Writing a Marking, Coating or Adhesion Specification a Supplier Can Be Held To

Writing a Marking, Coating or Adhesion Specification a Supplier Can Be Held To

Key Takeaways

• Every requirement needs a method and its conditions — naming a standard is not enough when the standard permits a range.
• Say where on the part the requirement applies, or it will be verified wherever results are best.
• Separate initial from aged performance, with the aging protocol stated, or delayed failures fall outside the specification.
• Test the specification before issuing it: two people, same parts, same document, compared verdicts.

A specification exists so that two organisations can agree, without discussion, whether a delivered part is acceptable. Most of the specifications written for marking, coating and adhesion cannot do that, because they state an outcome and leave the method to be inferred. The resulting disputes are not really disagreements about parts; they are disagreements about words.

The Anatomy of an Enforceable Requirement

Every requirement that could be contested needs six things attached.

• The property being specified, stated as a measurable quantity rather than an adjective.
• The test method, with the governing standard cited by number and revision.
• Every condition the standard leaves open. For a tape test: the specific tape product, application pressure, dwell time, removal angle and removal rate. For contrast: instrument, illumination geometry, aperture and measurement locations. For pull-off: dolly size, adhesive, cure and loading rate.
• Specimen preparation and conditioning, including elapsed time between manufacture and test, and the temperature and humidity at which testing occurs.
• Location on the part, identified on a drawing where the surface is not uniform.
• Acceptance criteria and sampling: the numeric threshold, the classification scheme where one applies, how many parts, drawn how, and what constitutes a lot.

Omitting any of the six creates an interpretation gap, and interpretation gaps are always resolved in favour of whoever is more motivated at the time.

Why Location Matters More Than People Expect

Marking, treatment and coating performance vary across a part. Skin thickness on filled materials changes with flow length. Treatment coverage varies with geometry and line of sight. Knit lines run across specific faces. Mould texture differs between regions.

A specification that does not say where the requirement applies is therefore satisfied by testing at the most favourable location, which is what will happen. Where the surface genuinely is uniform, saying so costs nothing. Where it is not — which is most three-dimensional parts — identifying the critical locations on a drawing is the single highest-value clause in the document.

Initial and Aged Are Two Requirements

Most of the failures that cause commercial damage are delayed: the coating that adhered at delivery and delaminated at four months, the mark that met contrast at the line and faded after a year of cleaning. A specification containing only an initial requirement has, by construction, nothing to say about any of them.

The fix is to write two requirements. Initial performance, tested at the line under stated conditions. Aged performance, tested after a stated exposure protocol, with its own acceptance criterion. The exposure protocol must be specified in the same detail as any other test — temperature, humidity, cycling profile, chemical contact, ultraviolet, duration — and it should bear a defensible relationship to the real service environment rather than being adopted because it was convenient.

This is also where durability language needs discipline. Statements about years of service life belong in requirements documents only when accompanied by the environment they assume and the test that represents it.

Specify the Process Where the Outcome Cannot Be Measured

Some properties are genuinely difficult to verify on finished parts, and surface treatment is the clearest example: by the time the part is coated, whether it was treated correctly is no longer directly measurable.

Where that is the case, the specification should reach into the process. State the required surface energy and the point at which it is measured. State the maximum interval between treatment and the subsequent operation. State the contamination controls that apply. Require notification of changes to treatment technology or parameters. These are process requirements rather than product requirements, and including them is what makes the product requirement achievable rather than aspirational.

Material Clauses Belong in the Specification

A large share of decorating and bonding failures originate in an undisclosed material change — a slip agent added, a colourant reformulated, a regrind fraction increased. Where the downstream operation is sensitive to these, the specification should say so: prohibit migratory slip agents and external lubricants on surfaces to be bonded or coated, state the maximum regrind content, require notification of any formulation change, and require lot traceability.

Suppliers can generally accommodate these constraints when they are stated at specification time. Retrofitting them after a failure is considerably harder, and doing so without a contractual basis is harder still.

Test the Document Before You Issue It

The most effective review is empirical. Take the draft specification and a set of parts spanning clearly good, clearly bad and marginal. Give both to two competent people independently, with no discussion, and compare their verdicts.

Every disagreement locates an ambiguity, and every ambiguity is a future dispute. The exercise takes an afternoon, requires no special equipment, and finds problems that would otherwise emerge two years later with a shipment on hold and a customer waiting.

It also produces a second benefit worth having: a rough measurement system analysis. If two trained people using the specified method on the same parts reach different numbers, the method itself is not capable of supporting the acceptance limits, and no amount of drafting precision will fix that. Better to discover it before the specification is issued than during the argument it was written to prevent.

Structure and Precedence

Specifications for decorated parts frequently exist in layers: a customer specification, a corporate standard it references, an industry standard that references, and a drawing note that contradicts all three. When they disagree, nobody knows which governs, and each party reads the stack in the way that favours them.

A short precedence clause solves this — stating explicitly which document controls where terms conflict, and requiring that conflicts be raised rather than resolved unilaterally. It costs two sentences and removes an entire category of dispute.

The related discipline is to keep the specification self-contained for anything genuinely critical. A requirement that can only be understood by following three references to documents the supplier may not hold is, in practice, a requirement that will be interpreted rather than met. Where a referenced standard is essential, name the revision and state the specific clauses and options that apply, so the supplier does not have to guess which parts of a long document were intended.

Related Reading

• Proving a Specification Was Met
• Cpk and Gage R&R for Mark Contrast
• IQ, OQ and PQ for Laser Marking
• Measuring Adhesion and Abrasion Durability, Part 2
• Writing an RFQ for a Marking System

Need help with this?

The Sabreen Group provides independent engineering support for writing and reviewing technical specifications for marking, coating and bonding. Our engineering services team works with manufacturers on process development, material qualification and production troubleshooting. Contact us to discuss your application.

Frequently Asked Questions

Is citing a standard sufficient?

Rarely. Most test standards permit choices — tape type, removal angle and rate, conditioning, specimen preparation — and different choices give different results. The specification must fix every option the standard leaves open, and cite the standard revision, because methods change.

How should durability requirements be written?

With the environment attached. “The mark shall remain legible for five years” is unenforceable; five years of indoor storage, outdoor exposure, autoclave cycling or daily alcohol wiping are entirely different requirements. State the exposure, the test that represents it, and the acceptance measurement afterwards.

What sampling detail is needed?

How many parts, drawn from where in the production run, at what frequency, and what constitutes a lot. Without this, one party tests three parts from the start of a run and another tests thirty across it, and both believe they followed the specification.

Can boundary samples replace numeric criteria?

They supplement rather than replace them. Physical limit samples are genuinely useful for appearance attributes that resist measurement, but they fade, get damaged and get lost, so the specification must state how they are controlled, stored and replaced. A numeric criterion remains the enforceable backbone.

How can a specification be checked before it is issued?

Give the draft and a set of parts to two people independently and compare their verdicts. Any disagreement identifies an ambiguity that will become a dispute later. It is an afternoon of work and it finds the problems that otherwise surface years afterwards with a shipment on hold.

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