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### 7.4.2.2 General conclusions from example

For the constant pressure case the ``common''19 assumption yields a constant velocity even for a zero gate area.

The solid line in Figure represents the gate velocity calculated based on the common assumption of constant while the other lines are based on calculations which take into account the runner geometry and the Re number. The results for constant represent ``averaged'' of the other results. The calculations of the velocity based on a constant value are unrealistic. It overestimates the velocity for large gate area and underestimates for the area ratio below 80% for the short runner and 35% for the long runner. Figure exhibits that there is a clear maximum gate velocity which depends on the runner design (here represented by the conduit length). The maximum indicates that the preferred situation is to be on the ``right hand side branch'' because of shorter filling time. The gate velocity is doubled for the excellent design compared with the gate velocity obtained from the poor design. This indicates that the runner design is more important than the specific characteristic of the die casting machine performance. Operating the die casting machine in the ``right hand side'' results in smaller requirements on the die casting machine because of a smaller filling time, and therefore will require a smaller die casting machine.

For the constant power case, the gate velocity as a function of the area ratio is shown in Figure . The common assumption of constant yields the gate velocity shown by the solid line. Again, the common assumption produces unrealistic results, with the gate velocity approaching infinity as the area ratio approaches zero. Obviously, the results with a constant overestimates the gate velocity for large area ratios and underestimates it for small area ratios. The other lines describe the calculated gate velocity based on the runner geometry. As before, a clear maximum can also be observed. For large area ratios, the gate velocity with an excellent design is almost doubled compared to the values obtained with a poor design. However, when the area ratio approaches zero, the gate velocity is insensitive to the runner length and attains a maximum value at almost the same point.

In conclusion, this part has been shown that the use of the ``common'' diagram with the assumption of a constant may lead to very serious errors. Using the diagram, the engineer has to take into account the effects of the variation of the gate area on the discharge coefficient, , value. The two examples given inhere do not represent the characteristics of the die casting machine. However, more detailed calculations shows that the constant pressure is in control when the plunger is small compared to the other machine dimensions and when the runner system is very poorly designed. Otherwise, the combination of the pressure and power limitations results in the characteristics of the die casting machine which has to be solved.

= 90 true mm

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