WO1996017499A1

WO1996017499A1 - Induction heating coil

Info

Publication number: WO1996017499A1
Application number: PCT/GB1995/002702
Authority: WO
Inventors: Andrew Cunningham Davies
Original assignee: Cheltenham Induction Heating Limited
Priority date: 1994-11-29
Filing date: 1995-11-17
Publication date: 1996-06-06
Also published as: GB9424051D0; GB2300096A; GB9614649D0

Abstract

This invention relates to a split return induction coil and in particular to a coil which is suitable for drying sealant deposited on can ends. Thus a conveyor (15) carried end caps (10) in an edge-to-edge configuration beneath a split return coil (16) which is fed from an R.F. source (17). The coil (16) has a central portion (18), which carries the full current (I), and two side return portions (19), which carry half the current. The central portion (18) has a non-linear section (20).

Description

INDUCTION HEATING COIL This invention relates to a split return induction heating coil and in particular, but not exclusively, to a coil which is suitable for drying sealant deposited on can ends.

Many different coil configurations exist which are suitable for creating preferential heating on wor pieces which are essentially static vis-a-vis the coil during the heating process. Split return induction heating coils have been developed for preferentially heating a linear band along a workpiece passing the coil, for example, the annealing of the butt weld on a tube or pipe. The basis of such split return coils is that one straight portion of the coil passes the full current over the area of the workpiece which is to be heated but that current is split to pass down a number of conductors, typically two, on its return so that the reduced current in the return paths does not create significant heating in other parts of the workpiece. The power is of course proportional to the square of the current.

In the can making industry can ends are usually preformed and put into store. Almost invariably these can ends have an upturned cut or raw edges, the thickness of which is uncoated and therefore vulnerable to corrosion. During the preparation of such can ends it is usual to deposit a sealant or seaming compound around the periphery of the can so that it lies just within the curled edge. These prepared can ends are then placed in stacks (or sticks as they are known in the trade) and wrapped in moisture- absorbent brown paper. They are then stored until required in the canning factory.

Until recently the sealant compound was solvent based and it was easy to flash off the solvent so that there was little or no water which might create corrosion on the raw edge of the can end. However the volatile components of the solvent tend to be extremely noxious and are unacceptable under present-day health and safety requirements. The industry has therefore substantially transferred to water based compounds but this has introduced its own problems, because the water must be removed before storage or else the raw edge will corrode. Typically the curing of the compound and the associated water removal has been carried out in hot air ovens operating on a continuous basis, because industrial standard machines produce can ends at about 1000 per minute. Hot air has many disadvantages including the length of time the ends have to be in the oven, and hence the oven length, and the slow turn on and turn off characteristics of such ovens. They are also extremely wasteful of heat. One of the reasons that the oven time is so long is that if the sealant compound is over-heated, then it will either burn or blister and as such is totally unacceptable particularly in the food industry, where most cans are used.

Attempts have been made to overcome this problem by passing the can ends through an induction coil once they have been assembled in a stack. This creates preferential heating around the rim of the can ends but introduces two fresh problems. Firstly, if the can ends are not separated as they are heated then the moisture given off simply becomes trapped within the stack, and secondly the centre of the can is very much colder than the rim and condensation occurs resulting in trapped moisture. The can ends can be magnetically separated after they pass through the coil to reduce the first problem, but this is difficult to achieve at the high production rates involved. From one aspect the invention consists in a split return induction heating coil having a central section for carrying the full current and at least two split return sections characterised in that the central section has a non-linear portion. In such a coil a band of heating is created on a workpiece passing under the non-linear portion and the band lies in the direction of the current. It will thus be appreciated that by creating a non-linear portion for the central section, the direction of the current varies relative to the workpiece as the workpiece passes along the non-linear portion. In this way an area of a moving workpiece can be preferentially heated, the exact characteristics being dependent, amongst other things, on the separation between the workpiece and the non-linear portion. This ability to preferentially heat an area on a moving workpiece may have many applications but it is particularly advantageous in the drying out of sealing compounds deposited on can ends for reasons which will be set out below .

Preferably the non-linear portion of the coil will be in the form of a zig-zag, a wave or other alternating formation. The coil preferably surrounds or is spaced from a workpiece's path of travel and conveniently the non-linear portion overlies the path, although those skilled in the art will appreciate that the portion could equally lie under the path.

The invention also consists in apparatus for preferentially heating portions of generally planar workpieces including, means for moving the workpieces in an edge-to-edge configuration along a path of travel, and a coil, as defined above, surrounding or spaced from the path such that the non-linear portion overlies or extends under the path. Means may be provided for moving the non-linear portion relative to the path.

Where the workpieces are can end caps with sealant deposited generally circumjacent the periphery of the cap, the coil is designed to preferentially heat the central portions of the cap which lie within the sealant.

With this arrangement the central portions can be rapidly heated to a temperature which is above the critical or damaging temperature for the sealant compound (which for one class of products is 85^CC) and the sealant compound is then heated by conduction from the central portion. This makes it possible to retain the sealant at a high, but not damaging, temperature for a longer period of time and so provides particularly efficient drying. Because the central portion is hot, there is little possibility of condensation and with the coil arrangement described above, the end caps can be dried in an edge to edge configuration before being placed into a stick and so moisture will not become trapped. The invention thus further consists in a method of drying sealing compounds deposited on can end caps circumjacent their periphery comprising, heating a portion of the end cap lying within the deposited compound so that the compound is dried by heat conducted from the portion to the compound.

In a particularly preferred embodiment the portion of the end cap is heated to a temperature which is above the critical or damaging temperature of the particular compound.

Although the invention has been defined above it is to be understood that it includes any inventive combination of the features set out above or in the following description.

The invention may be performed in various ways and a specific embodiment will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a view from above of a can end cap;

Figure 2 is a cross-section through the end cap of Figure 1 showing a deposit of sealing compound;

Figure 3 is a graph of temperature against time in an existing drying arrangement;

Figure 4 is the corresponding graph of the drying arrangement of the specification;

Figure 5 is a schematic side view of a drying apparatus;

Figure 6 is a schematic circuit diagram of the coil of the apparatus of Figure 5;

Figure 7 relates the heating in a workpiece to portions of the coil of Figure 6; and

Figure 8 illustrates a heating pattern on an end cap.

In Figure 1 and Figure 2 the can end cap is generally indicated at 10. This comprises a central portion 11 and a rim 12 having a raw edge 13. As can be seen in Figure 2 a sealing or seaming compound 14 can be deposited around the periphery of the end cap 10 just within the rim 12.

As has been explained above, a previous proposal using induction heating to cure and dry out the sealing compound 14 preferentially heats the rim of the end cap 10 with the result that the rim temperature is taken up to the critical temperature Δ for a very short period of time, and central portion 11 remains at a much lower temperature throughout with the resultant condensation described above.

The applicant proposes the completely contrasting arrangement illustrated in Figure 4 where the central portion temperature is taken significantly above Δ with the result that the rim temperature rises towards Δ, due to conduction, and falls away much more slowly. As has been described above this provides both good drying out of the sealing compound and reduces the chances of condensation.

Apparatus for achieving such sealing is illustrated in Figure 5. Here it will be seen that a conveyor 15 carries the end caps 10 in an edge-to-edge configuration beneath a split return coil 16 which is fed from an RF source 17.

As can be seen in Figure 6, the coil 16 has a central portion 18, which carries the full current I, and two side return portions 19, which each carry half the current. The central portion 18 has a non-linear section 20, which is generally in the form of a zig-zag, but can be configured in any alternating arrangement depending on the heat pattern required. As has been explained, above a band of heating is achieved in the end cap 10 whose orientation is determined by the direction of the coil. This is well illustrated in Figure 7 where the central portion 18 is divided into sectors A to H and the corresponding heat pattern in the end caps 10 is indicated by lines 21. When taken together these create a combined heating pattern as indicated at 22 in Figure 8 and thus the central portion 11 of end cap 10 is quickly and fully heated over a short length of travel.

Claims

1. A split return induction heating coil having a central section for carrying the full current and at least two split return sections characterised in that the central section has a non-linear portion.

2. A coil as claimed in Claim 1 wherein the non¬ linear portion is in the form of a zig-zag, a wave or other alternating formation.

3. A coil as claimed in Claim 1 or Claim 2 dimensioned and/or shaped to surround or be spaced from a workpiece path of travel.

4. A coil as claimed in Claim 3 wherein the non¬ linear portion overlie or lies under the path of travel.

5. A coil as substantially hereinbefore described with reference to the accompanying drawings.

6. Apparatus for preferentially heating portions of generally planar workpieces including, means for moving the workpieces in an edge-to-edge configuration along a path of travel, and a coil as claimed in any one of the preceding claims, surrounding or spaced from the path such that the non-linear portion overlies or extends under the path.

7. Apparatus as claimed in Claim 6 further including means for moving the non-linear portion relative to the path.

8. Apparatus for preferentially heating portions of generally planar workpieces substantially as hereinbefore described with reference to the accompanying drawings.

9. A method of drying sealing compounds deposited on can end caps circumjacent their periphery comprising, heating a portion of the end cap lying within the deposited compound so that the compound is dried by heat conducted from the portion to the compound.

10. A method as claimed in Claim 9 wherein the portion of the end cap is heated to a temperature which is above the critical or damaging temperature of the particular compound.

11. A method as claimed in Claim 9 or 10 wherein the heating is performed by a coil as claimed in any one of

Claims 1 to 5 or the apparatus of Claims 6 to 8.

12. A method of drying sealing compounds deposited on cam end caps circumjacent their periphery substantially as hereinbefore described with reference to the accompanying drawings.