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
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
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
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
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
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
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
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
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
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.