GB2318532A

GB2318532A - Electric heaters

Info

Publication number: GB2318532A
Application number: GB9621992A
Authority: GB
Inventors: Keith Barrie Doyle
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 1996-10-22
Filing date: 1996-10-22
Publication date: 1998-04-29
Anticipated expiration: 2016-10-22
Also published as: GB9621992D0; GB2318532B

Abstract

There is disclosed a method of producing an electrically insulating layer 2 on a support surface 4 comprising depositing an electrically insulating material to the support surface through a stencil 6, and firing the deposited material. A resistive heating track or an electrical contact may be produced in the same way.

Description

Electric Heaters The present invention relates to electric heaters and in particular a method of manufacturing heaters of the type comprising an electrically resistive heating track provided on an electrically insulating substrate.

Typical of such heaters above are so-called thick film printed heaters, such as disclosed, for example, in WO 96/18331 and WO 96/17497. In the manufacture of such heaters, a glass ceramic insulator ink is normally silk screen printed onto a metallic support, such as a stainless steel plate, which may have been pre-treated, e.g. oxidised, to receive the ink. The insulating ink is then dried and then fired. This process is repeated several times, typically three or four times in order to build up an insulating layer which is sufficiently thick to withstand mandatory voltage breakdown tests. For domestic heating appliances, such as kettles, hot water jugs and so on, the insulator must be able to withstand, typically 1250V rms at 50 Hz for 1 minute at room temperature without breaking down. Once a sufficiently thick layer of insulator has been produced, an electrically resistive track is printed onto the insulator, dried and fired. Contacts are then produced by selectively printing a silver or silver bearing ink onto the track in the desired locations and firing it to form contacts. A protective electrically insulating glass ceramic overglaze may then also be printed over the track and fired.

The multi-step process for producing the insulating layer is not only time consuming but also wasteful of energy since, typically, the ink has to be fired at up to 850to which requires heating the whole heater mass each time.

Attempts have been made to produce a sufficiently thick insulating layer in one application using silk screen printing techniques. The thickness of layer which can be deposited is determined largely by the mesh size of the screen, a coarser mesh producing a thicker deposited layer. However, the problem of using a larger mesh size is that it produces mesh marks corresponding to the openings in the mesh, in the printed layer, thus resulting in variations of thickness in the deposited layer. This is undesirable not only since it makes it almost impossible to produce an insulator with a minimum uniform thickness to meet voltage breakdown requirements, but also in that a non-uniform thickness of resistive ink may then be deposited in the next process stage, leading to variations in the power output and power distribution in the final heater.

The present invention seeks to mitigate the above problems, and from a first aspect provides a method of producing an electrically insulating layer on a support surface comprising depositing an electrically insulating material to the support surface through a stencil, and firing the deposited material.

Thus in accordance with the invention, the insulator e.g. an insulator ink, is applied using a stencil, i.e. a sheet of material having an aperture corresponding in shape to the shape of the insulating layer to be deposited on the support surface. This has the advantage that much thicker layers of insulating material can be deposited than by using a screen printing technique, without having undesirable mesh patterns produced in the deposited layer. For example, it has been found that layers of over 150 jim can very easily and satisfactorily be deposited.

The material is preferably applied using a squeegee or blade, as is commonly used in screen printing techniques, moving over the aperture.

Preferably the application portion of the blade at least is substantially rigid, so that as it moves across the stencil, it does not deflect into the stencil aperture, thereby leading to variations in the thickness of the deposited layer. This is particularly desirable where large apertures e.g. 100 mm in diameter are provided. Most preferably the blade is metallic, which gives the necessary rigidity and also improved tool life.

In order to produce an improved surface finish, the material may be applied to a greater thickness than desired in a first application step, and the thickness then trimmed to the desired thickness in a finishing step. This also permits surface defects to be removed in the trimming step. Although this could be achieved using sequential passes of a blade, the first pass with a desired clearance above the stencil, and the second in contact with the stencil, preferably, two separate blades are provided. Most preferably these blades may be moved together relative to the stencil, to produce the layer in a single pass. The clearance of the blade or blades from the stencil may be adjusted to give the desired results.

The stencil itself is preferably of metal, for example a metal foil typically of a thickness of 100-200 jim. Metallic stencils give consistent deposited thicknesses and good tool life. The aperture may be etched in the foil to give the desired insulator pattern.

Once the insulator has been deposited in accordance with the invention, it may be dried and fired, and a resistive track then deposited on it in the normal manner.

It will also be appreciated that the above technique can be extended to the application of a protective overglaze over the heater track. Whilst normally such layers are quite thin, in the order of 10's of microns, if a particularly thick layer is required say for additional electrical insulation, then rather than sequentially apply and fire a multiplicity of layers, a single layer may be applied in accordance with the invention.

Furthermore, the techniques described above may also be extended to the application of tracks and contacts, particularly where thick tracks or contacts may be required, to avoid the need for multiple screen printing. From yet further aspects, therefore, the invention provides a method of producing a heater of the type comprising an electrically resistive track deposited on an insulating substrate, wherein the track is deposited through a stencil and also a method of providing an electrical contact for an electrically resistive track wherein the contact is deposited through a stencil.

In the former case, the stencil may have a shape corresponding to the desired shape of the track, while in the latter, it has the appropriate contact shape.

For example the stencil may have a pair of apertures to allow a pair of contact pads to be deposited.

The invention also extends to a heater having an insulating layer, track or contact manufactured in accordance with the invention and to apparatus for carrying out the invention.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing, Figure 1.

With reference to Figure 1, a layer 2 of insulating material is shown, schematically, being deposited on a stainless steel plate 4, which may be pre-treated in known manner to improve adhesion of the insulating layer 2. The layer is circular in plan view, having a diameter of approximately 100mm.

A 150 jim thick metal foil stencil 6, having etched in it an aperture 8 corresponding in shape to the pattern of the insulating layer 2 to be deposited is attached to a conventional screen printing frame, tensioned and then fitted in a screen printer, and the plate 4 positioned in the appropriate position to receive the insulating layer 2.

The head of the printer is adapted to receive a double bladed squeegee 10. The clearance of the front blade 12 and rear blade 14 from the stencil 6 may be adjusted independently of each other by suitable means (not shown).

The print head is lowered until the squeegee 10 contacts the stencil 6, so that as it moves across the stencil, it will compress and seal it against the plate 4.

The rear blade 14 is intended to contact the stencil 6 while the front blade 12 is positioned so as to have a clearance of about 50 jim from the stencil surface.

The squeegee 10 is then moved across the stencil 6, in the direction of arrow A, so as to roll insulating glass ceramic ink 16 into the stencil aperture 8. The first blade 12 forms a preliminary layer having a thickness Tp, about 200 jim, as it moves across the stencil 6. This may, however, have air bubbles or other defects in its surface. The second blade 14, however, acts as a doctor blade, removing the surface of the preliminary layer, together with its defects, to produce a final layer thickness TF corresponding to the thickness, 150 jim, of the stencil 6.

The squeegee 10 is moved completely across the stencil 6 which may then be moved away to allow removal of the plate 4, with its insulating layer 2, for subsequent firing. After firing, a resistive track, contacts and overglaze may then be applied as appropriate.

It will be seen from the above that the invention allows a relatively thick layer of insulating material to be applied in a single process step. This saves both time and expense compared to the multi-step prior art processes. Furthermore the deposited layer is free of mesh marks, and is more tolerant of contaminants, such as dust particles, than multiple thinner layers as it is exposed to treatment conditions only once, rather than a number of times.

By using metal stencils and blades, excellent tool life and production tolerances can be obtained.

However, other materials may be used if found suitable.

Whilst the embodiment above describes essentially a contact printing process, in which the stencil 6 is laid in contact with the plate 4, a non-contact process in which the stencil is held at a spacing from the plate 4 and deflected into contact with it only as the blades pass over the stencil could also be used. A contact process, however, has the advantage that no bending stresses are applied to the stencil during operation, thereby prolonging its life.

Also, whilst the embodiment has been described with reference to providing an insulating layer on a metallic substrate, the techniques disclosed could equally be applied to applying an electrically insulating overglaze over the resistive track, depositing the track itself or depositing contacts for the track.

Claims

1. A method of producing an electrically insulating layer on a support surface comprising depositing an electrically insulating material to the support surface through a stencil, and firing the deposited material.

2. A method as claimed in claim 1 wherein the material is applied using a blade.

3. A method as claimed in claim 2 wherein the stencil contacting portion at least of said blade is substantially rigid.

4. A method as claimed in claim 2 or 3 wherein said blade is metallic.

5. A method as claimed in any preceding claim wherein the layer is applied to a greater thickness than required and trimmed to the desired thickness.

6. A method as claimed in claim 5 wherein the layer is applied using first and second blades movable with respect to the stencil, the first blade being arranged at a clearance from the stencil and the second in contact therewith.

7. A method as claimed in claim 6 wherein said blades are moved together across the stencil.

8. A method as claimed in any preceding claim wherein said stencil is metallic.

9. A method as claimed in claim 8 wherein the stencil aperture is etched in the stencil.

10. A method as claimed in any preceding claim wherein said support surface is metallic.

11. A method as claimed in any of claims 1 to 9 wherein said support surface is an electrically insulated substrate carrying an electrically resistive heating track and said layer is an overglaze layer.

12. An insulated support surface treated by a method as claimed in any preceding claim.

13. A method of producing a heater of the type comprising an electrically resistive track deposited on an insulating substrate, wherein the track is deposited onto the substrate through a stencil.

14. A method of providing an electrical contact for an electrically resistive track wherein the contact is deposited through a stencil.

15. A heater comprising an insulating layer, resistive track or contact manufactured by a method as claimed in any preceding claim.

16. Apparatus for providing an insulating layer on a support surface comprising a stencil, and blade means for depositing said material onto said surface through said stencil.

17. Apparatus as claimed in claim 16 wherein said blade means comprises a substantially rigid blade.

18. Apparatus as claimed in claim 17 wherein said blade is metallic.

19. Apparatus as claimed in any of claims 16 to 18 wherein said blade means comprises first and second blades coupled so as to move together over said stencil.

20. Apparatus as claimed in claim 19 wherein the clearance of at least one of said blades is adjustable.