Most plastic parts that crack in the field were never touched by a chemical agent. The cause is usually built in at the mold — or the moment of impact.
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Not every cracked plastic part involves a chemical agent at all. A weld line where two melt fronts meet and never fully fuse is inherently weaker than the surrounding material; a sink mark over a rib is also a stress concentration and often a void; a sharp internal corner is a notch that a ductile-looking resin will crack from regardless. These are mechanical and processing failures, and they are diagnosed differently than environmental stress cracking or chemical attack — the fracture surface, the mold-flow pattern, and the part’s own processing history are the record, and reproducing the loading condition that broke it is usually the fastest way to close the loop.
Most cracking traces to how the part was designed, molded, or loaded — not to any chemical the part was exposed to.
Incomplete fusion where two melt fronts meet around a core or through a multi-gate pattern, creating an inherently weaker plane in the part.
Shrinkage over thick sections or ribs concentrating stress and reducing effective wall thickness at the point of failure.
A sharp internal radius concentrating stress well beyond nominal, cracking a resin that would otherwise perform adequately.
A single overload event or repeated cyclic loading propagating a crack from an existing flaw or stress riser.
Excessive melt temperature, residence time, or regrind content degrading molecular weight and embrittling the resin before the part is ever loaded.
Uneven cooling or mold design locking in stress across the part that adds directly to whatever load it sees in service.
The investigation moves from the part’s design and molding history to the fracture surface itself.
The same mold or process usually made many more of the same part:
Fracture surfaces and processing records — resin lot, regrind ratio, molding parameters — are both part of the evidence. Keep every piece and pull the process traveler before it is purged.
Design defects tend to be geometric and repeatable across every part made from the tool — a sharp internal corner, an underbuilt rib, a weld line positioned exactly where load concentrates. Molding defects are more variable from part to part and lot to lot — inconsistent packing pressure creating sink and voids, degraded regrind content, or a cooling imbalance causing warpage. Comparing multiple failed parts, and ideally parts from different production dates, against the mold and process documentation is what separates a design issue baked into every part from a process issue affecting only some of them.
A weld line, or knit line, forms where two separate melt fronts meet inside the mold and re-fuse — around a hole, a boss, or downstream of multiple gates. Even with correct processing, that fusion plane is inherently weaker than the surrounding bulk material because the polymer chains do not fully entangle across it, and orientation effects can make it weaker still. A part that is adequately strong everywhere except along a weld line is a common and specific failure pattern, and its location is predictable from the mold-flow pattern and gate layout.
Yes. Excessive melt temperature, extended residence time in the barrel, or a high percentage of regrind can degrade a resin’s molecular weight before the part is ever molded, leaving it measurably more brittle than virgin material processed correctly — even though the certificate of analysis for the raw resin looks entirely normal. FTIR and thermal analysis on the finished part, compared against a known-good reference, can identify this kind of processing-induced degradation independent of the resin specification.
A handful of failed parts is usually enough to establish the failure mechanism. Determining whether it is systemic requires comparing those against unused parts from the same and different production lots, ideally spanning the full range of dates and tooling involved. That comparison is what tells you whether you are looking at an isolated outlier or a defect built into the mold, the process, or the material specification across the whole run.
The failed parts themselves, unmodified, along with the mold and process traveler — resin lot numbers, regrind ratio, melt and mold temperatures, and cycle time — before those records are purged or overwritten. Preserve exemplar unused parts from the same and different lots for comparison, and document the loading or assembly condition each failed part was under when it cracked.
Technical briefings from our work in this area.
A part can be measurably brittle while the resin certificate reads entirely normal. Thermal history in the barrel, moisture at the hopper and regrind content all degrade material before the tool sees it.
readA defect built into the geometry appears in every part from the tool. A process defect varies between lots and cavities. How failures distribute across a production population separates the two.
readWhere two melt fronts meet, the polymer never fully re-entangles. That plane is weaker than the material around it, its location is predictable, and it is a recurring source of field cracking.
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