Casting Gating System Choke Area Calculator
The gating system controls how quickly and how calmly metal enters the mold. This calculator sizes the choke from the Bernoulli equation and then scales the runner and ingate areas by the gating ratio you choose.
Your numbers
Casting plus gating plus risers filled through the system.
Approximate values: liquid steel about 6.9 to 7.0, gray iron about 6.9 to 7.0, aluminum about 2.4.
Typical 0.7 to 0.9 depending on sprue and choke shape.
Height from the pour basin level to the choke.
Pressurized systems are about 0.75; unpressurized systems often use 2 or more.
Pressurized about 0.5; unpressurized about 1 or more.
Your results
Uses the classic Bernoulli choke equation. It ignores friction losses, air entrainment limits and metal-specific maximum velocity limits; verify with simulation for critical castings.
Choke area vs pour time
Sweeps Target pour time from 5 to 60, holding your other inputs fixed.
Tie pour parameters to scrap data
Optional: a short review of how pour records and defect codes in your ERP could be analyzed together.
Optional. The tool and every export work without this. No spam.
How to use the calculator
Enter the total poured weight including gating and risers, the metal density, a pour time from your foundry practice, and the effective head. Then set the runner and ingate ratios. The tool shows the choke area, an equivalent round diameter, and the areas for the runner and the combined ingates.
- Divide the total ingate area by the number of ingates.
- Measure head from the basin liquid level, not the top of the flask.
- Round the choke to a standard sprue or ceramic filter size.
The method behind it
Flow through the choke is volume per second equals area times velocity. Velocity is the discharge coefficient times the square root of 2 g H, with g = 981 cm/s2. Rearranged, area = volume / (time x velocity). A ratio such as 1 : 2 : 1 (sprue : runner : gate) is unpressurized and slows metal in the runner to reduce turbulence; 1 : 0.75 : 0.5 is pressurized and fills faster but with more velocity at the gates.
Reading the result and common mistakes
The chart shows that doubling the pour time roughly halves the required choke area. Too short a pour time can cause erosion and cold shuts if it forces unrealistic velocity, while too long a pour time can let thin sections freeze before they fill. Stay within the pour time window your alloy and section thickness allow.
- Using casting weight alone and forgetting riser and gating weight.
- Mixing cm and mm units in the head.
- Applying a steel ratio to aluminum, which is sensitive to oxide entrainment.
- Not accounting for filters, which add flow resistance.
Frequently Asked Questions
What is the choke in a gating system?
The choke is the smallest cross-section in the gating system and controls the flow rate. In many designs it sits at the bottom of the sprue or at a filter. Sizing it correctly sets the pour time and prevents the mold from filling too fast or too slowly.
What is pressurized vs unpressurized gating?
In a pressurized system the choke is at the ingates, so the runner stays full and flow is fast. In an unpressurized system the choke is at the sprue base and runner and gate areas are larger, which slows metal and reduces turbulence, common for aluminum and some irons.
Why use 981 in the formula?
Gravity is 981 cm per second squared. Using centimeters for head and area gives velocity in cm per second and area in cm2, so the volume in cm3 divided by time matches. If you work in meters, convert consistently.
Can I use this for aluminum?
The equation applies to any liquid metal, but aluminum needs low velocity, often under about 0.5 m per second in the runner, to avoid oxide films. Use an unpressurized ratio and check velocities at the ingates, then confirm with simulation or a trial.
If gating practice and scrap causes sit in different systems, we can discuss connecting them for your engineers. This is optional and the calculator works on its own.
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