WO2000064218A1

WO2000064218A1 - Improvements relating to heating elements

Info

Publication number: WO2000064218A1
Application number: PCT/GB2000/001525
Authority: WO
Inventors: Robert Andrew O'neill; Robin Keith Moore
Original assignee: Otter Controls Limited
Priority date: 1999-04-19
Filing date: 2000-04-19
Publication date: 2000-10-26
Also published as: GB2349322A; CN1356017A; EP1172021A1; AU4131000A; GB9908918D0; CN1162047C; GB2349322B

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.