Last updated: 2026-09-02 · 5 min read

Weld Lines in Injection Molding — What Causes Them and How Do You Eliminate or Minimize Them?

Weld lines form where two melt fronts meet and fail to bond completely — caused by multiple gates, obstacles (cores, bosses, thickness changes) or fountain flow convergence — and they reduce part strength by 10–30% at the weld while creating a visible cosmetic defect. Elimination or minimization requires three actions: repositioning the gate to move the weld line to a non-critical area, increasing melt temperature and injection speed to improve bond strength, and adding overflow wells to capture the cold material at the convergence point.

Warpage — Part deformation after ejection caused by uneven shrinkage. Amorphous resins warp less (0.4–0.7% shrinkage). Semi-crystalline resins warp more (1.0–2.5% shrinkage). Fix: uniform wall thickness, balanced cooling, optimized gate location, adequate packing pressure. See also related guide.

Key Facts

  • Weld line strength depends on melt temperature, injection speed and material — at optimal processing, weld strength reaches 85–95% of base material strength; at poor processing, it drops to 50–70%.
  • Weld lines are unavoidable in parts with multiple gates, holes or bosses — the goal is to position them in non-structural, non-cosmetic areas through gate location optimization in mold flow analysis.
  • Overflow wells (also called weld line wells or overflow pockets) capture the cold, poorly bonded material at the convergence point — the weld line moves into the well, and the part body has full-strength material.

    • Warpage — Part deformation after ejection caused by uneven shrinkage. Amorphous resins warp less (0.4–0.7% shrinkage). Semi-crystalline resins warp more (1.0–2.5% shrinkage). Fix: uniform wall thickness, balanced cooling, optimized gate location, adequate packing pressure. The related guide covers adjacent topics.

    Weld Line Causes and Solutions

    Cause Solution Cost Impact
    Multiple gates Reposition gates; use sequential valve gating to eliminate convergence Gate relocation $500–$3,000; sequential valve gating +$5,000–$15,000
    Cores and bosses Add overflow wells downstream of the obstacle Overflow wells add 5–10% material; minimal mold cost
    Low melt temperature Increase barrel and nozzle temperature within resin spec No tooling cost; may increase cycle time 2–5%
    Slow injection speed Increase injection speed to maintain melt temperature at convergence No tooling cost; may require larger gate to reduce shear
    Thickness transition Redesign part to uniform wall or add flow leaders Part redesign (if possible); flow leaders add material

    How to Strengthen a Weld Line

  • Increase melt temperature: raising the barrel temperature by 10–20°C improves molecular diffusion across the weld interface, increasing bond strength by 5–15%. Stay within the resin manufacturer’s recommended range to avoid degradation.
  • Increase injection speed: faster fill maintains melt temperature at the convergence point — the melt does not cool as much before the weld forms. This is the most effective processing adjustment for weld line strength.
  • Add glass fiber reinforcement: glass fiber-filled resins show more pronounced weld line weakness (fibers do not cross the weld, creating a resin-rich zone). Increasing fiber length or using a coupling agent improves weld strength in fiber-filled grades.

    • What This Means for Your Quotation

    The DFM review should identify weld line locations from mold flow analysis before the mold is cut — showing whether the weld falls in a structural area, a cosmetic surface or a non-critical zone. When weld lines cannot be eliminated, the quotation should include overflow wells or sequential valve gating as options with cost impact documented. The T1 trial then verifies weld line strength by testing parts at the weld location.

    Frequently Asked Questions

    Can weld lines be made invisible?

    On textured surfaces, weld lines are often hidden by the texture pattern — the visual disruption blends into the surface. On polished or glossy surfaces, weld lines remain visible as a faint line. For cosmetic parts, the solution is to reposition the gate so the weld moves to a hidden area, or to use sequential valve gating which eliminates the weld line entirely by filling one gate at a time.

    How do you test weld line strength?

    Tensile test specimens are molded with the weld line at the center of the gauge section — the test measures the force at break compared to a specimen without a weld line. The ratio (weld strength / base strength) is the weld efficiency. For structural parts, the weld efficiency should exceed 80%; below 70%, the part design or processing must change.

    Does material selection affect weld line severity?

    Amorphous resins (PC, ABS, PS) show less visible weld lines than semi-crystalline resins (PP, PA, POM) because the molecular diffusion across the weld is more uniform. Fiber-filled resins show the weakest weld lines because fibers do not cross the weld interface. Material selection for weld-sensitive parts should consider both the base resin and the filler type.

    Key Takeaways

    1. Weld lines form where melt fronts converge — they reduce part strength 10–30% and create cosmetic defects, but are unavoidable in parts with multiple gates, holes or bosses.
    2. Three solutions: reposition the gate to move the weld to a non-critical area, increase melt temperature and injection speed to improve bond strength, or add overflow wells to capture the weak material.
    3. The DFM review should identify weld line locations from mold flow analysis — the quotation should document overflow wells or sequential valve gating options with cost impact before the mold is cut.

    Send aCatMould the part drawing, and the DFM review identifies weld line locations from mold flow analysis — with gate repositioning, overflow well and sequential valve gating options documented — so the weld line is managed before the mold is cut, not discovered at T1.

    Sources
    Plastics Technology — weld line formation and solutions
    SPE — weld line strength measurement and optimization
    MoldMaking Technology — mold flow analysis for weld line prediction

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