Why some caps loosen after sealing and how you can remedy the problem

We receive many calls concerning caps that loosen after induction sealing. To understand why this happens we need to quickly review the anatomy of a cap prior to sealing.

The side image reveals the inside of a cap with a two piece inner seal as it passes under the sealer.

Remember that the function of the induction sealer is to heat the foil liner so that the wax and polymer melts. The melting of the wax releases the liner from the cap. Melting the polymer activates its adhesive properties to create the actual seal.

So there are two changes to a cap after induction sealing. The polymer has sealed the liner to the lip of the container and the wax has been absorbed by the pulp board. The void created by the absorbed wax can cause a loose cap. It’s important to note that if the container is sealed properly the loose cap does not diminish the effectiveness of the seal. A loose cap will not result in a leak or damage to the product in the container.

However, a loose cap is unacceptable to some packagers. A loose cap can adversley affect customer perception of the safety of the product. A more significant problem related to a loose cap concerns Child Resistant Caps (CRC). In some cases a loose cap can cause the CRC locking feature to not function, making the cap easy to remove.

Retorquers to the Rescue

The most effective way to remedy loose caps is to install a retorquer downstream of the induction sealer. The key to successful retorquing is applying the correct amount of torque and properly locating the retorquer.

A generally accepted rule of thumb dictates that you should apply approximately half the cap metric diameter in inch-pounds. There are variations in the allowable amounts of pre-seal, or on-torque and post-seal, or off-torque. This is a matter of meeting individual requirements.

The location of the retorquer is as important as the amount of torque. Keep in mind that the cap requires a “cool down” time to allow the hermetic seal to "set up.” It is imperative that the bonding process is allowed sufficient time or the seal may be compromised. Any mechanical alteration prior to the polymer solidifying to the lip will either destroy or damage the seal.

It can take several seconds for the polymer to seal to the lip of the container. At speeds of several hundred fpm this can result in a considerable distance in a very short time.

For best results test your applications to determine the appropriate location of the re-torque.

If you have any questions about loose caps and retorquing please contact our service department at 262-255-6070.

Originally appeared in: eMaintenance: Induction Sealing Technology 1st Quarter 2003

Diagram of an induction sealing system head.