Induction's new gig: Capless sealing

Bill Zito
Enercon Industries
Menomonee Falls, Wis.
www.enerconind.com

Induction sealing is often used to hermetically seal containers for freshness and to thwart tampering. But until recently, the process was limited to plastic containers with circular openings and threaded caps. Advanced sealing heads and more compact, higher-wattage power supplies, however, are making it possible to use induction for capless sealing (containers sealed prior to capping) and nonthreaded snap-caps, as well as metal containers and nonround container openings.

Low or high volumes

Standard induction sealers operate as either table-top units for low production runs or mounted over conveyors for high-volume runs. An induction seal is placed inside the cap by the cap manufacturer so packagers need only to pass the container under a sealing head. Under the sealing head, an induction heater induces an eddy current into the seal material, causing a resistance-type heating effect. The hot seal material melts a polymer coating, bonding it to the container lip and creating a hermetic seal.

Sealing without a cap, however, is more complex. First there must be a way to deliver the seal to a container. Seals can be loaded from sleeves into a delivery system, or the seal material may be unrolled, punched out, and delivered in one operation. Then both the seal and container must be captured and controlled. This usually happens by articulating arms, indexing systems, and rotary-bottle tables. Line speed determines which technique is best.

Once captured, sufficient pressure must be applied on the seal to ensure contact around the container's lip. Depending on the system, there may be a single sealing head or multiple sealing head stations, such as with a rotary-bottle table. The sealing head normally requires only a fraction of a second to generate enough heat to seal the container.

Fortunately for packagers, most seal and container-handling techniques are already being used for conduction applications. This makes adapting induction into these systems a simple component swap. Typically induction power supplies replace conduction heating elements and induction-sealing heads replace the conduction versions.

Induction versus conduction

Induction holds many benefits over conduction. For example, induction sealers offer instant start-up while conduction systems must warm up before working. Also with induction, an electromagnetic field creates the heat as opposed to the conduction "hot-plate" process. The induction process minimizes ambient heat, keeps the sealing head at a cool temperature, and improves operator safety.

Induction also cuts clean-up time for product spillage because the product won't bake onto the sealing head. Maintenance is easier too because there's no cool-down time. Induction also relieves the headaches of maintaining a proper sealing temperature using elaborate temperature sensors and heaters. And, new compact air-cooled sealing heads and advanced power supplies can generate as much power on high-speed lines as conduction systems.

Another plus to packagers is energy savings: Induction-sealing systems use less power than conduction-sealing heads, which must maintain a constant high temperature. One company, SmithKline Beecham Consumer Healthcare, recently switched from conduction to induction sealing for its redesigned capped Tums package. Plant Supervisor Mickey Miramonti says the new system helped cut downtime and reduce scrap.

Induction sealing nuts and bolts

The principles of induction sealing stay the same for capped or capless sealing: Pressure, heat, and time are the keys to effectively sealed containers.

Standard induction sealers have two main components: a power supply and sealing head. The power supply produces the necessary current for creating an electromagnetic field in the induction-sealing head.

The power-supply rating for specific applications depends on closure size and production-line speed. The sealing head consists of a plastic housing with a conductor inside wound to form an inductive coil. When energized by the power supply, the head produces an electromagnetic field and the pretightened cap (in a cap-sealing application) provides the necessary pressure. As with conduction sealing, induction sealing of capless containers requires a third component to apply pressure to the foil liner. This could be an overhead belt, air pressure, or a capping press. Cap liners come in a variety of configurations as both secondary and primary seals. Induction-seal-liner manufacturers offer many configurations depending on the needs of packagers, such as tamper-evidence or easy opening.

When using a threaded cap, the seal configuration may include a layer of pulpboard or foam, a layer of wax, aluminum foil, and a layer of heat-activated polymer that's compatible with the bottle material. This type of innerseal, which leaves a pulpboard or foam liner inside the cap, is commonly called a two-piece innerseal. It's normally used when packagers want to leave some type of secondary seal in the cap to prevent leakage after removing the induction seal.

Another option, a single-piece innerseal, functions the same as the two-piece innerseal but leaves nothing inside the cap. Different combinations of innerseal materials, such as foam or paper backed, can be custom printed with a logo or trademark, or contain some type of generic message, such as "sealed for freshness."

Innerseals can be welded in so that they have to be destroyed during removal, or they may be peelable for easy removal. With so many varieties of innerseals, it's best to consult with a cap supplier or the manufacturer of the innerseal material before making a selection. The material used for conduction sealing in most cases can also be used for induction. The key requirement for induction sealing is that the sealing material include enough metal (aluminum) to generate sufficient heat for melting the polymers that bond to the container.