Weld lines: the plane a moulded part was always going to break on

When molten polymer flows around a core, through a multi-gate tool or past any obstruction, the melt divides and rejoins. Where the fronts meet, the material forms a plane that is chemically identical to the bulk and mechanically weaker than it. Polymer chains on either side never fully interpenetrate, contaminants and trapped air concentrate at the junction, and fibre or filler orientation turns parallel to the interface rather than across it. A part that is adequately strong everywhere except along one line is one of the more recognisable patterns in moulded-part failure, and its location is predictable from the tool.

Why the junction never fully heals

Bonding across a meeting melt front requires chains to diffuse across the interface and re-entangle, and that requires both temperature and time. Fronts arriving late in the fill, at the end of a long flow path, are cooler and under less pressure than material near the gate, so both conditions are marginal exactly where weld lines tend to form. Cold slugs and trapped volatiles at the junction reduce it further.

Filled materials suffer disproportionately

In a glass- or fibre-filled compound, reinforcement aligns with flow. Where two fronts meet head-on, fibres lie parallel to the weld plane rather than bridging it, so the reinforcement that carries load elsewhere contributes almost nothing across the junction. A filled material can therefore show a much larger strength reduction at a weld line than an unfilled one, which surprises designers who selected the filled grade specifically for strength.

Meeting and merging junctions differ

Fronts that collide head-on and stop produce the weakest condition. Fronts that meet at an angle and continue flowing together partially re-entangle as they travel and produce a stronger, less distinct junction. Distinguishing the two matters, because a part may contain several such features and only one of them is genuinely a weak plane. Reconstructing the fill sequence from gate positions and part geometry is what identifies which is which.

Recognising a weld-line failure

The signature is a crack that is unusually straight, that runs on a plane rather than wandering, and that sits at a location the fill pattern predicts — typically downstream of a hole, boss or core. Fracture-surface examination shows a comparatively flat, featureless plane with little of the drawing or fibre pull-out seen in bulk material fracture. Where the material is translucent, the line is frequently visible as a faint witness mark before any load is applied.

The design question is location, not existence

Weld lines cannot be eliminated from most parts with holes or multiple gates. The relevant design question is whether one was allowed to fall where load concentrates. A weld line in a low-stress region is unremarkable; the same feature crossing a fillet that carries bending, or across a boss that takes a fastener, is a design condition that gate placement could usually have moved. That is why the gate layout and any mould-flow analysis performed during development are worth obtaining.

Process conditions change how weak the line is

Melt temperature, mould temperature, injection speed and packing pressure all affect how well the junction knits, and venting affects how much trapped gas ends up in it. The same tool run at different settings produces weld lines of measurably different strength. This is what makes a weld-line failure sometimes a design issue and sometimes a process issue, and the discriminator is usually whether failures track production periods or appear uniformly across the entire history of the part.

Quantifying the loss

Weld-line strength is measured by testing specimens moulded with a deliberate junction against specimens without one, using the same tensile and impact methods applied to the resin generally. The ratio between them — the weld-line factor for that material and process — is what allows a comparison between the stress at the failure location and the strength actually available there. Without that factor, a stress analysis against the resin's published strength overstates the margin, sometimes badly.

Ruling out the alternatives

A straight crack near a hole is not automatically a weld line. Environmental stress cracking, an internal void, a sharp corner acting as a notch, and processing-induced degradation all produce cracking in similar regions. Establishing that the junction was where the fill pattern predicts, and that the fracture surface shows the flat morphology of a poorly knitted plane rather than a craze or a void origin, is what separates them.

What to preserve

All fragments, unmated and uncleaned, so the fracture plane can be examined and matched to the fill pattern. The tool information — gate positions, cavity number and any mould-flow study. The process record for the production period, including melt and mould temperatures, and unused parts from the same and different lots so weld-line strength can be measured on material that actually came off the tool.

This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.