Diameter comes first
The liner is die-cut to the bottle mouth, so diameter is the first number to pin down. We cut any diameter in the Ø 9–182 mm range, round, tabbed or to a custom outline. The cut size is not simply "the mouth opening" — it is set slightly against the lip so the foil covers the sealing land without overhanging into the thread area, where it would catch on the cap.
If you are unsure, measure the inside diameter of the cap or the outside diameter of the lip, and send a photo or drawing. We will confirm the correct cut size in your proposal rather than let you guess to the millimetre.
Thickness is the sum of four layers
An induction liner is not a single sheet. Its total thickness is the sum of four layers: the paperboard backing, the release wax, the 20 µm aluminium foil and the heat seal lacquer. The foil is the constant that matters — it is the active barrier and the layer the induction field heats — while the backing is the variable we tune.
- Paperboard backing. Chosen for stiffness and cap depth. A thicker board gives a firmer disc and a stronger reseal ring in two-piece builds; a thinner board suits shallow caps and one-piece finishes.
- Release wax. A thin separation layer that lets the membrane peel cleanly from the backing on first opening.
- Aluminium foil, 20 µm. The barrier and the heat target. It is what makes the seal hermetic and tamper-evident.
- Heat seal lacquer. The layer that melts onto the mouth; its gauge is matched to the polymer and the lip profile.
How the foil gauge is chosen
The foil is the constant everyone quotes — 20 µm — but it is worth understanding why that number matters. The foil is the barrier: it is what stops moisture and oxygen, and it is the layer the induction field heats. A heavier foil adds barrier and stiffness but needs more heat to seal; a thinner foil heats faster but offers less mechanical strength.
For most liquid fills, the standard 20 µm foil is the right balance of barrier, sealability and cost. If your contents are aggressive or your shelf life is long, we may recommend a heavier or coated structure. The gauge is chosen with the contents in mind, never picked at random.
How the cap and torque change the spec
Thickness and diameter are only half the story — the cap decides the rest. A deep cap can hold a thicker backing ring; a shallow cap cannot. A high-torque application needs a liner stiff enough to stay flat under load; a low-torque line needs a liner that seals without needing to be crushed into place.
Mouth flatness matters just as much. A lip with flash, sink marks or a moulding line leaves gaps where the weld cannot form. This is the difference between "sealed" and "actually sealed" — and it is the first thing we check when a customer reports a leak.
One liner, many line formats
Once the dimensions are fixed, the same liner can be delivered two ways: as pre-cut discs counted into cartons for manual or cap-lining insertion, or as punched liners rewound on cores for automatic insertion lines. The thickness and diameter are identical; only the delivery format changes. Tell us how your caps are lined and we will match the format and core to your equipment.
Validating a sample before you commit
A number on a drawing is a guess until a sample proves it. The fastest way to validate both thickness and diameter is to run free sample liners on your own line and look at three things:
- Fit. The liner drops into the cap flat and covers the sealing land without overhanging into the threads.
- Weld. After sealing, the weld ring is continuous and even all the way round.
- Opening. The membrane separates cleanly where it should, and the pack reseals if it is meant to.
Sample liners in your diameter are free — you cover only the courier cost — and a small trial batch lets you confirm the fit on the line before full production.
Get the dimensions right before you commit
The cheapest way to validate a spec is a free sample. Send the mouth diameter, the cap type and a photo or drawing of the lip, and we will send sample liners cut to your size — you cover only the courier cost. You can then run them on your own line and confirm the fit before a full production order.
See the full specification table on the induction seal liners page, or read how the liner is built on the manufacturing page.