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
2.3 FLOW AT THE METAL FILL FRONT
17
Figure 2.4 Graphical illustration showing the progression of a die cavity filling
with a planar metal front. Air Pocket
Being Entrapped
Direction of Metal Flow
Figure 2.5 Graphical illustration showing nonplanar flow.
18
MOLTEN METAL FLOW IN HIGH INTEGRITY DIE CASTING PROCESSES
Figure 2.6 Graphical illustration showing the progression of a die cavity filling
with a nonplanar metal front.
Figure 2.7 Graphical illustration showing the progression of nonplanar fill.
2.4 METAL FLOW IN VACUUM DIE CASTING
19
Runner Gate
Die Atomized
Liquid Metal
Figure 2.8 Illustration showing atomized flow typical in conventional die cast-
ing.
reaches the far side of the die cavity, gases are entrapped as the fill front doubles over on itself. The metal front continues to travel
along the surface of the die, filling the cavity from the outside inward. This results in additional pockets of entrapped gas.
When liquid metal is traveling at high velocities through a very small gate, the fill front breaks down, resulting in atomization.
This phenomenon is illustrated in Figure 2.8. Due to the high pressure and velocities, the metal becomes in effect an aerosol,
spraying into the die cavity. Shown in Figure 2.9 is the progres- sion of die fill, which occurs with atomized metal flow. Liquid
metal is sprayed into the die. Filling occurs from the surface of the cavity inward. Typically, the first metal to enter the die strikes
the far side of the die cavity and solidifies immediately.
2.4 METAL FLOW IN VACUUM DIE CASTING
In conventional die casting, high gate velocities result in atomized metal flow within the die cavity, as shown in Figures 2.8 and 2.9.
Entrapped gas is unavoidable. This phenomenon is also present in vacuum die casting, as the process parameters are virtually iden-
tical to that of conventional die casting.
20
MOLTEN METAL FLOW IN HIGH INTEGRITY DIE CASTING PROCESSES
Figure 2.9 Graphical illustration of die fill with atomized metal flow in con-
ventional die casting.
Example Calculation 2.4
Using the vacuum die casting process, a component is manufac- tured with a conventional aluminum alloy. Calculate the Reynolds
number for this process to determine if metal flow at the gate is laminar or turbulent given that the gate is 2 mm wide and 35 mm
in length. The velocity of metal at the gate is 50,000 cmsec. The density and viscosity of liquid aluminum are 2.7 gcm
3
and 1 ⫻ 10
⫺3
gcm 䡠 sec, respectively.
2
Solution For this case, the characteristic length is the width of the gate.
Using Equation 2.1, Dv
Re ⫽
3
0.2 cm 50,000 cmsec 2.7 gcm ⫽
⫺3
1 ⫻ 10 gcm 䡠 sec
⫽ 27,000,000 Due to the extremely high Reynolds number ⬎10,000, fluid flow
