The basic characteristic of high-density molding sand for gray iron castings is that the amount of coal powder added is also relatively low. Therefore, when mixing sand, the following points should be taken into consideration.
① Clay content and mud content. The strength of molding sand with high clay content also increases with the increase of specific pressure, and the effective bentonite is usually controlled at 7% to 10%. The sum of effective clay and dead clay in molding sand is equivalent to the mud content, which is generally controlled between 12% and 16%. The high or low mud content can affect the various performance indicators of molding sand.
② Moisture. The moisture in the sand used in the production of lost foam castings is the main factor determining the plasticity and bonding strength of clay. When the moisture content is too high, it is easy to cause a deterioration of the clay bonding force, a decrease in the flowability of the molding sand, and the inability to obtain a uniform casting density. When the moisture content is too low, the sand is not easy to mix evenly, the strength of the sand is low, the brittleness is high, and the moldability is poor, which can easily cause sand sticking defects in the casting. The general moisture content should be controlled to be 10% to 20% higher than the moisture content obtained at the peak wet compression strength.
③ Original sand particle size. During high-density molding, the sand mold has a higher density and expands greatly during pouring. Therefore, the original sand particle size should not be overly concentrated. The original sand particles are circular or polygonal, and generally three or four screen sand is selected.