What is a weld line in a molded plastic part?
A weld line is the plane inside a molded plastic part where two melt fronts meet after the molten polymer has divided and rejoined, as it does when it flows around a core, through a multi-gate tool or past any obstruction. The weld-line plane is chemically identical to the bulk polymer and mechanically weaker than it.
A weld line is weaker than the surrounding material because polymer chains on either side of it never fully interpenetrate, contaminants and trapped air concentrate at the junction, and fiber or filler orientation turns parallel to the interface rather than across it. A molded part that is adequately strong everywhere except along one line is one of the more recognizable patterns in molded-part failure, and the location of that weld line is predictable from the tool.
Why doesn’t a weld line in a molded part fully heal?
A weld line in a molded part does not fully heal because bonding across a meeting melt front requires polymer chains to diffuse across the interface and re-entangle, which requires both temperature and time, and both conditions are marginal exactly where weld lines tend to form.
Melt 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. Cold slugs and trapped volatiles at the weld-line junction reduce the bonding further.
Why can a glass- or fiber-filled plastic lose more strength at a weld line than an unfilled one?
A glass- or fiber-filled plastic can show a much larger strength reduction at a weld line than an unfilled plastic because the reinforcement in a filled compound aligns with flow. Where two melt fronts meet head-on, the fibers lie parallel to the weld plane rather than bridging it, so the reinforcement that carries load elsewhere in the part contributes almost nothing across the weld-line junction.
The larger strength reduction at the weld line surprises designers who selected the filled grade specifically for strength.
How do meeting and merging weld-line junctions differ?
Meeting and merging weld-line junctions differ in strength: melt fronts that collide head-on and stop produce the weakest condition, while melt fronts that meet at an angle and continue flowing together partially re-entangle as they travel and produce a stronger, less distinct junction.
Distinguishing meeting junctions from merging junctions matters because a molded 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 weld-line junction is which.
How do you recognize a weld-line failure in a molded part?
A weld-line failure in a molded part is recognized by 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 of a weld-line failure shows a comparatively flat, featureless plane with little of the drawing or fiber pull-out seen in bulk material fracture. Where the material is translucent, the weld line is frequently visible as a faint witness mark before any load is applied.
Can weld lines be designed out of a molded part?
Weld lines cannot be eliminated from most molded parts with holes or multiple gates, so the relevant design question is not whether a weld line exists but whether one was allowed to fall where load concentrates.
A weld line in a low-stress region is unremarkable. The same weld line 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 mold-flow analysis performed during development of a molded part are worth obtaining.
How do molding process conditions affect how weak a weld line is?
Melt temperature, mold temperature, injection speed and packing pressure all affect how well a weld-line junction knits, and venting affects how much trapped gas ends up in it, so the same tool run at different settings produces weld lines of measurably different strength.
Because process conditions change how weak a weld line is, a weld-line failure is sometimes a design issue and sometimes a process issue. The discriminator is usually whether the failures track production periods or appear uniformly across the entire history of the part.
How is the strength lost at a weld line measured?
The strength lost at a weld line is measured by testing specimens molded with a deliberate weld-line junction against specimens without one, using the same tensile and impact methods applied to the resin generally. The ratio between the two is the weld-line factor for that material and process.
The weld-line factor is what allows a comparison between the stress at the failure location and the strength actually available there. Without the weld-line factor, a stress analysis against the resin’s published strength overstates the margin, sometimes badly.
Is a straight crack near a hole in a molded part always a weld-line failure?
No. A straight crack near a hole in a molded part 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.
What separates a weld-line failure from those alternatives is establishing that the weld-line 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.
What should be preserved after a suspected weld-line failure in a molded part?
After a suspected weld-line failure in a molded part, preserve all fragments, unmated and uncleaned, so the fracture plane can be examined and matched to the fill pattern.
Also preserve the molding tool information — gate positions, cavity number and any mold-flow study — and the process record for the production period, including melt and mold temperatures. Keep 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.