of the key elements to making a metal casting of high quality is the design of a good gating system. This is even more important if a casting is produced by a gravitational process.
The process of pouring the molten metal has to be performed carefully; if not, there will be various casting defects directly traceable to pouring the molten metal incorrectly in the stage of mold filling. For example, too rigorous a stream could cause mold erosion; highly turbulent flows could result in air and inclusion entrapments; and finally, relatively slow filling might generate cold shuts. Thus, the design of the gating and venting overflow systems has to take into consideration the proper control of the liquid metal as it fills the mold.
An optimum gating system design can reduce the turbulence of the molten material’s flow, minimizing gas, inclusions, and dross. If poor gating techniques are used, invariably, lower casting quality is the result because of damage to the molten metal received during its flow through the gating system. It could be even worse if the molten material is a material sensitive damage (dross and slag formation) during mold filling. Aluminum and its casting alloys are such materials.
Aluminum alloys are very reactive to oxygen, and they form an oxide, Al2O3. When flow is smooth, this oxide tends to form and remain on the surface of the steam. However, when flow is turbulent, the oxide goes into the molten metal and may carry gas or bubbles with it. Then, to avoid damage to the molten metal, the gating system must be designed to eliminate the air problems by avoiding conditions that permit aspiration due to the formation of lowpressure areas.
In order to achieve an adequate gating system design it is necessary to follow basic principles. Molten metal behaves according to fundamental hydraulic principles. Applying those fundamentals to the design of the gating system can be an advantage.
Several mathematical and theoretical factors affect the flow of molten metal through the gating system and into the mold, and it is a good idea to have an understanding of their effect on the process. They are Bernoulli’s theorem, law of continuity, the effects of the force of friction, and Raymond’s number.
a) Bernoulli’s Theorem
The Bernoulli effect is simply a result of the conservation of energy in the steady flow of an incompressible fluid; Bernoulli’s theorem states that the sum of the energies in a flowing liquid is constant at any two points. This can be written in the following form:
| (1.2) |
where
| h | = | elevation above a certain reference plane, m (in.) |
| p | = | pressure at that elevation, Pa (psi) |
| v | = | velocity of the liquid at that elevation, m/s (in./s) |
| ρ | = | density of the fluid, kg/m3 (lbm/in.3) |
| g | = | gravitation constant, m/s2 (in./s2) |
| f | = | frictional loss in the liquid as it travels downward through the system |
Subscripts 1 and 2 indicate two different elevations in the liquid flow.
Let us simplify equation (1.2). If we ignore friction losses we can assume that the system remains at atmospheric pressure throughout. Point 1 is defined as being at the top of the sprue and point 2 at its base; if point 2 is used as the reference plane (h2 = 0) and the metal is poured into the pouring basin, then the initial velocity at the top is zero (v1 = 0). Hence, equation (1.2) further simplifies to
| (1.3) |
The velocity of the liquid metal flow at the base of the sprue is
| (1.4) |
where
| h 1 | = | height (length) of the sprue, m (in.) |
| v 2 | = | velocity of the liquid metal at the base of the sprue, m/s (in./s) |
| g | = | gravitation constant, m/s2 (in./s2). |
b) Mass Continuity
The law of mass continuity states that for incompressible liquids and in a system with impermeable walls, the volume rate of flow remains constant throughout the liquid. Thus,
| Q = A1v1 = A2v2 | (1.5) |
where
| Q | = | volume rate of flow, m3/s (in.3/s) |
| A | = | cross-section area of the liquid stream, m2 (in.2) |
| v | = | velocity of the liquid in the system, m/s (in./s) |
Subscripts 1 and 2 indicate two different locations in the liquid flow.
As the liquid metal accelerates during its descent into the sprue opening, equation (1.5) indicates that the cross-sectional area of the channel must be reduced. Hence, the sprue should be tapered.
c) Flow Characteristics
A molten metal’s flow characteristics constitute very a important consideration in the gating system because of the possible consequences of turbulence. However, there are two different types of real fluid flow: laminar and turbulent. In laminar flow the fluid moves in layers called laminas. Laminar flow need not be in a straight line. For laminar flow, the flow follows the curved surface smoothly, in layers. Moreover, the fluid layers slide over one another without fluid being exchanged between the layers.
In turbulent flow, secondary random motions are superimposed on the principal flow and there is an exchange of fluid from one adjacent sector to another. More important, there is an exchange of momentum such that slow-moving fluid particles speed up and fast-moving particles give up their momentum to the slower moving particles and themselves slow down.
The factor that determines which type of flow is present is the ratio of the inertial forces to the viscous forces within the fluid. This
