In sand casting process design, sand casting process designers often only pay attention to the design of sand casting process parameters, gating system, riser feeding system, chill system and so on. However, the exhaust system design is not paid enough attention, especially the coordination between the pouring and riser system design and the exhaust system design is not in place, and the exhaust capacity of the sand mold cavity and sand core is not checked. If the design of sand casting exhaust system is unreasonable, the exhaust of mold cavity and sand core is not smooth, and the design of pouring and riser system is not considerate, sand casting defects such as invasive air hole, explosive sand sticking, fire choking, gas shrinkage hole, insufficient pouring and cold insulation, and personal safety accidents caused by metal splashing will occur. The main waste rate of production products is mostly concentrated in porosity and sand sticking defects, which will cause great economic losses, and it is also one of the problems that sand casting workers urgently need to solve.
(1) The layout of exhaust system is unreasonable, the exhaust of sand mold cavity and sand core is insufficient, the structure design of sand core is improper, and the total exhaust area is too small, resulting in invasive pores, sand sticking, fire choking and other defects of sand castings.
(2) The effective air outlet area in the exhaust system is too small, the air outlet needle is not pierced, and the compactness of the sand mold is high and the air permeability is poor, which affects the exhaust capacity of the mold cavity, and the sand mold castings will produce air holes and sand sticking defects.
(3) When the high-pressure wet mold machine is used for molding and the hot core box and cold core box are used for core making, due to the deep mold lifting inside the mold cavity, the “air pocket” is generated at the dead corner, the intersection of sand shooting sand flow, the corner and the inside of the mold cavity, which is difficult to be discharged smoothly, resulting in poor mold lifting and sand shooting of sand core, which affects the quality of sand core and sand mold.
(4) It is generally believed that for sand castings without open risers, the minimum total cross-sectional area of the root of the air outlet hole should be equal to the total cross-sectional area of the choke, so as to ensure that the air outlet hole can discharge the gas in the mold cavity smoothly. In complex thin-walled parts, the sand core has large air generation, poor air permeability, inadequate drying, and no exhaust hole or drilling in place. During pouring, the gas generated by the sand core is large and difficult to be discharged smoothly, resulting in air hole and gas shrinkage hole defects in sand castings.
(5) Before designing the exhaust system, the sand core and process formula with low air generation and good air permeability were not selected according to the structure and quality requirements of sand castings, which increased the tendency of air hole defects in sand castings. The gas production index measured at 850 ℃: 10.4 ml / g for general clay sand, 6 ml / g for sodium silicate sand, 15 ml / g for resin sand, 15 ~ 17 ml / g for coated sand and 12 ml / g for cold core sand.
(6) The sealing between the core head and the core seat of the sand core is not in place, the metal liquid blocks the exhaust duct, and the sand core cannot exhaust, resulting in air holes and fire choking defects in the sand mold casting.
(7) The gating system is not designed properly, the temperature gradient from bottom to top is not formed, and the pouring is unstable. The pouring temperature is low, the pouring speed is too slow, and the oxide film on the surface of liquid metal is formed early, so that the original gas in the mold cavity can not be discharged, and the sand castings will have air holes, fire choking, insufficient pouring and cold insulation defects.
(8) For resin sand mold sand castings, the gas generation of resin sand mold is large. If the air outlet is too small and the overflow liquid metal is less, the exhaust of mold cavity is not unblocked, and sand mold castings will produce sand mold casting defects such as air hole, slag hole and gas shrinkage hole.
(9) When pouring, the ignition and air introduction are not timely, resulting in the increase of air pressure in the mold cavity, air holes and fire choking defects of sand castings, and even liquid metal splashing safety accidents.
