IMPROVEMENTS RELATING TO HEATING ELEMENTS

Abstract: A thick film heating element is manufactured by depositing a heating element track onto the convex surface of a slightly domed, thin, stainless steel substrate and then bonding the resultant structure to a planar surface to be heated, for example the base of a liquid heating vessel. The flattening of the domed substrate as it is mounted to the planar surface causes the dielectric structure(s) within the thick film heating element to be placed under compression which enhances their tolerance in use of the heating element of thermal expansion/contraction effects.

Description: IMPROVEMENTS RELATING TO HEATING ELEMENTS Field of the Invention: This invention concerns improvements relating to electric heating elements and, more particularly, concerns heating elements of the so-called thick film type comprising a substrate, commonly formed of stainless steel, carrying a resistance heating track or layer which, as appropriate having regard to the nature of the substrate, may be formed on an electrically-insulating layer, commonly of glass, provided on the substrate. An additional electrically-insulating layer may be provided over the resistance heating track or layer as a protective measure. Thick film heating elements are employed in a variety of applications and are currently becoming popular in the field of electrically-heated water boiling vessels, domestic kettles and hot water jugs for example, where their clean appearance as compared to the conventional immersion heating element of metal sheathed construction has aesthetic advantages. In addition it is possible with a thick film heating element to accommodate a greater power density than is readily accommodated with conventional sheathed heating elements, leading to more rapid boiling times. Background of the Invention: As mentioned above, thick film heating elements are commonly formed on a stainless steel substrate, for example by first providing an electrically-insulating layer of glass on one or both surfaces of a stainless steel plate or disc and then screen-printing a resistance heating track onto the glass surface using electrically conductive inks which are then fired. As abovementioned, a further layer of glass may then be provided over the resistive track. It is known that the manufacture of thick film heating elements by this process can give rise to problems of distortion of the heating element out of its normal planar configuration and that distortions can arise furthermore in use of the heating element on account of differential thermal expansion effects. In order to at least alleviate these problems, it has been proposed to select the materials deposited onto the stainless steel substrate to have compatible thermal expansion coefficients insofar as is possible and it has been proposed furthermore to provide layers on both sides of the stainless steel substrate so as to subject it to similar thermal expansion and contraction effects from both sides. All of these solutions give rise to cost implications which, when added to the basic cost of appropriate quality stainless steel substrates as are required for water boiling vessels, tend to render the product unattractive on considerations of cost irrespective of its other clear advantages. Objects and Summary of the Invention: It is accordingly the principal object of the present invention to overcome or at least substantially reduce the abovementioned problem. The present invention resides in the concept of forming the substrate with a slight domed curvature, forming the heating element track on the convex surface of the domed substrate, and bonding the thus formed thick film heating element to a planar surface to be heated by a process which flattens the domed thick film heating element onto the planar surface. In order to enable printing techniques to be employed in the formation of the heating element track, these techniques being most economical and most consistent with the objectives of the invention, the chord height of the domed substrate should preferably be no more than about 1% of the substrate diameter. This also facilitates flattening of the domed thick film heating element onto the planar surface without requiring such forces as might risk damage to the heating element track. The substrate, a stainless steel disc for example, can be very thin, which further reduces the forces required to flatten the formed thick film heating element and reduces the substrate cost. For example, whereas stainless steel substrates of the order of 1.2 to 1.5mm in thickness are commonly utilized in the formation of thick film heating elements at the present time, the present invention contemplates the use of 0.5 to 0.25mm thick substrates. The use of a very thin substrate provides a further advantage in a situation where the thick film heating element is bonded to the surface to be heated with the substrate between the heating element track and the surface to be heated. A thin substrate ensures a correspondingly reduced thermal resistance between the heating element track and the surface to be heated and this permits lower running temperatures, which will lead to lower stresses in the heating element during operation, and/or smaller heating elements with attendant cost reduction. By forming the heating element track on the convex side of the domed substrate, not only does the resultant thick film heating element become much more rigid with a lesser tendency to distortion than an equivalent flat plate heating element, but also any distortion that does occur tends to leave the element dished in the same direction. When the thus formed thick film heating element, with a domed configuration with the heating element track on its convex surface, is clamped to its mounting surface, the curvature is flattened and this places the dielectric in compression in a reliable manner. By maintaining the heating element dielectric in compression, cracking of the dielectric can be avoided. The teachings of the present invention can be applied to all manner of surfaces to be heated. The surface could, for example, be the exterior of a liquid heating vessel or it could be a cooker hob. It may be of any suitable material, for example stainless steel, copper or other metal, or it could be of ceramic or glass or even stone. For bonding the heating element to the surface any appropriate method may be employed, for example the thick film heating element substrate may be brazed or soldered to a metal surface and adhesive or a cement can be used with a non-metallic surface. Alternatively a sandwiched construction may be utilized, though this is not preferred because of the cost implications, where a backing plate is clamped to the surface that is to be heated and the thick film heating element is sandwiched between the backing plate and the surface to be heated. The use of a backing plate clamped to the surface to be heated, or vice versa, would normally be part of the manufacturing process of the invention, but only to ensure proper bonding of the thick film heating element to the surface to be heated, and the backing plate would not normally form part of the final structure. The thick film heating element according to the invention can be bonded by its substrate plate to the surface which is to be heated, leaving its heater track accessible for the use of any of the presently available heating element control systems that are responsive to the heating element temperature. However, the teachings of the invention can be applied to an element bonded by its heater surface to the surface to be heated. In this latter case, the heating element tracks would be between the substrate and the mounting surface and arrangements would be necessary to make electrical connections to the heating element track. This could be achieved by overhanging the thick film heating element from the mounting surface so as to leave contact portions of the heating element track exposed at the edge of the heating element, or alternatively spring contacts could be provided in the mounting surface (these would have to be insulated from the mounting surface in the case of the mounting surface being metallic). Control of the heating element could still be by use of conventional controls employing thermal sensors responsive, by way of the thin substrate material, to the temperature of the heating element track. It is noted that an arrangement wherein the heating element track was between the substrate and the surface to be heated would have superior heat transfer ability as compared to the alternative arrangement since, although a dielectric layer might still be required between the heater track and the mounting surface, depending upon the nature of the mounting surface material, the stainless steel substrate, which has a relatively low thermal conductivity, would not be located in the heat path between the heater track and the mounting surface. There follows a brief description of the invention given with reference to the accompanying drawings. Description of the Drawings: Figure 1 is an exemplary showing of the mounting of a thick film heating element to a mounting surface, the substrate of the heating element being next to the mounting surface; and Figure 2 shows an alternative arrangement wherein the heating element is mounted to the underside of a surface with the heating element track and its associated dielectric layer next to the mounting surface. Detailed Description of the Embodiments: Since thick film technology and the manufacture of electrical heating elements employing such technology is well documented, no description thereof will be provided herein. Figures 1 and 2 are substantially self-explanatory, it being appreciated that the curvature of the substrate in these figures is greatly exaggerated. In the figures, the chord height of the domed substrate is of the order of 15% of the substrate diameter, whereas a curvature of the order of just 1% is contemplated according to the teachings of the present invention. Figure 1 shows the thick film heating element 1 as comprising a domed substrate 2 of stainless steel having formed thereon a printed heating element track 3 having an overlying dielectric layer (not shown) and a further dielectric layer (not shown) between the heating element track and the substrate. The heating element track is formed on the convex side of the heating element and is uppermost in Figure 1 so that as the heating element is flattened into bonding engagement with a mounting surface 4 by application of a force 5, the dielectric layers of the thick film heating element are subjected to slight compression which provides enhanced capability to withstand thermal expansion effects during heater operation, the dielectric layers being stronger in compression than under tension. Figure 2 shows an alternative arrangement wherein the thick film heating element is secured to the mounting surface with the heating element track 3 sandwiched between the substrate 2 and the mounting surface 4. Also schematically shown in this figure is the provision of a spring contact 6 on the mounting surface 4 for making electrical contact with a terminal portion of the heating element track. Having thus described the invention, it will be appreciated that modifications and variations are possible without departure from the scope of the invention as set forth in the accompanying claims. For example, whereas stainless steel is the presently preferred material for the substrate of the thick film heating element, other materials could be employed for the same purpose. Furthermore, whilst mention has been made herein of the use of conventional thermal controls for controlling the heating element operation, alternative electronic controls are known including controls responsive to the resistance of track portions of the heating element and/or additional sensor tracks; such alternative arrangements can be accommodated by the present invention.

Claims: Claims: 1. A method of forming a heating element, said method comprising forming a thick film heater on the convex surface of a slightly domed substrate and mounting the resultant structure on a planar surface. 2. A method as claimed in claim 1 wherein said structure is mounted to said surface with the thick film heater sandwiched between the substrate and the surface. 3. A method as claimed in claim 2 comprising providing spring terminals on the surface for contacting terminal portions of the thick film heater. 4. A method as claimed in claim 1 wherein said structure is mounted to said surface with the substrate next adjacent to said surface. 5. A method as claimed in any of the preceding claims wherein the structure is bonded to the surface. 6. A method as claimed in any of the preceding claims wherein the slightly domed substrate has a chord height which is no more than about 1% of the substrate diameter. 7. A method as claimed in any of the preceding claims wherein the substrate thickness is less than 1.0mm and preferably in the range of 0.5mm to 0.25mm. 8. A method as claimed in any of the preceding claims wherein the substrate comprises stainless steel. 9. A method as claimed in any of the preceding claims wherein the planar surface is of metal, or ceramics or vitreous material (e.g. glass), or is of a natural material such as stone. 10. A thick film heating element manufactured by a method as claimed in any of the preceding claims. 11. An electrical appliance comprising a thick film heating element as claimed in claim 10. 12. An electrical appliance as claimed in claim 11 wherein the surface to which the thick film heater is mounted is the base of a liquid heating vessel.

Inventors: MOORE ROBIN KEITH, O'NEILL ROBERT ANDREW, MOORE, ROBIN, KEITH, O'NEILL, ROBERT, ANDREW
Applicants: MOORE ROBIN KEITH, ONEILL ROBERT ANDREW, OTTER CONTROLS LTD, MOORE, ROBIN, KEITH, O'NEILL, ROBERT, ANDREW, OTTER CONTROLS LIMITED
Application year: 2000
Main classification: H05B3
IPC codes: H05B3