A weld line (also called a knit line) forms wherever two melt fronts meet: behind a hole or core pin, between two gates, or where a thin area fills late. It shows as a fine line and it is a weak spot, most of all in glass-filled grades. Processing can make a weld line stronger and fainter but cannot remove it; only a change to how the part fills does that. Start with melt temperature, mold temperature and venting.
Also described as: knit lines, meld lines, weak lines where flow fronts meet, hairline marks around holes.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| A fine hairline behind every hole, boss or core pin, in the same place every shot | The flow splits around the core and rejoins. The two fronts have cooled by the time they meet. | Melt temperature with a probe; mold steel temperature near the line; fill speed |
| The line is a groove you can feel with a fingernail | The fronts were too cold to merge, or air was trapped where they met. | Venting at the weld; mold temperature; pack pressure and hold time |
| Parts crack along the line at assembly, in a pull test or in service | Poor fusion across the weld. In glass-filled grades the fibers lie along the line and do not cross it. | Whether the weld sits in a loaded area; glass content of the grade |
| The line shows as a streak of different shade or gloss, worst in metallic, pearl or dark colors | Flake pigments and glass fibers turn where the fronts meet and reflect light differently. | Pigment type with your color supplier; a short run in natural |
| A dark or burnt spot at the end of the line | Air trapped between the two fronts was compressed and overheated. | Vents at that spot. See burn marks. |
| The line got worse with a new lot, a new color or a colder day | A lower-flow lot or a colder melt. The fronts arrive stiffer. | Melt flow on the COA of the good lot against the bad lot; actual melt temperature |
| The line got worse after mold release was sprayed, or on a damp day | Release agent, moisture or other volatiles on the flow front keep the two fronts from fusing. | Stop the release spray; check the dryer |
Checks in order, cheapest first
- Find where the fronts meet. Run a series of short shots, each a little fuller than the last. You will see the two fronts approach and where the last air has to escape.
- Measure the melt temperature and raise it within the range. Use a probe in an air shot. A hotter melt fuses better. Stay inside the supplier’s range and watch residence time, because overheated resin makes a weaker weld, not a stronger one.
- Raise the mold temperature. Measure the steel near the weld. A warmer mold keeps the fronts fluid for longer after they meet, and it is often the most effective single change for appearance.
- Fill faster, then check pack pressure and hold time. A faster fill brings the fronts together hotter. Enough pack pressure, held until the gate freezes, presses them together. If faster filling brings burn marks, fix the venting first.
- Clean the vents and stop using mold release. A blocked vent at the weld leaves air between the fronts. Release agent on the cavity surface is carried to the weld by the flow fronts.
- Check the dryer and compare lots. Moisture and volatiles gather at the flow front. If the problem arrived with a new lot or color, compare the COAs before you change the process.
- Look at the tool. Add a vent or an overflow tab at the weld. Move or resize the gate, or change the number of gates, to shift the weld away from loaded or visible areas. A hot spot (a local heater or insert) at the weld is another option. These are the expensive fixes.
- Change the resin. A weld line is usually not a resin problem. Change the grade last, and only once you know the weld cannot be moved or strengthened enough.
The numbers
Typical melt and mold temperature ranges for resins where weld lines are a common complaint. For weld strength, run toward the upper part of each range.
| Resin | Melt temperature | Mold temperature |
|---|---|---|
| ABS | 425–525°F / 218–274°C (flame-retardant grades never above 475°F / 246°C) | 80–180°F / 27–82°C |
| Polycarbonate | 536–653°F / 280–345°C | 158–248°F / 70–120°C |
| Acrylic (PMMA) | 340–520°F / 171–271°C | 100–205°F / 38–96°C |
| GPPS | 356–500°F / 180–260°C | 50–150°F / 10–66°C |
| Polypropylene homopolymer | 374–525°F / 190–274°C | 50–203°F / 10–95°C |
| Nylon 6, 30% glass | 518–554°F / 270–290°C | 176–248°F / 80–120°C |
| Nylon 66, 30–33% glass | 536–581°F / 280–305°C | 176–248°F / 80–120°C |
| PBT, 30% glass | 460–527°F / 238–275°C | 140–212°F / 60–100°C |
| PPS | 572–644°F / 300–340°C (do not exceed 644°F / 340°C) | 275–320°F / 135–160°C |
Typical values from published processing guides. Ranges depend on the grade, so confirm against the data sheet for your grade. The full list is in the melt temp, mold temp and shrinkage chart.
Why a weld line shows and why it is weak
- The fronts have skinned over. The leading edge of the melt cools as it travels. When two fronts meet, the polymer chains have little time to mix across the joint before it freezes.
- There is a notch at the surface. Air and volatiles caught between the rounded fronts leave a small V-shaped groove. It catches the light, and it concentrates stress.
- Glass fibers do not cross the line. At a weld the fibers turn to lie along the joint. The weld gets little or none of the reinforcement, so its strength is closer to that of the unfilled resin than to the filled grade around it. The more glass, the bigger the gap between weld strength and the data sheet value.
- Head-on welds are the weakest. Where two fronts meet head-on and stop, fusion is poorest. Where they meet and keep flowing side by side, the joint mixes more and is stronger.
- Design with it in mind. Data sheet strength is measured on bars with no weld line. Keep welds out of snap fits, bosses under screw load and other highly stressed areas where you can.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A higher-flow grade of the same resin | The fronts arrive hotter and at lower pressure, and merge more fully. Melt flow index explained | Higher flow usually means lower toughness. The part may also flash more easily. |
| Lower glass content | A smaller gap between the strength of the weld and the strength of the rest of the part. | Lower stiffness, strength and heat deflection temperature everywhere else. Different shrinkage. |
| An unfilled or impact-modified grade for parts with a loaded weld | Unfilled resin loses far less of its strength at a weld than a glass-filled grade does. | Much lower stiffness and heat resistance than the glass-filled grade. A new grade for your customer to approve. |
| A different colorant | Solid pigments hide a weld line better than metallic and pearl flake pigments. | A change in appearance, a new color match and a new approval. |
Any resin change moves your dimensions. The tool was cut for the shrinkage of the resin you run now. A resin that shrinks more or less makes a different size part from the same steel, so check critical dimensions before you approve a swap.
Often confused with
- Flow lines. Wavy bands or ripples near the gate or at a change in wall thickness come from one front slowing and cooling. A weld line is a single sharp line where two fronts meet.
- Jetting. A snake-like squiggle starting at the gate, where melt shot across the cavity before the rest filled in around it. See jetting.
- A crack. A weld line is there on every part as molded and does not open when the part is flexed gently. A crack opens, grows, and often appears later. See brittle or cracking parts.
- Splay. Silver streaks fan out along the flow direction and change from shot to shot. A weld line stays in one place. See splay and silver streaks.
Quick answers
What causes weld lines in injection molding?
A weld line forms wherever two melt fronts meet: behind a hole or core pin, between two gates, or where a thin area fills late. The fronts have cooled and skinned over by the time they meet, so the polymer chains have little time to mix across the joint before it freezes.
How do I make a knit line stronger?
Start with melt temperature, mold temperature and venting. Raise the melt temperature within the supplier’s range, raise the mold temperature, fill faster and hold enough pack pressure until the gate freezes. Clean the vents and stop using mold release. Overheated resin makes a weaker weld, so watch residence time.
Why does my part crack at the weld line?
Cracking along the line means poor fusion across the weld. In glass-filled grades the fibers turn to lie along the joint and do not cross it, so the weld gets little or none of the reinforcement. Check whether the weld sits in a loaded area and the glass content of the grade.
Can a weld line be removed by changing the process?
No. Processing can make a weld line stronger and fainter but cannot remove it. Only a change to how the part fills does that, such as moving or resizing the gate or changing the number of gates. Those tool changes are the expensive fixes.
Is a weld line a resin problem?
A weld line is usually not a resin problem. Change the grade last, and only once the weld cannot be moved or strengthened enough. If the line got worse with a new lot or color, compare melt flow on the COAs before changing the process.
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