Casting Filling Analysis
Theory: gating systems and free-surface flow
Overview
If I'm running filling analysis, do I still need to hand-calculate the gating?
It's most efficient to draft the gating by hand and then analyse. Much of a gating system (sprue, runners and ingates) follows from Bernoulli's equation and continuity: flow rate from casting volume and fill time, velocity from the head, and the narrowest section (the choke) limits the flow. An unpressurised system puts the choke at the sprue base and widens runners and ingates to slow the metal, suiting easily oxidised metals such as aluminium. Filling analysis then checks how metal actually flows in that draft — uneven flow between ingates, jetting, trapped air, and misruns from cooling — and guides detailed fixes. Drawing a shape without a draft and running analysis leaves no direction for correction and prolongs trial and error.
Basic gating formulas
C_d is the discharge coefficient (losses from friction and bends), h the head, Q flow rate, v_max the maximum ingate velocity allowed, and h_top, h_base the heads to the sprue top and base.
The VOF method and casting simulation
The VOF method for free-surface flow was published in 1981 by Hirt and Nichols at Los Alamos National Laboratory. It tracks the fraction of each cell's volume occupied by liquid, handling interfaces that deform strongly, break up and merge. Casting filling is exactly that kind of flow, and from the late 1980s casting simulation software built on VOF came into practical use one after another.
Worked examples
This initial design assumes constant head and discharge coefficient 0.8. The 0.5 m/s guide concerns average gate speed, not a guarantee for transient gate velocity or entrainment. Three cells through a wall are only a starting resolution; mesh-sensitivity checks remain necessary.
Example: gating for a 5 kg aluminium casting (10 s fill, 250 mm head to sprue base)
| Item | Value |
|---|---|
| Casting volume (liquid density 2,385 kg/m³) | 2.10 L |
| Flow rate | 0.21 L/s |
| Sprue-base velocity (ideal / Cd 0.8) | 2.21 / 1.77 m/s |
| Choke area (sprue base) | 118 mm² (12.3 mm diameter) |
| Total ingate area (≤ 0.5 m/s) | 419 mm² |
| Choke : ingate | 1 : 3.5 |
| Sprue top area (50 mm head) | 265 mm² (2.24 × base) |
The 0.5 m/s ingate guide is for avoiding oxide-film entrainment in aluminium alloys. The discharge coefficient varies with runner bends and length, so check actual ingate velocities in the filling analysis.
I made the total ingate area 419 mm², but filling analysis shows 1.2 m/s jetting from the nearest ingate. Why?
Because the flow isn't dividing evenly between ingates. Metal in the runner carries momentum and tends to pass the nearest ingates, concentrating at the far ones; conversely, metal piling at the runner end can rebound and burst out of a near ingate. Even with enough total area, flow concentrated in one ingate exceeds the target velocity there. Remedies include extending the runner end to catch the first dirty metal and its momentum, sizing ingates differently to balance flow, and stepping down the runner to keep pressure ahead of each ingate. In filling analysis, monitor flow and velocity at each ingate over time to see which ingate works when.
Practical workflow
- Size choke, runners and ingates by hand from casting volume, fill time and head.
- Set up VOF filling with at least 3 cells across thin walls (≤ 1 mm for a 3 mm wall).
- Check flow and velocity at each ingate, free-surface velocity and trapped air.
- Include cooling and flow resistance to assess misruns and cold shuts.
- Pass filling results as initial conditions to solidification analysis.
“Flow seen through a glass-fronted mould”
Filling analyses are often validated with glass-fronted moulds or real-time X-ray of pouring. In reality metal splashes far more violently than one imagines, often breaking into thin sprays. Placing simulation and experiment frames side by side at the same times shows at a glance whether discharge coefficients, surface tension and mould venting are sensible, greatly increasing confidence in the analysis.
Common mistakes
Mistakes and fixes
| Mistake | Effect | Fix |
|---|---|---|
| Sizing ingates by total area only | Flow concentrates in one ingate | Check flow per ingate |
| Straight sprue | Air aspiration | Taper toward the base |
| Coarse cells in thin walls | Misjudged misruns | ≥ 3 cells across walls |
| Ignoring mould venting | Back-pressure delays filling | Model vents and gas escape |
| Not solving temperature | Misruns missed | Couple heat and flow |
I'd like to learn related topics.
Related pages include casting solidification analysis, casting defect prediction, investment casting simulation and casting residual stress analysis.
Frequently asked questions
What does casting filling analysis compute?
How molten metal flows through sprue, runners and ingates into the mould and displaces the air, as a transient free-surface flow. The VOF method usually tracks the metal-air interface, solving temperature and the increased resistance to flow as metal starts to freeze. It predicts misruns, entrainment of air and oxide films, and cold shuts (where cooled streams fail to fuse).
How are sprue and ingate sizes chosen?
From flow rate and velocity. Filling a 5 kg aluminium casting in 10 s needs 0.21 L/s; with 2.2 m/s at the sprue base from a 250 mm head (1.77 m/s with a 0.8 discharge coefficient), the narrowest choke is 118 mm². Slowing to 0.5 m/s at the ingates needs 419 mm², an unpressurised system with choke:ingate about 1:3.5.
Why is the sprue tapered toward the bottom?
Falling metal speeds up and its stream narrows. In a straight sprue the metal separates from the wall, creating suction that draws in air. With 50 mm from basin to sprue top and 250 mm to the base, making the top √(250/50) ≈ 2.24 times the base area keeps the metal against the wall.
References and scope of verification
Numbers are educational reference calculations under the stated assumptions. Sources document theory and setup, not proof that these numerical values were reproduced in each package. Engineering decisions require validation for the actual conditions.
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