When a weld line lands on a cosmetic surface, do two things before touching the mould: measure the meeting angle between the two melt fronts, and look at how far each front has travelled from its gate. If the angle is below 135°, the window for fixing it by adjusting mould temperature and injection speed is very narrow — the direction has to change at gate layout and wall-thickness balance. Our order of work on this class of problem is: move the gate if it can be moved, and only then go back to wall thickness, because changing wall thickness drags the entire cooling circuit and ejector layout with it. The cost is not on one drawing.
The core criterion: the position of a weld line is decided by gate position; how visible it is decided by the meeting angle and the temperature difference between the two fronts. Above 135° with a small temperature difference at the meeting point, the weld line can usually be faded until it is essentially invisible. Below 135°, even cranking mould temperature up will not rescue it — what needs to change is the gate layout.
Step one: work out whether the weld line is a head-on collision or a flow-around
Open the mould and look at where the weld line sits. There are two families of cause, and the fixes are completely different.
Meeting type: two gates feed two fronts that collide partway. The weld line sits on the line joining the two gates, usually a straight or gently curved line.
Flow-around type: a single gate feeds, and the melt flows around a core, an insert pin or a deep hole and then closes up behind it. The weld line appears on the back side of the hole or insert, and its shape follows the obstacle — one hole can produce two lines.
We had a blood-pressure monitor top cover with a single side gate. There was a ring of deep holes around the display window. When T0 came back there was a pale thin line to the right of the window, directly facing two snap-fit holes on the back. That is the classic flow-around case. The first instinct was more mould temperature: from 60°C to 80°C, several dozen shots. The line faded a little but never went away. We then moved each of the two snap holes by 6mm so they no longer sat on the same flow path, and the weld line dispersed into the non-cosmetic face.
Why does the distinction matter? The meeting type is solved by moving gates; the flow-around type is solved by moving holes and inserts. The directions are different. The test is simple: lay the weld line position over the structure on the back of the part. If it lines up with a hole, treat it as flow-around first.
Move the gate 10mm and the weld line follows — but the cost varies
The gate is the source of the weld line. Wherever the melt enters, that is where the front starts, and where the two fronts meet is the weld line. So on parts with cosmetic requirements, gate layout should be fixed at the structure review stage; changing it after T0 means reworking the mould cavity insert.
This table is the comparison we use internally when choosing a gate scheme; in a real project we rule options out in this order.
| Dimension | Single gate | Two symmetric gates | Multi-point / valve gate |
|---|---|---|---|
| Weld line count | Possible at flow-around holes | Always 1 | Increases with gate count |
| Position controllability | By avoiding with holes and inserts | Fixed between the two gates | Timing can be staggered to shift the meeting point |
| Injection pressure demand | Long flow path, high pressure | Medium | Low, but equipment demand is high |
| Mould rework cost | Change the sprue bush position | Rework runner + cavity | Rebuild the hot-runner system |
One tip that saves a lot of effort: if the cosmetic face has a hole or a recess, place the gate on the long side of that hole so the melt fills around the hole before moving to the far end. The back of the hole is then much less likely to form a closed flow-around line. We did exactly this on a handheld device shell: the gate moved from the short side to the long side, and the weld line moved from directly above a key to the bottom near the gate, where a structural rib hides it.
Gate adjustment priority: first see whether the weld line can be moved onto a non-cosmetic face (bottom, side face, an area covered by ribs or printing); if it cannot be moved, consider adding gates to shorten the flow path and push the meeting point out of sight; only then consider hot-runner timing control to stagger the two fronts — that is a step up in both equipment and mould cost.
Wall thickness difference is the quiet push behind it
When the gate cannot be moved, the problem is often wall thickness. In the same part, thick regions fill faster and thin regions fill slower; the arrival time difference at the meeting point grows, and the weld line becomes visible.
We keep the ratio of main wall thickness to local wall thickness within 1.5:1. Above that, the velocity difference between flow fronts becomes visible to the eye, and the weld line goes white or shiny, sometimes with a slight sink.
We took a wrong turn once that is worth remembering. It was a small appliance top cover, ABS, main wall 2.0mm, with six internal ribs. At T0 there was a very obvious weld line right in the middle of the cosmetic face. The first assumption was injection parameters, and we tested two rounds: mould temperature from 70°C to 95°C, injection speed from slow to fast. Nothing. Then we sectioned the cover and measured: the rib root thickness was 1.6mm, almost 0.8 times the main wall. It was like standing six low walls across the flow path — the melt split at each rib and had to re-merge after it.
Reducing rib thickness to 1.2mm faded the weld line by roughly seventy percent, and mould texturing covered the rest. There is a precondition: the rib's job is stiffness, and thinning it loses stiffness, so we made up for it in rib height — 0.5mm taller — and recalculated the section moment of inertia.
There is one more easily missed item: the transition fillet between wall thicknesses. A direct step between two thickness regions makes the melt decelerate or even stall, and the weld line lands right there. We generally use a transition fillet of 0.5mm or more so the melt keeps moving through.
Wall thickness criteria: a main-to-local wall ratio above 1.5:1, or a rib root thickness above 0.7 times the main wall, will tend to leave an obvious weld line where the flow splits. The order of change is to take the rib thickness down to 0.5-0.6 times the main wall first, recover stiffness with rib height, and only then consider mould temperature and injection speed.
Half an hour on site to decide what to change
Trial time is tight. Work through this order and in half an hour you will usually know the direction:
- Does the weld line line up with a hole or insert pin on the back? If yes, treat it as flow-around: move the hole, change the insert structure.
- Does the weld line sit on the line between two gates? If yes, treat it as meeting type: evaluate moving or adding gates.
- Measure the meeting angle between the two fronts. Below 135°, the direction is the gate; above 135° but still visible, look at wall thickness and ribs.
- Measure the ratio of main wall thickness to rib root thickness. Above 0.7, change the rib first.
- If none of the above, go back to injection parameters and look for the window.
This order runs from cheapest to most expensive. Parameter adjustments cost nothing; moving a gate means reworking the cavity insert; changing wall thickness pulls the cooling circuit along and is the biggest change. Do the cheap checks first — it saves a lot of mould rework money.
Questions we get most often
Q: Can a weld line be eliminated completely?
A: With a single gate and no flow-around obstacle, the part can be made essentially free of visible weld lines. With multiple gates or deep-hole inserts, the weld line can only be faded and relocated, not eliminated. In practice we do two things at once: move the weld line to a non-cosmetic face or hide it behind structural ribs, and cover what remains with mould texturing (depth of 0.02mm or more is our usual recommendation) or the direction of the grain. If the customer demands a perfectly clean cosmetic face, the fix has to come from part splitting and gate scheme redesign — not from downstream processing.
Q: Does raising mould temperature reduce the weld line?
A: It helps, but there is a ceiling. Higher mould temperature narrows the temperature difference between the two fronts, so the weld line fades. But when the meeting angle is below 135°, mould temperature alone will not rescue it. Raising mould temperature also has side effects: longer cooling, longer cycle time, and thin-wall parts that are prone to push-out whitening or distortion. Our approach is to try one step up in the parameter phase (say 70°C to 80°C); if the weld line only fades slightly, the direction is wrong and you should go back to the gate and wall thickness.
Q: How much do stiffening ribs affect the weld line?
A: A great deal. When the rib root thickness exceeds 0.7 times the main wall, the melt is clearly split as it passes the rib and an obvious whitened weld line tends to remain at the re-merge point. The usual practice is to hold rib thickness at 0.5-0.6 times the main wall. Thinning loses some stiffness, which is normally recovered by making the rib taller, and the rib root fillet should not be too small — 0.3mm or more — otherwise it creates stress concentration and sink marks.
Q: When must the gate position be changed?
A: When the meeting angle is below 135°, the weld line sits on the primary cosmetic face, and there is no room to adjust either hole positions or wall thickness — then moving the gate is the only effective direction. In that case, evaluate the cost first: repositioning a side gate usually only requires a new gate insert, which is relatively cheap; repositioning a hot-runner gate means rebuilding the whole system, which is a different order of cost. We normally lay the options and their costs in front of the customer before deciding which side changes.
Originally published by the Hezi Industrial Design team, based on our industrial design project practice in Dongguan. Learn more: Industrial design services
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