EP0906047A1

EP0906047A1 - Electric liquid heating vessels

Info

Publication number: EP0906047A1
Application number: EP97927260A
Authority: EP
Inventors: John Crawshaw Taylor; Stewart Leonard Huxley
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 1996-06-17
Filing date: 1997-06-17
Publication date: 1999-04-07
Also published as: WO1997048316A1; GB2329823B; GB2329823A; GB9827878D0

Abstract

A method of manufacturing a liquid heating vessel (2) is disclosed in which a sheet metal blank (110) is formed into a tubular shape, the opposed ends (10) of the blank are joined, e.g. by butt welding them together. A handle portion (44) is placed over the joint to hide it. A metal base (72; 200) having a heating element on its underside is attached to the tubular vessel wall, e.g. by soldering with a low-melt solder. A plastics skirt (20) may also be provided.

Description

Electric Liquid Heating Vessels

The present invention relates to electric liquid heating vessels, and in particular to a method of manufacturing such vessels having metal walls.

Metal walled electric liquid heating vessels, and in particular metal walled kettles and hot water jugs are popular in the market, particularly for their cosmetic appearance. Older vessels used chrome plating on polished copper, but more recently polished stainless steel has become widely used. With the difficulties of polishing inside a closed vessel, the inside is usually left with a brushed finish and the exterior of each vessel is brightly polished.

Up to now, however, the shapes available in such vessels have been limited by the manufacturing processes used, for example pressing, drawing and spinning. In particular the vessels are normally circular in section and have parallel walls. Further the vessels are also expensive by virtue of these processes, particularly since the vessels require polishing individually after manufacture, as the manufacturing process destroys any polished finish.

The present invention seeks to provide a less expensive and more versatile method of manufacturing electric liquid heating vessels, and from a first aspect provides a method of manufacturing an electric liquid heating vessel comprising shaping a sheet metal blank into a tube, joining the opposed ends of the blank to form an open ended tubular vessel wall, and closing the bottom of said tubular vessel wall with a base mounting or for mounting an electric heating means on its underside.

From a second aspect, the invention provides an electric liquid heating vessel comprising a tubular wall formed from a blank of sheet metal joined at opposed ends, and closed at its bottom end by a base, having electrical heating means provided on its underside, mounted to the bottom end of the wall. Thus in accordance with the invention, a sheet metal blank is shaped into a tube, its opposed ends joined to form the vessel wall, and a base for electrically heating the vessel mounted to the bottom end of the vessel wall. Preferably the electrical heating means is provided on the base prior to mounting the vessel wall, but in certain circumstances, particularly where a cold pressing method is used this could be provided on the base after mounting. The method of the invention is substantially less expensive than known methods as it avoids the need for drawing dies which are both extremely expensive and require regular maintenance to allow for wear.

The invention also allows vessels having a non- circular cross section and/or which are asymmetrical in elevation to be produced. In this way, the invention allows metal kettles and jugs to be produced with a wide range of novel shapes, for example an elliptical cone. Such shapes have not been hitherto possible using the prior art methods discussed above. From a further aspect, therefore, the invention provides an electric liquid heating vessel having a metal body, the metal body having a non-circular cross section or an asymmetric side elevation.

If a non-circular shape is required, the sheet may be folded into the desired configuration, for example around a suitably shaped mandrel.

Alternatively, it would also be possible to produce a vessel body having a non-circular cross section by first producing a cylindrical or conical body in any convenient manner, and then deforming the body, elastically or plastically, to the desired shape, for example by squeezing in the sides of the vessel body in the appropriate direction(s) at an appropriate position(s) . In this way, the whole body could be made elliptical along the whole of its height. Equally, the top of the body could, for example, be made elliptical along one axis at its top and along another axis, say at right angles to the first axis at its bottom. Thus the invention will allow a wide variety of shapes to be produced which could not be produced by existing methods. From a yet further aspect, therefore, the invention provides a method of manufacturing a liquid heating vessel comprising producing a tubular vessel wall and attaching a heating base to the vessel wall, the tubular wall being deformed after formation to produced a desired cross sectional shape. The tubular vessel body may be deformed before or after the base has been attached. For example, if only the upper part of the vessel wall is to be deformed, this can be done after attachment of the base. Clearly, however, if the lower end of the vessel wall is to be deformed, this must be done before the attachment of the base. The attached base will maintain the deformed shape of the lower end of the vessel wall after deformation, and a strengthening rim or the like be provided to support the upper end. Returning to the formation of the tubular body, the sheet metal blank may be shaped into a tube by rolling, for example, where a vessel of circular cross section is required. This is a working process in which a sheet is repeatedly passed between offset rollers which plastically deform the sheet into a tubular shape. However such a process has the problem that it is a contact method which may damage the surface of the sheet material being worked, meaning that a further finishing process may need to be performed on both the inside and outside of the formed blank. This is particularly so when the blank material is of stainless steel, as is preferred. In the preferred embodiment, however, the blank is formed into a tubular shape by gripping the end regions of the blank and then moving them together, for example in a generally circular path. In this way, the blank may be deformed essentially elastically, so that the ends of the blank will then try to spring apart from each other and they must be held together until they are joined. The advantage of this method is that once the ends are joined, the tubular blank is prestressed which means that it will be better able to withstand knocks and impacts which might otherwise dent the surface. This in turn means that thinner grade materials may be used, preferably less than 0.5mm thick and more preferably between 0.25 and 0.35mm thick stainless steel. This reduces significantly material costs.

Furthermore, this method involves minimal damage to the surface of the material since only the ends of the blank need be gripped. This in turn means that pre- finished, mill hard stainless steel (e.g. SS304) may be used which is less expensive than other steels normally used in vessel manufacture. Further, the steel blank as used may be provided with a protective plastics coating, which need only be removed locally along the ends of the blank to expose the weld region. This means that the coating may be left intact until a much later stage in manufacture, thereby reducing further the risk of damage to the surface finish. The above method of manufacture is particularly preferred, so from a further aspect, therefore, the invention provides a method of manufacturing an electric liquid heating vessel comprising gripping opposed end regions of a generally planar stainless steel sheet blank, bringing the end regions together in an elastic manner so as to form a tube, joining the opposed ends of the blank to form an open ended tubular vessel wall, and closing the bottom of said tubular vessel wall with a base mounting or for mounting an electric heating means on its underside. The opposed ends of the formed blank may be joined together in any suitable manner, for example by rolling together, or, more preferably, by welding or soldering together. In one embodiment, an outwardly facing lip or flange is formed on each end of the blank, the lips being placed together face to face and then welded or soldered together on the outside of the vessel to form a rib. This minimises the need for finishing within the vessel itself. Further, the welded or soldered rib so formed may be covered by a suitable cover.

Preferably, the joint or weld is arranged at the rear of the vessel so that it can be at least partially covered by a handle or a steam pipe of the vessel . In the presently preferred embodiment, the opposed ends of the tubular blank are butt-welded together e.g. by T.I.G. welding. In a preferred process, a backing plate preferably of a material having a good thermal conductivity e.g. copper or aluminium, is arranged against the inside surface of the formed blank. The backing plate will act to support the weld region as well as acting as a heat sink during welding to draw heat away from the inner side of the weld-site and surrounding region, thereby helping to prevent "blow through" during welding (particularly important where thin gauge materials are being used) and also discolouration of the metal. The unsightly discolouration or 'bluing' which is usually caused by welding would otherwise have to be abraded or polished off in an extra production step. Thus preferred embodiments of the present invention may avoid this step.

Preferably, inert gas such as Argon is passed over both sides of the weld site, to exclude oxygen thereby further helping to prevent discolouration. Preferably, therefore, passages are formed in the backing plate through which an inert gas such as Argon may pass. Preferably the passages are provided laterally on either side of the actual weld line, so that the weld itself is supported during welding.

Preferably, the vessel wall is provided with pouring means, such as a lip, spout or opening formed in the vessel wall prior to forming. Preferably the deformation of the vessel in forming the spout should be minimised to avoid damaging the surface finish of the vessel. Alternatively, it may formed in the vessel after forming or be a separate component, for example a metal or plastics component mounted to the vessel wall. A preferred form of metal spout would be a flanged spout whose flange is arranged inside the vessel, with a seal clamped between the flange and the vessel wall by a series of tack welds, preferably small laser spots which do not detract cosmetically from the finish. Preferably for a plastics spout, a pouring means may extend through an opening in the vessel wall.

For reasons of strength and cosmetic appearance, a plastics or other rim may be fitted over the upper edge of the vessel wall.

The rim may, for example, mount a lid for the vessel, and most preferably, the rim, the spout and the handle are formed as an integral moulding, for mounting to the vessel wall as a single component.

The vessel base may be of the same material, and generally of the same thickness as the vessel wall, and it may be rolled, welded, or more preferably soldered to the bottom of the vessel wall. The particular choice of material and thickness will depend to a large extent on the type of heater used. If a thick film element is used, the base may typically be in the range 0.75 to 1.0mm thick, which is thicker than the vessel wall thicknesses discussed above, but with a sheathed heating element attached to the base via a diffusion plate, the base may be thinner, of the same order as the vessel wall thickness e.g. 0.25 to 0.35mm. Soldering has a number of advantages over welding in this context. In a weld process, the temperatures experienced result in contraction of the metal on either side of the weld, leading to a visible witness line which would have to be polished out. As soldering is conducted at much lower temperatures, there is less thermal contraction around the joint, and thus less distortion. Furthermore, soldering may be used with thinner gauge materials to reduce the danger of the thin metal blowing through during welding.

There are a number of ways in which the soldering process may be conducted. In a simple embodiment, a collar may be arranged outside the tubular blank around the base part, which is suitably located inside the bottom of the formed blank. Solder is applied around the join between the base part and the joint heated to melt the solder, for example by rotating the base/blank assembly under a suitable dispenser. After heating for a predetermined time, the heating is discontinued and the joint allowed to cool to secure the base part in position. The collar acts to help prevent thermal distortion in the vessel wall during the soldering process. Preferably the collar itself is heated to melt the solder. The collar may act to support the vessel during welding, but equally, it may act just to apply the heat during welding. For example, one can envisage a collar being axially movable into and out of engagement with the soldering region as required. This would be useful, for example where the vessel wall is tapered, and the collar may be formed with same taper as the vessel wall so as to ensure good thermal contact therewith during soldering. Alternatively, the collar could be formed of a number of parts which may be moved radially in and out to effect the joint. This may be more appropriate when the vessel is untapered.

The base part can be positioned inside the tapered end of the vessel body in any convenient manner and, if necessary, the two held together by suitable clamping means to ensure a firm soldering operation.

The soldering operation described above is believed to be novel and advantageous in its own right, and from another aspect therefore the invention provides a method of attaching a metal heating base to a preformed metal tubular vessel body comprising positioning said base inside one end of said body, applying solder between the base and the vessel body, positioning a collar around the vessel body in the region of the base and heating said collar to melt said solder.

It will be appreciated by those skilled in the art that these steps may be carried out in any convenient order and suitably mechanised to reduce costs.

In a preferred embodiment, a silver-tin solder is used e.g. one have a melting point in the range 220- 235°C, and the collar is heated to approximately 240- 250°C. Using a solder material having a low melting point (relative to welding temperatures) is advantageous for the reasons discussed above, and also in that it reduces the amount of discolouration on the outside of the formed vessel, and thus the need for a further finishing step. The soldering operation may also be conducted preferably in an inert gas, e.g. Argon atmosphere to reduce further the risk of discolouration. It is possible to use a solder having a relatively low melting point since, in use, the temperature of the weld region will be kept considerably below the solder melting point by the liquid in the vessel. The vessel will also normally be provided with some form of overheat protection device such as that shown in WO 95/01342. Thus even if the vessel does boil dry or is switched on with no liquid in it, the power to the heating element will be disconnected before the joint overheats. In addition, heat conduction through the stainless steel base is poor, thereby further reducing the heat flowing into the joint.

The base of the vessel preferably Comprises a stiffening lip around its edge. Most preferably, the base is generally dish shaped having a generally planar central region, an upwardly angled rim extending from the central region, and a lip depending from said rim. This gives a particular stiff construction, and is novel in its own right, so from a further aspect, the invention provides a base for an electric liquid heating vessel comprising a generally planar central region, an upwardly angled, generally linear rim extending from said central region, and a lip extending downwardly from said rim.

The rim may also have a portion which extends generally parallel to the central region of the base, with the lip extending downwardly from the outer edge of that portion.

To facilitate the soldering of the base into the vessel wall, the free end of the lip of the base may be bent over so as to provide a location for receiving the solder.

Preferably the base is mounted to the vessel wall via the stiffening lip. Most preferably, the lip is arranged inside the end of the vessel wall facing downwardly and a weld or soldered joint effected between the lip and the vessel wall so that the joint cannot be seen inside the vessel. The exposed welded or soldered region can then be covered by a base cover which may also enclose electrical components such as the heating means, controls and connectors arranged under the vessel base.

By using a soldering technique as discussed above, an external cover may not be required to cover the joint. However, it is still desirable to provide a protective skirt at the base of the vessel wall to protect that region from knocks and other accidental damage. In a further embodiment, therefore, a plastics skirt is provided which is flush with the lower part of the vessel wall, and which locates inside the end of the vessel . This in itself is believed to be a novel arrangement irrespective of how the vessel wall is formed or how the base is attached to the vessel wall, and from a further aspect the invention provides an electric liquid heating vessel having a tubular body an_d a skirt mounted at the lower end of the tubular body, the skirt locating inside the end of the vessel wall an_d lying generally flush with the external surface of the vessel wall.

Preferably the vessel base is attached to the vessel wall at a location spaced from the end of the vessel wall. This allows control etc components to be provided under the base, while at the same time allowing the depth of the skirt to be minimised, which may be more aesthetically pleasing, and give improved stability. It also means that the skirt may locate inside the vessel wall rather than against a part of the heated base, which is advantageous in keeping the temperature of the skirt within acceptable limits. This is also thought to be a novel arrangement, irrespective of how the tubular body is formed, or how the base is attached to the tubular body, so from a further aspect, the invention provides an electric liquid heating vessel having a tubular body and a heated base attached to and closing the bottom of the tubular body, the base being attached to the body at a location spaced from the end of the body. If the vessel wall tapers outwardly towards the base, or is inclined to the vertical, then the lower edge of the vessel wall may be formed locally into a cylindrical shape so as to accommodate more accurately the base. Alternatively, to minimise distortion of the polished exterior of the vessel, the downwardly extending lip of the base need not be cylindrical, but formed with the necessary shape and angle to mate exactly with the vessel wall.

Furthermore, the base need not be horizontal, but may be inclined therefrom in side elevation. Most preferably the base is inclined upwardly towards the rear of the vessel away from the spout, and a thermal control of the vessel senses temperature near the highest section of the base, so that should the vessel boil dry, this area of the base will become exposed first, and thus overheat first. The shape of the base may be matched to the cross sectional shape of the base. For example, if the vessel base is elliptical, the base may also be elliptical in shape. The provision of a non-circular base for attachment in the base of a liquid heating vessel is in itself a novel departure from existing constructions, so from a further aspect the invention also provides a non- circular base for closing an opening in the base of a liquid heating vessel, said base having heating means provided on the underside thereof. This aspect of the invention applies not only to metal bodied vessels, but equally to plastics bodied vessels.

The electrical heating means provided on the vessel base may take a number of forms. In one embodiment it may comprise a sheathed heating element mounted to the underside of the base, preferably through a thermal diffusion plate of, typically, Cu, Al or a thermally conductive alloy. In another embodiment, however, the heating means may comprise a thick film printed element provided on an insulating substrate provided on the base. Examples of such elements are disclosed, for example, in our PCT application WO 96/18331.

Some preferred embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Fig. 1 shows a schematic exploded perspective of a hot water jug embodying the invention;

Fig. 2 shows a scrap section through the rear of a jug embodying the invention;

Fig. 3 shows the attachment of a base to a vessel wall;

Fig. 4 shows a horizontal cross section through apparatus for welding a blank in accordance with a second embodiment of the invention,-

Fig. 5 illustrates a method of attaching a base to a vessel wall in accordance with the invention;

Fig. 6 shows a base arrangement provided with a protective skirt; and

Fig. 7 shows a developed, further form of vessel body in accordance with the invention.

With reference to Figs. 1 and 2, a liquid heating vessel, more particularly a hot water jug 2, comprises a generally conical vessel wall 4 and an electrically heated vessel base 6 which is mounted, in a manner to be described below, to the bottom of the vessel wall 4.

The vessel wall 4 is made from a blank of polished stainless steel sheet material which has been shaped, eg rolled into the desired conical shape. The opposed ends 8 of the rolled sheet are folded outwardly to form lips 10 along the length of the wall 4. These lips 10 are welded together externally of the vessel to form a welded rib 12 extending along the back of the vessel. This not only facilitates the welding procedure by allowing accurate clamping of the edges prior to welding but also means that the weld will not extend into the vessel interior, which will accordingly need little, if any, finishing after the welding operation. Once the vessel wall 4 has been welded, the base 6 of the vessel is offered up to the bottom of the vessel wall 4 and secured there, for example by welding, so as to close the bottom of the vessel 2.

In the embodiment of Fig. 1, the base 6 comprises a generally flat plate 14 having a sheathed element 16 mounted to its lower surface via a diffuser plate (not shown) . A combined thermally sensitive control and electrical connector unit 18 is mounted to the underside of the base 6. Such a unit 18 is disclosed in our PCT publication WO 95/01342.

The underside of the base 6 is covered by a cover moulding 20 having openings 22 to allow the arms 24 of a control knob 26 of the control unit 18 to pass therethrough. The cover 20 may, for example, be mounted by mounting screws (not shown) extending into mounting locations provided on the base 6. The vessel 4 sits on a cordless base unit 28 having an electrical connector 30 on its upper surface for mating engagement with the connector of the unit 18.

The upper end of the vessel wall 4 has a pouring opening 32 which receives a locating spigot 34 of a plastics pouring spout 36. The spout 36 is formed as an integral part of a moulding 38 which also comprises a rim portion 40 having a downwardly open slot (not shown) which engages over the upper edge 42 of the vessel wall 4 with a seal (not shown) at the base of the slot . The moulding 38 further comprises a handle portion 44 and a pivot 46 for a hinged plastics lid 48. The moulding 38 may be retained in position on the vessel wall by engaging the slot over the upper edge 42 and the spigot 34 extending into the pouring opening 32 and bonding the components together, for example with adhesive, by ultrasonic welding or more preferably a large eyelet and seal as is conventional .

As can be seen more particularly from Fig. 2, when mounted to the vessel wall 4, a channel 50 of the handle moulding covers the welded rib 12, so hiding it from the user. A steam pipe 52 may also be formed within the channel 50 to conduct steam from the vessel 2 to the steam control of control unit 18 to allow operation of the control in the event that liquid in the vessel boils. The lower end of the handle 44 also provides a pivot 54 for the bearings 56 of the control knob 26.

Referring now to Fig. 3, a preferred form of vessel base 60 is illustrated.

In this embodiment, the base 60 is formed of a stainless steel plate 62 to which is mounted a sheathed heating element 64 via an aluminium diffuser plate 66 in a known manner. The diffuser plate 66 is provided with various mounting bosses 68 for a control unit or base cover 70.

The stainless steel plate 62 has a generally planar central portion 72 with an upwardly inclined rim 74 from which depends a lip 76. This gives a particularly strong and rigid base construction.

As can be seen, the base 60 is introduced into the open bottom of the vessel, so that the lip 76 overlaps with a lower edge portion 78 of the vessel wall 80. The lip 76 and edge portion 78 are then welded together, for example by a face ring weld 82 or an end weld 84. These form of welds, outside the vessel, avoid, or minimise damage to the finish of the vessel interior. The cover 70 is configured to fit over, and so hide, the weld 82,84. Alternatively, the lip 76 and lower edge portion 78 may be soldered together.

Fig. 4 shows a further embodiment of the invention in which the ends of a blank are butt welded together rather than being joined along a raised rib as in the embodiment described above. The blank is produced from 0.35mm thick mill hardened SS 304 stainless steel.

Prior to welding, the opposed ends of the flat blank 110 are gripped between respective pairs of clamping jaws 112, 114,-116, 118. If present, a protective coating provide on the blank may be removed from the ends to be welded together. The clamping jaws run along the entire length of the ends of the blank and have respective opposed gripping surfaces 120, 122;124, 126 which are arranged to grip the blank about 5-10mm inset from the respective ends of the blank 110. The lower clamping jaws 114,118 each have a chamfered cutout 130,132 to allow the ends of the blanks to be inserted between the respective jaws. Each end of the blank 110 is inserted to abut a stop (not shown) , and the clamping jaws then tightened by screws 134, 136 to grip both ends. One or both sets of clamping jaws is/are then moved in directions indicated by arrows A so as to fold the blank 110 into the appropriate shape, with opposed positioning surfaces 138,140 of the jaws in abutment. Dowels 142 set into these surfaces 138,140 accurately align the jaws, so that the ends of the blank are accurately abutted for welding along a weld line 152. It will be appreciated that this method of forming the blank into a tube results in minimal contact with the surface of the blank, which is desirable as it avoids the need for finishing operations. Furthermore, the forming operation does not plastically deform the blank material, so the opposed end regions will tend to spring apart and must be kept tightly gripped until after welding. Prior to welding, a copper backing strip 144 is positioned against the inner side of the weld site 146 in a groove 148 formed between the lower jaws 114,118. The backing strip may be slid into the groove 148, or may be fixedly attached to one or other of the jaws 114,118.

The backing plate 144 has two longitudinal channels 150 formed along its length to allow an inert gas such as argon to be passed over the weld site during welding. The channels are arranged on either side of the weld line 152 so that the weld is supported by the backing strip during welding.

The welding process is carried out from the outside, and the upper clamp parts 112,116 both have chamfered surfaces 154 to improve access to the weld area.

During welding, the backing strip 144 acts to support the weld, and to draw excess heat away from the weld region, thereby reducing the likelihood of discolouration or 'bluing' on the inside surface of the vessel. Furthermore, argon or some other inert gas is flowed through the channels 150 in the backing strip to exclude oxygen, thereby reducing further the risk of discolouration. Accordingly, the need for a further polishing or finishing step to be performed on the inside of the vessel may be obviated.

As the blank material was only elastically deformed during bending, the welded tube is prestressed, meaning that it will be better able to withstand impact damage.

Turning now to Figure 5, an further method of attaching a base 200 to the vessel wall 80 will now be described. Firstly the base 200, which is preferably of stainless steel and has a sheathed heating element or a thick film element provided on its underside is positioned inside the open bottom end 202 of a formed stainless steel tube 204 which forms a vessel wall. The base is generally similar in shape to the base shown in Figure 3, having a generally planar central portion 206 with an upwardly inclined rim 208. The rim includes a portion 210 arranged generally parallel to the planar central portion 206, and a lip 212 depends from that portion 210 of the rim. This provides a particularly strong and rigid base construction. As can be seen, the vessel wall 204 is frustroconical, and the lip 212 angled so as to engage and support the vessel wall 204. Once the base is positioned, a bead of solder paste 216, e.g. P40 solder paste from Johnson Matthey, is run around the lower edge 218 of the base lip 212 between the edge 218 and the vessel wall 204. Although not shown, the edge 218 may be bent over so as to provide a platform or groove for receiving the solder. The assembly is then positioned in a tapered annular collar 220 which has the same taper as the vessel wall 204, so as accurately to locate and support the vessel wall during welding, helping to minimise distortion. The collar 220 has been pre-heated by an embedded heating element 222 to approximately 250°C. The collar 220 may have a thermocouple (not shown) embedded in it to monitor this temperature.

Furthermore, it may be provided with channels on its inner surface so that, if required, inert gas can be passed over the outer surface of the solder site.

Since the melting point of the solder is below 250°C, the solder 216 melts evenly around the ring and is drawn into the gap between the lip 212 and the vessel wall to bond the base 210 to the vessel wall 204. The heating effect of the collar is concentrated in the solder region due to the poor thermal conductivity of the stainless steel vessel wall, which prevents heat being drawn away form the solder region. The assembly is then left to cool in the collar to reduce the possibility of disturbing the joint.

Since the process is carried out at relatively low temperatures, the risk of discolouring the vessel wall is decreased. This risk could be further reduced by passing inert gas such as argon over the assembly to exclude oxygen.

An advantage of using a soldering method as above is that the external surface of the vessel wall may not be deformed or discoloured. Accordingly, a skirt need not be provided which extends over the external vessel wall in that area. Figure 6 shows such an arrangement . In Figure 6 the soldered base construction of Figure 5 is provided with a plastics skirt 230 whose upper edge 232 engages inside the open end 202 of the vessel wall 204, so as to lie generally flush with the external surface of the vessel wall 204. The skirt is aesthetic and protects the lower edge of the vessel wall from accidental damage. As the heated base 200 is recessed from the open end 202, there is sufficient space to accommodate the upper edge 232 of the skirt, and a combined control and cordless connector unit 23₄ of the type described in WO 95/01342 arranged under t_he base 200. The vessel rests on a power base unit 236. The skirt may be mounted to the vessel base of the unit 234 in any suitable manner.

As has been mentioned above, the invention is applicable to the production of non-circular section vessels. Referring to Fig. 7, a developed stainless steel blank 250 for producing a tapering vessel which is square in horizontal cross section is shown The blank 250 comprises three trapezoidal panels 252, with a hal_f trapezoidal panel 254 attached to the respective outer trapezoidal panels 252. Strip panels 256 are formed on the outer edges of the panels 254. To form a vessel body from this blank, the panels 252,254 are each folded towards each other about fold lines 258 by approximately 90° so that the fold lines 260 become adjacent. The strip panels 256 are folded in the opposite direction about the fold lines 260 so as to form a pair of external lips, corresponding to the lips

8 in the first embodiment, which can be welded together to form a square cross section prismatic tubular body. The remaining components of the earlier embodiment may be suitably modified to engage with the square section of the vessel. Of course, the strip panels 256 may be dispensed with and the ends of the formed blank butt welded together as described with reference to Figure 4 above.

It will be clear to the skilled person that various modifications can be made to the above described embodiments without departing from the scope of the invention. For example the lip of the base may be rolled onto the bottom of the vessel wall instead of being welded or soldered. Furthermore, the whole of the base need not be formed of sheet metal. In an alternative embodiment, only the outer portion of the base is of sheet metal with the inner portion of the base being a die-cast element suitably mounted to the outer portion which acts as a mounting means for the element. Such elements are described in our co-pending application GB 9610097.9. Further any of the various methods of forming a tubular body may be combined with any of the methods of securing a base in the formed tube.

The vessel may also be formed so as to be asymmetric in side elevation and may lean forwardly or backwardly, for aesthetic reasons.

Claims

1. A method of manufacturing an electric liquid heating vessel comprising forming a sheet metal blank into a tubular shape, joining the opposed ends of the blank to form an open ended tubular vessel wall, and closing the bottom of said tubular vessel wall with a base mounting or for mounting an electric heating means on its underside.

2. A method as claimed in claim 1, wherein said sheet metal blank is formed into a tubular shape having a non- circular cross-section.

3. A method as claimed in claim 2, wherein the blank is first formed into a circular cross-section tubular shape and then deformed to produce the desired non- circular cross-sectional shape.

4. A method as claimed in claim 1, 2 or 3, wherein said sheet metal blank is rolled into said tubular shape.

5. A method as claimed in claim 1, 2 or 3 , wherein said sheet metal blank is folded into said tubular shape.

6. A method as claimed in claim 1, 2 or 3 , wherein the tubular shape is formed by gripping the ends of the blank and moving them together over a predetermined path so as elastically to deform the blank.

7. A method as claimed in any preceding claim, wherein said opposed ends of the blank are joined by butt- welding them together.

8. A method as claimed in claim 7, wherein during welding a backing member is provided on a surface, preferably the inside surface of the formed blank in the region of the weld, to support the weld.

9. A method as claimed in claim 8, wherein said backing member is of a material which has a good thermal conductivity.

10. A method as claimed in claim 8 or 9, wherein said backing member comprises a plurality of passages to allow inert gas to flow over the region of the weld during welding.

11. A method as claimed in any preceding claim wherein a handle is mounted to the vessel wall to at least partially cover said join.

12. A method as claimed in any preceding claim wherein said base is rolled, welded or soldered to the lower end of said tubular vessel wall .

13. A method as claimed in claim 12, wherein said base is soldered to the vessel wall and wherein a collar is arranged outside the tubular blank, around the base part during soldering.

14. A method as claimed in claim 13, wherein said collar is heated to provide heat for the soldering process.

15. A method as claimed in claim 16, wherein said collar is tapered.

16. A method as claimed in any preceding claim wherein said base is attached in the presence of an inert gas.

17. A method as claimed m any of claims 13 to 18, wherein said base is soldered with a low temperature solder.

18. A method as claimed m any preceding claim wherein said base has a strengthening lip around its periphery through which the base is joined to the vessel wall.

19. A method as claimed in claim 18 wherein said lip engages inside the end of said tubular vessel wall.

20. A method as claimed in claim 18 or 19, wherein the free end of said lip is bent to provide a region for receiving solder.

21. A method as claimed in any preceding claim wherein said vessel wall and/or said base is stainless steel .

22. An electric liquid heating vessel comprising a tubular wall formed from a blank of sheet metal lomed at opposed ends, and closed at its bottom end by a base, having electrical heating means provided on its underside, mounted to the bottom end of the wall.

23. An electric liquid heating vessel as claimed in claim 22 wherein said loin is arranged at the rear of the vessel, and the vessel further comprises a handle mounted over said join on the outside of said vessel wall .

24. An electric liquid heating vessel as claimed in claim 22 or 23, wherein a plastics rim is mounted to the upper end of said vessel wall .

25. An electric liquid heating vessel as claimed in claim 24 wherein said rim is moulded integrally with a handle part and/or pouring spout for the vessel.

26. An electric liquid heating vessel as claimed in any of claims 22 to 25 wherein said base has a strengthening lip formed around its periphery through which the base is mounted to the vessel wall.

27. An electric liquid heating vessel as claimed in claim 26 wherein said base is generally dish-shaped having a generally planar central region, an upwardly angled rim extending from said central region, and said lip depending from the said angled rim.

28. An electric liquid heating vessel as claimed in claim 27, wherein said rim has a portion which extends generally parallel to the central region of the base, said base lip extending downwardly from the outer edge of said portion.

29. An electric liquid heating vessel as claimed in any of claims 22 to 28, wherein the vessel base is attached to the vessel wall at a location spaced inwardly from the edge of the vessel wall.

30. An electric liquid heating vessel as claimed in any of claims 22 to 29, wherein a plastics skirt is arranged flush with the lower part of said vessel wall and which locates inside the end of the vessel .

31. An electric liquid heating vessel as claimed in any of claims 22 to 30, wherein said vessel has a substantially non-circular cross-section.

32. An electric liquid heating vessel as claimed in any of claims 22 to 31 wherein said electric heating means comprises a sheathed heating element.

33. An electric liquid heating vessel as claimed in any of claims 22 to 31 wherein said electric heating means comprises a thick film printed heating eiement .

34. An electric liquid heating vessel as claimed in any of claims 22 to 33 further comprising a control for said vessel arranged under said base.

35. A base for an electric liquid heating vessel comprising a generally planar central region, an upwardly angled, generally linear rim extending from said central region, and a lip extending downwardly from said rim.

36. A base as claimed in claim 35 wherein said rim has a portion which extends generally parallel to the central region of the base, said base lip extending downwardly from the outer edge of said portion.

37. An electric liquid heating vessel having a metal body, the metal body having a non-circular cross section and/or an asymmetric side elevation, and/or being inclined forwardly or backwardly from vertical.

38. A method of manufacturing a liquid heating vessel comprising producing a tubular vessel wall and attaching a heating base to the vessel wall, the tubular wall being deformed after formation to produced a desired cross sectional shape.

39. A method of manufacturing an electric liquid heating vessel comprising gripping opposed end regions of a generally planar stainless steel sheet blank, bringing the end regions together in an elastic manner so as to form a tube, joining the opposed ends of the blank to form an open ended tubular vessel wall, and closing the bottom of said tubular vessel wall with a base mounting or for mounting an electric heating means.

40. A method of attaching a metal heating base to a preformed metal tubular vessel body comprising positioning said base inside one end of said body, applying solder between the base and the vessel body, positioning a collar around the vessel body in the region of the base and heating said collar to melt said solder.

41. An electric liquid heating vessel having a tubular body and a skirt mounted at the lower end of the tubular body, the skirt locating inside the end of the vessel wall and lying generally flush with the external surface of the vessel wall.

42. An electric liquid heating vessel having a tubular body and a heated base attached to and closing the bottom of the tubular body, the base being attached to the body at a location spaced from the edge of the body.

43. A generally planar non-circular base for closing an opening in the base of a liquid heating vessel, said base having heating means provided on a side thereof.