Where Do Weld Lines Form — and How Do You Move Them Away from High-Stress Areas?

Last updated: 2026-09-03 · Reading time: about 5 min

A weld line forms wherever two melt fronts meet — past a hole, between two gates, or at a thickness change. Its location is predictable before steel is cut, and its strength runs 50–80% of the surrounding material. You rarely eliminate a weld line; you move it off the load path and prove the result on molded parts.
See also related article.

Key facts

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  • Weld lines land in three predictable places: downstream of a hole or boss, between two gates, and at a thin-to-thick transition.
  • Mold-flow analysis draws every seam — and the meeting angle — before the mold is cut; fronts meeting above 135° bond stronger than fronts meeting below 120°.
  • Strength at the seam runs 50–80% of bulk. Hotter fronts, a warmer mold and faster fill raise it; overflow wells carry the weak, fiber-rich melt away.

    • Runner system — The channel network that delivers melt from the machine nozzle to each cavity. Cold runner: solidifies between shots, produces runner waste. Hot runner: stays molten, eliminates runner waste but adds $15k–$40k to mold cost. See also related guide.

    Where do weld lines form — and how do you predict the location?

    Weld lines are geometry, not luck. Once the gate positions and the part shape are fixed, the meeting lines are fixed too — which means they can be drawn before any steel is cut. Three patterns cover almost every case.

    1. Downstream of a hole or boss. Melt splits around the obstruction and rejoins on the far side, leaving a seam 1–3 mm past the edge. A row of holes can merge these into one continuous weak line.
    2. Between two gates. Fronts from each gate meet where their pressures balance — at the midpoint for opposite gates, along a diagonal for corner gates.
    3. At a thickness change. The thin section fills faster, so its front arrives first; the seam sits at the thin-to-thick junction.

    Mold-flow analysis turns these rules into a picture. It traces every front, marks each meeting line, and reports the angle at which the fronts collide.

    That angle matters: fronts meeting above 135° entangle well and approach bulk strength, while a meeting angle below 120° leaves a shallow, weak seam. Because the study runs before the mold is cut, you can shift a gate and watch the seam move off the load path on screen — not on a rejected part.

    What reduces weld-line strength — and how do you recover it?

    The seam is weak for two reasons at once. The molecular chains never fully re-entangle across a cooling interface, and the meeting line leaves a shallow V-notch that concentrates stress on that same plane.

    Both effects scale with how cold the fronts were when they met, so most of the recovery is a temperature and speed story rather than a redesign.

    Factor Effect on the seam How to recover strength
    Low melt temperature Fronts too cold to entangle Raise melt 10–20°C within the resin window
    Low mold temperature Melt freezes at the interface Raise mold to 80–100°C for ABS and PC
    Slow injection Fronts meet at low pressure Increase speed to raise bonding pressure at the seam
    Meeting angle below 120° Minimal contact area across the seam Move the gate to push the angle above 135°
    Glass-filled resin Fibers align with flow and never bridge the seam Add overflow wells to carry the fiber-rich melt away

    Of these, mold temperature is the single strongest lever. A warmer mold keeps the interface fluid longer so chains can cross it, and for semi-crystalline resins such as PP, PA and POM it also drives crystallization that bridges the seam.

    Moving a mold from 40°C to 80°C typically lifts weld strength from about 60% to 70–80% of bulk — a process change, not a tool change.

    What design changes move a weld line off a high-stress area?

    Process tuning strengthens a seam; only design relocates it. When a seam must exist, the durable fix is geometric — decide where it lands, and make sure that place carries neither the peak load nor the eye.

  • Shift the gate. Moving a gate relocates every downstream meeting line — the simplest way to walk a seam off a load path before steel is cut.
  • Move the hole or boss. An obstruction that feeds two fronts can often be repositioned so the seam lands on a non-structural face.
  • Run the seam parallel to the load. A seam crossing the load direction pulls apart; one running with it is far less stressed.
  • Add a radius or a local wall increase. Where the seam must stay, thickening or radiusing moves the peak stress off the notch.
  • Use overflow wells on critical seams. A machined pocket past the seam collects the coldest, fiber-rich melt and is trimmed after ejection.

    • An overflow well is a trap, not a strengthener. It does not make the seam itself stronger — it relocates the weak, fiber-concentrated melt into a pocket that is cut away, so the seam left in the part is made of fresher, hotter material. It adds roughly $500–2,000 to the mold plus a trimming step, so it is reserved for structural seams that testing says are marginal.

    “A weld line is not a defect to hide — it is a load path to plan. We draw every seam in the flow study, hand the map to the engineer who owns the tool, and relocate the ones that cross a load before we quote. If a seam has to stay, we tell you where it sits and what it is worth.”

    — Project Engineer, aCatMould

    Frequently asked questions

    Can weld lines be eliminated entirely?

    Only on a simple part with one gate and no obstructions — a center-gated disc, for example. Any hole, boss, second gate or wall change creates a meeting line. On real parts the goal is to manage and relocate seams, not to chase a weld-line-free part that geometry will not allow.

    How is weld-line strength measured?

    Mold a tensile bar with a deliberate seam and pull it to ASTM D638 or ISO 527, then compare it with a bar that has no seam; the ratio is the weld strength percentage. As a guide, non-structural housings want above 60% of bulk, semi-structural brackets above 75%, and load-bearing parts above 85%.

    Why does our part keep breaking at the same seam?

    Because the seam is geometry — it sits in the same place on every shot until the gate or the shape changes. Process tuning can raise front temperature and help, but a seam crossing the load path at a thin wall is a drawing problem, not a molding one. Relocate it in design and the repeat break stops.

    Key takeaways

    1. Weld lines land downstream of holes, between gates and at thickness changes — all predictable with mold-flow analysis before steel is cut.
    2. A meeting angle above 135° bonds strong; below 120° is weak. Mold temperature is the strongest process lever, lifting strength from about 60% to 70–80% of bulk.
    3. Only design relocates a seam: shift the gate or hole, run the seam parallel to the load, radius the notch, and reserve overflow wells for marginal structural seams.

    If your part carries holes, bosses or multiple gates, it carries seams. Send the drawing and the load directions and you will get a seam map, the relocation options and the strength expectation, reviewed by a named tooling engineer, before any steel is ordered.

    Sources
    Autodesk Moldflow — injection molding simulation, reference for weld-line location and meeting-angle prediction
    ASTM D638 — Standard Test Method for Tensile Properties of Plastics, ASTM International
    Plastics Technology — melt and mold temperature effects on weld-line strength
    MatWeb — material property database, reference for glass-filled resin behavior at the seam
    Beaumont Technologies — mold filling analysis and flow-front balance

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