Above the water-cooled crystallizer, install a heat preservation cap made of thermal insulation film material, also called a hot cap for aluminum casting. The requirement for the hot cap is that it does not chemically react with molten aluminum, does not absorb moisture, does not absorb oil, does not wet aluminum, high temperature resistance, good insulation performance, thermal shock resistance, good strength, easy replacement and maintenance, and low price.
Traditional hot hat composition. Traditional hot top molds include hot top casting cap, heat-insulating sealing rings, molds, graphite rings, etc. The outer shell of the hot cap is welded with a 3mm thick iron plate, and the inner lining is made of 4-5mm asbestos board or aluminum silicate felt. The height of the hot cap is 80-100m. If the hot cap is too short, turbulence will occur and it is inconvenient to control the liquid level. When the metal static pressure is too high, aluminum leakage is easy to occur. At the same time, there is also a tendency to increase the segregation float, which is not conducive to the improvement of surface quality.
During casting, the metal level is controlled to be about 20mm away from the upper edge of the hot cap. The inner diameter of the hot cap is the same as the inner diameter of the aluminum silicate heat-insulating gasket, which is 3-5mm smaller than the graphite ring. The heat-insulating sealing gasket is made of aluminum silicate fiber felt with a thickness of 5-8mm. Because the aluminum silicate felt has good heat insulation performance, it can prevent the crystallization area from developing upward, and has a certain degree of elasticity. It does not infiltrate liquid aluminum and is convenient To achieve sealing. The protruding distance from the inner side of the overheated sealing gasket to the mold should be as short as possible, generally less than 3-5mm.
In order to reduce the impact of the boss on the surface of the ingot during production, an inclined plane can be used with an inclination angle of 45° to the horizontal. The mold is lined with a graphite ring, and an 8mm high aluminum table is left under the graphite ring to meet the needs of the multi-mold casting base. The inner diameter of the aluminum table is 1-1.5mm larger than the inner diameter of the graphite ring.
The total height of the mold is equal to the sum of the height of the graphite ring and the height of the aluminum table. The height of the crystallizer is the height of the effective crystallization zone, which is generally controlled at 15-50mm in production.
The height of the graphite ring is an important parameter of hot top casting. If it is too small, the crystallization area will rise and go deep into the sealing and heat insulation map, which will easily cause pull marks and cracks on the surface of the ingot. If it is too high, the contact area between the metal in the semi-solidified state of the casting edge and the graphite ring is large, and the surface of the ingot is not smooth. The inner diameter of the graphite ring plays the role of sizing. The thickness of the graphite ring should be as thin as possible, but considering the strength of the graphite itself and meeting processing needs, the thickness should be 4mm. The requirements for graphite rings are to have good thermal conductivity, lubricity, sufficient finish and strength.
New type hot top mold: When using multi-mode hot top casting, the mold is installed under the water tank pouring platform, and the hot cap for aluminum casting is located above the pouring platform to form a unified hot top distribution launder. The side of the mold is open to realize multi-mold casting.
