How Weak Is a Weld Line — and How Do You Design Around It?

Last updated: 2026-09-01 · Reading time: about 4 min

Where two flow fronts meet in a cavity, they form a seam — and a seam made of material that stopped moving is weaker than the material around it.
The strength loss is set by the temperature of the fronts when they meet and by the resin itself, filled grades losing more than unfilled ones; the design countermeasures are to move the seam away from load paths, soften its stress concentration, and prove the result on real parts. The seam cannot always be eliminated — it can almost always be relocated, and where it lands is a drawing decision.

Key facts

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  • Weld-line strength is a temperature story: fronts that meet hot re-entangle their molecules across the seam; fronts that meet half-frozen leave a boundary the load can pull apart.
  • Filled resins lose more: fibers orient along each flow front and cannot cross the meeting line, so glass- and mineral-filled grades pay a larger penalty at the seam than neat resins.
  • Location is designed, not fated: hole positions, gate placement and flow paths decide where fronts meet — moving any one of them moves the seam onto a face where its weakness does not matter.

    • Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. See also related guide.

    Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. See also related guide.

    Why is the seam weaker than the surrounding material?

    Two mechanisms stack. The molecular one: as the front travels, its surface layer cools and stiffens; when two fronts meet, those stiffened skins touch but their molecules do not intertwine the way they do inside a continuous flow — the interface is a plane of incomplete entanglement, and strength across it drops accordingly.
    The geometric one: the meeting line leaves a shallow V-notch on the surface, a built-in stress concentrator that sits exactly on the weak plane, so the seam loses strength twice — once in the material, once in the shape. Both effects scale with how cold the fronts were when they met, which is why the same part can show a negligible seam in summer production and a fragile one after a machine change.
    The companion page catalogs where seams come from; this page is about living with the one you cannot remove. Filled grades sharpen the problem — as the glass-filled page shows, fibers align with flow and stop dead at the seam line, so the reinforcement itself does not bridge it.

    How do you design around the weak seam?

    First, predict where it lands: the flow study draws the meeting lines before steel is cut, and the gate layout is where you negotiate them.
    Second, move the seam off the load path: a hole feeding two fronts can shift, a gate can relocate, a rib can split one front into a less critical meeting — the goal is a seam sitting in a lightly loaded, non-cosmetic region, oriented parallel to the load rather than across it.
    Third, soften the stress concentration where the seam must stay: a generous radius or a local wall increase moves the highest stress away from the notch, so the weak plane no longer carries the peak load — the same logic the rib rules apply to junctions.
    What design cannot fix, the process window can reduce — hotter fronts, faster fill — but the fix that survives product life is geometric, checked on parts from the first trial, not assumed from simulation alone.

    Which weld-line checks belong in the drawing review?

  • Predicted seam positions marked — from the flow study or experience, shown on the drawing so everyone sees where they land.
  • Seams compared against load paths — no predicted seam crossing a primary load direction at a thin section.
  • Seams compared against the cosmetic map — front-running seams on A-faces declared, texture or position adjusted where needed.
  • Structural compensation at unavoidable seams — local thickening or radiusing so the notch is not the peak-stress point.
  • Filled-grade penalty acknowledged — filled resins get more seam margin, not less, in the design review.
  • Strength proven on molded parts — functional testing on trial shots at the seams, before the design is accepted.

    • Frequently asked questions

    How much strength does a weld line actually lose?

    Enough to matter and too variable to quote honestly from a table — the loss depends on the front temperature at meeting, the resin, the fillers and the notch geometry, which is why the credible number comes from testing your part, not a brochure.
    What can be said in advance: unfilled resins at hot fronts lose little; filled resins at cold fronts lose the most. Design to the worst case, verify on parts.

    Why did our part always break at the same seam?

    Because the seam is geometry — it will sit in the same place on every shot until the geometry or the gate changes. Process tuning can raise the front temperature and strengthen the bond, and that sometimes solves the break; but if the seam crosses the part’s load path at a thin wall, the durable fix is relocating it in design. Repeated breaks at one line are a drawing conversation, not a molding one.

    Can a weld line be hidden on a cosmetic face?

    Often, yes. Hotter fronts and a glossy tool make the seam nearly invisible on unfilled resins; textures scatter the witness line well; and moving the seam to an edge or a transition where the eye expects a line hides what temperature cannot. The order matters: position first, process second, texture last — reversing that order buys a part that looks right only on the day it was tuned.

    Key takeaways

    1. The seam is weak twice — incomplete molecular re-entanglement plus a surface notch on the same line.
    2. Design moves the seam: hole and gate placement decide where fronts meet; radii and local wall shifts defuse the ones that must stay.
    3. Filled grades lose more at seams and get more design margin; strength is proven on molded parts, not simulation screenshots.

    If your part carries holes, bosses or multiple gates, it carries seams — send the drawing and the load directions. You will get the seam map, the relocation options and the structural compensation, reviewed by a named tooling engineer, before any steel is ordered.

    Sources
    Autodesk Moldflow — injection molding simulation, reference for weld-line prediction
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions, International Organization for Standardization
    MatWeb — material property database, reference for filled-resin properties

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