HEATING ELEMENT

Abstract: An electrical heating element is manufactured by applying an electrically insulating refractory layer (15) to a thermally conductive body (10) by plasma flame spraying, then applying an element track (20) to a surface of the coated body by plasma flame spraying a mixture of the refractory material and a resistive heating material using a mask (19). A final layer (21) of a refractory material is then applied over the entire region of the element track, excepting contact areas which terminate said track.

Description: HEATING ELEMENT TECHNICAL FIELD This invention relates to cooking elements and other devices requiring the electrical heating of a body presenting a continuous heated surface. The invention has particular application to cooking elements for stoves, but will find application elsewhere. BACKGROUND ART So called solid disk cooking elements have been well known for many years, normally consisting of cast iron. Such elements offer the advantages of greater control and more even heat distribution at low cooking temperatures, and greater ease of cleaning, but suffer in comparison with coils of sheathed tubular elements, in their slowness of response and their susceptibility to rusting. DISCLOSURE OF THE INVENTION An object of the present invention is to provide a cooking element which will combine the advantages of solid disk elements with some of those of the open coil type. The invention broadly resides in an electrical cooking element comprising an element body having an electrically insulating surface, an electrical heating element on said surface, and a continuous layer of electrically insulating refractory material applied to said surface and said element. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a bottom plan view of a hotplate disk; Fig. 2 is a side elevation of the disk; Figs. 3a to 3d are fragmentary cross-sections of the disk of Fig. 2 during successive steps in the manufacture of the hotplate; Fig. 4 is a bottom plan view of a completed hotplate assembly with the cover partly broken away; and Fig. 5 is a partly sectioned view of the completed hotplate assembly. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The manufacture of a hotplate assembly in accordance with the illustrated embodiment commences with a disk 10 of mild steel, this material being chosen for its thermal conductivity and appropriate mechanical properties. The disk 10 is provided with a peripheral step 11 for the reception of a cover and trim ring to be described below. Fixed to the disk 10 for example by spot welding is a central steel pin 12, which functions to provide an earth connection to the disk, a mounting point for components of the hotplate, and a thermal connection to the disk for thermostatic control of the hotplate if this is desired. Similarly attached to the disk near its periphery are a pair of pins 13 and-14, which serve to support the active and neutral connections to the resistive heating element applied to the disc as described below. After the attachment of the pins 12, 13 and 14, a thin layer 15 of electrically insulating refractory material is applied to the top surface of the disk 10. In the preferred practice of the present invention, this layer 15 is produced by the plasma flame spraying of alumina powder. As is known, in the plasma flame spray process the alumina in powder form is introduced by means of a carrier gas into a plasma flame produced by the electric arc excitment of an inert gas. A suitable form of alumina for this coating 15 is grey alumina powder containing 94% aluminium oxide, 2.5% titanium dioxide, 2% silicon dioxide, 1% iron oxide with the balance being other oxide materials. The layer 15 is applied with sufficient passes of a plasma spray to produce a depth of approximately 75 microns. The purpose of this upper layer of alumina is to protect the mild steel disk 10 from oxidation or other deterioration, and to provide an aesthetically pleasing finish for the hotplate. To the bottom surface of the disc 10 there is next applied, again by plasma spray, an alumina layer 16 in the region of 125 to 150 microns thick, which forms an electrically insulating base layer for the heating element track which is next applied. As shown in Fig. 3a, the region of the disk 10 immediately surrounding the pins 13 and 14 is masked by a sleeve 17 during the application of the alumina layer 16, to protect the thread on the pin (not shown) from the alumina spray. The masking sleeve 17 is next removed and a sleeve 18 of greater diameter is applied to the pins 13 and 14, as shown in Fig. 3b. Now applied over the entire area of the disk 10 is a mask 19, the purpose of which is to define the configuration of the heating element, which is to be applied as a plasma sprayed layer as described immediately below. The heating element layer may be applied in any suitable configuration, and a typical configuration is shown in ghost outline in Fig. 4. The heating element layer 20 preferably consists of a mixture of alumina and nickel chromium alloy, and is again applied by plasma spraying. In the preferred practice of the invention, alumina powder and powdered nickel chromium is supplied to a plasma spray gun from a dual powder feeder, the relative proportions of alumina and nickel chromium being controlled to achieve the desired resistivity for the layer 20. A suitable proportion of refractory to resistive material is 50% but of course this may be varied to suit the track dimensions and geometry, and the required dissipation. The conductive track 20 is applied to a thickness of approximately 100 microns by successive passes of a dual powder plasma spray gun relative to the workpiece, and the mask 19 is then removed. An important advantage of employing as the material for the resistive track, a mixture of the resistive heating material (in this case nickel-chromium) and the refractory material (in this case alumina) is the compatibility which is thus ensured between the superimposed layers of refractory and resistive track. Thus an excellent bond is obtained between these layers. A further advantage of this technique is the ease with which the resistivity of the track material may be adjusted by modifying the proportions of the two materials. There is next applied a mask 21 which covers the entire region of the resistive track 20, and exposes only a region around the pins 13 and 14, for the application of a layer 23 of copper, the purposes of which are to provide a region of conductive contact with the underlying track 20 for electrical connection to the track by the terminal assemblies of the hotplate, and to provide a region adjacent the terminals where heat generation does not occur. The layer 23 is applied by plasma spraying. The mask 21, and the masking sleeve 18, are then removed, the masking sleeve 18 being replaced by a masking sleeve 24 which is provided with a circular base 25, covering the copper layer 23. A final insulating layer 22 of alumina is now applied over the entire region of the element track 20, and after removal of the masking sleeve 24, the hotplate is ready for the application of appropriate termination and final assembly. As illustrated in Figs. 4 and 5, the assembled hotplate is provided with active and neutral terminal assemblies 26 and 27 connected with a thermostat 28 mounted on the central post 12. A cover 29 engages the disk 10 at the step 11 and is retained by a nut 30 on the post 12, while a trim ring 31 is locked between the edge of the cover 9 and the rearwardly facing face of the step portion 11. It will be appreciated that the present invention may be carried into effect in many ways other than that illustrated. Various materials may be used both for the body of the element, and for the coatings, and various forms of electrical heating devices may be employed, without departing from the scope of the present invention,

Claims: CLAIMS 1. A method of manufacturing an electrical heating element comprising the steps of: (a) applying to a thermally conductive body by plasma arc spraying an electrically insulating first layer of refractory material; (b) applying to said layer by plasma arc spraying an electrically conductive element pattern; and (c) applying to the area occupied by said pattern and said layer a second electrically insulating layer of refractory material. 2. A method according to claim 1 wherein said pattern comprises a mixture of a refractory material and an electrical conductor. 3. A method according to claim 1 wherein said first and second layers consist essentially of aluminium oxide. 4. A method according to claim 1 further comprising the step of applying to the remaining surfaces of said body, a layer of said refractory material. 5. A method of manufacturing an electrical heating element comprising the steps of : (a) applying to the surfaces of a body of thermally conductive material by plasma spraying a first layer of electrically insulating refractory material, (b) masking selected areas of said first layer on a surface of said body to form an unmasked area of said first layer on said surface said unmasked area extending continuously between first and second contact areas, (c) applying by plasma spraying to said unmasked area a resistive heating composition to form a resistive heating track, and (d) masking said contact areas of said track and applying over the remaining areas of said surface by plasma spraying of a further layer of said refractory material.

Inventors: VUKOVICH BRANKO, VUKOVICH, BRANKO
Applicants: EMAIL LTD, EMAIL LIMITED
Application year: 1986
Main classification: H05B3
IPC codes: H05B3