3.4 Effect of mold packing materials.
The variation of casting density with mold packing materials is shown in Fig.10. The melt was degassed and EPS foam densities were 12.5 and 20.0 kg/m3. Density of the CO2 mold casting is also marked in the figure for comparison. Density of the steel shots packed casting is higher than the other two cases, however lower than that of CO2 molding. Mold filling time for lost foam casting was about 8 s, while the molten metal was filled into CO2 mold within 3 s, because the presence of the polymer during mold filling acts as a resistance to the flow of liquid metal. Metal velocity at the ingate was 0.018 m/s in this experiment, which has a significant effect on the casting quality, primarily because the rate of foam replacement affect the type and numbers of defects formed. 0.015-0.02 m/s is recommended[8] generally that result in a minimum number of defects such as misrun, surface collapse, porosity, blister, folds and penetration in lost foam casting.
The variation of cooling capacity with mold packing materials. Properties of the silica sand were chose as the basis ratio 1. Cooling capacity of the mold is estimated by the solidification time or dendrite arm spacing of the casting or the heat diffusivity of the mold packing material[9] or square root of specific heat per unit volume in the lost foam casting in this experiment. The formation rate of polymer degradation products is thought to be almost the same because of the same mold packing material in this experiment. Therefore the elimination rate of polymer degradation products is seemed to be closely related to the pore formation. Density of the steel shots packed casting should be lower from the viewpoint of solidification rate, since solidification is the fastest in the steel shots packed casting. However the escape path of the gas evolved during pouring seems to be more important to remove the porosity. It depends on the various factors such as the mold packing ratio, particle size and shape of the packing material. The apparent densities of the compacted bed with silica sand, zircon sand and steel shots are 1.49×103,2.54×103, 4.1×103 kg/m3, respectively. Converting these densities into the mold packing ratio in volume percent by comparing them with their solid density, 57.0%, 60.1% and 55.6% are obtained respectively. Average diameters of the silica sand, zircon sand and steel shots are 0.25, 0.1 and 0.3 mm. Shape of the silica sand is almost round and that of steel shots is round. On the other hand, the zircon sand is composed of many angular particles. It suggests that packing density of zircon sand is high and size of the void between zircon sand grains is small. It can be concluded that the escape path in steel shots packing is wider than that in the other packing materials, and this results in higher density in the steel shots packed casting.
The variation of casting density with melt treatment. The melt was degassed and EPS foam densities was 20.0 kg/m3. Density of the CO2 mold casting is also marked in the figure for comparison. Density of the refined melt casting is almost the same as that of degassed melt casting, while that of no treated melt casting is much lower. The formation of porosity is also controlled by grain refining in aluminum casting. Among several advantages of grain refining, the primary one is an improvement in the amount and distribution of porosity and shrinkage in alloy that tends to form microporosity[10]. Refiner makes pores finely dispersed, but does not affect the amount of formation of polymer degradation products. It seems that refining the melt does not have any effect on the casting density.
