Sand hole is also a common defect of cylinder block (cylinder head) casting, which is often found on the upper mold surface of the casting and exposed on the inner surface of cylinder barrel after machining.
1.1 the design of pouring system is unreasonable.
1.2 the serial management of molding sand is poor, and the performance of molding sand is poor.
1.3 the cavity is not clean.
1.4 the surface of sand core is in poor condition or improper application and drying.
2.1 in terms of the setting of the pouring system, in order to avoid or reduce the sand hole defects, the following matters should be paid attention to;
a. Reasonable pouring speed is required. If the cross section is too small, the pouring speed is too slow, the rising speed of molten iron is too slow, and the baking time of the upper mold by molten iron at high temperature is too long, which is easy to burst the molding sand and cause flake falling off in serious cases. The proportion of the pouring system shall be such that the molten iron can be injected smoothly without turbulence or jet.
b. As far as possible, the whole passage through which the molten iron flows is formed in the sand core. Generally, the clay core sand (hot coated sand or cold core sand) is more resistant to high temperature molten iron erosion than the clay sand of the outer mold. And the runner is difficult to avoid in the external mold of the sand mold, then the filter can be placed at the junction between the gate and the cross gate (preferably foam ceramic), which can filter the molten iron in the sprue and remove the sand and slag.
c. The sprue is variable cross-section, so the variable cross-section should be as smooth as possible to avoid the formation of sharp sand which is easy to be washed away by molten iron.
d. The section proportion of the runner should be semi closed and semi open to reduce the velocity and impact of molten iron entering the mold cavity, while the position of the inner runner should avoid the direct impact on the mold wall and core as far as possible, and the expansion shape is better.
2.2 in order to prevent sand hole defects of castings, the main measures for molding sand are
a. It is to control the content of micro powder in molding sand. In the repeated use of molding sand, the content of micro powder will be higher and higher, which will reduce the wet compression strength of molding sand, increase the moisture content and compactness, and make the molding sand brittle.
b. When pouring, the core is broken and then mixed with old sand. The remaining resin film will make the toughness of molding sand worse and the possibility of sand hole increase. Therefore, it is necessary to improve the surface stability, reduce the brittleness and improve the toughness of molding sand. The method is to add appropriate a-starch in the molding sand, which can achieve good results, and can also apply the surface stabilizer (spray) on the cavity surface.
2.3 sand particles are easy to fall into the mold cavity in the molding, turning over, especially in the lower core, closing and other links, but they are not cleaned up, which is very easy to cause sand hole defects in castings. For this reason, the first is to select the proper clearance and inclination of the core head and ensure the accuracy of the lower core and the combined box, so as to avoid damaging the sand mold or damaging the core and scattering the sand in the cavity; the second is to clean up the sand that may fall into the mold before the combined box (the suction method is better than the blow out method).
2.4 it can not be ignored that the flash burr of the sand core shall be cleaned, and the sand dust on the surface of the sand core to be used after being coated and dried shall also be blown clean, otherwise it is easy to be washed by molten iron and concentrated in the casting to form sand holes. At the same time, it should be emphasized that the coating on the sand core should not be too thick, especially when the process requires that the coating should not be too thick for individual parts of the sand core or all two times of infiltration, and the coating should not be applied for the second time until the first coating is dried to a certain extent, otherwise the coating with too thick thickness will burst and form sand (slag).