Last updated: 2026-09-02 · 5 min read

What Is Sequential Valve Gating — and How Does It Move Weld Lines?

Sequential valve gating is a hot-runner control technique where electrically actuated valve pins open and close in a programmed order, so the melt front sweeps the part as one advancing wave instead of several fronts converging — and the weld line moves from wherever the flows happened to meet to wherever the opening sequence deliberately puts it. The weld line rarely disappears; it gets placed on purpose, in a non-critical location where it cannot fail a weld-strength test or show on a visible face.

Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. See also related guide.

Key Facts

  • Every simultaneous-fill multi-gate part has weld lines where the fronts meet: cold, partially relaxed, often trapping air, and measurably weaker than the surrounding material. Their location is fixed by geometry unless you control the opening order.
  • A valve gate is a needle that physically seals the gate; sequencing means opening gate two only after gate one has laid down a flow front. The second front rides over established material instead of colliding head-on, and the weak convergence point moves downstream — usually to a chosen, harmless spot.
  • The payoff shows in three places: weld lines relocated off visible faces (gloss and texture stay uniform), off structural load paths (strength tests stop failing at the same line), and off vent-starved corners (gas traps shrink because the front sweeps rather than pinches).

    • Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. The related guide covers adjacent topics.

    How Sequential Opening Changes the Fill Story

    Fill a long, flat part through three gates at once and three fronts race toward each other; where any two meet, the leading skins fuse cold and the air between them must escape or it traps. Fill the same part by opening the gate nearest the sprue first, then the middle gate once the front passes it, then the last — and the part fills as a single traveling wave.
    There is still a convergence somewhere when the wave closes, but the programmer decides where: typically at a non-visual edge near a vent, or over a rib where strength hardly matters.

    The same logic fixes a different problem: gates that drool or string between shots. Valve pins shut mechanically rather than by gate freeze, so cycle-controlled closure also cleans up gate vestige — a cosmetic benefit that compounds the weld-line control. On large parts, sequential opening also reduces the peak injection pressure, because each front moves against less resistance at its start.

    When It Earns Its Place

  • Visible Class-A surfaces where a weld line would force rework or rejected parts — automotive trim panels, appliance faces, large housings with long flow lengths.
  • Parts with a weld-line-driven structural failure: a load path crossing the old convergence point, where tests failed at the same line every time.
  • Long, thin parts with multiple gates where simultaneous fill creates an unpredictable meeting pattern that shifts with resin lot viscosity.
  • Gas-trap and burn-mark problems at front-convergence points that venting alone could not solve.
  • Family or multi-cavity tools where per-gate open timing doubles as flow-balance control, replacing trial-and-error gate sizing.

    • What the Control Looks Like in Practice

    1

    Map the fill with moldflow first

    The sequence is designed before the steel runs: simulate the wave with gate one alone, place the intermediate openings where the front passes each gate, and choose the closure point near a vent at the least critical feature. The simulation predicts where the residual convergence lands; the trial confirms it.

    2

    Program open times against fill, not the clock

    Open triggers are referenced to screw position or fill percentage, so the sequence tracks the front even when viscosity changes. Pure time-based sequencing works until the first resin lot shift — then the wave and the openings drift apart.

    3

    Tune the overlaps and verify on parts

    Short-shot studies at each sequence stage show exactly where the front is when each gate opens. The production check is visual and mechanical: weld-line location matches the plan, and strength at the new convergence point passes the part’s own specification.

    Frequently Asked Questions

    Does sequential gating remove weld lines entirely?

    No — a wave closing around a perimeter still converges somewhere. What changes is that the location is chosen: placed near a vent, off the visible face, and off the load path. Designed-in convergence at a harmless spot is a feature; accidental convergence at a critical one is a defect.

    What does the tooling cost difference look like?

    Valve-gated hot runners cost more than open thermal gates: needles, actuators, and a sequencing controller. The honest comparison is against the alternative costs — texture rework on visible parts, weld-line strength failures, or a design forced to single-gate with a longer, harder flow. On the right part class, the sequence control pays back in scrap and rework avoidance.

    Can existing hot-runner molds be retrofitted?

    Often, yes: manifolds designed with valve-gate ready nozzles can take actuators and a controller later, provided the plates have the space and the mold document anticipated it. Retrofitting open nozzles to valve gates is a larger surgery — nozzle and gate-steel changes — and deserves a fresh fill study rather than an assumption.

    Key Takeaways

    1. Sequential valve gating turns weld lines from accidents of geometry into decisions of the programmer: the front sweeps as one wave and converges where chosen.
    2. It earns its cost on visible faces, weld-line-driven structural failures, long multi-gate flows, and stubborn gas traps.
    3. Sequence triggers referenced to fill — not just time — keep the wave and the openings aligned across resin lot changes.

    Bring the part, its visible faces and its load paths to the DFM, and aCatMould will run the fill simulation that shows where a conventional multi-gate weld line lands — and what a sequenced valve-gate layout would move it to — so the decision about needles, actuators and control is made on the predicted weld-line map, not after the first strength test fails.

    Sources
    Husky Technologies — valve gate and hot runner systems
    Autodesk Moldflow — sequential gating simulation guidance
    MoldMaking Technology — hot runner sequencing practice

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