pulverized coal in the molding sand;
(5) The drying temperature of the template is too high, resulting in drying of the surface sand; or the drying temperature of the template is too low, and the molding sand adheres to the template.
3. Prevention methods
(1) When the air permeability is sufficient, use finer raw sand and appropriately increase the compactness of the molding sand;
(2) Ensure a stable and effective pulverized coal content in the molding sand;
(3) Strictly control sand moisture;
(4) Improve the pouring system, improve the pouring operation, and reduce the pouring temperature;
(5) Control the baking temperature of the template, which is generally equal to or slightly higher than the temperature of the molding sand.
trachoma
1. Features
Holes filled with molding sand inside or on the surface of the casting.
2. Causes
(1) The surface strength of the molding sand is not enough;
(2) There is no rounded corner on the pattern or the draft angle is small, which leads to the hook sand and the mold being damaged and closed without repairing or repairing;
(3) The sand mold is placed for too long before pouring, and the surface strength decreases after air-drying;
(4) The mold is damaged when closing the box or during handling;
(5) When the box is closed, the floating sand in the mold is not removed. After the box is closed, the sprue cup is not covered, and the broken sand falls into the mold.
3. Prevention methods
(1) Increase the viscosity of the molding sand, add new sand in time, and improve the surface strength of the molding sand;
(2) The finish of the appearance should be high, and the draft angle and casting fillet should be reasonably made. The damaged mold should be repaired before closing the box;
(3) Shorten the placement time of the sand mold before pouring;
(4) Be careful when closing the box or handling the mold to avoid damage or sand falling into the sand cavity;
(5) Before closing the box, remove the floating sand in the mold and cover the gate.
