The latest generation of kettles and hot water jugs, use so called"thick film"heaters."Thick film" heaters and their methods of manufacture are well known in the art and need not, therefore, be described in greater detail here. They are exemplified in WO 96/18331, and WO 96/17497. Briefly, however, a thick film heater comprises a resistive heating track applied, for example printed, onto an electrically insulating layer provided on a metallic substrate. The metallic substrate may form an integral part of the vessel itself or be a separate plate mounted to close an opening in the base of the vessel. This latter construction is used particularly in plastics walled vessels.

For reasons connected with their mechanical, thermal expansion and thermal conductivity properties, so-called 400 Series stainless steels have been used as the metallic base material in such applications. Whilst it might be assumed that such steels would exhibit satisfactory corrosion resistance, it has been found that they are, in fact, susceptible to both pitting and crevice corrosion and surface rusting when immersed in water unless they are thoroughly cleaned of surface oxides which occur during processing. These effects are exacerbated by the repeated heating and cooling of the stainless steel. Accordingly, unless treated, the water facing surface of the metallic base will corrode, which is unacceptable.

The present invention seeks to overcome the above

problem, and from a first aspect the present invention provides an electric liquid heating vessel having a heated wall part, the wall part being of a 300 series stainless steel, with a thick film heater laminated to its dry side.

Thus in accordance with the invention, a thick film heater is laminated to the dry side of a 300 series stainless steel wall part, preferably the base, of the vessel. 300 series stainless steels are more ductile and more corrosion resistant than 400 series stainless steels, but are unsuitable as substrates for thick film heaters due to a mismatch in their coefficient of thermal expansion relative to that of the insulating material which leads to overstressing of the insulating layer or even its separation from the steel. The insulating material can however be successfully laminated to other steels such as 400 series stainless steels which are suitable as thick film heater substrates. Thus the invention makes best use of the individual properties of 300 and 400 series stainless steels.

The wall part may be an integral part of the vessel, which may be made entirely of a 300 series stainless steel. Alternatively, however, the wall part may be a separate component mounted for example in an opening in the bottom of a plastics walled vessel. The laminated wall part thus itself constitutes a heater, so from a further aspect, the invention provides an electric heater for a liquid heating vessel comprising a plate of a 300 series stainless steel and a thick film heater comprising a metallic substrate, an electrically insulating layer provided on said substrate and a resistive heating track provided on the insulating layer, the metallic substrate of the thick film heater being laminated onto one side of the plate.

The wall part or heater plate is preferably of a 304 or 316 stainless steel, for example 316L stainless

steel. These materials are commonly used in metallic liquid heating vessels already and maintain a good stainless appearance in use.

The particular material of the wall part or heater plate will be determined to a large extent by the temperature and power density to which they will be subjected. For lower temperature, lower power applications, a 304 stainless steel may be used, but for higher temperatures and higher power densities, a steel more resistant to stress corrosion such as a 316 stainless steel, e. g. 316L may be used.

The metallic substrate of the thick film heater may be of any suitable material, such as a 430 stainless steel, preferably 430S17 stainless steel. An alternative material may be mild steel such a very low carbon (<. 01%) mild steel or VE grade mild steel.

The metallic substrate may be laminated to the vessel base or heater plate by any suitable process, for example brazing.

To improve thermal conduction over the vessel base or heater plate, a layer of a thermally conductive material such as aluminium, copper or alloys thereof may be sandwiched between the metallic substrate and the vessel base or heater plate.

Preferably, the thick film heater is provided with means to mount a thermally sensitive control, electrical connector means or the like. Up to now, this has been done typically by welding a plurality of mounting studs onto the metallic substrate of the heater. However, this is expensive and can leave a witness mark on the water side of the heater, for example when thin metal is used. By virtue of the invention, the metallic substrate may be provided with such mounting means without the problem of witness marks since the water facing side of the substrate is not exposed on the inside of the vessel. This also means that the substrate need not be continuous, so that the mounting

means for a control may comprise one or more lugs pressed out of the substrate. These lugs may be prepunched in the substrate, and kept flat during processing of the heater, and then pressed out of the plane of the substrate before the heater is laminated to the vessel base or heater plate.

This arrangement is believed to be novel in its own right, so from a further aspect, the invention provides a thick film heater having a planar metallic substrate, an electrically insulating layer provided on the substrate and an electrically resistive track on the insulating layer wherein the substrate is formed with one or more lugs which can be bent up out of the plane of the substrate to provide a mounting location for a control or the like.

It should be appreciated that forming the lugs in the above manner has the additional advantage that it allows gases generated during the laminating process, eg during brazing to escape, thereby reducing the possibility of gas bubbles forming between the layers which would not only weaken the bond between the layers but also act as a thermal insulation, leading to possible overheating of the heater.

This is in itself believed to be a novel and inventive concept, so from a yet further aspect, the invention provides a thick film heater assembly, comprising a support plate for the heater, a heater plate carrying the heater laminated to the support plate, the heater plate having one or more apertures formed therein for facilitating the escape of gases formed during the lamination procedure.

Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows a schematic view of a liquid heating vessel embodying the invention; Figure 2 shows a schematic exploded sectional view

through a first heater embodying the invention; and Figure 3 shows a schematic exploded sectional view through a second heater embodying the invention.

With reference to Figure 1, there is shown a water heating vessel 2 made from 0.4mm thick 304 or 316 stainless steel. The vessel body is drawn or spun into the appropriate shape in a conventional manner.

A thick film heater 4 is laminated onto the underside of the vessel base 6. As shown in the schematic enlarged section, the thick film heater comprises a metallic substrate 8 about 0.5 to l. Omm thick of 430S17 stainless steel, an insulating layer 10 about 50 to 100 Am thick deposited on the substrate 10 and a resistive heating track 12 about 10-15 Am, preferably 13 ym, thick deposited on the insulating layer 10. Typically, the insulating layer is screen printed onto the substrate in one or more steps and then fired, whereafter the track 12 is deposited on the insulating layer 10, again typically by screen printing, and the heater then fired again. An overglaze layer (not shown) may be provided over the track. Such thick film heaters and their processes of manufacture are well known in the art and need not, therefore, be described further here.

A plurality of tongue shaped lugs 14 are provided in the substrate 8 to mount a thermally sensitive control 16, electrical connector or such like to the heater. The lugs are punched or otherwise defined in the substrate 8 prior to manufacture of the heater 4 and are then bent down through 90° to the position shown in Figure 1 prior to laminating the heater to the base 6.

After the heater 4 has been laminated to the vessel base, for example by brazing, the control or the like can be mounted over the lugs which can be bent over to secure the control unit or the like in position.

Turning now to Figure 2, a heater 20 in accordance with the invention comprises a 0.4mm thick flat plate 22

of 304 or 316 steel, onto which is laminated a thick film heater 24 through a braze 26. The thick film heater is the same as that described in Figure 1 although it is now laminated to a plate 22 which is intended to be mounted in an opening in the base of a plastics vessel (not shown) by suitable clamping means.

The brazing can be effected in any suitable manner.

Inductive brazing is a particularly preferred method.

Thus a brazing powder or paste may be spread over one or both of the opposed surfaces of the plate 22 and the substrate 10 of the thick film heater, the two components clamped together under pressure (typically 1 tonne) and the interface inductively heated to melt the braze material. The braze material is preferably a high temperature braze, for example melting at 600-700°C.

Such temperatures will not damage the heater during the brazing process (as the heater will itself already have been fired to 850-900°C during its manufacture, and will be significantly higher than any temperature which might be experienced by the heater in use, even in a fault condition.

Figure 3 shows a further heater 30 in accordance with the invention.

This embodiment comprises a dished plate 32 of 0.3mm thick 316L stainless steel. The periphery of the plate is provided with a groove 34 which can be used to secure the plate 32 over an opening in the base of a plastics jug or the like, in the manner described in WO 96/18331.

As in the embodiment of Figure 2, a thick film heater 4 is laminated to the underside of the plate 32.

However in this embodiment, an aluminium plate 36 about 1-2mm thick is sandwiched between the plate 32 and the metallic substrate of the thick film heater. This layer may act to improve the distribution of heat across the heater. The various layers may be brazed together through respective layers of braze material 38,40, or

they may be joined by some other process, for example cold pressing.

It will be appreciated that the invention is not limited to the particular arrangements described above.

For example, while the invention has been described in the context of an open topped liquid heating vessel such as a kettle or jug, it may be applicable to other types of vessels having a liquid heating chamber. Also, the wall part need not be the base of the vessel, but any wall part which needs to supply heat to the vessel.