After sealing, you notice a few small bumps on the film surface. Not cracks, not peeling — just those little bubbles, not quite reassuring.

Where do they actually come from? Different bubbles have different origins — it depends on which type you're dealing with.

The most typical type is regular bubbles — they appear in the same spot every time, roughly the same size. This usually points to equipment parameters. Temperatures too high or uneven pressure cause localized overheating of the heat-seal layer, which shrinks inconsistently upon cooling, leaving bubbles on the film. If this is your issue, try lowering the sealing temperature by a few degrees, then check if the heating plate is level and the pressure is balanced on both sides.

The second type is bubbles in random positions — shifting left and right, varying in size. This is usually related to the condition of the bottle mouth. Residual water stains, oil, or dust on the bottle mouth create an isolation layer during sealing, preventing the heat-seal layer from fully bonding — and air gets trapped inside. Wiping the bottle mouth with a lint-free cloth before sealing can make an immediate difference.

The third type is large-area bubbles — when you're using non-separating gaskets, and the bubbles cover large patches with blurred edges. The problem often isn't with the film itself, but with the paperboard layer of the gasket. The paperboard has absorbed moisture, and during the instantaneous heating of sealing, the moisture turns to steam that can't escape, pushing up the paperboard surface. Trying a batch of gaskets stored in a dry environment usually resolves this.

There's also bubbles with tiny black spots — a small black dot at the center of the bubble, which is dust or debris. It gets trapped between the film and bottle mouth during sealing, lifting the aluminum foil layer to create a tiny void. Improving workshop cleanliness will reduce this type of issue.

If you've tried all these directions and still can't resolve it, Leadtra Packaging can help you troubleshoot — send us a sample, and we'll examine the position, shape, and distribution of the bubbles to determine whether it's a parameter issue, a bottle mouth issue, or a material issue — and then provide targeted adjustment recommendations.