Those tiny pinholes on the surface of sealing film may not be visible to the naked eye, but they can become pathways for oxygen and moisture to permeate. Studies have shown that even a pinhole with a diameter of just 0.3mm is enough to significantly reduce the barrier performance of aluminum foil sealing film.

The most common source of pinholes is the aluminum foil itself

The aluminum foil layer — the most critical component of aluminum foil sealing film — has an inherent tendency to have "holes." According to national standard GB/T3198-2003, pharmaceutical aluminum foil allows a maximum pinhole diameter of 0.3mm, with no more than 5 per square meter; industrial aluminum foil standards are more lenient, accepting diameters below 0.5mm. In other words, as long as the pinholes don't exceed the limit, the product is considered qualified — but "qualified" and "completely impermeable" are two different things.

And the thinner the aluminum foil, the more pinholes it has. When aluminum foil thickness drops to around 0.02mm, pinholes begin to appear; the thinner it gets, the higher the pinhole density.

The production process can also "create" pinholes

During aluminum foil rolling, hard particles in the rolling oil (such as diatomaceous earth) can be pressed into the aluminum foil surface, forming pinholes. There's also another scenario: oil pits left from rolling on the bright side of the aluminum foil, combined with depressions on the matte side, can penetrate under pressure, forming a type of "isolated pinhole" that shows distinct surrounding depressions under a microscope.

Post-processing and transport can also generate pinholes

Sealing film undergoes repeated bending during bag-making, filling, and transportation, causing the aluminum foil layer to fracture along the creases, forming new pinholes or micro-holes. These defects may initially be just breaks in the aluminum layer — with the outer plastic layer still intact, invisible to the naked eye. But after storage for some time, oxygen and moisture will seep in through these invisible channels.

Conclusion

Pinholes on sealing film come from various sources — some from the aluminum foil substrate, some from the production process, and some from damage during transport. Fortunately, experienced suppliers compensate for this through multi-layer composite structures — adding a layer of PET or PE film over the aluminum foil layer, so that even if the aluminum foil has pinholes, the outer plastic layer continues to block gases and moisture. When selecting products, rather than worrying about pinholes, it's better to focus on the supplier's quality control capabilities and lamination process expertise.