Under The Cap: How Induction Sealing Works

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Induction sealing technology is a widespread packaging technique, used in industries from pharmaceuticals to food production. There’s an interesting process behind those little foil seals on the necks of bottles, and some even more interesting history. Let’s take a look.

It was 1982 in Chicago, and a string of inexplicable deaths had the city on edge. There were seven victims in total, including a 12-year-old girl and several members of her family. After an investigation, it was discovered that all seven had ingested Tylenol-brand acetaminophen capsules. Samples from each offending bottle were lab-tested, and it was found that several of the capsules contained traces of potassium cyanide, a highly toxic compound.

It became apparent that someone had intentionally poisoned the pain medication and left it on store shelves to poison unsuspecting buyers. It was determined that the tampering occurred after manufacturing because the bottles came from different sources and casualties occurred exclusively in Chicago. The culprit had to have either carried out the act in-store or sabotaged the bottles after purchase and replaced them on shelves later. But one central question followed: how could someone open a bottle of medicine, contaminate the contents with cyanide, and replace it without someone noticing?

In fact, at this time in history, medicine bottles had incredibly simple packaging. Simple twist or pull-off caps were used (child-proof caps had been invented, but weren’t widespread yet). Under the lid, the only thing between the medicine and the person that opened the container was a wad of cotton to keep the contents safe during transport. In hindsight, it was easy for someone with nefarious intent to go into a drugstore and quickly tamper with almost anything they desired.

Johnson & Johnson, the company that produces Tylenol, immediately issued a nationwide recall of the medicine and switched out the hollow capsules in favor of solid tablets. However, measures were needed to make sure that no one could breach and tamper with pharmaceutical containers in the first place without it being apparent to whoever handled the item next.

To solve this, they (and many other pharmaceutical manufacturers) eventually introduced tamper-proof seals, the same foil layer underneath the cap that you have to peel away whenever you open a new medicine bottle. With these, consumers and distributors alike can immediately tell if any unwelcome fingers have been in their bottles. The technology used to apply these caps, in most cases, is called induction sealing.

Sealing the Deal

The concept and technology needed to create the seal they needed were made decades earlier in the ‘50s by a process engineer named Jack Palmer. He came up with induction sealing to solve the issue of liquid leakage from polyethylene bottles during shipment.

What appears to be a simple piece of foil covering the bottle opening is an ingenious combination of several elements that combine to make an efficient and safe tamper-proof seal. You might notice upon handling one of these seals that it feels relatively thick. If you were to take an x-ray look at a standard medicine bottle with an induction-sealed lid, you would see that it’s made up of different layers.

At the top, the first layer is the plastic cap which, nowadays, is usually of the childproof, push-to-twist variety. Underneath and directly glued to the cap, is a layer of pulpboard, a wood composite commonly used in products like bar coasters. Under the pulpboard is a layer of wax and beneath that, aluminum foil, and finally a thin layer of polymer.

Image source: Ptasia

The process of turning all these separate materials into one sealed container is quick but sophisticated: the pulpboard through polymer layers are placed precisely down on the rim of the bottle, and the cap is screwed down on top of them. The container then passes under something called a sealing head, which contains an induction coil. If you haven’t guessed, the induction coil is the key to this entire process.

The aluminum, being conductive, is heated by eddy currents from the coil. The hot foil melts the thin layer of wax above, which is then absorbed into the pulpboard. With the wax gone, the foil is freed from the pulpboard. On the bottom side of the foil, the polymer that touches the container begins to melt as well. It flows down and hardens again, forming the final seal with the lip of the bottle. In the packaging industry, this airtight seal is known as a hermetic seal.

A Versatile Solution

The uses of induction sealing now range far beyond pharmaceuticals, though that is by far one of its most crucial and common functions. Many food cartons take advantage of this technology, especially if the contents are perishable. Milk cartons and peanut butter jars are some of the more common items.

Most of the cosmetics you buy will bear an induction-sealed cover, as well. If you’ve ever casually picked up a bottle of facial scrub from the store, only to get home and realize it’s half empty, you’ll appreciate the containers that do have these seals. At the very least, they provide some inconvenience and visibility to those that think it prudent to squeeze hygiene products into a baggie rather than pay for them.

While the aforementioned induction sealing example is the most common, numerous other techniques and materials can be used depending on what you’re sealing and your end goal. You can choose a foam backing rather than a pulpboard one and can have them printed with a design or message. Seals can also be sturdier, so they are destroyed when the cap is opened, rather than just peeling off. Glass and metal containers can also be sealed, although they can be more challenging than plastic as these materials don’t melt or bond as well with the seal’s components.


plastic containers

Image source: Packaging Digest


Different sealer sizes and kilowatt amounts are also important considerations when sealing products. Depending on the bottles and containers you’re using, manufacturers need to ensure that they have precisely the right size for the job to ensure a safe and proper seal. Additionally, they need to take into account whether they’re dealing with one type of container, or many different ones. Traditionally, coil changeovers for different bottlenecks have been a critical issue for speed and efficiency because they require a special tool to switch out, causing long delays in the production chain. However, recent technology is poised to remove this speed bump and make induction sealing even more seamless than it already is.

According to Packaging Digest, manufacturing company ITW Pillar Technologies has created an induction sealing coil capable of sealing almost any container without needing a coil change. Rory Wolf, a business unit manager with the company, tells Packaging Digest:

“This is a two-coil system—meaning there are two different size inFinity coils that cover the waterfront in terms of being able to seal all container types/shapes/styles required of these tunnel-type coils without having to change out the coil. Previously, and with other competitor products, there is typically an individual coil required for each SKU [stock-keeping unit], and which must be removed by a tool.”

Improvements in sealing technology like this are only ensuring that it’s easier and more profitable for businesses to make sure that their goods remain safe and secure.

The Last Line of Defense

Induction sealing is only the last line of defense in a broad range of security measures meant to prevent products from falling into the wrong hands. Ideally, there are other safeguards in place that would stop a situation like the one in Chicago from happening before anyone would need to recognize a broken seal. Smart labels monitor products for quality and send information to consumers with a simple phone scan; and pharmaceutical containers and labels are embedded with dust-sized microtags that carry information about the contents, where they were manufactured, where they’ve been, and more.

We need all these interlocking security measures because the problems aren’t just limited to sabotage. Counterfeiting, fraud, and illegal resale are all extremely prevalent issues today, among many others. Monitoring such things is only becoming more difficult as we increasingly prefer online ordering and remote delivery for most of our products — pharmaceuticals included.

The less we rely on face-to-face and personal contact, the more comprehensive our security will have to be, especially concerning goods that have a direct effect on our health and well-being. So, the next time you’re struggling to remove the accursed foil seal from that bottle of painkillers, remember that it’s there for a good reason: keeping consumers like you safe.


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