Influence Of The Gas From Cavity On Quality Of The Crankshaft Casting During Mold Filling

There has been no research about the casting defects caused by the large air pressure in the cavity during mold filling, because there was no effective pressure testing equipment. In the present paper, WD615 crankshaft casting of sphere cast has been studied and a new set of pressure gauge with automatic storage and wireless transmission has been designed to measure the air pressure distribution in the cavity during mold filling. In a combination of the air pressure distribution, the quality of a casting and the result of numerical simulation, the influence of the gas from cavity on casting quality during mold filling was discussed.In addition, the properties of green sand was researched, which would have impacts on the gas of cavity.In the measurement of the air pressure distribution in the cavity during mold filling, it was found that the variation of the curve of the air pressure with time was similar in the different air hole over the top of cavity, the air pressure rose rapidly to the first peak from about 5 seconds to 10 seconds and then decreased sharply. When the air pressure fell to near 0 Pa, it rebounded rapidly and got to the second peak around the end time of pouring, and then declined gradually. After 150 seconds, there was still little pressure value left. The different air hole over the top of cavity had a different pressure, the closer to the defects, the greater the air pressure and the discharge capacity. The air holes at a distance of 160 millimeters from the defects play a major role in exhausting, when the air pressure of the No.3 air hole rose to 90 Pa, the casting defects appeared, after the exhaust techniques were improved based on the simulation results, the air pressure fell to 60 Pa and the casting defect disappeared.The viscosity of QT900-5 iron used in WD615 crank casting has also been studied, in the range of the composition of molten iron, when all the elements were changed, the viscosity varied in 9%, so the middle composition of the molten iron was selected as the object during the mold filling simulation. The air distribution in the cavity during mold filling was simulated by the soft of Procast, it was found that the variation of the curve of the air pressure was coincide with the air distribution calculated by numerical simulation. (1) As the air pressure rose to the first peak, the molten iron entered into the bottom of the cavity by ingate and covered over a large area of facing sand, which resulted in the vaporization and expansion of the water in facing sand, but the gas had no time to escape through the sand mould, so the air pressure increased rapidly.(2) Once the air pressure fell from the first peak, the late liquid iron came into contact with the small cavity wall because the earlier melted iron located on the maximum section of the cavity, which caused only a little area of facing sand covered. Therefore the gas-evolution speed was less than the exhaust speed, and then the air pressure began to fall to a figure. (3) In the second peak stage of the air pressure, three factors could account for this phenomenon. First, the open riser and blind riser were all blocked by liquid iron. The gas around the defects was entrapped, compressed and moved to the location of the defects. Second, the volume of the gas in cavity was decreased, because the gas was compressed by the rising surface of molted iron. The last, the decline of the air pressure was limited, because the gas escapes form sand mould slowly. These three factors lead to the increasing of the air pressure in the defects. (4) Around the end of pouring, the gas pressure curve fell because the gas-evolution was over and the gas constantly escaped though the sand.The performance datas of mold sand, for a totai of 129 datas, were analyzed with the soft of Spass. It is found that the moisture content, compactability, temperature, permeability, compression strength of backing sand were all under control. Besides, the permeability fitted the normal distribution.The moisture content, permeability and compression strength of facing Sand were also under control, The moisture content of facing sand is 0.67wt.% greater than backing sand, and the control range of water was also wider than sand back. The water in facing sand should be strictly controlled and even narrowed, because it was the main source of cavity gas.