Page 266 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 10.4 Fluidity of Molten Metal

If the liquid level has reached a height of x at the gate, then the gate velocity is

v = c\/Zgv la - x.
The flow rate through the gate will be the product of this velocity and the gate area
according to Eq. (10.4). The shape of the casting will determine the height as a func-
tion of time. Integrating Eq. (10.4) gives the mean fill time and flow rate, and divid-
ing the casting volume by this mean flow rate gives the mold fill time.
Simulation of mold filling assists designers in the specification of the runner di-
ameter, as well as the size and number of sprues and pouring basins. To ensure that
the runners stay open, the fill time must be a small fraction of the solidification time,
but the velocity should not be so high as to erode the mold material (referred to as
mold was/0) or to result in too high of a Reynolds number (see the following).
Otherwise, turbulence and associated air entrainment results. Many computational
tools are now available to evaluate gating designs and assist in the sizing of compo-
nents such as Magmasoft, ProCast, Quikcast, and Powercast.
Flow Characteristics. An important consideration of the fluid flow in gating
systems is the presence of turbulence, as opposed to the laminar /‘low of fluids.
Turbulence is flow that is highly chaotic; in casting systems such flow can lead to as-
piration. The Reynolds number, Re, is used to quantify this aspect of fluid flow. It rep-
resents the ratio of the inertia to the viscous forces in fluid flow and is defined as
Re = 3, (10.6)
D
TI
where v is the velocity of the liquid, D is the diameter of the channel, and p and 17
are the density and viscosity of the liquid, respectively. The higher the Reynolds
number, the greater the tendency for turbulent flow to occur.
In gating systems, Re typically ranges from 2000 to 20,000, where a value of
up to 2000 represents laminar flow. Between 2000 and 20,000, it represents a mix-
ture of laminar and turbulent flow. Such a mixture generally is regarded as harmless
in gating systems. However, Re values in excess of 20,000 represent severe turbu-
lence, resulting in significant air entrainment and the formation of dross (the scum
that forms on the surface of molten metal) from the reaction of the liquid metal with
air and other gases. Techniques for minimizing turbulence generally involve avoid-
ance of sudden changes in flow direction and in the geometry of channel cross sec-
tions in gating system design.
Dross or slag can be eliminated only by vacuum casting (Section 11.4.2).
Conventional atmospheric casting mitigates dross or slag by (a) skimming, (b) using
properly designed pouring basins and runner systems, or (c) using filters, which also
can eliminate turbulent flow in the runner system. Filters usually are made of ceram-
ics, mica, or fiberglass; their proper location and placement are important for effec-
tive filtering of dross and slag.

l0.4 Fluidity of Molten Metal

The capability of molten metal to fill mold cavities is called fluidity, which consists of
two basic factors: (1) characteristics of the molten metal and (2) casting parameters.
The following characteristics of molten metal influence fluidity:
Viscosity. As viscosity and its sensitivity to temperature (viscosity index) increase,
fluidity decreases.