WO1997001259A1

WO1997001259A1 - Printed heating elements

Info

Publication number: WO1997001259A1
Application number: PCT/GB1996/001511
Authority: WO
Inventors: John Crawshaw Taylor; Keith Barrie Doyle
Original assignee: Strix Limited
Priority date: 1995-06-21
Filing date: 1996-06-21
Publication date: 1997-01-09
Also published as: GB2318263A; GB9726658D0; AU6233796A; GB2318263B; GB9512559D0

Abstract

A method of manufacturing a heater comprising applying an unfired ceramic substrate and a resistive heating track to a support surface; and firing the track and substrate together on the support surface.

Description

Printed Heating Elements

This invention relates to printed electric heating elements and to a method of their manufacture.

Printed electric heating elements are used or have been proposed for use in a variety of electrical heating appliances, for example kettles, hot water jugs, washing machines, dishwashers, urns and other devices for the heating of water or other liquids. A printed element typically comprises a resistive heating track laid down, or printed, on an insulating, ceramic, glass ceramic or glass substrate (hereinafter collectively referred to as ceramic substrates) which is provided on a support surface, typically of metal. The ceramic substrate insulates the heater track electrically from the support surface. The track may, if required, be overlaid with a further insulating layer of a similar, or advantageously the same material as the substrate. This insulating layer may also be patterned so that terminal areas of the underlying element track are exposed to allow electrical contact to be made with the ends of the track. In one known method of manufacture, one or more thin "green" (ie. unfired) ceramic layers are first deposited on the support surface. Because of its porosity, the ceramic material must be built up layer by layer and fired after the deposition of each layer as described, for example, in EP-A-0485211. Thus, the process of depositing a layer of ceramic material on the article and firing the article together with the ceramic layers must be repeated several times until sufficient material has been deposited to provide an insulator of sufficient thickness and electrical insulation. Once sufficient ceramic material has been deposited, a resistive heating track is deposited, for example, screen printed, onto the surface of the ceramic using a metal loaded ink, for example containing palladium silver. The support surface together with the deposited ceramic and printed track is then fired again, so that the metal loaded ink on the ceramic develops stable electrical conductivity.

The above method has several disadvantages. For example, the support surface must undergo a firing process a number of times in order to fire each ceramic layer and the metal loaded ink. This necessitates either a number of separate firing units or a non- continuous production process in which the article is repeatedly returned to the same firing unit. Both of these arrangements are costly in terms of both time and energy.

Another disadvantage of the prior art method is that the ceramic and ink deposition methods are only ideally suited to flat, planar articles. This potentially limits the range of surfaces on which the element might be provided. For example, it would be extremely difficult with present methods to provide an element on a dish-like shape, which may be desirable in certain applications.

The present invention seeks to overcome these disadvantages, and in accordance with a first aspect of the invention there is provided a method of manufacturing a heater comprising applying an unfired ceramic substrate and a resistive heating track to a support surface; and firing the track and substrate together on the support surface.

The method of the invention has a number of advantages over prior art methods. It requires only a single firing of the support surface, ceramic substrate and resistive track which saves energy, and is therefore more energy efficient and thus less costly than known methods. Additionally, the single firing step enables production more easily to be carried out in a continuous manner, for example on an automated production line. Although it would be possible first to apply the unfired ceramic substrate to the support surface and then apply the resistive heating track to the substrate in-situ, preferably the resistive heating track is first applied to the substrate and the substrate and track then applied together to the support surface.

This allows the element to be fabricated in a dedicated location and only applied to the support surface at the location where the heater is finally to be assembled. This allows for greater quality control of the heater output.

Furthermore, the element may be applied to a recessed support surface. With known methods this is not a practical possibility, because the resistive heating track cannot be screen printed into a recess. Thus in accordance with a preferred embodiment of the invention, an unfired element comprising an unfired substrate with a deposited resistive heating track is effectively pre-fabricated and then applied to and fired in a desired position on a support surface such as a base in or for a liquid heating vessel.

From a second broad aspect, therefore, the invention also provides a heating element for application to a support surface, comprising an unfired ceramic substrate with a resistive heating track provided thereon.

The heating track is preferably printed, most preferably screen printed, onto the unfired ceramic substrate, the substrate preferably being flat. This is the optimum condition for screen printing of the element as the variation in resistance of the finished element in this case will be much lower than normally achievable. This is because the thickness of the element ink deposited onto the glass ceramic substrate by, for example, screen printing is very strongly influenced by the distance separating the screen from the substrate. Local variations in this distance result in undesirable local variations in the quantity of element ink deposited during printing, thereby varying the resistance of the heating track. Tight tolerances on the resistance of the printed track are desirable not only for consistency of performance but also because higher power elements can be produced without the risk of producing some with over-powerful ratings, which would be dangerous. With processes employing the invention, however, the unfired ceramic substrate may easily be made uniformly thick, and can easily be clamped against a perfectly flat bedplate in a printing machine. Thus the print screen to substrate separation is extremely uniform, and so is the element track thickness.

In prior art processes where the substrate is fired before the track is applied, such variations occur because during firing the metal and the glass ceramic will contract by different amounts when cooling from the high temperatures used in the firing process. The fired assembly accommodates this difference by conforming to either a spherical, or saddleback surface. Thus when printing the element track onto such a curved surface, the print screen to substrate separation is extremely non uniform, and element track thickness and thus resistance becomes highly variable.

Preferably the unfired ceramic substrate is flexible. This will allow the element or track to then be applied to a support surface which may not be flat, so as to follow the shape of the surface. The element could therefore, for example, be applied to flat, cylindrical or conical surfaces .

Preferably the ceramic substrate is provided on or with a carrier or backing, which may be flexible, and which will support it and allow it to be moved easily through the various process steps . Preferably the carrier is a film, for example a plastics film which may be of, for example, Mylar polyester. The ceramic substrate is most preferably supplied in the form of a continuous tape, for example, in rolls, such ceramic tape being sold by Du Pont under the trade name Green Tape. Using a roll of ceramic tape in the process also facilitates continuous production.

Preferably, the carrier is punched with registration holes which allow accurate and reproducible positioning of the substrate during the process . The carrier may be provided pre-punched or may be punched at a convenient point during the process.

As stated above, the resistive track may be applied to the ceramic substrate in any conventional manner, for example by silk screen printing. The term "printing" as used herein is intended to embrace all such methods. The track may be formed from a metal loaded ink containing, for example, palladium-silver, nickel or a 80/20 nickel chrome alloy. The use of an ink containing a 80/20 nickel chrome alloy is in itself a novel arrangement, and from a yet further aspect, therefore, the invention provides an ink for printing an electrical resistive heating track, comprising an 80/20 nickel- chrome alloy in an evaporative carrier.

The advantage of using a nickel-chrome alloy as the heater track material is that it is highly resistive to oxidation and corrosion during operation of the heater. Accordingly, tracks manufactured from such materials may not require an additional layer of glass or the like to protect them, thereby significantly reducing costs. Inks comprising mixtures of nickel and chrome particles are already known but these suffer from the disadvantage that the resistivity of the track may vary due to, for example, the particle sizes of the two components. By using an alloy, these problems are avoided, meaning that a reliable resistance may be obtained.

The track may be dried using infra-red radiation, hot air or microwave energy etc. before the next process step, which, in broad terms is the application of the printed substrate to the desired surface. It is envisaged that the track printing could be conducted at the factory where the substrate is to be applied to its support surface, but as stated above, it would also be possible and indeed it is preferred to supply unfired ceramic tape or the like to the factory site pre-printed with heating tracks. From a further broad aspect, therefore, the invention provides a sheet, strip, tape, roll or the like of unfired ceramic material printed with a series of spaced resistive heating tracks.

A protective ceramic layer may also be applied over the printed resistive track, either before or after firing. Advantageously, such a protective layer may be produced in the same way as the ceramic substrate, e.g. by pierce punching from green ceramic material . Furthermore, the protective layer may be applied to the printed resistive track before firing so that the substrate, track and protective layer may be fired together in a single step.

The prefabricated element formed as above must then be applied to its support surface. Where a carrier is used, the element will have to be separated from the carrier and transferred to the support surface.

In one embodiment, firstly, the shape of the element may be defined, for example by cutting the desired shape of substrate with its printed heating track from the green ceramic tape or the like. The desired shape may simply define the outline of the ceramic elements. However, cut-outs may also be pierce punched at the same time from the body of the ceramic substrate, for example to permit an electrical, mechanical or thermal connection to be made to the support surface, for example to allow an earth connection to be made thereto or a mounting stud to be welded thereto. Furthermore, the element may be shaped to accommodate projections on the support surface. This is an extremely advantageous feature since it would not be possible, for example, to print around such projections. Most preferably, the elements are 'kiss' cut or pierce punched from the tape leaving the underlying carrier intact. The excess ceramic material can then be removed and the elements passed forward on the carrier to the next process step.

Unless the carrier material is of a type which will remain stable and not disintegrate under subsequent firing, the elements must be removed from their carrier, so that the un-printed green ceramic surface may be placed directly on the article surface.

Preferably therefore the elements are removed from the carrier by transfer means. Preferably the transfer means comprises a low-tack adhesive on at least one side, which adheres releasably to the printed surface of the element . The transfer means is preferably lightly pressed into adhesive contact with the upper surface of the ceramic, for example by a roller. The transfer means may be in the form of a continuous tape, for example commercially available "NITTO" tape. Advantageously, the tape is punched with registration holes in a similar manner to the carrier, so as to permit accurate alignment of the elements as they are transferred from the carrier to the transfer carrier. Successive elements may be carried on the transfer tape to the application station where they are applied to the support surface. The support surface may be provided on, for example, a flat plate or a formed metal body such as the base of or for a liquid heating vessel . An example of such a base for a liquid heating vessel is disclosed in WO 96/18331. The support surface may be a metallic surface, for example a steel and most preferably a stainless steel surface. The elements are preferably applied to the article under pressure sufficient to adhere the green ceramic to the surface. The pressure also acts to exclude air bubbles and avoid entrapment of dirt particles, both of which could prevent intimate thermal contact between the element and the surface. The pressure may be applied by passing the surface and element between one or more rollers. Alternatively, the green ceramic substrate may be laminated to the support surface in an isostatic press.

The surface of the article to which the ceramic substrate is applied may be preoxidized. It is known that high temperature or chemical preoxidization of some stainless steels encourages the growth of a highly adherent and stable oxide film, which may form a strong interface between the unoxidized metal core and the applied element. Alternatively, the support surface may be pretreated with abrasive material, for example by blasting with an abrasive such as dry and clean alumina, to ensure effective keying of the ceramic substrate to the support surface. The supports for the elements may be supplied to the application station on a continuous conveyor, and they can be aligned with the elements being carried on the transfer tape, and then passed continuously through the roller(s) . Once passed through the rollers, the transfer tape may be peeled off before the assembly is fired.

The element and support may be removed from the conveyor for firing by means of a separation shoe, for example. The firing process may then be carried out in a conventional manner, either continuously or in a batch process .

It will be appreciated from the above that a major advantage of the present invention is that it permits non-flat surfaces, for example surfaces with projections and the like, to be provided with a suitably shaped ceramic substrate. This is made possible by use of the pre-formed "green" substrate. For example, a surface with a central projection may be provided with a substrate having a central cut-out to accommodate the projection. From a further broad aspect therefore, the invention provides a method of providing a ceramic substrate on a heater support surface wherein the substrate is applied in the form of a pre-formed "green" member. This may then be fired and provided with a resistive track in an appropriate location and manner, or fired with the track in one operation, as described above. The above method may also use the preferred features discussed above. The invention also extends to a heater manufactured in accordance with the invention and a heating appliance incorporating such a heater.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:

Figure 1 is a schematic perspective view of a green ceramic substrate before and after the printing of a resistive heating track;

Figure 2 is a schematic perspective view of the removal of excess ceramic material to leave unfired elements;

Figure 3 is a schematic perspective view of the removal of the elements from their carrier by means of a transfer tape; Figure 4 is a schematic perspective view showing the application of the elements to their support surfaces; and

Figure 5 shows a heater made in accordance with the invention. With reference to Fig. 1, there is provided a roll

1' of unfired ("green") ceramic tape 3. This roll 1' in fact comprises a thin layer of unfired glass or ceramic 1 on a smooth stable plastic carrier film 2, for example of MYLAR polyester. The ceramic layer typically has a thickness between 200 and 250 microns. Such tape is manufactured by Du Pont under the trade name GREEN TAPE.

As the tape is unrolled, registration holes 4 are punched through it at Station A to permit accurate and repeatable registration of the tape during subsequent processing. Individual resistive heating tracks 5 (shown only schematically in Fig. 1) , are then screen-printed onto the ceramic layer 1 using a metal loaded ink at Station B. The metal loaded ink may contain for example, nickel or a 80/20 nickel-chrome alloy. The 80/20 nickel-chrome alloy ink may, typically, contain 85 parts by weight of 10-20 micron 80/20 nickel-chrome alloy particles and 15 parts by weight of 5-15 micron glass frit in 15 parts by weight of polymeric printing medium. The form of such tracks 5 and the printing techniques used are already well known in the art and will not therefore be described in detail here. Typically, however, the thickness of the printed track 5 is approximately 12 microns. At this point, the ceramic layer 1 is still unfired and it is flat, which is the optimum condition for screen-printing of the tracks 5.

The tracks 5 are dried at Station C, for example by infra-red radiation, microwave radiation or hot air.

The above stages of the process may be effected at a different location from the stages to follow. Thus the tape 3 could be supplied with spaced pre-printed heating tracks applied to it.

At Station D, individual heating elements 6, each having a heating track 5 are pierce punched from the main body of the ceramic layer 1 in a desired shape. The punching process only pierces the layer 1 of unfired ceramic, the plastic carrier film 2 remaining intact so that it may transport the cut elements to the next station .

As shown in Fig. 2, the residual ceramic layer 7, i.e. the portion of the tape 3 surrounding the elements 6 is peeled from the plastic carrier film 2 by a roller 8 at Station E around which the residual ceramic 7 is wound, thus lifting it from the plastic film. The elements 6 remain on the plastic carrier film 2 for further processing.

As shown in Fig. 3, at Station F, a second plastics film 9, for example 'NITTO¹ tape, having a low tack adhesive on its underside is applied to the upper, printed, surface of the elements 6. The adhesive film 9 is provided with registration holes 12 in a similar manner to the plastic carrier film 2. The adhesive film 9 is lightly pressed into contact with the elements over the top of the printed tracks by a pressure roller 10, in order to ensure satisfactory adhesion between the elements 6 and the adhesive film 9. The plastic carrier film 2 is then rolled downwardly onto a roller 11 away from the adhesive film, the elements 6 remaining removably adhered to the underside of the plastic film 9. The elements 6 are thus separated from the plastic carrier film 2.

As shown in Fig. 4, at Station G, the unfired elements 6 are applied to metal, for example stainless steel, pressings 13 which are transported on a conveyor 14 having registration holes 15 corresponding to the registration holes of the adhesive tape, for accurate alignment of each element 6 with its respective metal pressing 13. The pressing 13 may, for example, be a flat or dished plate acting as a base for a water heating vessel. The elements 6 are pressed into contact with the pressings 13 by passing between two laminating rollers 16,17. The rollers 16,17 laminate the pressing 13 to the element 6 and exclude air bubbles and avoid entrapping any dirt particles between the pressing and ceramic disc, either of which would prevent intimate thermal contact between the element and the metal surface. The metal pressings are preferably pretreated with abrasive material to ensure effective keying between the ceramic substrate and the metal surface. Alternatively, they may be pre-oxidised chemically or at high temperature which encourages the growth of highly adherent and stable oxide films on their surface. These oxide films form a strong interface between the unoxidised core of the metal and the applied ceramic material.

After lamination, the metal pressing is held in a separation shoe 18 and the adhesive film is removed upwardly onto a roller (not shown) . There is sufficient keying between the surface of the metal pressing 13 and the unfired ceramic as a result of the lamination process to ensure that the adhesive film can be removed without weakening the interface between the ceramic and the surface of the metal.

The laminated metal pressing and element 19 is then fired at a sufficiently high temperature, for example

850°C to 950°C, to form a strong permanent bond between the ceramic and the pre-treated metal pressing. Simultaneously, the metal loaded inks on top of the ceramic will develop stable electrical conductivity. The resistance of an element track formed in this way will typically be in the region of 30 ohms and will have a current carrying capacity of typically 10 amps. The sheet resistivity of the resistive heating tracks formed by the 80/20 nickel-chrome alloy ink is typically 0.5 Ohms/ square as compared to 0.05 Ohms/square for nickel loaded inks . The sheet resistivity is however dependent on the printed thickness of material and the firing conditions of the furnace. Thus a desired resistance can be achieved by choosing the appropriate material, thickness and firing conditions.

Figure 5 shows a further heater 19 in accordance with the invention. In this embodiment the stainless steel base 13 of a liquid heating vessel has a peripheral channel 21 for engaging over the bottom edge of a plastics wall of the vessel. This type of base is also described in WO 96/18331. This arrangement creates a recess 22 on which is applied an unfired element 6 prefabricated as discussed above, base and element then being fired together. Vacuum or suction means, for example, may be used to transfer the unfired element to the base, and pressure may be applied, if required, by piston means or the like. Without using the invention it would not be possible to provide an element in this position.

It will be apparent that many variations to the above embodiment can be made within the scope of the invention. For example, whilst the pressing described above is flat, it will be seen that it could, for example be dished. Also, other means may be provided for transferring the elements to their support surfaces, for example vacuum means. Furthermore, while a continuous process has been described, using a tape of green ceramic material as a starting material, this is not essential. Moreover, although in the process described herein the track is printed onto the ceramic substrate before the ceramic is cut to the desired shape, the printing operation may also occur after the cutting operation. Also it will be appreciated that the term track as used herein is used broadly and may include any suitable pattern. It should also be appreciated that the term "heater" as used herein should be interpreted to include apparatus such as motor protectors etc. where a resistive track which, although it is not used specifically to heat, does produce a significant amount of heat during its operation.

The skilled person will also recognise that by using a pre-formed "green" substrate, non-flat support surfaces may be provided with an insulating surface in a much easier manner than heretofore.

Claims

1. A method of manufacturing a heater comprising applying an unfired ceramic substrate and a resistive heating track to a support surface; and firing the track and substrate together on the support surface.

2. A method as claimed in claim 1 wherein the resistive heating track is first applied to the substrate and the substrate and track then applied together to the support surface.

3. A method as claimed in any preceding claim wherein said support surface is provided on a metallic member.

4. A method as claimed in claim 3 wherein said metallic member is a metallic plate.

5. A method as claimed in claim 3 or 4 wherein said metallic member is stainless steel.

6. A heating element for application to a support surface, comprising an unfired ceramic substrate with a resistive heating track provided thereon.

7. A heating element or method as claimed in any preceding claim wherein the heating track is printed onto the substrate.

8. A heating element or method as claimed in any preceding claim wherein said substrate is flat.

. A heating element or method as claimed in any preceding claim wherein said substrate is flexible.

10. A heating element or method as claimed in any preceding claim wherein said substrate is provided on or with a carrier or backing.

11. A heating element or method as claimed in claim 10 wherein said carrier backing is flexible.

12. A heating element or method as claimed in claim 10 or 11 wherein said carrier or backing is a plastics film.

13. A heating element or method as claimed in claim 11 or 12 wherein said carrier or backing is in the form of a continuous tape.

14. A method as claimed in any of claims 10 to 13 wherein a series of spaced resistive heating tracks is provided on said carrier or backing.

15. A method as claimed in any of claims 10 to 14 wherein the shape of the element (s) is defined on the carrier or backing.

16. A method as claimed in claim 15 wherein the element (s) is (are) kiss cut or pierce punched on the carrier.

17. A method as claimed in claim 15 or 16 wherein the element (s) is (are) removed from the carrier prior to its (their) application to a support surface.

18. A method as claimed in claim 17 wherein the element (s) is (are) removed from the carrier or backing by transfer means which transfers the element (s) to the surface.

19. A method as claimed in claim 18 wherein the transfer means comprises a low-tack adhesive on at least one side, which adheres releasably to the printed surface of the element (s) .

20. A method as claimed in claim 19 wherein the transfer means is in the form of a continuous tape.

21. A method as claimed in any of claims 17 to 20 wherein the element (s) is (are) applied to the article under pressure sufficient to adhere the unfired substrate to the surface.

22. A method as claimed in any preceding claim wherein the surface to which the elements are applied is pre¬ treated with abrasive material.

23. A method as claimed in any of claims 16 to 21 wherein a plurality of element support surfaces are supplied to an application station on a continuous conveyor and are aligned with the unfired elements carried on the carrier or backing, so that the elements may be applied thereto.

24. A method as claimed in any preceding claim, wherein a protective ceramic layer is applied to the resistive heating track.

25. A method of providing a ceramic substrate on a heater support surface wherein the substrate is applied in the form of a pre-formed "green" member.

25. A sheet, strip, tape, roll or the like of unfired ceramic material printed with a series of spaced resistive heating tracks.

26. A heater comprising a support surface having a heating element as claimed in any of claims 6 to 13 applied thereto.