Design · Weld lines

A weld line is not where the obstacle is. It is where two flow fronts happen to meet.

Melt entering a cavity travels as a front. Put anything in its way — a hole, a boss, a core pin, a second gate — and the front divides. The two halves travel on, and somewhere downstream they run into each other and stop. That meeting is the weld line, and it can be a long way from the feature that caused it. On the plate in the animation the hole sits 30 mm from the gate, and the fronts do not meet until well past it, about 10 mm off the part centreline, because the gap on one side is 8 mm and on the other 22 mm.

A 90 x 40 x 2 mm plate with one 10 mm through-hole, placed 7 mm off the centreline. A centred hole gives two symmetric paths and the fronts rejoin exactly behind it, which is the picture everyone already carries. Offsetting the hole makes the two gaps unequal — 8 mm on one side, 22 mm on the other — so the fronts arrive out of step and meet late and off-axis. That is the part of it worth showing: the meeting point is a consequence of where you gated, not a property of the hole.The blue line is where the two fronts close, derived from the fill field by one rule: a cell is on it when its arrival is a local maximum along some axis, meaning both opposite neighbours filled earlier. That is the definition of two fronts meeting, read off the solver’s own output. It locates 23 cells out of 3,729 — a line, not a region — and the coarse and fine grids put it in the same place (y = −0.50x + 14.5 against y = −0.36x + 13.6).Solved at 0.965 mm for this render — half the grid spacing the service uses by default, and thirty-one minutes rather than seconds, because the fill kernel is roughly quadratic in cell count. Both grids were run and they agree on where the fronts meet, so the finer one is the same answer better resolved.

Meeting is not the same as fusing

Two fronts meeting is not the same as two fronts fusing. By the time they touch, each has a skin that has been cooling against the steel, and the surfaces that press together are the coldest, most oriented material in the shot. How much strength that costs depends on the resin, the melt and mold temperature, and how much of the shot is left to pack behind it — it is a real effect, and it is not one you can read off a fill pattern.

Knit line, weld line, flow line

Knit line and weld line get used interchangeably. Where the distinction is drawn at all, a knit line is two fronts meeting at a shallow angle and continuing to flow together, and a weld line is two fronts meeting head-on and stopping. The head-on case is the one that costs you strength.

Flow lines are a different defect with the same reputation. They are cosmetic streaks left by the front hesitating or by a viscosity gradient as the melt fans out from the gate — a surface artefact, usually answered with gate size, melt temperature or injection rate. A weld line is a structural join. Searching for one and reading about the other is how a fix gets applied to the wrong problem.

What the part decides

These are settled in CAD, before any steel is cut, and they are the levers with real range. Everything after this is trimming at the edges.

Geometry

Move the gate

The single largest lever, and the cheapest one while the part is still a model. Where the melt enters decides which features split the front, how far apart the two halves travel, and therefore where they meet. Gating so that the fronts meet on a non-critical face, or so that one front reaches the obstacle well ahead of the other, moves the weld line somewhere it does not matter.

Where it stops: The gate has to live somewhere that can be fed, sheared off, and lived with cosmetically, and it is also the last thing holding the packing path open. Optimising gate position for the weld line alone will cost you somewhere else.

Geometry

Move or reshape the obstacle

A hole that can shift a few millimetres, or a boss that can become a teardrop rather than a cylinder, changes how the front divides and how quickly it closes back up. A streamlined obstacle leaves the two halves meeting at a shallower angle, which is the difference between a knit and a weld.

Where it stops: Most holes are where they are for a reason, and the ones that can move are rarely the ones causing the problem.

Geometry

Give one side a thicker path

Thickening one route by a fraction of a millimetre — a flow leader — speeds that front up and slows the other in relative terms, which slides the meeting point along the part. It is a way to steer a weld line without moving the gate.

Where it stops: A thicker path is also a slower-cooling path, so you are trading a weld line against a sink and against cycle time. This is a scalpel, not a hammer.

Geometry

Run the meeting point off the part

An overflow well — a small tab past the point where the fronts meet — pulls the join out of the part and into something that gets trimmed off. Common on cosmetic parts where the weld line cannot be tolerated anywhere visible.

Where it stops: It adds a trim operation and a scrap stream on every shot, and it only works if the tooling has somewhere to put it.

What the press can still do

Once the tool exists the weld line is where it is, and these change how much it costs you rather than where it sits.

Process

Hotter melt

The two fronts fuse better when they are still hot and mobile where they touch. Raising melt temperature is the most direct process answer and often visibly reduces the line.

Where it stops: Every resin has a ceiling before it degrades, and on a micro shot the material has a long residence time in a barrel sized for something bigger — the degradation limit arrives sooner than the datasheet suggests.

Process

Hotter mold

A hotter cavity keeps the skin thinner for longer, so the surfaces that meet are less solidified. On demanding cosmetic parts this is what variotherm cycling is for.

Where it stops: Cycle time. Every degree of mold temperature is paid for on every shot, forever.

Process

Faster injection

Filling faster gives the fronts less time to cool on the way, so they arrive hotter. It also moves the meeting point, because the balance between the two paths shifts with rate.

Where it stops: Faster fill raises shear and can burn the material at the gate, and on a part with a trapped-air problem it makes that worse rather than better.

Process

Vent where they meet

Two fronts closing on each other compress whatever is between them. If that air cannot get out, it burns, and the weld line gets a scorch mark on top of being a weld line. Venting the meeting point is often what actually fixes the cosmetic complaint.

Where it stops: You have to know where the fronts meet before you can vent it, which is the whole point of looking at the fill first.

A weld line lands on a thick–thin junction as often as on a hole, which is the same geometry that produces sink marks — and what the join is worth dimensionally is part of the tolerance stack. Otherwise, send the part and we will tell you where the fronts meet.