Last updated: 2026-09-05 · 5 min read
Weld Line Strength in Injection Molding — How Much Does a Weld Line Reduce Part Strength, Where Does It Fall on Your Part and What Design Changes Can You Make to Move It?
A weld line forms where two melt fronts meet — polymer chains do not fully entangle across the joint, so the weld is weaker than the bulk. Tensile strength at a weld line is typically 50–80% of bulk for unfilled resins, 30–60% for glass-filled. Impact is worse: 30–50% (unfilled), 15–30% (glass-filled). The weld line location depends on gate position — and it can be moved by changing the gate or adding overflow wells.
Key Facts
- Weld line strength depends on melt temperature, injection speed and resin type. At the weld, the two melt fronts meet at low temperature — if below the resin’s no-flow temperature, chains do not interdiffuse and the weld is weak. Raising mold temperature by 20°C improves weld strength by 10–15% because melt fronts arrive hotter. Increasing injection speed by 50% improves weld strength by 5–10% because the fronts meet with more kinetic energy.
- Glass-filled resins have the weakest weld lines — glass fibers do not cross the weld. At the weld, fibers align parallel to flow on each side but no fiber crosses the joint. The joint is held only by the resin matrix. For PA66 GF30: bulk tensile is 170 MPa, weld drops to 60–100 MPa (35–60%). For PBT GF30: bulk 150 MPa, weld 55–90 MPa. Unfilled resins have better weld strength because polymer chains interdiffuse across the joint.
- An overflow well (also called a weld line trap or fishbone) captures the cold weld material past the weld line — the actual weld moves into the well, away from the part. The well is a small pocket (5–10mm wide, 1–2mm deep) placed at the expected weld location. After molding, the well is broken off or trimmed. This improves apparent weld strength by 30–50% because the weak weld material is removed with the overflow well.
How Does Weld Line Strength Vary Across Common Resin Families?
| Resin | Bulk Tensile (MPa) | Weld Tensile (MPa) | Weld/Bulk Ratio | Impact Retention |
|---|---|---|---|---|
| ABS | 40 | 28–32 | 70–80% | 50–60% |
| PC | 60 | 42–48 | 70–80% | 40–50% |
| POM | 65 | 50–55 | 75–85% | 60–70% |
| PA66 GF30 | 170 | 60–100 | 35–60% | 15–30% |
| PBT GF30 | 150 | 55–90 | 35–60% | 15–25% |
| PP GF20 | 70 | 35–50 | 50–70% | 25–40% |
POM has the best weld strength ratio because its crystalline structure allows good chain interdiffusion. PC and ABS have moderate weld strength — the amorphous structure allows some chain entanglement. Glass-filled resins are the worst because fibers create a physical barrier — no fiber crosses the joint, and the resin matrix alone must carry the load.
Overflow well (weld line trap) — a small pocket (5–10mm wide, 1–2mm deep) placed at the expected weld line location. The cold, weak weld material flows into the well — the weld interface ends up inside the well, not on the part. After molding, the well is broken off. Weld strength improves by 30–50%. The gate vestige guide covers how gate type and location determine where the weld line forms.
What Three Design Strategies Improve Weld Line Strength?
- Move the weld line away from high-stress areas — the easiest fix is a gate relocation. If the weld line falls on a snap-fit root, a screw boss or a drop-test corner, the part fails at the weld. Run a mold flow analysis to predict the weld location — then move the gate so the weld shifts to a low-stress area. For a rectangular housing with two gates on opposite sides: the weld is at the center. Move one gate to a corner — the weld moves to the opposite corner, away from snap-fit roots along the side walls. The screw boss design guide shows why weld lines at boss bases are especially dangerous.
- Add an overflow well at the weld line — this captures the weak material outside the part. If the weld line cannot be moved from a critical area, place an overflow well at the expected weld. The well is a pocket (5–10mm wide, 1–2mm deep, connected by a thin land 0.5–1.0mm) that fills after the main cavity. The cold, weak weld material flows into the well — the weld interface ends up inside the well, not on the part. After molding, the well is broken off. Weld strength improves by 30–50% because the weak material is removed. Trade-off: additional waste and trimming.
- Increase mold temperature and injection speed at the weld zone — hotter melt meets with more energy. If the weld line is in a fixed location, increase the mold temperature in the weld zone by 20–30°C using a separate cooling circuit. This keeps melt fronts hotter when they meet, improving chain interdiffusion and weld strength by 10–15%. Also increase injection speed through the nearest gate — faster flow means the melt arrives hotter with more energy. The combination improves weld strength by 15–25%.
Chain interdiffusion (at weld lines) — the process where polymer chains from two meeting melt fronts entangle across the weld interface. In amorphous resins (PC, ABS), chains interdiffuse 10–30%, creating 70–80% of bulk strength. In glass-filled resins, fibers block interdiffusion — no fiber crosses the joint, and the weld is held by the resin matrix alone (35–60%). Raising mold temperature by 20°C improves interdiffusion 10–15%. The corner radius guide shows how sharp corners near weld lines amplify stress.
Weld line: tensile 50–80% of bulk (unfilled), 30–60% (glass-filled). Impact: 30–50% (unfilled), 15–30% (GF). POM best (75–85%). PA66/PBT GF30 worst (35–60%). Fix: move gate, add overflow well, raise mold temp +20°C, increase injection speed.
- Mold flow analysis run before mold build — predicts weld location and strength ratio.
- Weld kept out of high-stress areas — snap roots, screw bosses, drop-test corners.
- Overflow well at critical welds — captures weak material, improves strength 30–50%.
- Mold temp +20°C at weld zone — hotter melt fronts improve interdiffusion 10–15%.
- GF resins: expect 35–60% weld strength — fibers don’t cross the joint; design for lower load.
Frequently Asked Questions
How can I tell if my part will fail at the weld line during drop testing?
Run a mold flow analysis first — it predicts the weld line location and estimates the weld strength ratio. Compare the estimated weld strength to the drop test stress at that location. For a housing surviving a 1m drop: impact stress at corners is 20–40 MPa. If the weld is at a corner and weld strength is 30 MPa (PA66 GF30 at 40% of 170 MPa), the part fails. Move the weld to a flat wall where stress is below 15 MPa, or add an overflow well.
If the mold flow analysis is not available, mold trial parts and test them — the parts will almost always break at the weld line first if the weld is in a high-stress area.
Does the weld line affect cosmetic appearance as well as strength?
Yes — the weld line is visible as a faint line or color difference, especially on glossy or dark parts. On Class A surfaces (automotive exterior), the weld line is unacceptable — it must be moved to a non-visible area or hidden by texture. On a textured surface (MT-11010 or coarser), the weld is less visible because the texture breaks up visual continuity. If the weld must be on a cosmetic surface, raise the mold temperature for a smoother weld surface and match the color to minimize contrast.
Can welding parameters be adjusted after the mold is built to improve weld strength?
Yes — within limits. Raising melt temperature by 10–20°C improves weld strength by 5–10% because melt fronts arrive hotter. Increasing injection speed by 30–50% improves weld strength by 5–10% because the melt front has more kinetic energy. However, higher melt temperature may cause degradation (especially for PVC, POM and flame-retarded resins), and higher injection speed may cause flash or gas traps. The practical range is ±10–20°C and ±30% speed — beyond that, new defects appear. For larger improvements, the mold must be modified.
Key Takeaways
- Weld tensile: 50–80% bulk (unfilled), 30–60% (GF). Impact worse.
- GF resins: fibers don’t cross the weld — matrix carries all the load.
- Move gate, add overflow well, raise mold temp +20°C, faster injection.
- POM best weld ratio (75–85%). PA66/PBT GF30 worst (35–60%).
Send the part drawing with the load conditions and the resin — the DFM review predicts the weld line location by mold flow analysis and specifies the gate position and overflow well needed to keep the weld out of high-stress areas.
Sources: BASF — Weld Line Strength Data for Engineering Resins · DuPont — Weld Line Design Guidelines for Glass-Filled Resins · Plastics Technology — Weld Line Strength Measurement and Improvement Methods · Society of Plastics Engineers (SPE) — Weld Line Technical Papers · MatWeb — Material Property Database with Weld Line Strength Ratios
— Arthur, Tooling Engineer at aCatMould