An induction seal liner is a laminated disc that sits inside a cap; when the capped container passes under an induction sealer, the aluminium foil layer heats up and melts a polymer coating onto the container rim, giving a hermetic, tamper-evident seal. This guide explains how the foil and the other layers work, how to choose a structure for your bottle and product, and how to test and troubleshoot the seal. Metline supplies the soft-temper 8011, 8079 and 1235 foil that liner converters laminate, to customers worldwide.
Induction sealing is a non-contact way of bonding a foil membrane to the mouth of a plastic or glass container after it has been filled and capped. The liner is pressed against the container land (the flat top of the neck) by the cap torque. The capped container then runs under a sealing head that produces a high-frequency alternating magnetic field. That field induces eddy currents in the aluminium foil, the foil heats within a second or two, and the heat melts the sealing layer underneath it, which fuses to the container rim. As the assembly cools, the foil membrane is welded across the opening.
The aluminium layer does three jobs at once. It is the heating element (only an electrically conductive layer couples with the induction field; paper, foam and plastics do not). It is the barrier: a continuous aluminium foil blocks oxygen, water vapour, light and aroma far better than any polymer film of similar thickness. And it is the visible tamper indicator: once the membrane is punctured or peeled, the consumer can see it.
Because the heat is generated inside the liner and nowhere else, the product, the cap shell and the container walls stay close to room temperature. That is why induction sealing works on hot-filled sauces, pharmaceuticals and chemicals alike, and why it is fast enough for lines running hundreds of containers a minute.
Every induction liner is a laminate. The exact build varies by supplier and by product, but the functional layers are always the same family. Reading from the cap downwards to the container:
| Layer | Typical materials | Function |
|---|---|---|
| Backing | Pulpboard, polyethylene foam, or none | Fills the cap, transmits torque evenly to the land, and in two-piece liners stays in the cap as a reseal gasket |
| Release layer | Wax or a temporary polymer bond | Two-piece liners only: melts during sealing so the foil separates from the backing |
| Support film | PET (polyester) film | Adds puncture and tear resistance, keeps the membrane flat, and gives a smooth surface for print |
| Aluminium foil | 8011-O, 8079-O or 1235-O foil, roughly 12 – 40 µm | Induction susceptor, gas and moisture barrier, tamper indicator |
| Heat-seal layer | PE, PP, PET-compatible or glass-compatible coatings and films | Melts and fuses to the container rim; chosen to match the container polymer and the product |
The heat-seal layer is the part most often specified wrongly. Polyethylene will bond to HDPE and LDPE but not to polypropylene; a PP-compatible sealant is needed for PP necks; PET bottles need a sealant formulated for polyester; glass requires a coating that wets a mineral surface. Some suppliers offer "universal" sealants that bond to several polymers, usually with a peel strength compromise. The foil layer, by contrast, rarely changes between these structures: the same soft-temper foil works under all of them, with gauge chosen for the cap size and barrier needed.
Foil gauges above are indicative of common practice. The right gauge for a given liner is set by the converter after sealing trials on the actual container and sealer.
Foil, usually with a thin foam or paper backing, bonded entirely to the container. The whole liner stays on the bottle after the cap is removed. Simple and economical for products used up quickly or where a reseal gasket is not needed, such as many powders and dry goods.
Pulpboard or foam backing joined to the foil by wax. Induction heat melts the wax, which is absorbed into the board, so the foil stays on the container and the board stays in the cap as a secondary seal after opening. The normal choice for liquids, creams and products used over weeks.
Part of the foil is folded or laminated over itself to form a tab that is not sealed to the rim. The consumer pulls the tab rather than puncturing the membrane. Common on supplements, coffee, infant formula and premium foods.
A sealant formulated to peel cleanly from the rim without leaving foil or fibre behind. Used where consumers open the pack by hand and where torn foil residue would be unacceptable.
A score or partial seal leaves a pull ring or flap; often used on engine oil and lubricant bottles so the user can remove the membrane without tools.
Incorporates a microporous membrane that lets gas escape while holding back liquid. Used for products that off-gas, such as some cleaners, bleach-based chemicals and agrochemicals, to stop containers from bulging or collapsing.
Induction liner foil is supplied in the fully annealed O temper. Soft foil conforms to the land without springing back, laminates without wrinkling, and does not crack at the edge of the container mouth when the cap is torqued. Annealing also burns off rolling oil, leaving a clean, dry surface that adhesives and coatings wet properly; converters normally check wettability before lamination.
| Alloy | Family | Key composition (wt %) | Why converters use it |
|---|---|---|---|
| 8011 | Al-Fe-Si | Fe 0.60 – 1.00, Si 0.50 – 0.90, Al remainder | The workhorse packaging foil: good strength and dead-fold in thin gauges, widely available |
| 8079 | Al-Fe | Fe 0.70 – 1.30, Si 0.05 – 0.30, Al remainder | Lower silicon gives better ductility and fewer pinholes in very thin foil; preferred for high-barrier laminates |
| 1235 | Commercially pure | Al 99.35 % minimum, Si + Fe 0.65 max | High purity, excellent formability and conductivity; used where softness matters more than strength |
| 1145 | Commercially pure | Al 99.45 % minimum | Occasionally specified for laminates in North America |
Composition limits are those registered with the Aluminum Association and repeated in EN 573-3. Mechanical property limits for foil depend on gauge and are set by the order specification (ASTM B479 or EN 546-2); the mill certificate reports actual values for the lot.
Three foil properties matter most to the liner converter. Pinholes: thin foil always contains a few, and the count rises steeply below about 9 µm, which is one reason liner foil is rarely that thin. Gauge uniformity: induction heating is proportional to the amount of metal under the coil, so a foil that varies in thickness heats unevenly and gives patchy seals. Surface cleanliness: residual oil weakens the bond between foil, adhesive and sealant, and can cause the laminate to delaminate in service, particularly with oils and solvents in the product.
| Container material | Typical products | What to watch |
|---|---|---|
| HDPE / LDPE | Agrochemicals, lubricants, cleaners, dairy, supplements | PE sealants bond strongly; check resistance to solvents and surfactants in the product |
| PP | Food jars, pharmaceutical and supplement bottles | Requires a PP-compatible sealant; PE sealant will not bond reliably |
| PET | Edible oil, sauces, beverages, personal care | Needs a PET-specific sealant; land flatness on blow-moulded necks is critical |
| PVC | Some chemical and personal-care packs | Sealant must be confirmed by trial; less common today |
| PS | Specialty food, laboratory and pharmaceutical containers | Bonds with selected sealants; test peel and leak performance |
| Glass | Coffee, honey, supplements, pharmaceuticals | Glass-specific coatings; neck finish tolerances are wider, so a cushioning backing helps |
The product inside matters as much as the container. Essential oils, solvents, fragrances and some surfactants can migrate through the sealant and attack the adhesive between the sealant and the foil, causing delamination weeks after filling. Strong acids and alkalis can corrode the foil itself if the sealant is thin or pinholed. Oils and fats can soften some sealants. For any aggressive product, the converter should run an accelerated storage test (for example, inverted containers held at elevated temperature) before approving the liner.
Food-contact compliance is normally addressed at the laminate level: the sealant and any layer that may touch the food must comply with the regulations of the market, such as EU Regulation (EC) No 1935/2004 and the plastics rules in Regulation (EU) No 10/2011, or the relevant parts of US 21 CFR. For the aluminium itself, EN 602 sets the composition requirements for aluminium intended to contact food, and 8011, 8079 and 1235 meet it.
Because the membrane is the first thing a consumer sees after twisting off the cap, many brands use it as a small display surface. The print is normally applied to the top face of the foil or to the PET layer, then protected by a lacquer or by lamination so that ink does not transfer to the backing board during sealing.
Common options include a brand logo or "sealed for your protection" message, opening instructions on tabbed liners, gold or coloured lacquers, embossed patterns that give a premium feel, and holographic or micro-text features that are hard to copy and help distributors spot counterfeit products. Agrochemical, lubricant and pharmaceutical brands in markets with a counterfeiting problem use these features routinely.
Two practical points apply. First, inks and lacquers must tolerate the brief temperature spike during sealing without blistering or discolouring, so they should be part of the sealing trial. Second, heavy embossing reduces the flat foil area in contact with the land and can create leak channels if it extends into the seal zone; keep relief patterns inside the bore diameter.
A visually perfect seal can still leak, so liner approval and routine production checks use a combination of the methods below. None of them is specific to induction seals; they are general package-integrity tests applied to sealed containers.
| Test | What it shows | Typical method |
|---|---|---|
| Visual inspection after removing the cap | Complete fusion around the land, no wrinkles, no unsealed segments | Every shift, on samples from each sealing head |
| Peel test | Bond strength and the failure mode (clean peel, fibre tear, foil tear) | Hand peel against reference samples, or a tensile tester following ASTM F88 principles |
| Vacuum leak test | Leaks through channels, wrinkles or pinholes | Container submerged in water under vacuum, watching for bubbles (bubble-emission method, as in ASTM D3078) |
| Squeeze or inversion test | Gross leaks in liquid products | Inverted or squeezed containers stored on absorbent paper |
| Accelerated storage | Delamination or corrosion from product attack | Inverted containers held warm for several weeks, then peeled and inspected |
| Removal-torque check | Whether the cap can be opened after sealing | Torque meter; sealing can raise removal torque on some closures |
Many sealing lines also use inline detection: sensors that confirm the presence of a foil liner in each cap before sealing, and systems that reject containers whose liner did not draw the expected power. These catch missing liners and severe under-sealing but not marginal seals, so they supplement manual sampling rather than replace it.
| Symptom | Likely causes | What to check first |
|---|---|---|
| Weak seal or no seal | Power too low for line speed; liner upside down; wrong sealant for container; insufficient cap torque | Liner orientation, sealant type, power and speed settings |
| Partial seal (one side only) | Container not centred under the coil; cap cocked; land not level | Guide rails, cap application, neck finish quality |
| Burnt or scorched liner, melted cap | Power too high or dwell too long; foil gauge too heavy for the settings | Reduce power or increase speed; confirm foil gauge with the converter |
| Wax residue on foil, backing stuck to foil | Overheating in two-piece liners, or underheating so wax did not release | Adjust power window; check backing board grade |
| Wrinkled membrane | Excessive torque, liner diameter too large for the cap, or foil without PET support | Liner size and torque |
| Seal fails after days or weeks | Product attacking adhesive or sealant; caps removed before cooling; back-off of torque during storage | Chemical compatibility and storage test results |
| Difficult to remove membrane | Over-sealing, or a sealant designed for weld rather than peel | Sealant type and power settings |
A common mistake is to fix a weak seal by turning the power up until it seals, without asking why it was weak. If the root cause is a non-flat land or low torque, more power simply moves the problem: the high spots overheat and scorch while the low spots still do not bond. Fix the mechanical cause first, then establish a power window with margin on both sides.
Foil for converters. Liner foil is supplied as slit coils, with width, core inside diameter (commonly 76 mm or 152 mm) and maximum roll diameter set to the laminator. Each coil is identified with alloy, temper, gauge and lot number so it can be traced through the laminate to the finished liner.
Finished liner formats. Converters supply laminated liner stock in rolls or sheets for punching, or pre-cut discs that are inserted into caps by the closure maker. Diameter is matched to the cap, from small pharmaceutical necks to wide-mouth food jars.
Storage. Keep foil and liner stock in sealed packaging in a dry, temperature-stable store, away from direct sun and chemicals. Condensation on cold foil causes staining and weakens lamination bonds, so let coils reach room temperature before unwrapping.
Shelf life. Laminated liners with wax release layers or heat-activated sealants are best used within the period the converter specifies, typically measured in months, and rotated first-in, first-out.
Metline supplies the aluminium foil, not finished liners or sealing equipment. Liner converters, closure makers and packaging buyers who need 8011, 8079 or 1235 soft foil for lamination can request slit coils to their width and core, with a mill test certificate for each lot.
Yes. The seal forms only when the foil is heated by an induction field, which needs a handheld, benchtop or conveyor induction sealer. Simply screwing the cap on leaves the liner as an unbonded gasket. Conduction heat sealers are a different process and use different lidding foils.
8011-O is the most widely used because it combines strength with good dead-fold and is readily available. 8079-O is preferred for thin, high-barrier laminates because its lower silicon content gives fewer pinholes. 1235-O suits liners where maximum softness matters. All are supplied fully annealed.
In a one-piece liner the whole liner bonds to the container and comes off with the membrane. In a two-piece liner a wax layer melts during sealing so the foil stays on the container while the pulpboard or foam backing stays in the cap, giving a reseal after opening.
Most often because the liner has a polyethylene sealant, which does not bond to polypropylene. Specify a PP-compatible or universal sealant. Also check that the liner is not upside down and that cap torque is high enough to hold the liner flat against the land.
Yes, they are standard on both. Oils can soften some sealants, so the structure should be confirmed with storage tests. Engine oil bottles often use ring-pull or partial-peel liners so the user can remove the membrane without a tool.
No. Metline supplies the aluminium foil, in 8011, 8079 and 1235 alloys and O temper, slit to width for liner converters and laminators worldwide, with a mill test certificate for each lot.
Tell us the alloy, gauge, slit width, core size and destination. We reply with availability, certification and a delivered price.
