Before shipment, a PPGI coil should be checked with a documented plan: sampling frequency agreed in the contract, coating mass or thickness verification, dry film thickness readings on both faces, T-bend and impact adhesion tests, MEK solvent rub, gloss meter reading, color Delta E measurement, coil weight and dimension checks, and an eye-to-sky visual inspection of the outer wraps and exposed laps. The acceptance criteria — numeric limits, test methods, sampling plan and who bears the cost of a failed lot — belong in the purchase contract, not in a general guide. This article explains what each test tells you, how to write it into a contract, and where buyers most often lose money.
Nothing below is a substitute for the exact edition of the standard your project references. Standards such as those from ASTM, EN, ISO, JIS or GB are revised; confirm the required edition and the exact contract values with your technical team and supplier before you sign.
Why Pre-Shipment Inspection Is a Contract Issue, Not a Factory Favour
Inspection performed only as a courtesy has no remedy attached to it. If a coil arrives with coating adhesion failure, the buyer’s position depends on whether the contract named a test method, a sampling frequency, an acceptance limit and a consequence. Verified general industry practice is that pre-shipment inspection is most useful when it is written as a specification annex to the purchase order.
A practical annex usually covers four things:
- Test list and methods — which test, which standard, which instrument class.
- Sampling plan — how many coils per lot, how many positions per coil.
- Acceptance limits — numeric values with tolerances and units.
- Non-conformance route — retest, concession, replacement, or third-party arbitration.
Buyers sourcing coated coil alongside uncoated material often run the same inspection logic across both product families; the base-metal checks overlap heavily with those used for GI coil, while the paint-film checks are specific to prepainted product.
Sampling Frequency: How Much to Test
Sampling frequency is the single most negotiated item, because it drives inspection cost. There is no universal number. Three approaches are common in industry practice:
Lot-based sampling
Define a lot as coils produced in one continuous campaign with the same substrate, coating mass, paint system and colour. Sample a fixed number of coils per lot — for example, the first, middle and last coil — and treat the lot as the inspection unit. This suits repeat orders where process stability is already established.
Coil-based sampling
Test every coil, but at reduced positions per coil. This suits high-value or colour-critical orders where a single off-tone coil is expensive to sort downstream.
Tiered sampling
Start with tighter sampling, then reduce frequency if consecutive lots pass. Write the reduction rule into the contract so it is not a matter of goodwill. If any lot fails, revert to the tighter level.
For coating thickness, a defensible default is multiple readings per face at both the head and tail end of the coil, since coating weight can vary along the length. For destructive tests such as T-bend and impact, samples are usually taken from the coil ends or from a test piece supplied with the lot, because the test destroys the material.
Coating Mass, Dry Film Thickness and the Difference Between Them
These are frequently confused, and the confusion causes disputes.
Metallic coating mass refers to the zinc or zinc-alloy layer on the base steel, usually expressed in g/m² or as a coating designation. It is normally verified by stripping tests or by the supplier’s process records, and it is a property of the substrate, not the paint.
Dry film thickness (DFT) refers to the cured paint system — primer plus topcoat, and back coat if specified. It is measured non-destructively with a magnetic or eddy-current gauge calibrated to the substrate and to the paint system.
Practical points for the contract:
- State whether DFT limits apply to the front face, the back face, or both, and give separate values.
- State whether the limit is a minimum, a range, or a nominal with tolerance.
- State the number of readings per position and how the average is calculated.
- Note that gauge readings on a coated coil can be affected by curvature, surface profile and calibration foil choice; a single reading is not evidence.
If the DFT is below the specified minimum, corrosion protection and colour durability may both be reduced. If it is far above, forming behaviour can change and the buyer may be paying for paint that was not required.
Adhesion Tests: T-Bend, Impact and MEK Rub
Adhesion is where prepainted coil most often fails in service, and it is the area where contract wording matters most.
T-bend
The coated sample is bent back on itself through progressively tighter bends. The result is reported as the tightest bend at which the coating shows no cracking or flaking — commonly written as 0T, 1T, 2T and so on. The lower the number, the tighter the bend the coating survives. The correct acceptance value depends on the forming severity of the end application, the paint system, the substrate thickness and the direction of bending relative to the coil rolling direction. Specify the value your forming operation actually needs, not a generic figure.
Impact
A falling weight or similar impact test deforms the coated panel. The coating is then examined, often with tape applied and removed, for cracking or detachment. Report the impact energy and the pass/fail criterion, and state which face is tested.
MEK rub
A solvent rub test uses methyl ethyl ketone to assess the degree of cure of the paint film. The result is typically reported as the number of double rubs before the coating breaks through or the substrate becomes visible. This is a cure indicator, not a durability prediction. Specify the solvent, the rubbing method, the cloth or felt used, the load if any, and the pass threshold.
For all three tests, the contract should name the test method and edition, the specimen location, the number of specimens, and the acceptance value. Where the buyer’s forming process is severe, it is worth agreeing a forming trial on the buyer’s own equipment rather than relying on a laboratory bend value alone.
Gloss, Colour and Delta E
Appearance disputes are common because they are easy to see and hard to quantify without agreed instrumentation.
Gloss
Gloss is measured with a gloss meter at a specified geometry — commonly 60°, with 20° or 85° used for very high or very low gloss surfaces. The contract should state the geometry, the measurement angle, the number of readings, the average, and the tolerance band. Gloss measured at different angles is not comparable.
Colour and Delta E
Colour is measured with a spectrophotometer, and the deviation from the approved reference is expressed as Delta E. The contract should state:
- The colour space and illuminant/observer combination used.
- Whether the reference is a physical master panel or a numeric target.
- The Delta E limit, and whether it applies to the coil body, the ends, or both.
- Whether metamerism — colour shift under different light sources — is addressed.
A tight Delta E limit sounds rigorous but can be unachievable across a long production run and may simply transfer cost to the buyer. Set the limit based on how the finished product is viewed and assembled, not on the tightest number available.
Coil Weight, Dimensions and Eye-to-Sky Inspection
Physical checks catch problems that laboratory tests miss.
- Coil weight — verify against the packing list and the agreed tolerance; weight affects crane capacity, uncoiler loading and freight calculation.
- Inner and outer diameter — confirm compatibility with the buyer’s uncoiler mandrel and handling equipment.
- Width and thickness — measure at several positions; confirm whether thickness is measured on the coated or uncoated product.
- Eye-to-sky inspection — with the coil axis horizontal, view the outer wraps and the exposed lap edges. Look for edge damage, telescoping, coil break, scratches, paint rub-through, moisture staining, rust spots and lap marks.
- Edge condition — burrs and edge cracks can propagate during slitting and roll forming.
- Packing and protection — eye of the coil, wrapping, desiccant use where relevant, and edge protection.
Eye-to-sky inspection is a visual method and therefore subjective. Photograph any defect, record the coil number and position, and reference the agreed defect classification in the contract so that “minor” and “rejectable” are defined before the coil leaves the works.
Practical Specification Table
The table below shows how a contract annex can be structured. The values are placeholders — the buyer and supplier must agree actual numbers for the specific application.
| Check | Typical method | Sampling guidance | Contract must state |
|---|---|---|---|
| Metallic coating mass | Stripping or supplier process record | Per lot, per agreed frequency | Target mass, tolerance, test method and edition |
| Dry film thickness | Magnetic/eddy-current gauge | Multiple readings per face, head and tail | Front and back limits, averaging rule, instrument class |
| T-bend | Progressive bend | Per lot or per sampled coil | Bend rating, specimen direction, pass criterion |
| Impact | Falling weight | Per lot or per sampled coil | Energy, face tested, tape test inclusion |
| MEK rub | Solvent rub | Per lot | Solvent, method, double-rub threshold |
| Gloss | Gloss meter | Multiple readings per sample | Angle, target value, tolerance |
| Colour | Spectrophotometer | Multiple readings per sample | Reference, colour space, Delta E limit |
| Weight and dimensions | Scale, tape, micrometer | Every coil | Tolerances, measurement basis |
| Visual | Eye-to-sky | Every coil | Defect classification, rejection criteria |
RFQ and Inspection Checklist
Send this list with your enquiry so the quotation and the inspection plan are aligned from the start.
- Product: PPGI coil, with substrate specification and metallic coating mass.
- Paint system: primer type, topcoat type, back coat requirement, total DFT target with tolerance.
- Colour: reference panel or numeric target, colour space, Delta E limit, gloss value and angle.
- Mechanical: T-bend rating required by your forming process, impact requirement, MEK rub threshold.
- Dimensions: thickness basis, width, inner diameter, coil weight range and tolerance.
- Sampling: number of coils per lot, positions per coil, destructive test specimen source.
- Inspection: who inspects, at what stage, whether third-party inspection is required and who pays.
- Documentation: mill test certificate, coating records, inspection report format, photographs of defects.
- Non-conformance: retest rights, concession procedure, replacement or credit terms, claim window.
- Packing and marking: coil eye orientation, wrapping, edge protection, labelling and traceability.
- Commercial: quantity, delivery basis, required date, and a dated quotation with validity period.
Price for PPGI coil is driven mainly by substrate thickness and grade, metallic coating mass, paint system type and total DFT, colour and gloss specification, coil dimensions, order quantity, packing and delivery terms. Because these inputs move, ask for a dated quotation that states the specification basis and validity period rather than comparing headline numbers.
Common Mistakes Buyers Make
- Specifying tests without methods. “Adhesion test” is not a specification; a named method with an edition and a numeric pass value is.
- Copying Delta E and gloss limits from another project. Limits should follow how the finished product is viewed and assembled.
- Confusing coating mass with dry film thickness. They describe different layers and are tested differently.
- Testing only the coil head. Coating condition can vary along the length; sample both ends.
- Ignoring the rolling direction. Bend and impact results change with specimen orientation relative to rolling direction.
- Leaving sampling frequency to the supplier. Frequency determines both cost and confidence; agree it in writing.
- No claim window. Without a defined period and procedure for raising non-conformance, a defect found after arrival has no clear route.
- Treating a lab pass as a performance guarantee. Actual service performance depends on substrate, metallic coating, paint system, environment, forming and maintenance, and cannot be reduced to a single test result.
FAQs
How many coils should be inspected before shipment?
There is no universal number. Common practice is to define a lot and inspect a fixed number of coils per lot, or to test every coil with fewer positions per coil. The choice depends on order value, colour criticality and how established the supplier’s process is. Agree the frequency in the contract.
What Delta E limit should I write into the contract?
Set the limit from the visual requirement of the finished product and the assembly context, then confirm it is achievable across a full production run. Also state the colour space, illuminant and observer, and whether the reference is a physical panel or a numeric target.
Is MEK rub a durability test?
No. MEK rub primarily indicates the degree of cure of the paint film. It does not predict how long a coating will last in a given environment. Actual performance depends on substrate, metallic coating, paint system, environment, forming and maintenance.
Can I rely on the supplier’s inspection report alone?
A supplier report is useful evidence, but its value depends on whether the methods, sampling and limits match your contract. Where the order value justifies it, buyers appoint an independent third-party inspector and specify in the contract who bears that cost.
What is eye-to-sky inspection and is it objective?
It is a visual inspection of the coil with the axis horizontal, examining outer wraps and exposed laps. It is inherently subjective, so pair it with a written defect classification, photographs and a defined rejection threshold.
Should I reference a standard in the contract?
Yes, where one applies — but confirm the required edition and the exact contract values, because standards are revised and different editions can carry different limits. The contract value governs in a dispute, not a general recommendation.
Next Step
If you are preparing an enquiry, send your application details, forming severity, colour and gloss requirements, and the inspection items you want covered. ChinaCNU Steel can respond with a specification-based quotation and an inspection plan aligned to your contract annex. Start from the PPGI coil page for product parameters, compare base-metal options on the GI coil page, and submit your RFQ through contact us so the technical team can confirm achievable values before you finalise acceptance criteria.
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