Darren Dodd explains how new technology has overcome the difficulties involved in induction heat sealing of glass containers for fatty foodstuffs

Consumer demand for total confidence in the sealing of food and drink containers is growing. As well as containing the product and maintaining its freshness, seals now need to give clear visual evidence of any tampering.

With the advent of induction heat sealing, otherwise known as induction cap sealing, customers enjoy peace of mind. This non-contact technique quickly and economically applies a hermetic seal, and is ideal for fast-moving production lines. The aluminium foil seal liner prevents leaks, keeps air and bacteria out, and confirms that the contents are untouched.

The trouble with glass
While induction heat sealing is widely used for plastic containers, it is not so easy to create a good seal on glass. The first problem is that glass is a difficult substrate for adhesion. It becomes even harder if the product contained is high in fats or oils, which can penetrate and degrade adhesive bonds.

For those foodstuffs, glass bottles and jars are still preferred to plastic. 23Glass is relatively inert, so foods are never tainted, while its superior impermeability to gas ensures a longer shelf life. It is also perceived as being easier to recycle.

Given this preference, and the susceptibility of induction heat seals to weakening by fat or oil, most manufacturers opt for steam-applied metal caps instead. However, these are more expensive, and for anyone with limited grip strength they are impossible to open by hand.

To bring the cost-saving, easy-opening and tamper-evident benefits of induction heat sealing to glass containers for fat-rich or oil-rich products, stronger and more oil-resistant adhesion is needed. At the same time, to comply with FCM regulation EC 10/2011, the seal’s layered composition must be free from petroleum-based or synthetic hydrocarbon-derived waxes.

In addition, there is a requirement for seals to be easily removable, leaving no residue, to enable recycling. The good news is that induction heat seals meeting these needs are now available, although not all manufacturers’ products comply fully.

How induction heat sealing works
Before going into specifics on the new technology for glass and oily foods, it would be worth briefly outlining the general principles of induction heat sealing.

It uses highly engineered liner assemblies incorporating several layers. Prior to capping, the liner is positioned inside the ‘closure’ – the cap or lid. The seal itself consists of an aluminium foil layer laminated to an appropriate material for bonding with the container’s substrate.

On the packaging line, the closure is screwed onto the neck of the container and then passed within the field of an electromagnetic coil. This generates an induction current which heats up the aluminium. As the heated seal cools, it adheres to the rim of the container. Tight seals produced in this way cannot be broken without leaving obvious evidence of tampering.

For products which need to be resealed after opening, a two-piece seal assembly is used. The additional part, often made from pulp board, adheres to the inside of the closure. A coating of wax or polyolefin normally holds the two pieces together before capping, and this melts away when heated, during sealing, so they can separate. After opening, the soft material stuck to the inside of the closure forms an effective second seal when the cap is re-tightened.

Specific adaptations
In the latest development, to solve fat and glass issues, aluminium foil is coated with a heat seal polymer layer instead of wax. White folding box board is used for the backing material.

The closure’s structure includes a retention bead – a ridge running around its internal perimeter – which holds the seal assembly in place. Bead and liner dimensions are crucial, as the liner must be retained but allowed to rotate freely.

Consumers hear a reassuring ‘crack’ when they twist off the closure, as the liner materials separate. They then see the attractive foil liner, tightly stretched over the container’s neck. A secondary resealing liner or wad remains in the closure to maintain the product’s integrity and avoid contamination.

A question of balance
As in any induction heat sealing application, successful bonding depends on using compatible materials and finding the right balance between pressure, heat and time.

The capping machine’s torque heads, delivering a rotational force to screw the closure into place, should be checked at least once per production. Correct pressure must be applied evenly to the entire rim. If it is too low, there will be insufficient contact for bonding. Excessive torque may cause uneven pressure by damaging the screw threads. Inconsistency in pressure distribution may also result from poor matching of threads in the container and closure, or from defects, irregularities and high or low spots in the rim surface.

Achieving sufficient heat for effective bonding depends on carefully specifying the size and power level of the induction seal equipment. Since it is impractical to measure temperatures or temperature rise profiles at the seal interface, the best approach is to run trials of different systems, liners, sealers and power settings under production conditions.

Temperature and timing are firmly linked. The closure needs to be under the induction coil long enough for the sealing surface to melt and bond with the rim. Variables affecting this include conveyor line speed, container size, liner design and whether the product is filled hot or cold. Cooling time is important too, as full bond strength is not reached until it is cool. Rough handling during this critical stage may cause seal displacement.

Ask for advice
With so many factors to consider, you should seek expert help. Induction equipment and lining material suppliers will be happy to advise and to co-operate in trials, ensuring the optimum set-up for your operation.

Darren Dodd is Marketing and Service Director for Selig, a specialist manufacturer of tamper-evident and easy-open cap and closure lining materials. Selig’s latest aluminium foil and heat seal combination products for glass containers and fatty foodstuffs are fully compliant with the relevant EU legislation.