The basic distinctions in material types are those between welded foil seals and peelable foil seals and between single piece liners and 2-piece liners.
Welded foil seals are aimed at the market where tamper evidence and/or anti-counterfeiting are required. Access to the container requires that the foil is punctured or cut and the bond between the container and foil is sufficiently strong that it can not be removed without damaging the neck or leaving a residue making it obvious to a consumer that tampering had occurred. Welded foil seals tend to be used mainly in pharmaceutical applications, chemicals, pesticides and motor oils. Peelable seals represent the majority of material used in FMCG applications and provide a hermetic, leak resistant closure which can be easily and cleanly removed from the pack before use leaving no residue on the container neck. Applications for peelable foil seals range from food & beverage through cosmetics & personal care products. Peelable foil seals require some form of tab by which the foil can easily be gripped for removal, this presents other challenges such as interference with cap/thread fit and heat-loss into the tab.
The second major distinction in different kinds of induction sealing liners is that between single piece and 2-piece materials. A 2-piece liner typically comprises a re-seal material such as pulp board or a high temperature resistant foam (so that it is not melted as the foil is heated) which is bonded via a low temperature medium to the foil seal, wax is often used for this purpose. As this kind of liner is induction heated, not only does the heat-seal layer bond the foil to the container but the low temperature medium melts releasing the re-seal liner from the foil seal such that when the cap is removed, the reseal remains in the cap and the foil is sealed onto the container.
A single piece liner has no re-seal component and as such is transferred from the cap to the container during the sealing process. Single piece liners do however have to possess sufficient structure and rigidity to retain them inside the cap from the point where they are inserted into the cap right through the supply chain and until they are finally applied to the container. This means that for most welded seals, a 2-piece liner is the only appropriate choice since they have to be generally easy to break through before use and therefore can’t be provided with the necessary structure to remain in the cap.
Whether you are using a single piece liner, 2 piece, welded or peelable seal, one constant requirement is that the heat-seal layer has to be compatible with the container material. It is recommended that users work with a recognized manufacturer of induction heat sealing material to determine the correct heat-seal facing for their container. Once suitable materials have been identified, it is also important that these are tested to ensure that there are no product-contact issues. Some products can react either with the aluminum layer in the sealing material or with the heat-seal. The result of this can be either to degrade the seal or break down the bond between the container and the foil, neither of which are desirable. Options are usually available to avoid these problems such as barrier layers between the heat-seal and the aluminum foil or different heat-seal formulations however testing of the actual product and container along with the foil to be used is strongly recommended.