Department of Mechanical Engineering,Sharif University of Technology
The present study is undertaken to calculate the rate of change of pressure and residual air mass in die casting for vacuum venting under choked flow conditions. In these calculations, the influence of friction factor, due to roughness and vent air velocity change through the Mach number, has been taken into account. The results show that there is a critical area ratio below which the pressure and vent inlet Mach number increase with time and above which decrease with time. In addition, for an area ratio less than the critical area, the rate of change of residual air mass seems to be more changed at the late stages than at the early stages of the filling time. The picture is reversed for a larger area ratio. This critical area ratio depends on vent area, filling time, evacuated volume, the initial pressure and temperature of the air in the die cavity.