Consistent induction sealing results from consistency in the process. Indirect or seemingly unrelated functions impact the outcome of an acceptable induction seal. In order to achieve a proper seal, the foil liner must fully contact the lip of the container with sufficient torque, there must be material compatibility between the liner and the container, and there must be sufficient exposure to the magnetic field to heat the foil to accomplish the seal. A failure at any point in the process can result in inconsistent or poor seals. Almost always, the apparent culprit is the induction sealer.
The most common cause for inconsistent seals is the lack of sufficient force to ensure complete surface contact of the foil liner to the lip of the container or insufficient exposure of the liner to the magnetic field. Incompatible material problems generally become suspect when there are changes in suppliers or different lots of either the closures or the containers used.
Many times the induction sealing system is referred to as a heat sealer. The actual process of induction sealing results from energy conversion induced into the closure. By transforming energy from electrical to magnetic, magnetic to electrical, and finally from electrical to thermal or heat energy, the seal is accomplished. The induction power supply converts energy supplied by the power company to a high frequency, regulated AC current. The sealing head transforms the electrical energy into a magnetic field. The foil liner is exposed to the magnetic field. The resistance of the foil disc in the liner interacts with the electrical energy called circulating currents which results in heating of the foil. The foil disc must heat sufficiently to melt the polymer layer on the disc. When cooled, the polymer adheres to both the liner and the lip of the container creating a hermetic seal.
The liner is comprised of a foil with a polymer coating applied to the bottle's contact side. To hold the liner in place in the closure, wax is commonly used as an adhesive. When the foil disc is heated, two functions are performed. The melting of the polymer accomplishes the seal, and the wax layer is melted, releasing the foil from the pulp backing. The wax is absorbed into the pulp, and the liner adheres to the lip of the container. The vacant area left by the wax, even though ever-so-thin, loosens the closure.
Because loose caps create a perception of unsafe product, many filling applications incorporate a re-torque system in line. Because the foil disc retains the heat, it must be allowed to dissipate the energy before disrupting the polymer. If the polymer is not allowed to "cure", a seal failure will result. Because of this, the ideal placement of the re-torque unit should as far away or as far down the line as possible.
The sealing head design is based on the point where the foil liner is placed in relation to the top of the closure. As mentioned previously, the magnetic field developed by the AC current applied to the sealing coil interacts with the foil disc. The distance which the magnetic field can propagate through air is very limited. Ideally, the foil disc should be approximately 1/8" to 1 / 4" away from the bottom of the sealing head. This area provides the maximum field strength. When the foil liner is further away, as in spout type closures or CRC containers, the field must be applied from the side of the closure. These types of sealing heads are referred to tunnel or channel designs. The locations of the components that develop the magnetic field are well below the top of the container, adjacent to the position of the foil liner in the closure.
Enercon Industries Corporation
W140 N9572 Fountain Blvd. | Menomonee Falls, WI 53051 | 262.255.6070
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