Troubleshooting · Defect guide
Weak weld lines and cracks along them
When flow fronts meet but do not fuse, what to fix first
A weld line forms whenever the melt splits around a hole or core, or arrives from several gates, and the fronts meet again. The line itself is not the problem; a line that is too weak or too visible is. This guide covers the mechanism, four groups of causes, checks ordered from cheapest to most expensive, and an honest answer on how little a formulation change can do.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 22 October 2026 · about 7 min read
Confirm it is a weld line
A weld line is a thin line or shallow V-shaped groove on the surface, usually behind a hole, core or insert or halfway between two gates. It appears in the same place on every shot, and when the part is loaded the crack runs along it.
- Weld line: two flow fronts run head-on into each other and stop. This joint is the weakest and the most visible.
- Meld line: the fronts meet at an angle and keep flowing side by side. The melt mixes more, so it is usually stronger and less visible.
- Flow marks: wavy bands near the gate, not where fronts meet. See Flow marks and tiger stripes.
- Stress cracks: if the crack does not follow the line where the fronts meet, look at moulded-in stress or chemical attack instead.
Mechanism: why the joint is weak
As the melt flows, its front touches the cold mould wall and the air in the cavity, so its surface keeps cooling, and the polymer chains at the front lie parallel to it. When two fronts meet, the chains need heat and time to diffuse across the interface before the joint is strong. If the fronts are too cold, the pressure afterwards is too low, or trapped air sits between them, the joint only bonds partly and shows as a V-groove on the surface.
The practical takeaway: whenever the melt has to split, a weld line will form. You have three options: make it stronger (hot fronts, enough pressure, air can escape), move it somewhere harmless (gate position, wall thickness), or stop the melt from splitting, which is a design change.
Four groups of causes
1. Part design
Holes, cores, bosses and inserts split the flow, and parts with several gates always have a line between them. Thin sections before the meeting point cool the front quickly. The real risk is a weld that lands on a load point, such as a snap fit or the ring around a screw boss.
2. Mould
No vent where the fronts meet, or vents clogged with additive residue and wax. Gate positions that put the weld in the wrong place, long flow lengths, and a mould area around the weld that runs colder than the rest.
3. Process
Low melt or mould temperature, injection that is too slow so the front cools before it arrives, too little holding pressure or time to press the joint together, and switching from velocity to pressure (V/P) too early.
4. Material
A very viscous grade (low MFI) whose front cools before the end of the flow. Glass fibre and high filler loadings weaken the joint. Over-dosed lubricant or masterbatch, or a carrier that does not suit the base resin, gets in the way of fusion, and moisture or volatiles turn into gas at the flow front.
Step-by-step checks, cheapest first
- Run a short-shot series. Reduce shot size in steps to see where the fronts meet and whether that matches the crack line. See Short shots for how to read the results.
- Inspect and clean the vents at the weld location. A clogged vent is a common cause and the quickest fix. A burn mark at the same spot points even more strongly to trapped air; see Burn marks.
- Mould the base resin alone for 20–30 shots without masterbatch or regrind. If the joint gets clearly stronger, the problem is in the blend.
- Step up melt temperature within the resin supplier's range, then mould temperature around the weld.
- Increase injection speed so the fronts arrive while still hot. Watch for burn marks and flash.
- Check holding pressure and the V/P switchover. The joint needs pressure after the fronts meet.
- Test strength the same way every time, for example breaking or pulling parts with one fixed method, before and after each change, so you know it is genuinely better rather than just looking better.
- Change the mould if none of the above is enough: add a vent, add an overflow well, move the gate or adjust wall thickness to shift the weld. This is the most expensive step.
Cause → check → fix table
| Cause | Check | Fix |
|---|---|---|
| Clogged or missing vent at the weld | Look for residue in the vent · burn mark where fronts meet? | Clean on a schedule; add a vent at the weld location |
| Flow fronts too cold when they meet | Short-shot series; check flow length and thin sections | Raise injection speed, melt temperature or mould temperature |
| Too little pressure after the fronts meet | Step holding pressure, compare joint strength | Raise holding pressure or time; check V/P switchover |
| Weld sits on a load point | Compare weld position with where parts break | Move the gate, adjust wall thickness or add an overflow well |
| Over-dosed masterbatch or lubricant | Mould base resin alone · weigh actual dosing | Reduce dosing; use a carrier that suits the base resin |
| Glass fibre or high filler | Is the fracture flat along the joint? | Move the weld off load points, add an overflow well, review the grade |
| Grade too viscous for the flow length | Compare TDS MFI with flow length | Discuss an easier-flowing grade with the resin supplier |
| Moisture or volatiles | Dry resin and masterbatch, mould again and compare | Dry as the resin maker specifies |
| Flake or metallic pigments | Line clearly visible only in certain colours | Move the weld or change pigment type; process changes help little |
How much can formulation help?
The honest answer is only a little. Joint strength depends mostly on heat, pressure and air. On the material side, this is what matters:
- Remove what blocks fusion. Over-dosed masterbatch or lubricant, and a carrier that does not suit the base resin, weaken the joint. Getting the dosing right and using a compatible carrier genuinely helps.
- An easier-flowing grade keeps the fronts hotter when they meet, but a higher MFI usually trades off some mechanical properties. Discuss it with the resin supplier and test real parts. See Polymer basics for MFI.
- Glass fibre and high filler loadings always leave the joint weaker than the rest of the part. Formulation changes do little here; manage it through weld position and design.
- Flake or metallic pigments align with the flow and make the joint show as a clear line. The fix is pigment choice or moving the weld, more than machine settings.
What a formulation change cannot fix: clogged or missing vents, gates that put the weld on a load point, a mould that is too cold, and a design that forces the melt to split. For the bigger picture of injection settings, see Injection molding.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
FAQ
Weld line questions we hear often
Can a weld line be eliminated completely?
If the melt has to split around a hole, core or insert, or arrives from more than one gate, a weld line will always form. What you can do is make it stronger and less visible with the right temperatures, speed, holding pressure and venting, or move it to a spot that carries no load and is not a cosmetic surface. Truly removing it means changing the design or the gating, for example fewer gates or sequential valve gates on a hot runner, which is mould work.
Why do parts crack along the weld line only when we add colour or regrind?
Masterbatch and regrind change melt viscosity and can carry lubricants, pigments or volatiles to the flow front, where they get in the way of the two fronts fusing. Mould 20–30 shots of the base resin alone. If those parts do not crack, the problem is in the blend: weigh the actual dosing, and ask the masterbatch maker whether the carrier is compatible with your base resin and flows at least as easily.
Faster injection improved the weld line, but now there is a burn mark at the same spot. Is that related?
Directly. Air trapped between the two flow fronts is compressed faster and gets very hot. With no way out, it scorches the plastic (the diesel effect). The fix is not to slow down again but to vent the weld location properly, or to clean vents that have clogged with additive residue.
Why do glass-filled parts break along the weld line so easily?
Fibres at the flow front lie parallel to the front. When two fronts meet, the fibres do not cross the joint, so the weld relies on the polymer alone and can be far weaker than the rest of the part. The answer is to move the weld away from load points through gate position, add an overflow well so the fronts keep flowing past the joint, or design the part so the weld sits in an unloaded area.
Parts still cracking along the weld line?
Send photos of the broken part, your gate positions and machine settings on LINE. Our technical team will help you separate mould, process and blend causes.