There are many casting production processes, so it is easy to produce various defects. Some of the defective products can still be used after repair, while serious defects make castings waste. In order to ensure the quality of castings, it is necessary to correctly judge the defect category of castings, analyze and find out the causes so as to take improvement measures. The common defects of sand casting are porosity, cold shut, insufficient pouring, sand sticking, sand inclusion, sand hole, sand expansion, etc.
Air hole is a kind of hole defect which is formed in the casting when the gas does not escape in time before the liquid metal crusts. The inner walls of the pores are smooth, bright or slightly oxidized. When porosity occurs in the casting, the effective bearing area will be reduced, and stress concentration will be caused around the porosity, which will reduce the impact resistance and fatigue resistance of the casting. Porosity can also reduce the compactness of castings, resulting in the scrapping of some castings requiring hydrostatic test. In addition, porosity also has a bad effect on corrosion resistance and heat resistance of castings.
The effective methods to prevent the formation of pores are: reducing the gas content in the molten metal, increasing the permeability of the sand mold, and adding a gas outlet at the highest part of the mold cavity.
Sand hole is a kind of hole defect that is filled with molding sand inside or on the surface of casting. It is mainly caused by the low strength of molding sand or core sand, the loose sand in the cavity is not blown out, the mold is damaged, and the casting structure is unreasonable.
The methods to prevent sand hole are: to improve the strength of molding sand; to design the structure of casting reasonably; to increase the compactness of sand mold.
There is a layer of sand on the surface of casting which is difficult to remove. Sand sticking not only affects the appearance of castings, but also increases the workload of cleaning and cutting, and even affects the life of the machine. For example, if there is sand sticking on the surface of casting teeth, it is easy to be damaged. If there is sand sticking in pump or engine parts, it will affect the flow of fuel oil, gas, lubricating oil and cooling water, and will stain and wear the whole machine.
The methods to prevent sand sticking are: adding pulverized coal into the molding sand, and applying anti sand coating on the surface of the mold, etc.
Sand inclusion is a kind of groove and scar defect formed on the surface of the casting, which is easy to be produced in the wet casting of thick flat castings. Most of the parts in the casting that produce sand inclusion are in contact with the upper surface of the sand mold. The upper surface of the mold cavity is affected by the radiant heat of the metal liquid, which is easy to arch and warp. When the warped sand layer is continuously scoured by the metal liquid flow, it may break and break, stay in the original place or be brought into other parts. The larger the upper surface of the casting is, the larger the volume expansion of the molding sand is, and the greater the tendency of sand inclusion is.
The way to prevent sand inclusion is to avoid large plane structure.
Sand expanding is a defect formed by mold wall moving and local expanding of casting under the pressure of liquid metal during pouring.
In order to prevent sand expansion, it is necessary to increase the strength of sand mold, the rigidity of sand box, the pressing force or fastening force when the box is closed, and properly reduce the pouring temperature, so that the surface of the metal liquid can be crusted earlier, so as to reduce the pressure of the metal liquid on the mold.
Insufficient cold insulation and pouring
Liquid metal has insufficient filling capacity or poor filling conditions. Before the cavity is filled, the liquid metal will stop flowing, which will lead to insufficient casting or cold shut defects. When the casting is not poured enough, the complete shape of the casting will not be obtained; when the cold shut, although the complete shape of the casting can be obtained, the mechanical properties of the casting will be seriously damaged due to the existence of incomplete fusion joints.
The methods to prevent insufficient pouring and cold separation are: increasing pouring temperature and pouring speed; reasonably designing wall thickness.