June 10, 2025 | Quality Control
When a customer reports "a surface problem", the first move is neither apology nor defence — it is deciding which defect this actually is. The same appearance can have opposite causes, and fixing in the wrong direction makes it worse.
| Defect | Appearance | Usual cause | Prevention |
|---|---|---|---|
| Missed coat | Bare patch, sharp edge | Damaged roll, paint supply interruption | Roll inspection, level interlocks |
| Particles | Small raised spots | Dust in the booth, poor filtration | Positive booth pressure, scheduled filter changes |
| Colour drift | Shade varies across or between sheets | Oven temperature swings, uneven film | Cure curve monitoring, first-off colour check |
| Scratches | Linear marks down to substrate | Debris on rollers, handling impact | Roller cleaning, edge protection |
| Craters | Small round pits with bare centre | Oil contamination, insufficient defoamer | Thorough degreasing, paint filtration |
| Orange peel | Uneven, peel-like surface | Paint viscosity too high, poor flow | Viscosity control, controlled ramp rate in oven |
| Roll marks | Periodic stripes | Damaged coating or pressure roll | Roll inspection and regrinding |
| Edge build-up | Thicker film at the edges | Uneven roll pressure | Roll pressure adjustment, edge scrapers |
1. Calling orange peel an emboss. Emboss has a regular, directional pattern; orange peel is irregular relief from poor flow. Run a hand over it — emboss has a direction, orange peel does not.
2. Calling roll marks colour drift. Roll marks are periodic with fixed spacing; colour drift is diffuse with no period. Measure the spacing and the question answers itself.
3. Calling transit scratches production scratches. Look at direction and location: marks running with the coil direction and spread across the face are usually the line; transverse marks concentrated at edges and corners are usually handling.
Preserve evidence first — photographs, batch number, sealed sample. Then classify using the table. Only then discuss responsibility and remedy. Reverse the order and the evidence is gone, the cause is unclear, and both sides split the loss.
Coil coating is a continuous process; absolute zero is not realistic. What works is an agreed acceptance standard: what area, how many points, and which zone counts as a reject. Writing that into the acceptance clause does more than repeating that your quality is good.
