14
MOLTEN METAL FLOW IN HIGH INTEGRITY DIE CASTING PROCESSES
a
Direction of Metal Flow
b
Figure 2.1 Comparison of a laminar flow and b turbulent flow.
flow of a fluid with specific properties in a conduit with a specific geometry, a critical velocity exists below which the flow is laminar
and above which the flow is turbulent. This phenomenon was ascertained as the result of studies conducted in 1883 by Osborne
Reynolds.
1
Figure 2.2 graphically presents the experiments conducted by Reynolds in which a colored dye was injected into a liquid flowing
in a glass tube. At low flow rates, the dye flowed with the fluid without mixing, as shown in Figure 2.2a. When increasing the
flow of the fluid Figure 2.2b, the colored dye rapidly broke up and mixed with the fluid from its point of injection. As a result
of these experiments, Reynolds established the criterion for the transition from laminar to turbulent flow in terms of the dimen-
sionless quantity presented in the following equation:
Re ⫽ Dv 2.1
in which D is the characteristic geometry of the conduit and v, ,
2.3 FLOW AT THE METAL FILL FRONT
15
a
b Direction of Fluid Flow
Figure 2.2 Illustration of the experiment demonstrating the difference between
a laminar flow and b turbulent flow.
and are the velocity, density, and viscosity of the fluid, respec- tively. This dimensionless number Re is known as the Reynolds
number. Turbulent flow is often perceived as detrimental to casting pro-
cesses while laminar-type flow is preferred. This misconception stems from the confusion between liquid metal flow and the liquid
metal fill front that progresses within a mold or die cavity. The liquid metal fill front has a much greater effect on casting integrity
than the type of flow within the bulk liquid metal.
2.3 FLOW AT THE METAL FILL FRONT
Although an understanding of the bulk liquid metal flow is often useful, the flow of the metal at the fill front is of most concern.
Three distinct metal fill fronts are encountered in die casting pro-
16
MOLTEN METAL FLOW IN HIGH INTEGRITY DIE CASTING PROCESSES
Direction of Metal Flow
Figure 2.3 Graphical illustration of planar flow.
cesses: planar fill, nonplanar fill, and atomized fill. Each of these phenomena will be discussed separately in this section.
Traditional thinking regarding fluid flow tends to assume that the liquid metal fill front progresses as a uniform plane throughout
the die. A graphical illustration of this phenomenon is shown in Figure 2.3. This form of planar fill does occur in some casting
processes. However, planar fill during die casting occurs only un- der very specific conditions. The complex geometries of most
components cause the liquid metal fill front to separate. When planar filling of the die cavity does occur, gases trapped within
the die are pushed ahead of the metal fill front. In Figure 2.4, the progression of a die cavity filling with a planar metal front is
shown. By locating vents and overflows at the farthest point from the gate, gas entrapment can be virtually eliminated.
Nonplanar flow is also observed in many casting processes. Unlike planar metal flow, the fill front is not uniform, as shown
in Figure 2.5. Often metal fronts converge and surround a pocket of air, resulting in entrapped gases in the component being pro-
duced. When die casting, nonplanar fill often results in the die cavity being filled from the outside inward. This fill behavior is
shown in Figures 2.6 and 2.7. In Figure 2.6, the metal front enters the die as a single stream that changes direction only after con-
tacting the far side of the die cavity. The metal front continues to hug the surface of the die, filling the cavity from the outside in
creating large pockets of entrapped gas. In Figure 2.7, the metal stream begins to fan out after entering the die cavity. As the metal
