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9.3 General Discussion

When a noncompressible liquid such as water is pushed, the same amount propelled by the plunger will flow out of the system. However, air is a compressible substance and thus the above statement cannot be applied. The flow rate out depends on the resistance to the flow plus the piston velocity (piston area as well). There could be three situations 1) the flow rate out is less than the volume pushed by the piston, 2) the flow rate out is more than the volume pushed by the piston, or 3) the flow rate out is equal to the volume pushed by the piston. The last case is called the critical design, and it is associated with the critical area.

Air flows in the venting system can reach very large velocities up to about 350 []. The air cannot exceed this velocity without going through a specially configured conduit (converging diverging conduit). This phenomena is known by the name of choked flow''. This physical phenomenon is the key to understanding the venting design process. In air venting, the venting system has to be designed so that air velocity does not reach the speed of sound: in other words, the flow is not choked. In vacuum venting, the air velocity reaches the speed of sound almost instantaneously, and the design should be such that it ensures that the air pressure does not exceed the atmospheric pressure.

Prior models for predicting the optimum vent area did not consider the resistance in the venting system (pressure ratio of less than 2). The vent design in a commercial system includes at least an exit, several ducts, and several abrupt expansions/contractions in which the resistance coefficient, , is of the order of 3-7 or more. Thus, the pressure ratio creating choked flow is at least 3. One of the differences between vacuum venting and atmospheric venting occurs during the start-up time. For vacuum venting, a choking condition is established almost instantaneously (it depends on the air volume in the venting duct), while in the atmospheric case the volume of the air has to be reduced to more than half (depending on the pressure ratio) before the choking condition develops - - and this can happen only when more than 2/3 or more of the piston stroke is elapsed. Moreover, the flow is not necessarily choked in atmospheric venting. Once the flow is choked, there is no difference in calculating the flow between these two cases. It turns out that the mathematics in both cases are similar, and therefore both cases are presented in the present chapter.

The role of the chemical reactions was shown to be insignificant. The difference in the gas solubility (mostly hydrogen) in liquid and solid can be shown to be insignificant [#!poro:metals!#]. For example, the maximum hydrogen release during solidification of a kilogram of aluminum is about 7 at atmospheric temperature and pressure. This is less than 3% of the volume needed to be displaced, and can be neglected. Some of the oxygen is depleted during the filling time [#!poro:genickreac!#]. The last two effects tend to cancel each other out, and the net effect is minimal.

The numerical simulations produce unrealistic results and there is no other quantitative tools for finding the vent locations (the last place(s) to be filled) and this issue is still an open question today. There are, however, qualitative explanations and reasonable guesses that can push the accuracy of the last place (the liquid metal reaches) estimate to be within the last 10%-30% of the filling process. This information increases the significance of the understanding of what is the required vent area. Since most of the air has to be vented during the initial stages of the filling process, in which the vent locations do not play a role.

Air venting is the cheapest method of operation, and it should be used unless acceptable results cannot be obtained using it. Acceptable results are difficult to obtain 1) when the resistance to the air flow in the mold is more significant than the resistance in the venting system, and 2) when the mixing processes are augmented by the specific mold geometry. In these cases, the extraction of the air prior to the filling can reduce the air porosity which require the use of other techniques.

An additional objective is to provide a tool to combine'' the actual vent area with the resistance (in the venting system) to the air flow; thus, eliminating the need for calculations of the gas flow in the vent in order to minimize the numerical calculations. Hu et al. poro:jia and others have shown that the air pressure is practically uniform in the system. Hence, this analysis can also provide the average air pressure that should be used in numerical simulations.

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