Polymers Under Pressure: Why Plastics Fail More Often Than People Think
Plastics are everywhere now. They’ve replaced metals, wood, and a long list of other materials over the last couple of decades. The shift makes sense because polymers are light, inexpensive, and easy to shape. But they also fail more often than people expect; and when they do, the problem usually starts long before the crack, split, or deformation appears.
It often begins at the design stage.
Companies sink significant cost into injection molds, and those molds rarely get modified once they’re built. If a design includes a thin wall, a thick wall, or creates an unnecessary stress concentration, that feature often stays locked in for the life of the product. You see this across plumbing components, consumer goods, car parts and even medical devices. The part functions as intended until real-world loading, temperature, or installation conditions expose the weakness.
Beyond mold design, many polymer issues come from how the material is selected. Engineers often rely on handbook values for strength, chemical resistance, or temperature limits. But polymers don’t behave like metals. Their performance depends on how they were processed, how they cool, what they contact, and what kind of loads they see over time. A material that looks good on paper can fail quickly if real service conditions aren’t considered.
A quick look at the failures that are precipitated at the design stage includes:
- Designs locked in place by costly molds
- Reliance on generalized handbook values
- Limited testing under real-world conditions
- Part sections too thick or too thin for the thermodynamics of the injection molding process
When Plastics Fail: What Really Drives Polymer Breakdown
Most polymer failures fall into familiar categories, even though they show up across very different industries. Sometimes the polymer simply wasn’t the right choice. Sometimes the environment—heat, moisture, chemistry—changes the material, once in service, more than expected.
Other failures trace back to additives that strengthen one property while softening another. In medical applications, for example, polyethylene blended with vitamin E reduces inflammation, but the same additive also softens the polymer. Softer materials handle load differently. That pattern repeats in industrial and consumer applications where tradeoffs aren't fully understood.
Even with all the ways a polymer can fail, the causes constitute a handful of categories:
- Wrong polymer selected for the environment
- Interaction with surrounding chemicals or materials
- Loads or flexing underestimated during design
- Processing variations introduced during molding
- Environmental degradation over time
These failures are predictable when you understand how the polymer behaves and how design and environment influence it.
Forensic Polymer Investigation: Uncovering the Root Cause of Failure
When a failed plastic part reaches my desk, the investigation starts with the material itself. Plastics tell their story through chemistry and structure. To read that story, we rely on tools such as infrared spectrometry, differential scanning calorimetry, melt index testing, hardness measurements, and microscopy.
Each method reveals something different: how the polymer was made, how it was processed, or where the fracture began.
These clues narrow down the possibilities until one explanation fits all the evidence. It’s the scientific method: gather the facts, develop hypotheses, test them, and let the strongest explanation stand. That disciplined approach matters in insurance claims, product liability cases, and expert witness work. The goal is always the same—identify whether the failure came from design, manufacturing, installation, misuse, or material choice.
Plastics may seem unpredictable, but they aren’t. They follow the rules of chemistry and mechanics. When a part fails, the evidence is there if you know how to interpret it. And once you understand why something broke, you can prevent the next one from doing the same.
About the Author
Richard Edwards, P.E., is a forensic engineering consultant at EDT with a background in mechanical, materials and metallurgical engineering. He brings decades of experience in metallurgical analysis, polymer and plastic failure investigation, fracture evaluation, and materials processing. His work supports root-cause determinations in product failures, industrial accidents, and complex materials cases across multiple industries.