Mold Flow Balancing for Multi-Cavity Molds — Why Do Some Cavities Fill Before Others, and How Do You Make Every Cavity Produce the Same Part?

Last updated: 2026-09-03 · Reading time: about 6 min

Flow balancing makes every runner branch deliver melt at the same pressure drop — not merely the same length. Three levers get you there: geometric (a symmetric layout), rheological (runner-diameter and gate-size compensation), and active (sequential valve gating). Which lever you for depends on the cavity count and the part size.
See also related article.

Key facts

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  • The real target is equal pressure drop at every gate, not equal runner length. A geometrically symmetric H-pattern still runs unevenly, because each split sees a different shear rate and melt temperature — so length is the starting point, not the finish line.
  • Cavity count sets the strategy: 2 / 4 / 8 / 16 cavities (powers of two) balance by geometry; 3 / 5 / 6 / 12 need rheological compensation; very high counts or large parts need sequential valve gating.
  • Three design levers, in rising order of cost: runner length (layout), runner diameter and gate size (ΔP ∝ 1/D⁴ — doubling a diameter cuts its pressure drop 16×), and valve-gate opening sequence (active timing control).

    • Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. See also related guide.

    How do you actively balance a runner — and what are the three design levers?

    Balance is designed, not discovered at the press. Three levers act on the same goal — equal pressure drop at each gate — but they work at different stages and different costs.

    1. Geometric — make the flow paths equal. Lay the runner out symmetrically so the distance from sprue to every gate is identical. This is free at the design stage and needs no simulation, but it only closes cleanly for powers of two (2 / 4 / 8 / 16), where branches split evenly. Odd counts force one branch to be longer.
    2. Rheological — compensate with diameter and gate. When the paths cannot be equal, balance the pressure drop instead. Widen the runner to the far cavity — pressure drop scales as 1/D⁴, so a modest diameter increase recovers a large pressure loss — and trim the gate at the near cavity so it does not over-fill. This needs a flow simulation to size correctly.
    3. Active — sequence the valve gates. On a hot runner, open each valve gate on a timed sequence rather than all at once. You control fill order and timing directly, which is the only way to balance a very large part across several gates or a mold at 32–128 cavities without a huge simultaneous clamp-force peak.
    Lever What it changes Best for Cost / complexity
    Geometric Runner length to each gate 2 / 4 / 8 / 16 cavities, symmetric layout Lowest — set at layout, no simulation
    Rheological Runner diameter and gate size Odd counts, fishbone layouts, unequal paths Moderate — needs flow analysis to size
    Active (valve gate) Gate opening sequence and timing Large parts, 32–128 cavities, weld-line control Highest — hot runner, actuators, controller

    Which balancing strategy fits your cavity count?

    The cavity count and the part size decide which lever is even available. Match the strategy to the layout before you commit to steel.

    Cavity count / part Strategy Why
    2 / 4 / 8 / 16 (power of two) Geometric natural balance Branches split symmetrically; equal length gives near-equal pressure drop
    3 / 5 / 6 / 12 (not a power of two) Rheological artificial balance One branch is always longer; compensate with diameter and gate size
    Fishbone / non-symmetric layout Rheological, simulation-led Path lengths differ by design; only pressure-drop matching balances them
    32–128 cavities or a large part Sequential valve gating Simultaneous fill needs excessive clamp force; timed fill lowers the peak

    Rheologically balanced. A runner is rheologically balanced when the pressure drop from sprue to each gate is equal — not when the runner lengths are equal. Because pressure drop scales as 1/D⁴, diameter is the strongest correction: doubling a branch diameter cuts its pressure drop by 16×. Length symmetry is the simple first approximation; pressure-drop matching is the real balance.

    “A drawing can show a perfectly symmetric runner and still mold eight cavities that weigh three percent apart. Symmetry gets the lengths equal, not the pressure drops — the shear at each split and the melt temperature do the rest. So we design to pressure drop, correct with diameter and gate, and reserve valve-gate sequencing for the layouts geometry cannot .”

    — Project Engineer, aCatMould

    Frequently asked questions

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • What is the difference between gate balancing and runner balancing?

    Runner balancing equalizes the path from the sprue to each gate — the length and diameter of the branches.
    It is the coarse adjustment, done in the layout, and it gets the melt to every gate at roughly the same pressure.

    Gate balancing is the fine adjustment at the gate itself: sizing each gate land so the cavities fill and pack alike. Because a gate is a small replaceable insert, it is the last and least costly lever to tune — you correct residual imbalance by opening a starved gate or restricting a heavy one, without touching the runner.

    How does sequential valve gating lower the clamp force a mold needs?

    When every cavity or every gate on a large part fills at once, the projected area under pressure peaks in a single instant, and the machine must hold that whole peak. On a 32–128 cavity mold this can push the tonnage past what is available.

    Sequential gating opens the valves one zone at a time, so only part of the projected area is under peak pressure at any moment. The clamp-force requirement drops to the largest single zone rather than the sum of all of them — and the timed fill also moves weld lines off cosmetic faces.

    How do you size a runner diameter to correct a pressure drop?

    Work from the 1/D⁴ relationship: pressure drop falls with the fourth power of diameter, so small diameter changes have large effects. As a starting range, 1.5–3.0 mm for parts under 10 g, 3.0–6.0 mm for 10–100 g, and 6.0–12.0 mm above 100 g.

    Keep the runner weight under about 30% of the total shot, or the material you are balancing with becomes the biggest cost in the shot. A flow simulation sizes the final diameter by trading pressure drop against runner weight.

    Key takeaways

    1. Balance to equal pressure drop at each gate, not equal runner length — a symmetric H-pattern can still run unevenly from shear and temperature at the splits.
    2. Let the cavity count pick the lever: powers of two balance by geometry, odd counts need rheological compensation, large parts and 32–128 cavities need sequential valve gating.
    3. Tune in rising cost order — layout, then runner diameter (1/D⁴), then gate size, then valve sequence — so the cheapest lever that fixes the imbalance is the one you use.

    Every cavity producing the same part is a design outcome, decided before the steel is cut. Set the layout to the cavity count, correct the pressure drop with diameter and gate, and reserve active sequencing for the molds geometry cannot — then the weight, dimensions and quality hold cavity to cavity.

    Sources
    Autodesk Moldflow — runner balancing and pressure-drop simulation
    Synventive — sequential valve-gate hot runner control
    Husky Technologies — hot runner systems
    Plastics Technology — runner and gate balancing practice
    Society of Plastics Engineers — injection molding rheology

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet