Heat-seal lacquer (strong seal)
A thin coated layer, often vinyl- or polyolefin-based, that forms a permanent seal. Used for induction seals and tamper-evident closures where the foil must be torn to open.
Sealing foil
Heat Seal Aluminium Foil: Lidding, Induction and Blister Grades
Heat seal aluminium foil is foil carrying a lacquer or film that melts and bonds to a container when heat and pressure are applied. The alloy, temper and gauge set the foil's strength and formability, while the heat-seal layer decides what it seals to (PP, PE, PS, PET, PVC or paper) and whether it peels cleanly or welds shut. Metline supplies bare, lacquered and laminated sealing foil in reels for converters making cup lids, bottle liners, blister lidding and pouches.
- Alloys
- 8011, 8021, 8079, 1235
- Gauge
- About 20 – 90 µm
- Seal layers
- Lacquer, film, laminate
- Seals to
- PP, PE, PS, PET, PVC, paper
How heat seal foil works
Bare aluminium does not bond to plastics. A heat-seal foil therefore has at least two functional layers: the aluminium, which provides the barrier to oxygen, moisture, light and aroma, and a thermoplastic sealing layer on the product side. When a heated die presses the foil onto the container rim, the sealing layer softens, wets the rim and solidifies into a bond as it cools.
The outside face is usually printed or protected with a primer or overlacquer so it does not stick to the hot sealing head. In laminated structures, a PET or paper layer on the outside adds stiffness, printability and puncture resistance.
Three variables control a heat seal: temperature, pressure and dwell time. Too little heat leaves a weak or leaking seal; too much burns the lacquer, distorts the container rim or makes a peelable lid tear. A good foil supplier gives the converter a sealing window, not a single number, and the packer fine-tunes it on the line.
| Sealing method | How heat is applied | Typical uses |
|---|---|---|
| Conduction (heat) sealing | Heated die presses foil onto the rim | Yogurt and dairy cups, food trays, blister packs, capsules |
| Induction sealing | Electromagnetic field heats the aluminium inside a closed cap | Bottles and jars for food, beverages, chemicals and pharmaceuticals |
| Impulse and band sealing | Short pulse or moving heated band | Pouches and bags with laminated foil |
Foil grades by sealing application
| Application | Usual alloys | Typical gauge | Temper | Sealing layer |
|---|---|---|---|---|
| Cup and dairy lids | 8011, 8021, 8079 | About 30 – 50 µm | O (soft) | Peelable lacquer for PS, PP or PET cups |
| Induction bottle liners | 8011, 8079, 1235 | About 20 – 50 µm in the liner | O or H18 depending on structure | PE, PP or PET heat-seal film; universal grades for mixed bottles |
| Push-through blister lidding | 8011, 8021, 1235 | About 20 – 25 µm | H18 (hard) | Heat-seal lacquer for PVC, PVDC-coated PVC or PP |
| Peel and peel-push blister lidding | 8011, 8079 | Varies with laminate | O | Foil laminated to PET or paper, peelable lacquer |
| Cold-form (Alu-Alu) blister base | 8021, 8079 | About 45 – 60 µm | O | Laminated OPA / Alu / PVC; sealed to lidding foil |
| Food trays and containers | 8011, 3003 | Lid foil about 30 – 70 µm | O or H22 | Lacquer for aluminium, PP or PET trays |
| Pouches and flexible packs | 8011, 8079 | About 7 – 12 µm in laminate | O | PE or CPP sealant film in PET / Alu / PE laminate |
Gauges are typical ranges used in the industry; the right gauge depends on rim size, filling and sealing equipment, and the barrier and puncture requirements of the pack.
8011 is the general-purpose packaging alloy and the most common choice for lids and liners. 8021 and 8079 contain more iron, which gives finer grains and higher strength and elongation in soft temper, so they resist pinholes at thin gauges and form well in cold-form blisters. 1235 is a high-purity foil alloy used where a clean surface and consistent pinhole performance matter, such as pharmaceutical and printed foil.
Heat-seal lacquers, films and laminates
Peelable lacquer
Formulated so the bond fails cleanly at the interface under a controlled force. The standard for yogurt and dessert lids and peelable food trays.
Heat-seal film
A coextruded PE, PP or PET film laminated to the foil. Thicker than lacquer, it tolerates rough or contaminated rims and gives higher seal strength.
Universal sealants
Formulations that seal to several polymers, useful where a packer runs different bottle materials on one line.
Paper or PET laminate
An outer layer that adds stiffness, print quality and puncture resistance, and protects the foil in handling.
Primers and overlacquers
Primers anchor print and lacquer to the foil; overlacquers protect print and prevent sticking to the sealing head.
The sealing layer must be compatible with the container polymer. PP lacquer will not bond reliably to PS, and a PE film will not seal to PET. Always name the container material, rim finish and whether the pack is filled hot, retorted, pasteurised or frozen when specifying foil.
Induction seal liners for bottles and jars
In induction sealing, a foil liner sits inside the cap. After capping, the bottle passes under an induction head; eddy currents heat the aluminium, which melts the heat-seal layer onto the bottle neck. The foil gauge must be heavy enough to carry the induced current without overheating locally, and light enough to tear cleanly when the consumer opens it.
| Liner type | Construction | After opening |
|---|---|---|
| Two-piece liner | Pulp or foam backing, wax, foil, heat-seal layer | Wax melts and the backing stays in the cap as a reseal; foil stays on the bottle |
| One-piece liner | Foam or film backing laminated to foil and heat-seal layer | Entire liner stays on the bottle and is peeled off |
| Tabbed or lift-and-peel liner | One-piece liner with a tab for easy removal | Consumer lifts the tab to peel the seal |
Metline supplies the foil and foil laminates that converters punch into liners. See our induction seal liner foil guide for liner-specific constructions.
Sealing conditions and how to test seals
Sealing temperatures commonly fall in the range of roughly 150 – 250 °C, depending on the lacquer, foil gauge and container. Thin peelable lids seal at the lower end; heavy laminates and high-speed lines run hotter. Dwell times on cup lines are often a fraction of a second.
- Agree the windowUse the lacquer supplier data as a starting point for temperature, pressure and dwell.
- Run a sealing curveMake test seals across the temperature range, following a practice such as ASTM F2029, and plot seal strength.
- Measure seal strengthTest peel force per unit width with ASTM F88 or a cup-lid peel method. For peelable lids, aim for a force that holds in distribution but peels without tearing.
- Check integrityUse burst or leak tests (for example dye penetration or vacuum decay) for packs needing hermetic seals.
- Validate in real conditionsRepeat after retort, pasteurisation, freezing or transport, because heat and moisture change seal behaviour.
Quality, food contact and pharmaceutical requirements
- Pinholes: pinhole counts rise as gauge falls; 8021 and 8079 help at thin gauges. Specify the limit and test method.
- Surface cleanliness: annealed foil should be free of rolling-oil residue that can weaken lacquer adhesion; a wettability test is common.
- Food contact: aluminium foil composition per EN 602; coatings must meet the applicable rules, such as EU Regulation 1935/2004 and FDA 21 CFR 175.300 for resinous coatings.
- Pharmaceutical: blister lidding is qualified against the medicine pack specification, including seal strength, lacquer compatibility and printed surface quality.
- Dimensions: gauge tolerance, reel width, core diameter, outer diameter and splice limits stated on the order.
- Documents: mill test certificate (EN 10204 3.1) for the foil and declarations of compliance for coated grades.
References and standards
- EN 546 – Aluminium and aluminium alloys: Foil
- EN 602 – Wrought products: chemical composition of semi-products used for articles in contact with food
- ASTM F88 – Standard Test Method for Seal Strength of Flexible Barrier Materials
- ASTM F2029 – Standard Practices for Making Heatseals for Determination of Heatsealability
- Regulation (EC) No 1935/2004 on materials and articles intended to come into contact with food
Heat seal aluminium (aluminum) foil: frequently asked questions
What is the difference between heat sealing and induction sealing?
Heat (conduction) sealing presses the foil onto the container with a heated die, as on cup and tray lines. Induction sealing heats the foil liner inside a closed cap with an electromagnetic field, so the seal forms after capping. Both melt a heat-seal layer on the foil.
Which alloy is best for yogurt cup lids?
8011 or 8079 in soft O temper, typically around 30 – 50 µm, with a peelable lacquer matched to the cup material (PS, PP or PET). 8079 gives better pinhole resistance and elongation at thinner gauges.
Why is blister lidding foil hard temper?
Push-through blister lidding uses about 20 – 25 µm H18 foil because hard foil ruptures cleanly when the tablet is pushed through. Soft foil would stretch instead of breaking. Peel-push and child-resistant packs use laminated soft foil instead.
Can one foil seal to both PP and PET containers?
Universal heat-seal lacquers and films exist that bond to several polymers, but performance is always a compromise. For best results, match the sealing layer to the container material and test on the actual line.
Does Metline supply finished lids or liners?
No. Metline supplies the aluminium foil, bare, lacquered or laminated, in reels slit to width. Die-cut lids, liners and blister packs are made by converters and packers from this foil.
What sealing temperature should I use?
Start from the lacquer or film data, commonly somewhere around 150 – 250 °C, then run a sealing curve on your equipment to find the window that gives the required seal strength without burning or tearing.
Sourcing heat seal foil?
Tell us the application, container material, gauge, seal type and reel format. We reply with a foil proposal and a delivered price.
