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Induction Sealing Explained

Induction sealing is a simple and reliable method of bottle sealing. It welds an aluminium foil seal across the bottle neck:

prevent product leakage.
provide tamper evidence.
improve product shelf life due to the excellent barrier properties of the foil and its airtight seal onto the container.
improve pack presentation and customer acceptance.

Bottles are filled and capped with a cap fitted with an auluminium foil seal (wad) and then passed under the induction sealer. This transmits an electromagnetic field, somewhat similar to a microwave oven, that heats the aluminiium seal and bonds it to the container neck. An automatic filling, capping and sealing line is represented diagaramatically as follows:

The bottle is filled at position 1 and the cap, complete with aluminum foil seal, is applied at position 2. The capped bottle is then passed under the induction sealing machine which causes the induction seal liner in the cap to heat up and bond to the container neck by position 5. When the bottle is later opened (6) the aluminum foil liner is released from the cap and left in place across the bottle neck.

The Induction Sealing Machine

Induction sealers transmit an electro magnetic field which create eddy currents in a metallic element, such as an aluminium foil induction innerseal, placed in the magnetic field. This current heats the aluminium foil seal and its plastic coating bonds to the container neck.

Induction sealing machines comprise a power source, a sealing head or coil and a cooling system. Manual hand held units are available for laboratory or low volume work but high power automatic cap sealers are usually mounted above conveyorised production lines.

Some induction sealers, especially older ones, are water cooled and have the power generator at floor level connected to the sealing head, mounted above the conveyor, by combined electrical and cooling cables.

Increasingly, modern sealing machines are air cooled which makes them much more compact so that the entire unit, including the power supply, can be mounted above the conveyor.

The frequency of the electromagnetic field varies slightly from manufacturer to manufacturer but is typically in the range of 30-100 KHZ. Lower frequencies give a more uniform heating across the diameter of the induction seal liner, which can help wax melt on large closures, and higher frequencies gives a greater edge heating effect, which can help with fast sealing rates on smaller diameters.

Induction sealing machine heads or coils

The induction sealing machine's head, or coil, transmits the magnetic field. There are two main types, flat coils and tunnel coils.

Tunnel Sealing Head or Coil

Flat Sealing head or Coil

The tunnel coil has the advantage that it locates the bottle centrally and often has a more uniform and deeper field because the coil windings can be incorporated in the sides around the cap. This makes it particularly suitable for closures where the induction seal liner is deep inside, e.g. sports caps and child resistant closures. Usually different sized tunnel coils are needed for different size caps.

Flat coils are more universal and good for fillers using different cap sizes but special care must be taken to ensure that the cap is centrally positioned beneath the head. See requirements for a good seal below.

Requirements for a good induction seal

For a good seal exactly the right amount of heat must go into the foil - too little and the seal will lack strength and may leak, too much and burning or degradation of the foil will occur.

There are three main factors that determine the amount of heat created in the foil liner:

1. The power setting on the heat sealer.
2. The distance between the cap and the heat sealer's coil. This is critical as the strength of the field varies inversely with the distance from the source, doubling the distance reduces the field strength to one quarter
3. The conveyor speed which determines the dwell time of the foil seal under the sealing head

The advantage of the LINE PATROLMANTM is that it measures the combined effect of these three variables in a single reading to make setting and monitoring much easier and more accurate..

As well as these three main variables there are other factors, relating to the strength and distribution of the magnetic field that have to be taken into consideration. This is because the field from the induction heat sealer is usually stronger down the centre line than at the edges, particularly with flat coils.

This can give rise to:

A. Overheating of the induction foil down the centre line and/or under-heating at the edges, particularly with large diameter closures.
B. Uneven heating side to side if the bottles are not correctly centered under the sealer
C. Variable results if guide rails set too wide allowing bottle position to vary under the sealing head.

Again the LINE PATROLMANTM will detect and measure all of these effects because it measures from the position of the seal. Indeed we have had comments from customers that varying PATROLMAN reading are obtained when what is happening is that the guide rails are set too far apart, allowing the bottles with PATROLMAN attached to be sent down the line in different positions, when it correctly records the different heating effects at these different positions.

Manufacturers of induction sealing machine are aware of these potential problems and one way of overcoming them is to slew the coil round at an angle to the conveyor. This ensures that the cap innerseal passes through the hot and cold areas of the coil but brings another variable into play because the dwell time of the cap under the heat sealer is reduced.

Slewing the coil requires the sealer output to be increased to compensate for this reduced dwell time. This is very easy using the LINE PATROLMANTM as the heat sealer power just needs increasing until the original PATROLMAN value is restored.

Induction seal liners

Induction seal liners, or innerseals or wads, come in two main types, with and without a wax bonded backing liner These are often referred to as one piece and two piece:

The facing of both types is similar, an aluminium film (D) laminated to a heat seal layer (E); the two piece type then has an additional backing liner or 'wad' (B) which is bonded to the facing layers D and E by a layer of 'wax' adhesive (B).

The one piece type of liner is removed from the cap entirely after the sealing process whereas the two piece leaves the backing liner (wad) in the cap for improved resealing after the initial opening.

The two piece liner has a 'wax' layer bonding the sealing layers to the backing board. The heat of the induction sealing process causes this wax layer to melt and it is absorbed into the backing board or other layer so that on cooling the two parts are separate.

The heat from the sealer causes the wax to melt progressively inwards from the edge. If correctly done the wax will completely free the liner facing, alternatively a small central area of unmelted wax may deliberately be left to retain the cap in place after sealing. This can avoid the need for subsequent cap re-tightening.

In all cases it is the heat seal layer which bonds to the container not the aluminium itself. The exact nature of this heat seal determines whether the induction liner will be peelable or full weld type and the types of container the foil will adhere to e.g. HDPE, PP, PET, glass etc.

Induction Seal Quality Tests

The simplest methods of testing bottle seal quality are the 'squeeze', 'stand on' or 'lay on side' leak tests. Full evaluation of cap seal quality is much more complex as other factors, such as foil degradation, long term product compatibility, wad stick etc. may be potential problems even though the seal passes the initial leak test.

The cap or foil supplier should be consulted for their recommendations but the following list may be helpful: