Quick answer: An induction foil liner is not defined by its aluminium foil. The aluminium is only the heating element. Sealing performance is determined by the polymer film that faces the container rim, the wax or adhesive layer that releases the foil from the closure, and the board or foam backing that holds everything in place under the cap. Specifying a liner by foil thickness alone is the single most common reason a liner that passed a bench test fails on a high-speed filling line.
Hangzhou Xinxing Pharmaceutical Packaging has manufactured sealing liners since 2001, and in that time the same conversation has repeated with almost every new customer: a liner was selected on foil thickness and diameter, the sample sealed acceptably by hand, and then the production line produced a leak rate the bench test never predicted. The cause is almost always that the liner was treated as a single material rather than a four-layer system.
What is actually inside an induction foil liner
A one-trip induction liner for a pharmaceutical or cosmetic bottle is built up from four functional layers, each doing a different job:
| Layer | Typical material | Function | What fails if it is wrong |
|---|---|---|---|
| Sealant film (faces product) | PE, PP, PET, EVA or a co-extrusion | Bonds to the container rim under heat and pressure; provides the chemical barrier | Poor bond, channel leaks, or a film that is attacked by the product |
| Aluminium foil | 20–40 µm household-grade foil | Converts the alternating magnetic field into heat | Weak or uneven heating; sealing only at the rim edge |
| Wax or adhesive release layer | Microcrystalline wax, hot melt, or a peelable adhesive | Holds the liner in the cap before sealing; releases cleanly after | Liner drops out in the hopper, or tears when the cap is removed |
| Backing | Paperboard, EPE foam, or PET film | Gives the stack rigidity for automatic cap sorters | Jams in the cap feeder; inconsistent seating pressure |
The sealant film is the layer that matters most, and it is the layer a foil-thickness specification does not describe. A 25 µm foil paired with a PP sealant behaves completely differently from the same foil paired with a PE sealant, because the two polymers have different sealing windows and different resistance to the product behind them.
Why the sealant film choice decides the application
Three product variables drive the film selection, and they are not interchangeable.
Container material. The liner seals to the rim of the bottle, not to the cap. An HDPE bottle rim and a PET bottle rim need different films because the two plastics soften at different temperatures. A PE sealant that forms a strong bond to HDPE may not bond reliably to PET at the same induction setting.
Product chemistry. Aqueous formulations, alcohol-based cosmetic lotions, essential oils and aggressive solvents interact with polymers differently. A film chosen for a water-based syrup can be swollen, plasticised or extracted by a fragrance oil, which shows up months later as a leaking or delaminating seal rather than as an immediate failure.
Thermal process. If the filled bottle is terminally sterilised, the seal must survive the autoclave cycle. A liner designed for ambient storage and a liner designed to withstand 121 °C steam sterilisation are different products, even when both seal correctly on the filling line.
The filling line decides as much as the product does
Two parameters on the sealing machine determine whether a correct liner actually performs:
- Induction power and dwell time. Too little energy and the bond is incomplete; too much and the wax layer boils, the sealant film scorches, or the foil burns through. The correct setting is specific to the liner stack, the cap material, and the line speed.
- Top-load pressure during sealing. The sealant film must be held against the rim while it is molten. Insufficient head pressure produces a seal that looks complete but has thin or unbonded regions.
Both are reasons why a hand-held sealer result cannot be transferred directly to a rotary line. The hand sealer applies more dwell and more pressure than most high-speed lines, so a liner that seals perfectly by hand can fail at 200 bottles per minute.
A practical specification checklist
| Specify this | Not this |
|---|---|
| Container material and rim geometry (neck finish) | "Standard 28 mm" |
| Product chemistry and pH | "Liquid" |
| Maximum process temperature (autoclave or pasteurisation) | "Ambient" |
| Sealant film polymer | "Aluminium foil" |
| Peel type required (peelable / weld / clean peel) | Not stated |
| Line speed and induction sealer model | Not stated |
| Certification requirement (GMP, food contact, migration limits) | "Pharma grade" |
A specification built on the left column can be quoted immediately and sampled with confidence. A specification from the right column produces a sample that will need two or three rounds of adjustment before it can be approved.
Frequently asked questions
Does a thicker aluminium foil give a stronger seal?
No. Foil thickness affects how quickly the stack heats and how evenly heat spreads, not how well the sealant film bonds. Once the foil is thick enough to heat uniformly across the rim, additional thickness adds cost and can increase dwell time requirements without improving seal strength.
Can I use the same liner for HDPE and PET bottles?
Only if the product chemistry and process conditions are identical, and even then the induction settings will differ. The sealant film bonds to the rim material, so the two container plastics should be validated separately.
My liner seals on the bench but leaks on the line. What changed?
Almost always head pressure or dwell time. Hand sealers apply more of both. Measure the actual top-load pressure at the sealing head and confirm the dwell time against the line speed before changing the liner.
Do I need a different liner for terminally sterilised product?
Yes. A liner that must survive 121 °C steam sterilisation needs a sealant film and backing chosen for that cycle. Heat-stable backing materials and a film that does not embrittle in the autoclave are both required.
What information do you need to quote a custom liner?
Container material and neck finish, product chemistry, process temperature, required peel behaviour, line speed, annual volume, and any certification or migration limits. A drawing or a physical sample of the closure speeds this up considerably.
Specify the liner as a system, not as a material
Hangzhou Xinxing Pharmaceutical Packaging manufactures induction foil liners, hot melt adhesive liners, foam liners and pressure sensitive liners in a 100,000-class cleanroom, and has supported pharmaceutical, cosmetic and food packaging customers since 2001. If you send us the container material, product chemistry and process conditions, our engineering team can recommend the correct layer stack and provide a sample for line trial.