Factors affecting the quality of casting sand

Production conditions and existing problems The improvement of the quality requirements of stainless steel castings has promoted the development of new casting modeling methods and equipment. The new modeling methods and equipment put forward higher requirements for sand performance, and sand can solve the problem of gas defects in casting production.

At present, some stainless steel castings produced by some foundries in China have poor surface quality and high scrap rate. One of the main reasons is that the quality of the used sand raw materials is not good. So what are the factors that affect the quality of casting sand?

1. Bentonite. Bentonite acts as a binder in the molding sand, and its wet-state cohesive force, hot-wet cohesive force and reusability are the main indicators reflecting the quality of bentonite. If the quality of the bentonite used is poor, in order to ensure the strength of the sand type, more bentonite needs to be added, and the non-bonded dead clay accumulated in the molding sand will gradually increase.

2. The compaction rate of the molding sand, that is, the dryness and wetness of the molding sand. The sand is too dry, the bonding force of the bentonite can not be fully exerted, which will result in insufficient strength of the molding sand, and is not conducive to the compaction of the sand type by the molding method such as air punching, the sand type is easily damaged, and the stainless steel casting is prone to defects such as sand washing and sand holes; Too wet will cause the fluidity of the molding sand to be poor, and the sand type is not easy to be tight and uniform. At the same time, excessive moisture will cause defects such as pores, pinholes, and smoldering of the casting. Under normal circumstances, high pressure and impact modeling require a compaction rate of 35% to 40% at the molding station.

3. The water content of the molding sand is a major factor causing defects such as pinholes and pores in the casting. When the molding sand is in the most suitable dry and wet state, the lower the water content, the better. The quality of bentonite and pulverized coal and the process of sand mixing directly affect the water content of the sand. Most of the water content of molding sand in gray cast iron and ductile iron foundry factories producing high-pressure molding and air-punching modeling methods in foreign countries is between 2.3% and 3.8%.

4. The wet strength of the molding sand reflects the ability of the sand to withstand various external forces. The sand type must undergo the processes of mold-forming, handling, core-down, and forming in the production process. In order to avoid damage and slump, it must have sufficient wet strength. In the casting process, the sand type is also subjected to the scouring and impact of the molten metal and the expansion force when the graphite is deposited, which may cause defects such as sand holes, looseness, and sand expansion. Especially for complex castings such as engine blocks and cylinder heads, the requirements for the wet strength of the molding sand are high. Under normal circumstances, when the compaction rate is 40%, the wet compressive strength is at least 0.18 MPa.

5. The mud content of the molding sand includes the effective and failed bentonite pulverized coal, the dust and ash which are brought in by the raw materials such as new sand, bentonite and pulverized coal, and the fine powder formed by the breakage of the silica sand particles, wherein the bentonite and the pulverized coal are molding sand. The main source of the mud. Excessive mud content will increase the water content of the molding sand and reduce the gas permeability. Stainless steel castings are prone to pinholes and pore defects. Moreover, when the same wet compressive strength level is reached, the toughness and hot wet tensile strength of the molding sand are lowered, and the casting is prone to sand holes and sand inclusion defects.