Quick answer: Peel strength is not a property of a liner. It is the result of four interacting variables: the sealant polymer, the energy delivered by the induction field, the geometry of the sealing land, and the contact pressure from cap torque. A liner data sheet can state the first of those. The other three belong to your line and your container. This is why a liner that "tests at 12 N" can leak on a production line and a nominally weaker liner can seal perfectly — the number describes a system that does not exist until the container arrives.
This distinction matters commercially because it is the ground on which liner specifications are usually argued. A buyer asks for a peel strength figure, a supplier provides one, and both parties treat the question as answered. At Hangzhou Xinxing Pharmaceutical Packaging we have spent twenty years managing the consequences of that assumption, and the pattern is always the same: the argument resurfaces later, on a production line, with containers in it.
What a peel strength number actually describes
When a laboratory reports a peel strength for a sealing liner, it has done the following: sealed a liner to a specific substrate under specific conditions of temperature, pressure and dwell, cooled it for a specific time, and pulled it at a specific angle and rate. Every one of those parameters changes the number.
The substrate is the biggest of them. A laboratory will typically use a standard test cup or a ring cut from a container of a known material. Your container is a different object — a moulded part with a rim radius, a wall thickness tolerance and a surface that may carry mould release residue. Change the substrate and the number changes without the liner changing at all.
The dwell and the pressure matter just as much. A bench sealer can apply a generous dwell because it seals one container at a time. A production sealer applies a dwell determined by line speed and coil length, and it applies pressure determined by cap torque rather than by an operator's hand. Neither of those is a liner property.
The four variables of a real seal
Variable one — the sealant polymer. This is the only one the liner supplies. PE bonds readily to HDPE and LDPE rims and forms an adequate bond to PET at the right energy level. PP bonds best to PP and tolerates higher temperatures. PET bonds well to PET and resists oily fills. The polymer sets the ceiling on achievable bond strength; it does not set the achieved one.
Variable two — energy delivered. A function of coil power, coil geometry, the gap between coil and cap, and the dwell time. Of these, the gap is the least controlled and the most consequential, because cap height tolerance means the gap varies from container to container within a single run. A sealer tuned at the top of the height tolerance will under-seal at the bottom.
Variable three — sealing land geometry. Measured, not nominal. Outer diameter, inner diameter and rim flat width. A narrow rim needs a thinner sealant film and a tighter diameter tolerance, because there is less area over which to distribute the bond and less margin for a diameter error. A rim with a radius rather than a flat is the single most common cause of a seal that looks complete and tests weak.
Variable four — contact pressure. Cap torque, applied by the capping head, holds the liner against the rim while the field melts the sealant. Too little and the liner floats; too much and it buckles. Both produce a seal that is visually acceptable and functionally unreliable.
Building a sealing window instead of a spec sheet
A sealing window is a range, not a number, and it is the only deliverable from a qualification run that has operational value. It has three axes:
Power, at fixed line speed. Sweep the power setting upward in steps. At each step, seal a set of containers and test them. The lower bound of the window is the setting at which every container in a batch of a defined size passes. The upper bound is the setting at which the sealant begins to scorch or flow outside the sealing land.
Speed, at the chosen power. Sweep upward until containers begin to fail. The maximum reliable line speed is below that point, and the margin between them is the line's safety buffer.
Torque, at both limits. Repeat at the low and high ends of the closure manufacturer's torque range. A window that only holds at nominal torque is not a window; it is a coincidence.
Record all three and put them on the line's setup sheet. A window that lives in a project folder is lost within two staff changes; a window on the setup sheet survives.
Why the same liner stops working after a bottle change
When a liner that has run reliably for a year suddenly produces leaks, the liner is the first suspect and almost never the cause. The usual sequence is a change of bottle mould, a change of moulding machine, a change of cap supplier, or a change of line speed — and each of those changes one of the four variables above.
A new mould cavity can shift the sealing land diameter by a fraction of a millimetre and the rim flatness by more. A new cap supplier can change the cap height, which changes the coil-to-cap gap. A speed increase reduces dwell. In each case the liner is unchanged and the seal is different, which is exactly what the four-variable model predicts.
The practical defence is a reference container. Keep a set of containers from the batch that was qualified, sealed and tested, and when a problem appears, run one of them through the current line settings. If the reference container still seals, the line is fine and the container changed. If it does not, the line changed. That single test resolves in ten minutes what a specification argument takes weeks to resolve.
What to ask a supplier for instead of a peel number
Ask for a material declaration naming the sealant polymer and its thickness, a dimensional report on the die-cut diameter against your drawing, and a statement of which container materials the construction has been qualified against. Ask, separately, for the recommended sealing window as a starting point for your own qualification. Then qualify it yourself, on your line, and keep the reference containers.
A supplier who offers a peel strength number as the answer to a sealing question has answered a different question. It is not dishonesty; it is the industry's shorthand. But shorthand is a poor basis for a specification that has to hold for two years of shelf life.