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1.6 ``Integral'' Models

Unfortunately, the numerical simulations of the liquid metal flow and solidification do not yield reasonable results at the present time. This problem has left the die casting engineers with the usage of the ``integral approach'' method. The most important tool in this approach is the diagram, one of the manifestations of supply and demand theory. In this diagram, an engineer insures that die casting machine ability can fulfill the die mold design requirements; the liquid metal is injected at the right velocity range and the filling time is small enough to prevent premature freezing. One can, with the help of the diagram, and by utilizing experimental values for desired filling time and gate velocities improve the quality of the casting. The gate velocity has to be above certain value to assure atomization and below a critical value to prevent erosion of the mold. This two values are experimental and no reliable theory is available today known to the author [#!poro:genickthesis!#]. The correct model for the diagram has been developed and will be discussed in Chapter [*]. A by-product of the above model is the plunger diameter calculations and it is discussed in Chapter [*].

It turns out that many of the design parameters in die casting have a critical point above which good castings are produced and below which poor castings are produced. Furthermore, much above and just above the critical point do not change much but costs much more. This is where the economical concepts plays a significant role. Using these concepts, one can increase the probably significantly and, obtain very quality casting and reduce the leading time. Additionally, the main cost components (machine cost and other) are analyzed and have to be taken into considerations when one chooses to design the process with will be discussed in the Chapter [*] on the economy of the die casting.

Porosity can be divided into two main categories; shrinkage porosity and gas/air entrainment. The porosity due to entrapped gases constitutes a large part of the total porosity. The creation of gas/air entrainment can be attributed to at least four categories: lubricant evaporation (and reaction processes), vent locations (last place to be filled), mixing processes, and vent/gate area. The effects of lubricant evaporation have been found to be insignificant. The vent location(s) can be considered partially solved since only qualitative explanation exist. The mixing mechanisms are divided into two zones: the mold, and the shot sleeve. Some mixing processes have been investigated and can be considered solved. The requirement on the vent/gate areas is discussed in Chapter [*]. When the mixing processes are very significant in the mold other methods are used and they include: evacuating the cavities (vacuum venting), Pore Free Technique (in zinc and aluminum casting) and squeeze casting. The first two techniques are used to extract the gases/air from the shot sleeve and die cavity before the gases have the opportunity to mix with the liquid metal. The squeeze casting is used to increase the capillary forces and, therefore, to minimize the mixing processes. All these solutions are cumbersome and more expensive and should be avoided if possible.

The mixing processes in the runners, where the liquid metal flows vertically against gravity in a relative large conduit, are considered to be insignificant. The enhanced air entrainment in the shot sleeve is attributed to operational conditions for which a blockage of the gate by a liquid metal wave occurs before the air is exhausted. Consequently, the residual air is forced to be mixed into the liquid metal in the shot sleeve. Now with Bar-Meir's formula one can calculate the correct critical slow plunger velocity and this will be discussed in Chapter [*].


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