GB2306873A

GB2306873A - Insulating heater substrate by electrostatic deposition

Info

Publication number: GB2306873A
Application number: GB9622393A
Authority: GB
Inventors: John Crawshaw Taylor; Keith Barrie Doyle
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 1995-10-26
Filing date: 1996-10-28
Publication date: 1997-05-07
Anticipated expiration: 2016-10-28
Also published as: GB2306873B; GB9622393D0

Abstract

An insulating layer or overglaze of an electric heater is deposited using an electrostatic spraying technique. Selected areas of a substrate may be masked to prevent deposition of powder in unwanted locations. Also, selected areas may be exposed by vacuum means which locally removes deposited material prior to firing.

Description

Manufacture of Heaters The present invention relates to the manufacture of electric heaters and in particular to the manufacture of electrical substrate heaters.

Substrate heaters, as may be used in liquid heating vessels, immersion heaters, shower heaters, and other heating applications typically comprise a metallic substrate onto which is deposited one or more layers of a ceramic, glass or glass ceramic (hereinafter collectively called "ceramic") electrically insulating material, with a resistive heating track deposited onto the insulating layer, for example by screen printing or flame spraying.

Typically the ceramic insulating layer is screen printed onto the metallic substrate. The ceramic material is carried in a suitable binder which, once the layer has been printed, is evaporated and the layer then fired. This process may have to be repeated two or three times in order to produce an insulating layer of sufficient thickness and electrical insulation, since evaporation of the binder may lead to porosity in the individual layers. This process is therefore relatively expensive. The present invention seeks to provide an improved and less expensive method of manufacturing such heaters.

In accordance with the present invention there is provided a method of manufacturing an electric substrate heater comprising a substrate and an electrically insulating ceramic layer provided on said substrate for receiving a resistive heating track, wherein the ceramic layer is applied to the substrate by electrostatically spraying a ceramic powder onto the substrate and firing the deposited layer.

Such a process has the advantage that ceramic materials can be applied directly without an evaporable carrier which, as stated above may cause porosity in the deposited layer. The ceramic is applied in a powder form, the particles being treated with a suitable charge-carrying coating, and thereafter sprayed in a manner known per se. Such materials are considerably less expensive than the ceramic "inks" used in earlier processes. Typically the powder particle size is between 5-12 microns, thus allowing a closely packed layer to be deposited.

Preferably the desired layer thickness is laid down in a single operation, thereby avoiding the need for multiple firing processes. This is possible because of the close packing of the deposited ceramic particles.

Preferably the substrate is metallic and most preferably generally planar, for example plate-like, but other shapes of substrate can be envisaged. That is a further advantage of the invention since unlike printing techniques, the ceramic may be satisfactorily applied to 3-dimensional surfaces.

Preferably only one side of the metallic substrate is coated. This is advantageous in that it not only reduces the amount of ceramic material required for the process, but also keeps the thermal resistance through the coated substrate to a minimum, since there is only one layer of thermally insulating ceramic rather than two. This has the practical advantage that the track temperature may be kept lower for the same watts density. Of course selected areas only of a side of the substrate can be coated, for example by masking other areas or removing the deposited material before firing, for example using a vacuum.

Furthermore, regions of the substrate which may be used in sealing are preferably masked to prevent ceramic material collecting there. This is potentially important if, for example a portion of the substrate is deformed to effect a seal, since in such circumstances any fired ceramic present may crack, thereby reducing the effectiveness of the seal.

In one particularly preferred embodiment, the substrate may be a plate having a peripheral channel formed to receive the depending edge of a wall of a liquid heating vessel. Such an arrangement is described in our International patent application W096/18331. In this case, the channel should be suitably masked to prevent ingress of ceramic to the sealing portions of the channel.

In preferred embodiments of the invention, therefore, the side of the substrate not to be coated may be suitably masked during spraying, and most preferably, the substrate may be arranged on a carrier, such as a conveyor, which masks the surface lying thereon. Preferably, the conveyor is horizontal, passing under suitable spraying means.

In the embodiment described above, with a platelike substrate having a peripheral channel, the outer wall of the channel may be dimensioned so as to rest on the conveyor to prevent ingress of material thereunder.

Preferably excess ceramic material deposited on the conveyor is re-cycled from the carrier for re-use.

Once the electrostatically deposited ceramic layer has been fired, a resistive heating track may be deposited thereon, for example by a silk screen printing process. Thereafter a protective overglaze may be provided over the track, for example by electrostatic deposition in the same way as discussed above. This is itself a novel proposal, and from a further aspect, the invention provides a method of providing an overglaze on a resistive heating track comprising electrostatically spraying a ceramic powder over said track and firing the deposited layer.

Again selected areas of the track may be masked (for example to provide locations for electrical contacts) or selected areas of the deposited layer removed to expose the track. Most preferably, the selected areas of the track are removed by vacuum means, as this is a non-contact process which will minimise disturbance to adjacent areas of the deposited material.

The vacuum means can most simply be a miniature suction head brought into position over the deposited layer and activated. To produce complex shapes, the vacuum means may be computer, robotically or mechanically controlled to travel a defined path to expose the desired area. Alternatively, the vacuum means may itself have a suction opening defining the desired shape.

The above idea of selectively removing a part of a deposited layer of insulating material using vacuum means could also be applied in the deposition of the original insulating layer, for example if an opening needs to be provided to allow the fixing of a mounting stud or the like. From a further broad aspect, therefore, the invention provides a method of manufacturing a heater wherein a ceramic layer of said heater is deposited by an electrostatic spraying process, and a selected portion of said deposited layer is then removed by suction means.

The invention also extends in broad terms to a method of manufacturing a substrate heater wherein a ceramic layer of said heater is deposited by an electrostatic spraying process, and also to a heater manufactured in accordance with the invention.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings in which: - Figure 1 shows a schematic representation of an apparatus for carrying out the process invention; and Figure 2 shows a preferred form of substrate.

With reference to Figure 1, a number of stainless steel plate-like substrates 2 are loaded onto an earthed, metal or electrically conductive rubber continuous conveyor 4. The conveyor 4 passes through a spray booth 6 into which dry air is introduced through a filter 8. The pressure within the booth 6 is maintained slightly above the ambient pressure to prevent the ingress of contaminated air from outside the booth 6.

A glass ceramic powder is aerated in an aerator chamber 10, and the aerated powder passed to a number of electrostatic spray guns 12 through a conduit 14.

The ceramic powder is typically an MZB glass ceramic powder produced by CDTh treated in a known manner with a charge-carrying coating. Typical materials would be MZB 550 and MZB 639, typically with a particle size in the range 5-12 micron.

The spray guns 12 are at a high voltage, for example at 15-70 kV, relative to the earth conveyor 4, and a charge is transferred to the air-suspended ceramic powder in a known manner as it flows through the guns 12. Typically the separation between the guns 12 and the conveyor 4 is 10 to 40cm. The charged particles are attracted to the metal substrates 2 (which are at earth potential) and are deposited thereon.

The thickness of layer deposited upon the substrates 2 can be controlled by the speed of movement of the substrates through the booth 6. Typically a layer of up to 200 microns most preferably between 70100 microns may be deposited, for mains voltage applications where the layer must be able to withstand a flash test of 1500V. Thinner layers, may be used for lower voltage applications, for example around 20 microns for use in 12V DC automotive applications. If necessary, a substrate may be passed through the apparatus more than once to achieve a desired thickness.

The coated substrates 2 after leaving the booth 6 may be removed from the conveyor 4 for firing in an appropriate furnace. Excess powder deposited on the belt is removed by a brush and vacuum system 16 to be returned to the aerating chamber 10 for further use.

This also prevents transfer of powder from the conveyor 4 to the underside of further substrates 2 subsequently deposited on the conveyor.

After the ceramic layer has been fired, a resistive heating track may be deposited thereon by a known technique, and thereafter a protective overglaze applied to the track. This overglaze may also be deposited by electrostatic spraying as discussed above. If contact pads or the like are to be provided on the track, then the appropriate areas may have deposited material removed by vacuum means, thereby resulting in unglazed areas after firing.

As shown in Fig. 2, the substrates 2 are generally circular and are each formed with a peripheral channel 20 which engages, via sealing means, on the lower edge of a liquid heating vessel wall, the outer channel wall being crimped to mount the plate in position. The lower edge 22 of the outer channel wall 24 sits on the conveyor 4 to prevent ingress of ceramic material into the channel 20 or onto the inner face 26 of the plate during spraying. If ceramic were to get into the channel 20, this might crack during crimping of the channel wall 24 onto the vessel wall, thereby destroying the integrity of the seal therewith.

It will be appreciated that the Figure 1 is merely schematic. Suitable electrostatic spraying equipment is known in other fields and may be obtained from a number of sources. For example suitable electrostatic spray guns are available from various manufacturers such as Gemma Volkstatic and Nordson.

Claims

1. A method of manufacturing an electric substrate heater comprising a substrate and an electrically insulating ceramic layer provided on said substrate for receiving a resistive heating track, wherein the ceramic layer is applied to the substrate by electrostatically spraying a ceramic powder onto the substrate and firing the deposited layer.

2. A method as claimed in claim 1 wherein the desired layer thickness is laid down in a single spraying operation.

3. A method as claimed in claim 1 or 2 wherein the ceramic powder particle size is between 5-12 microns.

4. A method as claimed in claim 1, 2 or 3 wherein the substrate is metallic, more preferably a metallic plate.

5. A method as claimed in claim 4 wherein said plate is formed to have a peripheral channel.

6. A method as claimed in any preceding claim wherein only one side of the metallic substrate is coated.

7. A method as claimed in any preceding claim wherein a portion of the substrate used for sealing purposes is masked.

8. A method as claimed in claim 6 or 7 wherein the substrates are arranged on a conveyor which masks the surface of the substrate in contact therewith.

9. A method as claimed in claim 8 wherein said substrate has a peripheral channel, the outer wall of said peripheral channel locating against said conveyor to prevent ingress of ceramic material into said channel.

10. A method as claimed in claim 8 or 9 wherein said conveyor is generally horizontal and passes under suitable spraying means.

11. A method as claimed in claim 8, 9 or 10 wherein excess ceramic material deposited on the conveyor is removed therefrom and re-cycled.

12. A method as claimed in any preceding claim wherein a resistive heating track is laid down on the ceramic layer.

13. A method as claimed in claim 12 wherein a ceramic overglaze is provided over the heating track, by electrostatically spraying a layer of ceramic powder thereover.

14. A method of providing an overglaze on a resistive heating track comprising electrostatically spraying a ceramic powder over said track and firing the deposited layer.

15. A method as claimed in claim 13 or 14 wherein selected areas of the track are masked to prevent deposition of material there.

16. A method as claimed in claim 13, 14 or 15 wherein selected areas of the track are exposed after deposition of the ceramic powder for example by vacuum means.

17. A method of manufacturing a heater wherein a ceramic layer of said heater is deposited by an electrostatic spraying process, and a selected portion of said deposited layer is then removed by suction means.

18. A method of manufacturing a substrate heater wherein a ceramic layer of said heater is deposited by an electrostatic spraying process.

19. A heater manufactured by a method as claimed in any preceding claim.

20. A method of manufacturing a substrate heater substantially as hereinbefore described with reference to the accompanying drawings.