Teachers like to say there’s no such thing as a “dumb” question. And when it comes to induction sealing I’ve heard them all. In this article, I’ll answer some of the more common questions.
What occurs inside the cap or “heat” sealer?
Many times induction sealing systems are referred to as a heat sealer. 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 to which the foil liner is exposed. The resistance of the foil in the liner interacts with the electrical energy which results in heating of the foil. The foil disc must heat sufficiently to melt the polymer layer on the liner. When cooled, the polymer adheres to both the liner and the lip of the container creating a hermetic seal.
Why is the cap sometimes loose after the induction sealing process?
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 liner 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.
What is the proper application torque for my cap/container?
Of all the factors that go into achieving a consistent induction seal, application torque is one of the most overlooked. However, it is one of the most important. In the majority of situations where a packager is getting inconsistent seals, it is normally some type of mechanical problem involving the closure and the bottle. This is where application torque becomes critical. To insure a good seal, the foil innerseal must be in intimate contact with the lip of the container prior to entering the induction sealer. This intimate contact is the result of the proper application torque, which provides the necessary downward pressure to ensure good contact between the foil and the lip of the container. The industry “rule of thumb” for proper application torque is one-half of the diameter of the cap, in inch pounds. Therefore, for a 38mm closure, your target application torque should be 18-20 inch pounds. Conversely, a 63mm cap would require 30-32 inch pounds of application torque and so on. This “rule of thumb” has been used since the inception of induction sealing, back in the mid-1960’s. Normally, you will not go wrong by using this formula in setting up your cappers for application torque. However, because of the rapid development of closures and innerseal materials, Enercon is embarking on a study to review this “rule of thumb” and we plan to publish our findings in the not too distant future.
Which sealing head design is best?
Sealing head design is based on the location of the foil liner in relation to the top of the closure. The distance which the magnetic field can propagate through air is very limited. Ideally, the foil liner should be 1/8″ to 1/4″ away from the bottom of the sealing head. This area provides 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 as 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.