Creating a perfect package

Induction heat sealing can ensure product protection while providing visible proof of tampering, says Darren Dodd, Marketing and Service Director at Selig

The malicious tampering of packaging is, thankfully, rare, but the risk — however slight — must be taken seriously.

For a premium product such as a nutraceutical, consumers and retailers expect faultless product protection and the highest level of security.

A common way of providing tamper evidence for rigid packaging such as jars and bottles is to use induction heat sealing (sometimes called induction cap sealing).

This non-contact method uses a laminated structure to apply a hermetic seal over the mouth of the container. The most important component in induction heat sealing is the seal liner — which must be applied with the correct balance between pressure, heat and time to create a perfect seal.

Liner notes

The induction liner is a highly engineered, laminated film, available in one- or two-piece designs that have different bonding capabilities. One-piece liners, when bonded, leave no residual part of the liner bonded to the closure. They are suitable for products that do not need to be resealed.

Two-piece liners are initially bonded to the inside of the closure. The two parts enclose a bonding layer of wax that disperses after heating. The main two layers separate on heating: one stays inside the closure, whereas the other becomes sealed to the neck of the container.

For pharmaceutical or nutraceutical products, the upper layers can be printed to provide a clear visual indication of tampering. Several technologies offer enhanced security and tamper-evidence, including etched foils, holographic films and inks that change colour.

Pressure’s on

Once the appropriate liner has been chosen, it must be processed correctly: it needs to be exposed to the correct pressure and the right level of heat for sufficient time to form a secure bond between seal and container. The correct pressure ensures that the liner creates an even seal around the container. This is achieved by the torque heads of the capper when placing the closure onto the filled container.

The design of the neck and threads of the container must be matched to the closure, so that it gives even pressure to the container rim. If the rim of the container has any defects — such as high or low spots — there will be uneven pressure around the seal, leading to uneven heating and potential leakages.

During production, it is important to check the torque heads at least once per shift, to ensure that each closure receives the right amount of rotational force.

Hot stuff

Heat is applied using an electromagnetic induction process, which raises the temperature of the seal’s metallic foil liner by inducing eddy currents.

It’s important to get the size and power of the induction seal equipment right as it must produce enough heat around the induction liner to seal effectively for a particular line speed.

Measuring the temperature or temperature rise profile of the liner at the seal interfaces is not practical. For this reason, it makes sense to work with induction equipment suppliers to run timing and power trials with induction liners and sealers.

Timing is everything

The induction seal liner needs enough time under the coil to ensure that it’s heated to the right temperature, allowing the polymer sealing surface to melt and bond to the rim of the container.

This timing is dictated by conveyor speed. The time required to heat the induction liner depends on several factors, including the size of the container, whether it is hot- or cold-filled and the type of liner chosen.

Achieving a satisfactory bond will also be determined by the cooling time interval, as the bond only develops properly on cooling. This is a critical process during which smooth handling of the containers avoids any relative movement of the seal before it has fully bonded to the container.

Choosing a liner and induction system is of vital importance if the process is to be successful, which makes it more important to seek expert advice from an experienced lining material supplier before installing an induction heat sealing line.