Pinholes in aluminium foil: causes, testing and what to accept
A pinhole is a hole you cannot see, in a material whose only job is to be a barrier. It is also the defect buyers complain about most and understand least.
Pinholes are micro-perforations that form as foil is rolled thin, caused by inclusions, roll-surface defects or micro-folds. They increase sharply below 12 μm, which is why thin gauge requires online light-testing. For household wrap they are a quality indicator, not a safety issue — the real consequence is barrier loss, not contamination.
Ask a supplier "how many pinholes?" and you will usually get a shrug. Ask "do you light-test online?" and you will learn something useful in one sentence, because a mill that tests knows its number and a mill that does not test has never had to.
What causes them
Gauge
The dominant factor. As foil thins, the sheet's cross-section approaches the size of the defects already present in the metal and on the roll surface. Below roughly 12 μm, pinhole counts rise steeply; at 9–10 μm, they become the defining quality characteristic rather than an occasional flaw.
Inclusions and impurities
Non-metallic particles in the melt create discontinuities as the sheet is reduced. This is a melt-quality issue — which is why the alloy and the mill's filtration discipline matter, and why 1235 and 8011 do not perform identically at the same gauge.
Roll surface defects
A scratch, a mark or a build-up on the work roll is transferred to the sheet on every revolution. This produces pinholes in a repeating pattern, which is how a mill engineer identifies the cause quickly.
Folds and handling
A micro-fold during rewinding can crack the sheet along the crease. This is a converting defect rather than a rolling one, and it is the one most likely to appear in a slitting operation that is running too fast or under uneven tension.
How they are detected
| Method | Who uses it | What it tells you |
|---|---|---|
| Online light transmission | Mill, at production speed | Continuous count per square metre; the real quality control |
| Light box, offline | Converter, QA | Spot check of a sample length |
| Dye penetration | Laboratory | Confirms a specific suspect area |
| Air permeability | Laboratory, packaging spec | Barrier performance rather than hole count |
| Backlit visual | Anyone, in a dark room | A rough indication, useful for supplier comparison |
The last one is the one you can do yourself, today, with a phone torch. It will not give you a number per square metre, but it will tell you the difference between a supplier whose 10 μm is controlled and one whose 10 μm is a lottery.
In a dark room, hold a one-metre length of foil against a bright lamp or phone torch and count the points of light. Do it for three samples from three different suppliers at the same stated gauge. The comparison is the result — absolute counts mean little without a reference.
What to accept
There is no universal number, because acceptance depends on what the foil is for. The useful framing is by application:
The practical rule for importers: specify the test, not the number. "Online light-tested, defect count declared on the MTC" gives you a mechanism for accountability. "No pinholes" gives you a claim that nobody can verify and that will be quietly ignored.
When a customer complains
A complaint about "the foil tears" is usually not a pinhole complaint, even when the customer says it is. Distinguish three failure modes before responding:
- Tears from the edge inward — a slitting or core problem, not pinholes
- Tears along a line — a fold or crease from rewinding
- Tears randomly in the sheet — genuine gauge or pinhole issue
This matters commercially. The first two are converting problems the supplier can fix on the next run; the third is a mill issue. Diagnosing it correctly is the difference between a corrective action and an argument.
Frequently asked questions
At what thickness do pinholes become a real problem?
Below about 12 μm the count rises quickly, and by 9–10 μm it is the dominant quality characteristic. Above 15 μm pinholes are rare enough that they stop being a useful differentiator between suppliers.
Does a pinhole make food unsafe?
Aluminium itself is not a hazard in this context. The problem is that the barrier is compromised — moisture escapes and oxygen enters, so food dries, oxidises or spoils faster. That is a quality issue, not a safety one, except in sterile packaging where it becomes critical.
Can I demand zero pinholes?
Only at 15 μm and above, and only with an agreed test method. At 10 μm, demanding zero is a way of ensuring the supplier either refuses or quietly ships something and hopes you do not test. Specify the test instead.
Sources
- EN 546-4 — Aluminium foil, special requirements (including porosity testing).
- Related: Thickness guide.