Page 264 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 10.3 Fluid Flow 243

l0.3 Fluid Flow

To emphasize the importance of fluid flow in casting, let’s briefly describe a basic
gravity-casting system, as shown in Fig. 10.8. The molten metal is poured through a
pouring basin or cup; it then flows through the gating system (consisting of sprue,
runners, and gates) into the mold cavity. As also illustrated in Fig. 11.3, the sprue is
a tapered vertical channel through which the molten metal flows downward in the
mold. Runners are the channels that carry the molten metal from the sprue into the
mold cavity or connect the sprue to the gate (that portion of the runner through
which the molten metal enters the mold cavity). Risers (also called feeders) serve as
reservoirs of molten metal to supply any molten metal necessary to prevent porosity
due to shrinkage during solidification.
Although such a gating system appears to be relatively
simple, successful casting requires proper design and control of Top riser
Pouring cup
the solidification process to ensure adequate fluid flow in the
Side riser
system. For example, an important function of the gating system
in sand casting is to trap contaminants (such as oxides and other
inclusions) and remove them from the molten metal by having
the contaminants adhere to the walls of the gating system, there-
by preventing them from reaching the mold cavity. Furthermore,
a properly designed gating system helps avoid or minimize prob- Casting
Gate
lems such as premature cooling, turbulence, and gas entrapment. Runner
Even before it reaches the mold cavity, the molten metal must Well
be handled carefully to avoid the formation of oxides on molten-
metal surfaces from exposure to the environment or the intro- FIGURE l0.8 Schematic illustration of a typical
duction of impurities into the molten metal. riser-gated casting. Risers serve as reservoirs,
Two basic principles of fluid flow are relevant to gating supplying molten metal to the casting as it shrinks
design: Bernoulli’s theorem and the law of mass continuity. during solidification.

Bernoulli’s Theorem. This theorem is based on the principle of the conservation
of energy and relates pressure, velocity, the elevation of the fluid at any location in
the system, and the frictional losses in a system that is full of liquid. The Bernoulli
equation is
2
h + 3 + L = constant, (10.2)
pg 23
where h is the elevation above a certain reference level, p is the pressure at that
elevation, 1/ is the velocity of the liquid at that elevation, p is the density of the
fluid (assuming that it is incompressible), and g is the gravitational constant.
Conservation of energy requires that, at a particular location in the system, the fol-
lowing relationship be satisfied:
2 2
/@,+&+@=/@,+@+3+,§ (10.3)
pg Za pg 2g
where the subscripts 1 and 2 represent two different locations in the system and f
represents the frictional loss in the liquid as it travels through the system. The fric-
tional loss includes such factors as energy loss at the liquid-mold wall interfaces and
turbulence in the liquid.
Mass Continuity. The law of mass continuity states that, for incompressible liq-
uids and in a system with impermeable walls the rate of flow is constant. Thus,
Q = A11/1 = A21/2, (10.4)