Material Selection Matrix

Material Selection Matrix: Balancing Performance, Cost, and Litigation Exposure in Automotive Plastic Parts

Automotive plastics drive innovation—from lightweight bumpers to under-hood brackets—but each resin choice carries distinct mechanical behaviors, compliance obligations, and potential liability exposures. A structured Material Selection Matrix helps align engineering goals with legal risk management. Below is a deep-dive into how polymer class, regulatory overlay, real-world failure data, and expert-witness considerations coalesce into informed material decisions.

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  1. Risk Factors by Polymer Class

Polymer Class Key Attributes Typical Failure Modes Litigation Concerns Polypropylene (PP) Low cost, excellent chemical resistance  Creep under sustained load; brittle overload in cold  Recall risk when substituted for tougher resins without re-validation (e.g., 2020 Ford Mustang brake bracket) Nylon (PA 6/6) High strength, toughness, moisture-sensitive  Hydrolysis accelerated embrittlement; fatigue crack-growth Warranty suits over engine-compartment clips that absorb water and crack under vibration Polybutylene Terephthalate (PBT) Good dimensional stability, electrical properties Stress-cracking under chemical exposure Claims tied to switch housings that craze when exposed to solvents Polycarbonate (PC) Exceptional impact resistance, transparency UV-induced yellowing; notch-sensitivity fractures Headlamp lens fractures leading to water intrusion and electrical shorts Acrylonitrile Butadiene Styrene (ABS) Tough, easy to mold, moderate heat resistance Crazing under paint or solvent exposure Litigation when sprayed-on coatings cause stress-cracks in dashboards

Table compiled from common-automotive-plastics industry surveys .

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  1. Compliance Overlay: Flammability & Safety Standards

Even a mechanically robust resin can trigger recall if it fails fire-safety requirements. Under FMVSS 302, interior-trim materials must not burn faster than 102 mm/min when exposed to a 15-second flame  . Designers substituting a lower-cost polymer must ensure pass/fail data accompany any spec change—otherwise, a legally compliant label is no bar to strict-liability claims.

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  1. Use Case: PP Brake-Pedal Bracket Recall (20V-573)

In late 2020, Ford recalled certain automatic-transmission vehicles after nylon brake-pedal brackets were swapped to polypropylene, which fractured under spike-stop loads . Though FMVSS 302 didn’t apply (bracket isn’t an interior surface), the insufficient design margin for PP’s brittle overload exposed Ford to costly field actions and expert-driven root-cause analyses. This underscores that material substitutions demand full re-qualification under both mechanical and legal lenses.

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  1. Expert Analysis: From Fractography to Failure-Mode Mapping

When a plastic part fails in the field, expert witnesses employ a toolkit of techniques: • Scanning Electron Microscopy (SEM) to distinguish ductile vs. brittle fracture facets • Fourier-Transform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC) to verify resin grade and additive packages • Fatigue-Crack Growth Testing (e.g., ASTM E 647) to quantify threshold stress-intensity factors in moisture-sensitive nylons • Non-Destructive Evaluation (CT scans) for porosity or void detection in thick-section parts

Courts require that these methods satisfy Daubert reliability factors—and under Kumho Tire Co. v. Carmichael, that gatekeeping extends to all engineering experts, not just “scientific” ones  . A well-documented validation study, peer-reviewed test protocols, and known error-rate analyses bolster admissibility and persuasive impact.

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  1. Implementing the Material Selection Matrix
    1. Define Weightings: Assign quantitative scores (1–10) for performance (strength, fatigue life), cost, compliance difficulty, and litigation risk.
    2. Populate Options: List candidate resins—PP, PA 6/6, PBT, PC, ABS—with their scores based on lab data and historical claim rates.
    3. Calculate Composite Scores: Use a weighted sum to highlight the optimal balance for the intended application.
    4. Document Decisions: Maintain a “decision-record” dossier, linking each cell in the matrix to test reports, supplier certifications, and legal-risk summaries.
    5. Periodic Review: Update the matrix with new field-failure data, recall bulletins, and evolving regulations to keep risk assessments current.

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  1. Doctrinal Foundations & Precedents • MacPherson v. Buick Motor Co. (1916) removed privity barriers, establishing that manufacturers owe a duty of care to end-users of inherently dangerous products, a principle courts apply equally to plastic component claims . • Restatement (Third) of Torts § 402A imposes strict liability for defective products—meaning a failed PP bracket that departs from intended design spec can trigger liability even absent negligence. • Kumho Tire Co. v. Carmichael (1999) confirmed that trial judges must scrutinize engineering-based testimony under Daubert’s gatekeeping framework, ensuring expert-driven material analyses meet rigour and reliability thresholds .

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Conclusion

A Material Selection Matrix marries engineering metrics with legal foresight—guiding plastic-part designers through the risk/reward landscape of performance, cost, compliance, and liability. By systematically scoring polymers, embedding compliance tests (FMVSS 302, ASTM standards), and preparing Daubert/Kumho-ready expert validation protocols, manufacturers and counsel can confidently innovate while minimizing downstream recalls and litigation.

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