BIZOMAT MAKEOS · Fabrication
Casting Gating System Choke Area
What choke (sprue) area fills this mould in the planned pouring time?
Velocity of metal at the base of the sprue from the effective head, the choke area needed to pour a casting in a set time, the equivalent round sprue diameter and gate areas for a chosen gating ratio.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Poured mass (casting plus runners) | kg | Mass of metal poured, including runners and risers. |
| Liquid metal density | kg/m³ | Density of the liquid metal (grey iron about 7000, steel about 7000, aluminium about 2400 kg/m³). |
| Effective metallostatic head | mm | Effective head at the choke (for top gating, the sprue height; reduce it for bottom gating as the mould fills). |
| Pouring time | s | Target pouring time from foundry practice for the casting weight and section (often found from an empirical rule). |
| Flow efficiency (discharge coefficient) | Flow efficiency of the gating system (about 0.7–0.9). | |
| Gating ratio: runner area ÷ choke | Runner area relative to the choke (e.g. 1 : 2 : 2 unpressurised gives 2). | |
| Gating ratio: total gate area ÷ choke | Total gate area relative to the choke. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Poured mass (casting plus runners)
- What it is
- The total mass of liquid metal that flows through the choke during pouring: the casting plus runners, gates and any risers filled through the same system.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 1000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The choke area rises in direct proportion to the poured mass.
- Get it right
- Include the gating and risers, not just the finished casting weight.
What this means: Liquid metal density
- What it is
- The density of the metal in its liquid state at pouring temperature, from alloy data. Liquid metal is less dense than the same alloy when solid.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 25000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7000 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The choke area is inversely proportional to density.
- Get it right
- Use the liquid density, not the room-temperature solid density.
What this means: Effective metallostatic head
- What it is
- The effective height of liquid metal above the choke that drives the flow. For top gating it is the sprue height; for bottom gating it is reduced to allow for the mould filling up against the flow.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The choke area falls with the square root of the head.
- Get it right
- Measure from the pouring basin level down to the choke, not to the bottom of the casting.
What this means: Pouring time
- What it is
- The time allowed to fill the mould, from foundry practice or an empirical rule for the casting weight and section thickness.
- Units and format
- Enter a number in s: seconds. The form accepts 0.000001 s to 10000 s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12 s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The choke area is inversely proportional to the pouring time.
- Get it right
- Enter seconds; a shorter time needs a larger choke and gives faster, more turbulent flow.
What this means: Flow efficiency (discharge coefficient)
- What it is
- The discharge coefficient of the gating system: the share of the ideal free-fall flow it actually delivers once friction and turns are allowed for.
- Units and format
- Enter a plain number with no unit. The form accepts 0.1 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.8 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The choke area is inversely proportional to this coefficient.
- Get it right
- Type it as a fraction, not a percentage. More bends and steps in the system lower it.
What this means: Gating ratio: runner area ÷ choke
- What it is
- The total runner cross-section area divided by the choke (sprue base) area, the middle number of the gating ratio.
- Units and format
- Enter a plain number with no unit. The form accepts 0.5 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The runner area is the choke area multiplied by this ratio.
- Get it right
- Ratios above one slow the metal down (unpressurised); below one keep the system pressurised.
What this means: Gating ratio: total gate area ÷ choke
- What it is
- The combined cross-section area of all ingates divided by the choke area, the last number of the gating ratio.
- Units and format
- Enter a plain number with no unit. The form accepts 0.5 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Total gate area is the choke area times this ratio; a larger ratio lowers the gate velocity.
- Get it right
- Add up the areas of every ingate, not just one.
Method and formulas
- Velocity v = √(2 g h).
- Choke area A = W ÷ (ρ t C_d v).
- Runner and gate areas = choke area × gating ratios; round sprue diameter = √(4A/π).
- Example: 40 kg poured in 12 s from 200 mm head needs a choke of 300.54 mm² (19.562 mm diameter).
Limitations
- Simple Bernoulli model; friction, bends and turbulence are lumped into the flow efficiency.
- Pouring time rules and gating ratios depend on the alloy and casting; use foundry practice.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size sprues and gates for a new pattern
- Check pouring time for thin-walled castings
- Compare pressurised and unpressurised gating
Who it is for
- Foundry engineers
- Pattern makers
What you get
- Metal velocity at the choke
- Choke area and equivalent sprue diameter
- Runner and total gate areas
Interpreting the result
- Unpressurised systems (gates larger than the choke) give calmer filling for aluminium; pressurised systems suit iron.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1404–1420. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Gating: filling the mould
In Metal Casting by the Numbers
What it covers: Sizing a gating system so the mould fills in time without turbulence: metal velocity from the head, the choke area for a pouring time, gating ratios, gate velocity, and oxide films from fast metal.
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