Key Takeaways
- Find the interface before you change anything — whether the failure is adhesive, cohesive or substrate decides which third of the process to investigate.
- Most “paint problems” are surface problems. Low surface energy, mold release and additive bloom cause far more delamination than the coating itself.
- Delayed failure is a different disease from immediate failure — migration, moisture and stress take weeks, and a launch-day adhesion test will not see them coming.
- Change one variable at a time and re-test with the same method. Shotgun fixes cure symptoms and leave the root cause in the process.
“The paint is peeling” is a symptom, not a diagnosis, and treating it as a coating problem is the reason so many delamination investigations run for months. Coating and ink failure on molded plastics is almost always a failure of the interface between two materials rather than a failure of either material, and the interface has a history: resin selection, additive package, molding conditions, handling, surface preparation, ambient environment, application and cure. Any of those can put the defect there. The efficient path is not to start changing coatings; it is to establish where the failure plane sits, then work backwards through the short list of mechanisms that can produce failure at that plane.
Step One: Locate the Failure Interface
Before any process change, determine what separated from what. Three answers are possible and each points at a different third of the process.
- Adhesive failure leaves clean substrate on one side and clean coating on the other. The bond between coating and plastic never formed properly. Investigate surface energy, contamination and pretreatment.
- Cohesive failure leaves coating on both faces — the coating tore through its own thickness. The interface held; the coating is under-cured, over-thick, embrittled or the wrong chemistry for the stress it sees.
- Substrate failure pulls polymer away with the coating. The bond and the coating are both stronger than the surface layer of the plastic, which usually means a weak boundary layer — degraded material, a bloomed additive layer, or a resin-poor skin.
A stereo microscope at 10–40x settles this in minutes. Where the visual answer is ambiguous, infrared spectroscopy on both fracture faces will identify what is actually present on each side. Skipping this step is the single most expensive mistake in coating investigations, because adhesive and cohesive failures have almost no causes in common and every hour spent optimising the wrong one is wasted.
Step Two: Measure the Surface, Not the Part
If the failure is adhesive, surface energy is the first quantity to establish. Untreated polyolefins sit near 29–31 mN/m; acetal, TPEs and fluoropolymers are similarly unreceptive. Most coatings and inks want the low-to-mid forties before wetting is reliable. Take dyne readings at the actual coating location on production parts — not on a plaque, not on the flat area that is easy to reach — and take them at the same interval after treatment that production experiences.
Two readings tell most of the story. A part measured immediately after treatment establishes whether the pretreatment equipment is capable at all. A part measured after the real dwell between treatment and coating establishes whether the treatment survives the process. Treatment decay is routine and its rate depends on polymer, additive package, storage temperature and humidity; a line that works in winter and fails in summer is often a treatment shelf-life problem rather than a coating problem.
Step Three: Rule Out Contamination
Contamination defeats good treatment and good coatings alike, and it is easy to miss because the contaminant is usually invisible and present in quantities measured in monolayers. The usual suspects are silicone-bearing mold release, hand oils, airborne overspray from adjacent operations, plasticiser and slip additive that has migrated to the surface, and residue from a cleaning solvent that left more behind than it removed.
The diagnostic that costs nothing is the fresh-part comparison. Mold a part, treat it and coat it within minutes, under supervision. Good adhesion on that part with poor adhesion in production isolates the problem to what happens in between. It converts an open-ended materials question into a bounded handling question, and it can be run on a single shift.
Step Four: Separate Immediate Failure From Delayed Failure
Failure timing carries diagnostic information that most investigations discard. A coating that fails a cross-hatch test at the end of the line has a wetting, cure or chemistry problem. A coating that passes at end of line and fails at four weeks has a different disease entirely, and the candidates are specific: additive migration to the interface, moisture ingress and hydrolysis, residual moulded-in stress relaxing, thermal cycling driving differential expansion, or continued cure embrittling the film.
Delayed failures are the ones that reach customers, and they are the reason adhesion specifications built solely around immediate testing are inadequate. Every specification worth writing pairs an immediate test with an aged test on the same parts, in conditions that bracket the real service environment.
Step Five: Change One Variable at a Time
The temptation when parts are failing and a shipment is due is to change several things at once — more treatment power, a different primer, a longer flash, a hotter oven. It sometimes works, and it always destroys the information the investigation needed. If a stack of simultaneous changes fixes the line, nobody knows which one mattered, nobody knows how much margin the process now has, and the failure returns as soon as one of them drifts.
The disciplined alternative is a structured screening experiment across the two or three factors the earlier steps implicated, holding the rest fixed and measuring adhesion with a single consistent method — cross-hatch to a defined classification, pull-off, or a tape test with a specified tape and angle. It takes days rather than hours, and it produces a process window with known limits rather than a configuration that happens to work today.
The Short List, Ranked
Across most investigations the causes cluster in a predictable order. Insufficient or decayed surface treatment comes first by a wide margin. Mold release and silicone contamination is second. Additive migration — slip agents, plasticisers, flame retardants blooming to the surface — is third and disproportionately common in delayed failures. Incorrect coating chemistry for the substrate is fourth, cure schedule problems fifth, and moulded-in stress and design geometry sixth. Weak boundary layers from thermal degradation during moulding round out the list, and are worth suspecting whenever the failure is substrate-side.
None of that ranking substitutes for the interface determination in step one. It is a prior, not an answer — useful for deciding what to test first, dangerous if it becomes what you assume.
Related Reading
- Adhesive Bond Failures
- Surface Pretreatments and Surface Quality
- The Science of Solving Plastics Adhesion Problems
- Mold Release and Silicone Contamination
- Reading a Fracture Surface
Need help with this?
The Sabreen Group provides independent engineering support for diagnosing coating and ink adhesion failures on molded plastics. Our engineering services team works with manufacturers on process development, material qualification and production troubleshooting. Contact us to discuss your application.
Frequently Asked Questions
Why does paint adhere in testing but peel in the field?
Laboratory panels are usually molded, treated and coated in quick succession under controlled conditions, while production parts sit in bins, absorb moisture, bloom additives to the surface and experience thermal cycling. Migration and stress-driven failures need days or weeks to develop, so a launch-day cross-hatch test cannot see them. Correlate every adhesion specification with an aging protocol that reproduces the real exposure.
Is low surface energy always the cause of delamination?
No, but it is the most common single cause on polyolefins, acetal, fluoropolymers and TPEs. A quick dyne check separates the two families of problem. If the surface reads in the low thirties the treatment is the issue; if it reads in the forties and coating still fails, look at contamination, cure schedule, coating chemistry or the substrate itself.
How do I tell contamination from insufficient treatment?
Treat a fresh part and coat it immediately. If adhesion is good, the surface preparation is capable and the problem is what happens between treatment and coating — handling, storage, airborne silicone, release agent transfer. If a freshly treated part still fails, the treatment window or the coating chemistry is wrong.
Can regrind or recycled content cause coating failure?
Yes. Regrind carries whatever was on the surface of the previous part, including mold release, printing inks and coatings, and repeated heat history changes the additive package. Recycled content can also carry slip agents and unknown contaminants. Trial a virgin-resin lot as a diagnostic control before rebuilding a coating line.
When should we bring in outside failure analysis?
When the failure crosses departmental boundaries — molding blames coating, coating blames the resin, the supplier blames handling — or when the same defect returns after a fix. An independent analysis identifies the failure interface and the causal mechanism rather than the department, which is usually what unlocks a stalled investigation.
