Whilst induction sealing provides an efficient, clean and extremely convenient means of applying a foil seal to a container and is in widespread global use, it is not without limitations and does expose the user to a number of potential issues of which they should be aware.
Since induction sealing is completely reliant upon the torque of the cap to provide the pressure which is necessary for the sealing process to work, any loss of cap torque whether through cap/bottle fit, crossed threads or insufficient application torque will lead to weakened or failed seals.
Cross-section photos showing poor cap/bottle fit whereby foil is not being uniformly pressed onto the bottle neck
As mentioned above, since delivery of the foil and sealing pressure are both provided by the cap, there are certain performance related limitations on the design of the bottle neck, cap and foil liner. The bottle neck and cap have to have a sufficient thread to allow the production of the required sealing pressure, this is especially problematic on wider mouth containers where both parts need a high degree of rigidity. The sealing liner has to be re-enforced either with a stiffening layer such as EPE foam if single piece or the backing/re-seal liner in order to retain it within the cap preventing the use of light-weight unsupported materials and the cap itself has to have design features in order to hold the liner in place. This all adds up to components which are heavier, more complex and more expensive than equivalent parts which are not designed to be used as part of the induction sealing process.
Since the heat provided is a function both of the setting on the machine and the spacing between the top of the cap and bottom of the induction sealing head whilst the time is a relationship between the conveyor speed and the length of the sealing head, it is clearly critical that good control is exercised over the production line. The height of the sealing head must be checked before starting a production run likewise the induction coil must be centralised over the conveyor to ensure that the foil has an even exposure to the induction field. Conveyor speed must also be checked and as a general rule, higher line speed requires a higher power setting on the generator.
The consequences of incorrect line set up could be either under-sealing, burning of the foil liner or un-even sealing where one side has been exposed to a more concentrated part of the seal than the other, all of these are potential seal failures. Some sealing problems can be the result of various problems such as a scorched foil seal could be due to insufficient cap torque, too much power or incorrect sealing head alignment.
Finally, with a screw on cap in place over the top of the liner during and after the sealing process, it is extremely difficult to inspect and verify the correct application of the seal short of removing the cap to access the seal below. In critical applications, the lack of an effective means of 100% seal validation may deter the end user from utilising induction sealing.