The induction seal equipment must produce sufficient heat around the induction liner to seal effectively for a given conveyor line speed. Measuring the temperature or temperature rise profile of the liner at the seal interfaces is not a practical proposition and it may be more appropriate to engage with induction equipment suppliers at an early stage to run timing and power trials with induction liners and induction heaters – preferably at the supplier’s premises.
As well as maintaining a correct balance between pressure, heat and time, achieving a successful induction seal will also depend on the choice of liner. An induction liner is a highly engineered, laminated structure, available in one- or two-piece options and with different bonding capabilities. When bonded, one-piece liners leave no residual part of the liner bonded to the closure, whereas two-piece liners are initially bonded to the inside of the closure. The two parts of a two-piece liner enclose a bonding layer (usually wax, though polyolefin is also used). The parts separate on heating, one part staying bonded to the closure, the other becoming bonded to the lip of the container.Two-piece, wax bonded liners have a secondary backing board structure ensuring that, once opened, the container can be resealed. These liners are complex laminates comprising sequential layers of pulp board (the secondary reseal liner); wax or polyolefin (the bonding material); a polymer external contamination barrier; a polymer/paper easy-open ‘lift-and-peel’ device; foam (ensuring even pressure during sealing); aluminium (the induction heated foil); a further polymer layer compatible with the container material (protecting the seal from the container contents), and finally a heat seal specific to the container type. A one-piece induction liner is of similar construction but without the wax/polyolefin and pulp board secondary reseal layers.
Clearly, the choice of liner and the induction system engineering are not trivial tasks to be undertaken lightly. There is an excellent case for trialling both liner and induction equipment at customers’ premises; however, obtaining some initial advice from an experienced supplier such as Selig is an important first step that potential users of induction heat sealing are strongly advised to take when considering the installation of such equipment.