Good gating leads to good castings.

One of the keys to making a good metal casting is to design a good gating system. Poor gating techniques invariably lower casting quality and often result from poor shop floor practice.

Basically, a gating system is the series of channels through which molten metal travels from the ladle to the mold cavity.

Good gating design involves three steps (placement, design and size) that always conform to the physical principles of a free flowing liquid. By using proven rules derived from Bernoulli's theorem (dealing with the relationships influencing the behavior of molten metal), one can anticipate and compensate for problem areas normal in gating design.

In general a good gating system:

* reduces molten metal turbulence;

* eliminates gas and air entrapment;

* decreases liquid velocity;

* prevents premature solidification;

* develops proper thermal gradients.

Turbulence

Turbulence is the agitation of the molten metal stream. It will cause air, gas and oxides to be entrapped in the metal flowing into the mold and cause a defective casting.

Typically the area of highest turbulence within any gating system tends to be at the location of highest velocity. Decreasing velocity reduces turbulence, which in turn aids in the flotation and entrapment of oxides.

Velocity vs. Fill Time

Velocity must not be confused with flow or fill rates. Velocity is a measurement of speed; flow rate is a gauge of volume. It is possible to reduce metal speed and yet retain a rapid fill rate.

A gating system should allow the mold cavity to be filled fast enough to eliminate misruns, premature freezing and/or cope-related sand expansion defects but slow enough to eliminate inclusions. The foundryman's dilemma, then, is how to fill a mold both slowly and fast.

Analysis shows that there is an optimum fill time for each casting to be poured including adjustments to accommodate flow losses caused by friction and fluidity. A well-designed gating system will fill a mold quickly and quietly. It also will aid directional solidification by developing proper feed paths within the mold cavity to allow risers to better compensate for solidification shrinkage.

System Elements

A good gating system removes the "art" from pouring by scientifically controlling the flow rate and always performing properly regardless of who is manning the ladle.

Essential components that contribute to an effective gating system include:

Pouring basin--The initial receiver of metal from the ladle, it directs the metal to the downsprue, establishes a constant metallostatic head and the initial separation of oxides.

Sprue--The vertical passage from the pouring basin to the runner system.

Sprue base--A well, or enlargement at the bottom of the sprue that reduces turbulence in the liquid entering the runner.

Runner--The main horizontal channel that directs the liquid metal from the sprue to the ingate(s). In a properly designed system, it also entraps oxides by reducing velocity.

Ingate--The ingate(s) directs the metal from the runner into the mold cavity.

A primary design consideration is the choke. It is defined as the smallest total cross-sectional area in a system that, given a fixed head pressure, regulates the fill rate of a casting. It also is the point of the highest velocity or most turbulence and could cause several problems in certain alloys.

It is important not to confuse velocity and fill rate. Increasing the velocity of a metal stream entering a mold cavity will not result in a faster mold fill time. To increase the fill time, the volume or fill rate must change. Changing the fill rate is the sole function of the system's choke.

For example, think of a gating system as a garden hose used to wash a car. After turning on the faucet, the water's flow rate is determined by how far the faucet valve is opened. The faucet is the smallest total cross-sectional area in the hose example and corresponds to the choke.

To properly clean the vehicle, a jet of water is created by restricting the cross-sectional area at the end of the hose. By reducing this area, the velocity is increased to a point at which it can loosen any dirt or dust.

The important point to note is that restricting the end of the hose increases water velocity, but the flow rate doesn't change. If there was any change in the flow rate, the hose would swell and act as a reservoir.

The only way to change the flow rate at the end of the hose would be to reduce the total cross-sectional area of the hose to make it smaller than that of the faucet valve opening. The choke may be located in different parts of the gating system depending on the type of system used.

For those wanting to learn more about gating, CMI will offer three courses (May, July & September '92). For more information call CMI at 800/537-4237.