Bill Zito Vice President - Sales Enercon Industries Corp.

As with the Laws of Gravity, the principles of induction sealing do not change over time. The basic principles pertaining to induction sealing are the same today, as they were 10 years ago when this article was first published. The intent of this article is to separate fact from fiction concerning induction sealing in a way that anyone can understand and to offer suggestions to assist in selecting the proper equipment for your application.

Since the inception of induction sealing in the mid-60's, there has been an almost mysterious aura surrounding this phenomenon. Most of the mysteries and misinformation have been generated by the manufacturers and sellers of this equipment, with a helping hand from cap suppliers and innerseal producers.

In the past twenty years there has been a dramatic change in the electronic equipment and sealing heads available. Power supplies and sealing heads have become much smaller and more efficient. Applications that formerly required a 5kW-power supply, in many cases, can now be handled with a much smaller power supply. (Kilowatt ratings have become passe; however, I will cover that further along in the article.)

Figure 1: Cap Sealers, from left-to-right: circa 1980, 1990, 1995 & 1998 with author behind.

Let's begin with the basics of induction sealing. Just what is induction sealing and what can it do for you? Induction sealing is a non-contact heating process that accomplishes the hermetic sealing of a container with a closure that includes a heat sealable foil laminate inside of the closure. It may consist of a layer of pulpboard or foam, a layer of wax, aluminum foil and a layer of heat activated polymer that is compatible with the bottle material. This type of innerseal, which leaves a pulpboard or foam liner inside of the cap, is commonly referred to as a 2-piece innerseal. It is normally used when a packager desires to leave some type of secondary seal in the cap, to prevent leakage, after the induction seal has been removed.

Figure 2: Flat Sealing Head with 2-Piece Innerseal in Cap.

Another option is the single piece innerseal, which functions the same as the two-piece innerseal, but as its name suggests, nothing is left inside of the cap. There are various combinations of innerseal materials, some are foam backed and some paper backed. They can be custom printed with a customer's logo or trademark, or can contain some type of generic message, such as "sealed for freshness." They can be very aggressive (welded) and have to be destroyed to be removed, or they may be peelable, for easy removal. Because of the many varieties of innerseal, it's a good idea to consult with your cap supplier or go directly to the manufacturers of innerseal material before making your selection.

Figure 3: Welded Seal

Figure 4: Peelable Seal

What are the major components of an induction sealer? The standard induction sealer has two main components, the power supply and the sealing head.

The power supply is an electrical generator capable of operating at the medium to high frequencies required for the induction sealing process. It supplies the induction sealing head with the current necessary to create the electromagnetic field. The power supply rating required for a specific application will depend upon the size of the closures and the speed of the production line.

The sealing head consists of a plastic housing with a conductor wound to form an inductive coil inside. The head produces an electromagnetic field when energized by the power supply. The most common shapes used in induction sealing are the flat head and the tunnel head. A tunnel head concentrates the current around the sides and above the cap, creating a more uniform electromagnetic field, and a more consistent seal. A flat head disperses the magnetic field more widely, allowing a larger area (and larger cap) to be sealed.

There is a third possible component to the induction sealer, which is a water recirculator. The water recirculator is a water-to-air heat exchanger, which cools the sealing head by pumping water through the sealing head coil via leads connecting the two.

The sealing process takes place after the filling and capping operation. The capped containers simply pass underneath the induction sealing unit, which is mounted on the conveyor. As they do so, they pass through the electromagnetic field created by the induction heater. An electromagnetic current, called an eddy current, is induced into the foil, resulting in a resistance type heating effect. The hot foil melts the polymer coating, causing it to bond to the lip of the container, creating a hermetic seal. In cases where the liner is two-piece, the heated foil also melts the wax layer, which is absorbed into the pulpboard, releasing the foil from the pulpboard.

Perhaps you've noticed I referred to the induction system as a heater, and not a sealer. This reveals the first misconception. Everyone who manufactures induction systems for affixing a foil innerseal to a container, refers to their equipment as induction sealers. The truth of the matter is we do not seal anything. The only function of the induction system is to heat the foil. You can heat the foil as much as you want, but if the liner is not in intimate contact with the lip of the container, you will not achieve a seal.

Occasionally, I'll receive a call from a customer who tells me something is wrong with his induction "sealer." He goes on to tell me he has run 100 containers through the system and only 97 of them sealed. I explain that if 97 of them sealed, there is nothing wrong with the induction unit and suggest he look elsewhere for the problem.

Figure 5: Tunnel Sealing Head with 1-Piece Innerseal in Cap

Further examination usually uncovers the fact that there was insufficient torque on the three containers that did not seal: either the foil was not in intimate contact with the lips of the containers, the lips of the containers were deformed or the caps were cocked and therefore not exerting enough downward pressure on the foil to achieve a good seal.

If a series of identical containers are put through an induction field and one of them seals, then all of them should seal. You must realize that when you are dealing with hundreds or thousands, if not millions, of containers and caps, you will experience an occasional bad lip, insufficient torque or cocked cap. When this occurs, poor seals cannot be blamed on the induction equipment.

Many people who are packaging liquid products are concerned that they will not achieve a good seal if there is product on the lip of the container when it is capped. Not to worry; this is not a problem. Normally the torque applied to the cap will squeeze out most of the liquid, and the heat generated by the induction process will eliminate whatever is left between the lip and the innerseal. To prove the point, many times I have taken WD-40 lubricant and sprayed the innerseal and lip of containers and then induction sealed them without any problems.

What types of products and containers can be sealed? With the correct type of container and innerseal material, virtually all products can be sealed. Plastic containers with plastic caps are easiest to seal. Glass may need to be treated before the lip of the container will accept a seal, especially for liquid products. While it is possible to induction seal containers with metal caps, it is not recommended. The metal cap is heated by the induction field and the innerseal is heated by conduction from the heat in the metal cap. The hot metal cap presents a safety problem to workers who may inadvertently touch it. In addition, the cap may become so hot it melts the plastic threads on the container.

In the last five years, there has been a gradual shift from the use of water-cooled systems to air-cooled systems. Because of advances in power supply technology, air cooled power supplies are now able to achieve speeds that were once only in the realm of water-cooled systems. The benefits of air-cooled systems are obvious: a lower priced system and none of the hassles of maintaining a water recirculation system. There are some applications where water cooled systems still have a slight advantage, but generally air cooled systems will satisfy most production requirements.

So when do you want to purchase a water-cooled system? A major reason would be to duplicate existing systems, and thus maintain spare parts for only one type of system. Another major reason for purchasing a water-cooled system would be if the production environment has an extremely high ambient temperature or high humidity. In this type of situation, a water-cooled unit would be able to perform better and have fewer failures.

Figure 6: State-of-the-Art, Water-Cooled Induction Sealing System (Note: Water Recirculator under the conveyor.)

As a general rule, water-cooled systems have greater longevity and fewer failures than air-cooled systems, but not substantially so.

Some manufacturers of induction sealing systems tout their use of ferrites in the sealing head, as if this is something new and radically different. Ferrites are nothing more than dense homogeneous ceramic structures made by mixing iron oxide with oxides or carbonates of one or more metals such as manganese, zinc, nickel or magnesium. They are pressed, then fired in a kiln at 2000ºF and machined as needed.

How and why are they used in induction cap sealing? If you examine the cross section of an induction-sealing coil without ferrites (Figure 8), the electromagnetic field radiates equally in all directions.

Figure 8: Electromagnetic Field without Ferrites

By surrounding the coil with a ferrite material (Figure 9), the dense ferrites prevent the electromagnetic ferrite material field from radiating and actually concentrate and direct the field, making it more efficient. Ferrites have been in use as flux concentrates by all major sealer suppliers for over fifteen years and are certainly nothing new.

Figure 9: Electromagnetic Field with Ferrites

How do you select the right induction sealer for your application? There are two major factors that determine which induction sealer is appropriate for a particular application: the size of the cap, and the speed of the production line (measured in feet or meters/minute). If it is a food application, a washdown enclosure may be necessary. Other factors to consider are the type and composition of the container, the type of innerseal material used, and the type of product (wet, dry, flammable).

What size power supply is best for your application? There appears to be a misconception in the packaging industry concerning the relationship between the kilowatt ratings of induction sealing systems and sealing capability. While it is true that a higher kilowatt rating generally means a more powerful system, this doesn't necessarily result in higher sealing rates.

Kilowatt rating is only part of the equation. The real secret to creating efficient and consistent seals time-after-time is the energy transfer from one part of the system to the other.

All sealing heads are not created equal! There is both an art and a science involved in coil design to achieve efficient and effective power transfer from the power supply into the innerseal foil. All suppliers of induction systems do not share that design capability. If you do not have the correct coil design, increasing the power of the power supply only wastes energy and does not significantly improve the quality or rate of your sealing.

The major manufacturers of induction systems feel the emphasis should be on sealing performance and system efficiency rather than kilowatts. Because of all of the aforementioned reasons, it's not unusual for a 1or 2 kW rated system to outperform a higher kW rated unit. Don't be fooled by suppliers that try to convince you that kilowatts are the best way to rate induction systems. This is simply not true.

So how can you tell which system is right for you? Most people will receive quotations from three or four different manufacturers, all saying their equipment is the best. To further confuse things, the prices are generally not more than a few hundred dollars apart. The power supplies of most major manufacturers of induction systems are very similar. In addition, they all provide satisfactory seals.

This is not to say that there are not special applications where one manufacturer has an advantage over the others because of special coil design or other application knowledge. However, these special applications are such a small percentage of the overall industry, they are insignificant.

So what's a buyer to do? How does he know whom to believe? Little is accomplished by asking a manufacturer for references. Do you think they are going to refer you to someone who is going to give them a bad reference? Not likely!

Good sources of information are your suppliers of closures, bottles or induction innerseal materials. They have no axes to grind and, for the most part, can be objective. They are constantly in the field and usually know if a company has a reputation for reliability and good service, which is really what you are looking for.

Figure7: State-of-the-Art, Waterless Induction Sealing System

You should also take a very close look at the warranties offered by the various suppliers. Service can be extremely expensive if you have a problem with your equipment. Don't fall into the parts-and-service trap.

Several companies advertise free parts and service on equipment for a period of one year after your installation. However, the customer pays the travel and living expenses of the service technician. This can amount to more than a thousand dollars for a so-called free service call. Another company offers two different warranties in its quotations; one covers the equipment for six months and the other for eighteen months. So, you see, it can be very confusing. It's to your benefit to ask questions about each company's warranty. It could save you a bundle of money. One excellent way of comparing equipment is to visit each supplier personally.

Major differences between suppliers do exist in terms of pre- and post-sale service and in some areas of warranty. The rules haven't changed. To be a smart buyer, read the proposals carefully, investigate the vendor's reputation for product quality and service and ask a lot of questions.