The defects of sand casting castings are: cold insulation, insufficient pouring, stomata, sticky sand, sand-hole, sand-expanding and so on. 1 cold insulation and insufficient liquid metal filling capacity, or poor filling conditions, in the cavity is filled before the metal liquid will stop flowing, casting will be caused by inadequate or cold insulation defects.
When the casting is insufficient, the castings can not obtain the complete shape, while the castings can obtain the complete appearance when the casting is cold, but the mechanical properties of the castings are seriously damaged because of the incomplete fused seams.
Prevent pouring and cold insulation: increase pouring temperature and pouring speed. (2) Stomatal gas in the metal liquid crust before the failure to escape in time, the formation of holes in the casting defects. The inner wall of the stoma is smooth, bright or with a slight oxidizing color. When the hole is produced in the casting, the effective bearing area will be reduced, and the stress concentration will be caused around the stomata to reduce the impact resistance and fatigue resistance of the castings. The pores also reduce the compactness of castings, causing some castings to be scrapped that require pressure tests.
In addition, the pores of the casting corrosion resistance and heat resistance also have adverse effects.
To prevent the formation of pores: reduce the gas content in the metal, increase the permeability of sand, and at the top of the cavity to add air vent riser. 3) Sticky sand castings on the surface of the adhesion of a layer of difficult to clear sand particles known as sticky sand. Sticky sand not only affects the appearance of castings, but also increases the working load of casting cleaning and cutting, and even affects the life of the machine.
For example, the cast tooth surface is prone to damage when bonded sand, pumps or engines and other machine parts if there is sticky sand, will affect the fuel oil, gas, lubricants and cooling water and other fluid flow, and will tarnish and wear the entire machine.
Prevent sticky sand: Add pulverized coal in the molding sand, and brush the anti-sticky sand coating on the mold surface.
4 sand in the casting surface formed by the groove and scar defects, in the use of wet casting thick plate type castings are very easy to produce. The parts of the castings that produce the sand are mostly in contact with the surface of the sand type. The surface of the cavity is affected by the radiant heat of molten metal, it is easy to arch and warp, and when the sand layer is constantly washed by the molten metal flow, it may break, stay in place or be brought into other parts.
The larger the upper surface of the castings, the larger the volume expansion of the molding sand, the greater the tendency of forming the sand.
5 sand hole in the casting inside or the surface is filled with the type of holes defects. 6 The mold wall moves under the pressure of molten metal during pouring, and the defect of casting local swell is formed. In order to prevent the expansion of sand, it is necessary to improve the strength of sand, the rigidity of the sand box, the pressure of the box or the fastening force when the box is enlarged, and the pouring temperature should be lowered, so that the surface of the metal liquid will be crust early so as to reduce the pressure of molten metal