Failure mode
Pinholes in Aluminium Foil
Hold the foil up to a light and you see bright points. On a packaging line they show up as barrier failure; in pharmaceutical blister they are a rejection, not a defect to be negotiated.
What it actually is
Hold the foil up to a light and you see bright points. On a packaging line they show up as barrier failure; in pharmaceutical blister they are a rejection, not a defect to be negotiated.
Mechanism
Why it happens
Four causes account for most of it. They are not equally common, and they are not equally within the mill's control.
Melt cleanliness
Hydrogen and non-metallic inclusions carried over from the casting stage. If melt filtration is coarse, oxides survive into the rolled gauge and become pinhole nuclei.
Rolling oil filtration
Metal fines in the rolling oil get pressed into the surface and tear out at thin gauge. Filtration quality matters more than the mill admits.
Reduction per pass
Aggressive reduction at the final passes stretches the metal past what the inclusion population can survive.
Gauge threshold
Pinhole probability rises sharply below 0.009 mm. Double-zero foil is a different process discipline from 0.02 mm, not the same product rolled thinner.
Verification
How it is measured
Pinhole count is measured by the transmitted-light method: a defined area of foil is placed over a light box in a dark enclosure and the bright points are counted. ASTM B479 and GB/T 3198 set the method and the acceptance limits by gauge. Ask for the count per unit area, not a pass/fail statement.
Specification
How to specify around it
Most of these cost nothing to write into an order and are difficult to enforce afterwards. This is the part worth copying into your next RFQ.
1
Write the limit into the order
State maximum pinholes per square metre at your gauge. "Thickness 0.006 mm" alone does not control this.
2
Name the application
Household, container, pharmaceutical and capacitor foil have different tolerances for it. Tell the mill which one runs on your line.
3
Require degreasing for pharma
Annealed and degreased foil with a clean surface is a separate specification from mill-finish foil.
4
Ask for the batch record
A mill running pinhole counts per coil has the data. One that only offers a certificate of analysis does not.
We ask which line the foil runs on before quoting, and we ask the mill for the pinhole count record for that batch. Where the application is pharmaceutical or capacitor, we say so on the enquiry so the allocation goes to a foil mill rather than a general rolling mill.
We do not claim this never happens. Metal is rolled, shipped across an ocean and unloaded by someone else. What we do is ask the question before the order runs, put the answer in writing, and be the single party that answers for it afterwards.