A weld line — also called a knit line, and on the floor a “dam water line” — is what you get when two flows of melt meet and don’t fully fuse back into one. It shows up as a fine line on the surface, usually downstream of a hole, a window, an insert, or wherever the flow had to split and come back together. It looks cosmetic. The dangerous part is that it often isn’t only cosmetic: where the fronts met too cool to weld properly, the part is genuinely weaker, and it can crack along that line under load.
The single fact that explains most weld-line trouble is temperature at the meeting point. When two melt fronts arrive and the plastic there is still hot and fluid, they fuse and the line nearly disappears. When they arrive cool — at the end of a long flow path, behind an obstruction, with trapped air between them — they butt together without truly knitting, and you get a visible, weak line. Everything that follows is about keeping that confluence hot, clean, and vented, or about putting it somewhere it doesn’t matter.
Where weld lines come from
A weld line is a flow event, not a random defect — it appears wherever the melt is forced to divide and rejoin:
- Around holes and windows. The flow splits to go around the opening and meets on the far side. A hole essentially guarantees a weld line behind it.
- Around cores, inserts, and bosses. Anything the melt has to flow past creates two fronts.
- In multi-gate parts. Two gates feeding one cavity meet somewhere in the middle — that meeting is a weld line whose location you chose (or didn’t) when you placed the gates.
- At thick-to-thin junctions and obstructions that make the flow break apart and recombine.
Because a hole or an insert forces a split, you usually can’t eliminate the weld line. That reframes the whole problem: the goal isn’t to make it vanish, it’s to make it strong and to land it where it won’t show or won’t carry load.
The cause tree: why this weld line is bad
When a weld line is too visible or too weak, the cause sits in one of four places. Naming which one keeps you from raising temperature when the real problem is a blocked vent.
| Cause area | What’s happening | Typical fix |
|---|---|---|
| Process — too cold | Melt or mold temp too low; fronts meet already chilled and won’t fuse | Raise melt temperature; raise mold temperature; improve flowability |
| Process — too slow / weak | Slow fill or low pressure lets the fronts cool before they meet | Raise injection speed and pressure so the fronts arrive hot |
| Venting | Air, volatiles, or mold-release trapped at the confluence keep the fronts apart and can burn | Add or clean venting at the weld zone; add an overflow well |
| Design / mold | Gate location or count puts the weld line in a bad spot; thin sections chill the flow | Move, add, or remove gates; thicken the flow path; relocate the line |
The most common reflex error is treating every weld line as a temperature problem and cranking the barrel. If the real cause is air trapped where the fronts meet, more heat just gets you a burned weld line instead of a cold one.
Make it hotter: process levers
When the weld line is weak or visible because the fronts met cool, the process moves all push the same direction — get more heat to the meeting point:
- Raise melt temperature so the plastic arrives with more heat to fuse. Good flowability can make a weld line almost invisible.
- Raise mold temperature so the fronts don’t chill against cold steel before they meet. This is one of the most effective levers for weld-line appearance.
- Raise injection speed so the melt reaches the confluence before it loses heat — slow fill is a frequent hidden cause.
- Raise pack/fill pressure to push the fronts together firmly while they’re still hot.
These have limits and trade-offs — too hot or too fast brings burning, flash, and degradation — which is exactly why the diagnosis matters before the adjustment.
Vent it: the cause people forget
Two melt fronts closing on each other trap whatever is between them: air, moisture-driven volatiles, and any mold release. If that pocket can’t escape, the fronts can’t fully meet, and at the pressures and temperatures involved the trapped gas can compress and scorch — a burned weld line. Adequate venting right at the weld zone lets that gas out so the plastic can knit. An overflow well (a small trap just past the weld area) takes the idea further: it pulls the cool leading edges of both fronts — and the trapped gas — out of the part and into a tab you trim off, so the weld that remains in the part is hot, clean, and strong.
Control where it lands: the design and gate decision
Since a hole or insert forces a weld line, the highest-leverage move is often upstream of the press: decide where the line goes. Gate location and gate count determine where fronts meet. By moving a gate, adding one, or removing one, you can:
- Push the weld line onto a hidden or non-cosmetic surface.
- Move it off a load-bearing feature so the weak point isn’t where the stress is.
- Shorten the flow distance to the confluence so the fronts arrive hotter.
This is a qualification-stage conversation, ideally informed by a flow analysis that shows the predicted weld-line locations before steel is cut. Choosing where the weld line lands is far cheaper at the design stage than fighting its appearance at the press for the life of the job.
A practical sequence
- Confirm it’s a weld line and where it is relative to holes, inserts, and gates.
- Decide whether it’s cosmetic, structural, or both — that sets how hard you fight it.
- Rule in the cause: cold (temp), slow (speed/pressure), gas (venting), or location (gate/design).
- For cold/slow: raise melt and mold temperature and fill speed, one change at a time.
- For gas: vent the weld zone; add an overflow well if needed.
- For location: treat it as a gate/design change — relocate the line rather than chase its appearance forever.
FAQs
Are weld lines just cosmetic, or do they weaken the part?
Both, depending on how well the fronts fused. A weld line is where two melt fronts met, and if they met cool, the plastic there never fully knitted — so the part can be significantly weaker along that line and may crack there under load. Where the fronts met hot, the line is mostly a faint cosmetic mark. That’s why the meeting-point temperature matters so much: it determines whether you have a surface blemish or a genuine structural weak point.
Can I eliminate a weld line completely?
Usually not, if the part has a hole, window, insert, or anything the melt must flow around — the split-and-rejoin that creates the line is unavoidable there. The realistic goal is to control it: make it strong by keeping the confluence hot and well vented, and put it where it doesn’t matter by choosing gate location and count. Trying to make a weld line vanish entirely is often the wrong target; landing it on a hidden, non-structural surface is the achievable one.
Why does my weld line look burned?
Because gas is trapped where the two fronts meet. Air, moisture-driven volatiles, or mold release caught at the confluence can’t escape, and as the fronts close and pressure rises, that gas compresses and scorches — burning the weld line. The fix is venting, not more heat: add or clean a vent at the weld zone so the gas can get out, and consider an overflow well to pull the trapped gas and the cool leading edges out of the part. Raising temperature on a gas-trapped weld line usually makes the burn worse.
Which process changes reduce weld-line visibility?
The ones that get more heat to the meeting point: raising melt temperature, raising mold temperature, and increasing injection speed so the fronts arrive before they chill, plus enough pressure to press them together while still hot. Higher mold temperature in particular does a lot for weld-line appearance. All of these have limits — too hot or too fast brings burning and flash — so change one at a time and confirm. If temperature and speed don’t help, the cause is more likely trapped gas or gate location than heat.