Proving a Marking or Adhesion Specification Was — or Was Not — Met

Proving a Marking or Adhesion Specification Was — or Was Not — Met

Key Takeaways

• Most disputes are specification failures, not process failures. If the requirement was never measurable, neither party can prove compliance.
• A test method without conditions is not a specification — tape, angle, rate, temperature and timing all change the result.
• Sampling and acceptance criteria must be stated, or every party reads the same data differently.
• Retained samples decide these cases. Without them the argument reduces to competing recollections.

When a customer rejects parts because the mark rubbed off or the coating peeled, and the supplier maintains that the parts met specification, the argument that follows is rarely about the process. It is about a document. And in the majority of these disputes, examination of that document shows that the requirement was never capable of being proven met or unmet by anyone.

The Failure Happens at Drafting

Requirements such as “the marking shall be permanent”, “the coating shall adhere satisfactorily” or “the print shall not be removable under normal handling” appear in real purchase specifications constantly. Each is untestable. Permanence over what period, under what exposure? Satisfactory by what measurement? Normal handling by whom?

Because no measurement can settle them, both parties can hold entirely sincere and completely incompatible positions. The customer applies a thumbnail and sees material transfer; the supplier applies a specified tape test and records a pass. Neither is being unreasonable, and no amount of further testing resolves it, because the disagreement is about what the words meant.

What a Provable Requirement Contains

A specification becomes provable when a competent third party, given the document and a sample, would reach the same verdict as either party. That requires more than naming a standard.

• Method and standard, precisely cited — including revision, because methods change.
• Every condition the method leaves open. For a tape test that means the specific tape product, application pressure and dwell, removal angle and removal rate. Each of these changes the result materially, and standards frequently permit a range.
• Sample preparation and conditioning: temperature, humidity, and elapsed time between manufacture and test. A coating tested one hour after cure and one tested a week later are different measurements.
• Location on the part. Marking and adhesion vary with geometry, flow and treatment coverage, so a requirement that does not say where it applies is satisfied by testing wherever results are best.
• Sampling plan and acceptance criteria: how many parts, drawn how, and what classification constitutes a pass.
• Aged as well as initial performance where the failure of interest is a delayed one, with the aging protocol stated.

Marks Need Numbers, Not Adjectives

Marking specifications suffer from the same problem in a distinctive form, because appearance is subjective and lighting-dependent. A mark judged excellent under a bench lamp can be judged unacceptable under diffuse daylight, and both observers are describing what they see honestly.

The usable alternatives are measurement-based. Optical contrast between mark and substrate, measured with a stated instrument, illumination geometry and aperture, produces a number both parties can obtain. Machine-readable codes have established verification standards that produce a letter or numeric grade under specified conditions, which is why code requirements are far less often disputed than human-readable ones. Approved boundary samples — a physical best-acceptable and worst-acceptable part, signed and controlled — are a useful supplement, provided the specification states how they are stored, how often they are replaced, and that they do not themselves fade.

Testing an Existing Dispute

Where the specification is already inadequate and parts are already rejected, the analysis shifts. The first question becomes what a competent supplier in this industry would reasonably have understood the requirement to mean, and what method would ordinarily be applied to verify it. Industry practice, the supplier’s own qualification records, the customer’s incoming inspection procedure and any pre-production approval activity all inform that.

The second step is comparative testing under identical conditions on both parties’ retained samples. Identical conditions is the operative phrase: two laboratories running nominally the same tape test with different tapes and different removal rates will produce different verdicts, and each will believe the other made an error. Agreeing the method before testing — ideally jointly, or through a mutually accepted third party — is what makes the results decisive rather than merely additional.

Retains Decide These Cases

The single most valuable practice for anyone who supplies or buys decorated, marked or bonded plastic parts is a disciplined retention program. Samples from each lot, sealed, labelled with lot and date, stored in stated conditions, held by both parties independently.

Their value is that they answer the question the argument actually turns on: were the parts as delivered acceptable, or did something happen afterwards? Without retains that question is unanswerable, and the dispute degenerates into competing recollections about handling, storage and use. With them, it is a laboratory exercise. The cost of the program is trivial against a single rejected shipment, which is why its absence is so often the most expensive omission in the whole supply arrangement.

Preventing the Next One

The lasting fix is to treat the specification as a deliverable in its own right and to test it before production. Take the draft requirement, give it to two people independently, have each measure the same set of parts, and compare their verdicts. Disagreement means the specification is ambiguous and will be disputed eventually.

This exercise takes an afternoon and finds the gaps that otherwise emerge two years later with a shipment on hold. It also produces a useful by-product: the measurement system analysis that shows whether the chosen method is even capable of distinguishing acceptable from unacceptable parts reliably, which is a question surprisingly few specifications have ever had asked of them.

When the Parts Changed After Delivery

A distinct class of dispute arises where both parties agree the parts were acceptable on delivery and disagree about what happened next. Marks fade, coatings degrade, bonds weaken — and the question becomes whether the product was inadequate or the conditions were outside what was specified.

These are answerable, but only with information that has to be gathered deliberately. Retained samples from the same lot, stored in controlled conditions, establish what the product was capable of at delivery. Exposure records — temperature, humidity, chemical contact, ultraviolet, cleaning regime — establish what the delivered parts actually experienced. Comparative testing of retains against returned parts then shows whether the difference is consistent with the recorded exposure or requires an additional explanation.

The lesson for specification writing is that durability requirements need an environment attached. A requirement that a mark remain legible for five years means nothing without stating five years of what: indoor storage, outdoor exposure, repeated autoclave cycles and daily contact with alcohol wipes are entirely different requirements, and only one of them was probably intended.

Related Reading

• Writing an Enforceable Specification
• Measuring Adhesion and Abrasion Durability
• Reading a Fracture Surface
• When to Bring In a Plastics Expert Witness
• Cpk and Gage R&R for Mark Contrast and Adhesion

Need help with this?

The Sabreen Group provides independent engineering support for resolving supplier disputes over marking, coating and adhesion specifications. 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 do adhesion specifications cause so many disputes?

Because they are frequently written as an outcome without a method. A requirement that a coating “shall adhere” or “shall not be removable by normal handling” cannot be tested objectively, so both parties can hold sincere and incompatible views about whether it was met. The dispute is created at the drafting stage, not at the failure.

What must a testable adhesion requirement include?

The test method and its governing standard, the exact conditions including tape type, removal angle and rate where applicable, sample preparation and conditioning, where on the part the test is performed, how many samples, how results are classified, and the acceptance threshold. Any of these left unstated becomes an argument later.

How is mark quality specified so it can be proven?

Through measurable optical parameters rather than appearance: contrast measured with a defined instrument and geometry, or for machine-readable codes, grade to an established verification standard using a specified aperture and illumination. Approved physical boundary samples supplement but do not replace numeric criteria.

What if the specification is already inadequate and parts are in dispute?

Establish what a competent supplier would reasonably have understood the requirement to mean, and what industry-standard method would ordinarily apply. Then test both parties’ retained samples by that method under identical conditions. This does not always resolve liability, but it converts an unfalsifiable disagreement into a technical finding.

Who should hold retained samples?

Both parties, independently, from the same production lots, with sealed packaging and documented storage conditions. Retains held by only one side are always open to the objection that they are unrepresentative, and that objection is difficult to answer.

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