A COPE-on-PC overmold that passes every initial peel test in the mold shop can still generate a wave of field returns eight months after launch — and the cause is almost always one of four process variables that initial testing never touched.
Failure Pattern: Delamination That Only Shows Up at the Parting Line
When bond failure concentrates specifically along the parting line or gate-adjacent region rather than across the whole overmold surface, the usual cause is a weld line in the elastomer coinciding with the bond zone. Gate placement that routes flow across the bond surface, rather than along it, avoids this — but a design that was gated for moldability rather than bond-line integrity will reliably show this exact failure pattern months into production, not on day one, since a marginal weld-line bond degrades gradually under service stress rather than failing immediately.
Failure Pattern: Whitening or Crazing at the Bond Interface Months After Molding
Chemical stress cracking (CSC) is polycarbonate’s most distinctive and most delayed failure mode, and it’s frequently misdiagnosed as an adhesion problem when it’s actually a chemical-compatibility problem. Plasticizers, solvents, or aromatic compounds in a TPE’s additive package can migrate into stressed PC and initiate crazing over weeks or months — well after the part has shipped and passed every initial QC check. Any crazing or whitening at a bond line disqualifies that specific material combination regardless of how strong the initial bond tested, and the fix requires reviewing the full additive formulation from the TPE supplier, not just retesting peel strength with a fresh sample.
Failure Pattern: Adhesion That Was Never Actually Reliable, Just Inconsistent
SEBS on PC without an adhesion promoter produces results that can look adequate in a small qualification batch while hiding wide bond-strength scatter across a full production run. Low-end samples from that scatter fall below structural requirements even though the average result looked acceptable — this is a statistically different problem than a uniformly weak bond, and it requires sampling enough parts across a production run (not just the first article) to actually see it. Where SEBS is preferred for cost reasons despite PC’s ester-dominated surface chemistry, adhesion-promoting tie-layer compounds or silane-based coupling agents applied before overmolding are what convert an inconsistent bond into a reliably strong one.
Failure Pattern: A Bond That Only Fails After the Product Has Been in Service
Thermal cycling reveals bond weaknesses that as-molded testing never exposes, because COPE and PC have different coefficients of thermal expansion, and every thermal cycle in service adds a small increment of cumulative interface stress on top of whatever bond quality existed at molding. A part that passes room-temperature peel testing on day one can still delaminate after the several hundred thermal cycles a real product sees over its service life — which is why thermal-cycling validation, not just as-molded peel strength, needs to be part of the qualification plan before tooling is finalized rather than added after a field-failure investigation.
A Pre-Launch Diagnostic Checklist
Before committing to production tooling, four checks catch the large majority of PC-TPE field failures before they happen: confirm gate location routes elastomer flow across, not along, the intended bond surface; obtain the TPE’s full additive formulation and screen it for CSC risk on the specific PC grade in use; sample bond strength across a statistically meaningful number of parts from a full production-representative run, not just a handful of qualification samples; and validate bond integrity through a realistic thermal-cycling protocol before finalizing the process. Email Us if you’d like help scoping this checklist against a specific PC grade and TPE sub-class combination.
Failure Pattern: A Bond That Looks Fine Until a Repair Technician Uses the Wrong Cleaning Solvent
Field service and repair technicians sometimes clean or degrease a PC-TPE assembly using whatever solvent is on hand, rather than one confirmed compatible with polycarbonate. Aggressive solvents can craze or soften PC at the exposed bond edge during routine field maintenance, well after the original molding and qualification process is long finished — a failure mode that never shows up in mold-shop testing because it’s introduced downstream, in the field, by a process the original design team never controlled. Specifying an approved cleaning-solvent list as part of the product’s service documentation closes this gap.
Process Controls That Reduce All Four Failure Patterns at Once
Pre-drying PC substrates before overmolding (typically several hours at an elevated temperature) reduces moisture-related surface defects that compound with any of the above failure modes, and stress-relieving PC inserts before overmolding lowers residual molding stress that acts as a CSC accelerant when combined with TPE additive exposure. Cleaning PC surfaces with isopropyl alcohol rather than a stronger solvent avoids introducing a second stress-cracking trigger before the overmold even begins. For applications where an adhesive-bonded joint fits better than overmolding, Incure’s Uni-Weld™ plastic bonder line covers PC among its supported substrates, and Incure’s broader TPE compatibility guide across substrate families is a useful reference when a design spans more than just PC.
Most TPE-on-PC field failures trace back to a process variable that initial mold-shop testing never exercised — gate location, additive chemistry, sample size, or thermal history. Contact Our Team for help building a pre-launch validation plan that catches these before tooling commitment.
Visit www.incurelab.com for more information.