Conventional “foil-in-cap” induction sealing relies upon the principle of induction heating to heat a foil seal which is usually pre-loaded into a cap. The cap is screwed onto the bottle such that it delivers the foil to the correct location on the container and provides the required sealing pressure. After the cap has been applied to the correct torque, the container is passed through the induction sealing machine where the foil is heated to the temperature necessary to melt the heat-seal. This induction heating process provides the bond between seal and container such that when the cap is removed, the foil remains sealed to the bottle.
Induction sealing is very much like other means of heat sealing except that the heating medium is via electro-magnetic field rather than conduction. Thus, the requirements for forming a seal onto a container are the same as with any kind of thermo-contact sealing:
: delivering the contact between the seal and the container
: raising the temperature of the seal surface to activation point
: allowing the bond to be formed between the seal & container
In the case of induction sealing the main difference is that the sealing pressure which forces the foil membrane against the neck of the bottle is produced by the torque of the cap. The foil sits in the cap with the heat-seal surface facing downward, when the cap is applied, the foil is forced down by the screw-thread such that it is pressed against the sealing surface of the container.
Since no contact is required for the induction heating to take place, exposure of the cap with the foil inside to the electromagnetic field coming from the induction head of the sealing machine will produce very rapid heating of the seal. Due to the power of the induction equipment currently available, foil membranes can reach sealing temperature anywhere from 0.2 seconds to around 1 second (depending upon foil diameter and thickness). It is also worth noting at this point that the power delivered to the foil is not only a factor of the power setting on the machine but also the spacing between the induction sealing head and the cap. There is an incremental reduction in delivered power as the distance between the induction coil and the sealing foil membrane increases so it is important in machine set up to ensure the correct relative position of cap and seal head.
Whilst the foil may reach temperature very quickly, it is necessary to have some exposure time in order for the heat generated within the foil to melt the heat-seal layer on the underside of the membrane so that this can then form a bond with the container. On linear induction sealers this is determined by the relationship between the speed of the conveyor belt and the length of the induction sealing head. On dedicated head sealers (whether semi-automatic or those on auto machines) the sealing time is simply the length of time for which the induction generator remains activated.
Given the above, it may reasonably be assumed that Conventional induction sealing is a straight-forward proposition. What must always be remembered is that with capped induction sealing, both the machinery and the components/materials play a critical role in forming the seal. The machinery is actually just a particular kind of induction heater designed to deliver the required energy in such a way as to raise the temperature of the foil inner-seal to the point where the polymer layer will melt and bond to the container. If there are any defects in the bottle, cap or foil liner then the process will not work, likewise if the cap has not been correctly applied, if the capping torque is too low or if there are material non-compatibility issues, the induction sealing machine can be functioning perfectly but a seal will not be formed.
In practice once everything is in place and correctly set up the process is relatively simple and does work well however at first, the range of material types, cap & container combinations and equipment specification and set-up issues can present the inexperienced with a challenge.