GB2338632A

GB2338632A - Metal sheathed planar element: Edge connector with shutter

Info

Publication number: GB2338632A
Application number: GB9812876A
Authority: GB
Inventors: James Henry Sharples; Paul Harden
Original assignee: Pifco Ltd
Current assignee: Pifco Ltd
Priority date: 1998-06-16
Filing date: 1998-06-16
Publication date: 1999-12-22
Also published as: GB9812876D0

Abstract

An electrical heating element 1 comprises a first plate 2, at least one dielectric layer 3 applied to at least one side 4 of the plate 2, a thick film resistive track 5 printed onto the layer 3, at least one encapsulating dielectric layer 8 and a second plate 9 of similar size to the first plate 2 said second plate having one side 10 thereof covered with one or more dielectric layers 11 and bonded to the encapsulating dielectric layer 8 by means of the layer or layers 11 to seal the resistive track 5 between the plates 2 and 9. Electrical, contact pads 6 may be formed for the track 5 on a protruding portion 7 of the plate 2. The construction of the element 1 enables it to be plugged into appliances for use and then removed therefrom for cleaning. The portion 7 can form a male plug-in electrical connector enabling the heating element 1 to be inserted into a suitably formed female electrical connector 14, said connector having a displaceable shutter 20 which additionally operates the contacts 16, means (not shown) being included to releasably retain the element in the connector.

Description

1 2338632 IMPROVEMENTS TO ELECTRICAL HEATING ELEMENTS The present

invention relates to a thick film resistive heating element such as can he used Particularly but not exclusively in a domestic appliance such as a water boiler, kettle, toaster, grill, oven and the like, and to a method for its production.

Thick film resistive heating elements conventionally comprise a thick film resistive track which has been printed onto the surface of an electrically insulative suhstrate such as a..5tanle,.;s steel plate tbet has been covered by a glass or ceramic dielectric layer. The track is itself then protected hy being covered by a. fl)rther dielectr5c layer.

Such heating elements are used in kettles and ii- is convenient in these cases for the element to be integrated into the bottom of the body of the vessel defining the kettle body. Typically, the beating element is mounted such that the side of the substrate on which the resistive track is printed faces to the exterior of the vessel in order that appropriate electrical control devices and thermal cut-nut mechanisms can be connected thereto. Such an arrangement is also advantageous because the resistive track comprising the element is located in a base portion of the kettle which is not normally to the liser. Tt is, therefore, protected from tampering and damage, which could occur if the protective dielectric layer were to be scratched to expose a Dnrt-ien of the resistive track.

2 The object of the present invention is to provide a thick film resistive heating element which is not readily susceptible to damage by scratching or rough treatment and which as result can be used in a more versatile way than hitherto.

According to a first aspect of the present invention there is provided an electrical heating element comprising a first plate defining a substrate which is covered by at least one dielectric layer on which is formed a thick film resistive track, and at least one encapsulating dielectric layer which covers the track, and characterised in that a second plate is bonded to said encapsulating dielectric layer to seal the resistive track between the first and second plates.

Preferably. the second plate is bonded to the encapsulating layer by a third dielectric layer which covers the surface of the second plate apposed to the encapsulating layer.

Preferably also, the outer surfaces of the first and the second plates are covered by a protective or non-stick coating. Advantageously, this coating comprises a layer of a conventional heat resistant non-stick, wipe-clean material. Alternatively, the protective coating comprises a glass or ceramic layer.

Preferably also, the second plate defines a substrate which is covered by at least one dielectric layer and on which is formed a second thick film resistive track that is apposed by the encapsulating dielectric layer and that is sealed between the first and second plates.

3 Preferably also, the second resistive track comprises a thermistor whereby the temperature of the element can be measured. Alternatively. the second resistive track comprises a heating track.

Preferably also, a plurality of thick film resistive tracks are formed on the first plate, at least one of which tracks comprises a thermistor.

Preferably also, electrical contact pads are formed at the ends of the track which are located on a protruding portion of the first plate that is not covered by the encapsulating dielectric layer and the second plate.

Preferably also, the protruding portion of the first plate comprises a male electrical connector whereby the heating element can be plugged into a complementary female electrical connector.

According to a second aspect of the present invention there is provided the combination of an electrical heating element according to the first aspect of the present invention with a complementary electrical connector, the heating element comprising electrical contact pads located on a protruding male portion of the first plate which can be plugged into a recess defined by a complementary female connector, and characterised in that the female connector comprises a releasable retaining means, which when engaged operates to retain the protruding portion in the recess with the electrical contact pads in electrical connection with correSDonding electrical contacts located within the recess to supply current to the resistive track and when released frees an associated mechanism which isolates the electrical contacts from the respective electric contact pads and permits the heating element to be removed from the connector.

4 Preferably, the contacts within the recess are spring-loaded into connection with the electrical contact pads of the heating element.

Preferably also, the mechanism associated with the retaining means comprises a carriage into which the protruding portion of the first plate locates and which is movable between a first position, wherein the retaining means is engaged and the protruding portion is retained within the recess, and a second position, wherein the retaining means is released and the heating element can be removed from the connector, movement of the carriage between the first and second positions acting against the spring-loading of the contacts in order to move the contacts out of electrical connection with the contact pads of the heating element.

Preferably also, the carriage is spring-loaded and operably connected to the retaining means whereby release of the retaining means frees the carriage for movement under the influence of its spring-loading from the first position into the second position.

Preferably also, in its second position the carriage covers the entrance to the recess.

According to a third aspect of the present invention there is provided a method of manufacturing an electrical heating element comprising the steps of applying one or more dielectric layers to a surface of a first plate defining a substrate; forming a thick film resistive track on the exposed surface of the dielectric layer; applying an encapsulating dielectric layer over the resistive track; and characterised in that a second plate is bonded to the encapsulating dielectric layer to seal the resistive track between the first and second plates.

Preferably, the method comprises the additional step of applying one or more dielectric layers to a surface of the second plate and using these layers to bond the second plate to the encapsulating layer applied to the first plate.

Preferably also, the method comprises the additional step of applying an electrically conducting layer to part of the thick film resistive track to form electrical contact pads for the track.

Preferably also, the method comprises the additional step of coating the outer svrfaces of the first and second plates with a protective or nonstick layer.

The present invention will now be described by way of example with reference to the accompanying drawings. in wh i eh: - Fig. 1 is a perspective view of an electrical heating element according to the first aspect of the Dresent invention; Fig. 2 an end view of the element in the direction or arrow II in Fig. 1:

Fig. 3 is a partial cross-sectional view to an enaIrged scale along the line III-III in Fig. 1; 6 Fig. 4 is a perspective view of from above of a female electrical connector for use with the element shown in Figs. 1 to 3 in accordance with the second aspect of the invention; Fig. 5 is a cross-section along the line V-V in Fig. 4 and shows the connector when in use in combination with the electrical element; and Fig. 6 is a view similar to that of Fig. 6 but when the connector is not in use and the electrical element has been unplugged therefrom.

Referring to Figs. 1 to 3, an electrical heating element 1 comprises a first plate 2 which forms a substrate for at least one dielectric layer 3 that is applied to cover at least one side 4 of the plate 2. A thick film resistive track 5 is printed onto the layer 3 and electrical contact pad. s 6 are formed for the track 5 on a protruding portion 7 of the plate 2, as is described in more detail below. At least one dielectric layer 8 is then applied to cover the track 5. A second plate 9. . of similar size to the first plate 2 and which has also had at least one side 10 thereof covered with one or more dielectric layers 11, is bonded to the encapsulating dielectric layer 8 by means of the layer or layers 11 to seal the resistive track 5 between the plates 2 and 9. In this way the track 5 is sandwiched between the plates 2 and 9, hermetically sealed within the dielectric layers 3, 8 and 11. The resulting heating element 1 is then capable of being used in a variety of ways. In particular in domestic kitchen appliances it can be used as an immersion heater by being submerged in liquid.. or be partially covered by liquid - for example in the manner of a conventional thick film element in the base of a kettle, or it can be suspended in air - for example in the manner encapsulating 7 of a conventional toaster element, or it can be placed in direct contact with i.tems to be heated - for example food to be heated, toasted or cooked in a sandwich toaster.. griddle, hot plate or simila-r, Advantageously, the outer surfaces 12 and 13 of the plates 2 and 9 respectively, or indeed the whole element 1 apart from the electrical contact pads 6. is coated with a protective or non-stick layer of material. For use in food preparation appliances, this coating preferably comprises a layer of a heat resistant non-stick material in order that the element 1 can simply be wiped clean. Alternatively, the protective coating may comprises a glass or ceramic layer. As the resistive track 5 is sealed between the plates 2 and 9 it is in no danger of damage even if the element 1 is subjected to comparatively rough handling and cleaning, for example in a dishwasher or similar.

In a modification, not shown. the second plate 9 may also have a second thick film resistive track applied. thereto over the top of the dielectric layer 11. This track can then be covered with a further encapsulating layer or apposed directly by the encapsulating dielectric layer 8 and sealed between the first and second plates 2 and 9.

The resistive track applied to the plate 9 may comprises a thermistor, whereby the temperature of the element can be measured. Alternatively, the second resistive track could comprise a second heating track. It will also be appreciated that a plurality of thick film resistive tracks can be formed on the first plate 2, at least one of which tracks comprises a thermistor.

8 The inclusion of a thermistor track in the element 1 is advantageous in some applications of the element 1. For example in a toaster the thermistor of the element 1 could be used to sense the condition of the bread, for example fresh or frozen, and the operation of the element 1 adjusted by suitable control means accordingly. Similarly, the use of a thermistor with suitable electronic controls will provide keep warm and re-heat facilities in a wide range of appliances.

It will be appreciated that the element 1 could be permanently connected into an appliance in the manner of conventional electrical elements. In this case, the plates 2 and 9 can be earthed to ensure user protection. However, the robust construction of the element 1 enables it to be used in a novel manner for domestic appliances as a detachable element 1 which can be plugged into appliances for use and then removed therefrom for cleaning.

The element 1 is connected to a power supply by means of the electrical contact pads 6 which are formed. at the ends of the track 5 on the protruding portion 7 of the first plate 2 in the manner of card-edge connectors on printed circuit boards. It will be appreciated that. the pads 6 are not covered by the encapsulating dielectric layer 8, the second plate 9 nor any outer protective coating or layer applied to the element 1. The protruding portion 7 thereby comprises a male plug-in electrical connector enabling the heating element 1 to be inserted into a suitably formed female electrical connector 14 as will now be more described in more detail with reference to Figs. 4 to 6.

The female connector 14 defines a recess 15 of a suitable size to accommodate the protruding portion 7 of the element 1 as shown in Fig. 5. Within the recess 15 are 9 located electrical spring contacts 16, which can be connected to an electrical power supply, an earth connector and electronic control means via cables 17, and a movable carriage 18 into which the protruding portion 7 can locate. In a first position, as shown in Fig. 5, the carriage 18 supports the element 1 when the portion 7 is plugged into the connector 14. In this position the contacts 16 make electrical contact with respective contact pads 6 of the element 1 to power the resistive track 5. The carriage 18 is biassed by a spring loading 19 into a second position, as shown in Fig. 6, wherein it covers the entrance to the recess 15 and. a releasable retaining means (not shown) is provided to hold the carriage 18 in the first position when the element 1 is inserted into the connector 14.

The retaining means not only operates to retain the carriage 18 in the first position but also to retain the protruding portion 7 in the recess 15. The reason for this is to prevent the element 1 from simply being pulled out of the connector 14 whilst it is still connected to an electricity supply. This would result in the spring-loaded contacts 16 from being dragged over the area of the protruding portion 7 adjacent the pads 6 and could result in them being short-circuited. Under normal circumstances this is prevented as follows.

The carriage 18 comprises a platform 20 which supports the element 1 and an upstanding rim 21 at the same side of the recess 15 as the contacts 16. The contacts 16 themselves comprise bowed leaf springs which are mounted so as normally to he biassed towards the interior of the recess in order that they will make good electrical contact with the pads 6. However, as the carriage 18 moves between its first and second positions, the rim 21 comes into contact with one or other end of the i 0 bowed contacts 16 and pushes them outwardly to one side of the recess away from element 1. The free end of the contacts 16 is pushed into a cut- away region 22 of the connector casing and the contacts 16 are thereby isolated from the pads 6 and other areas of the element 1.

Release of the retaining means therefore not only releases the element 1 to enable it to be removed from the connector 14 but also frees the carriage 18 in order that the spring-loading 19 operates to eject the element 1 by urging the carriage to move into its second position. However, as the carriage 18 moves its rim 21 moves the contacts 16 away from the element 1.

The retaining means may comprise any conventional. clipping mechanism which is automatically engaged when the protruding portion 7 of the element 1 is inserted into the connector 14. Typically in these arrangements a release button is provided to enable the user to disengage the mechanism to release the element 1 and the carriage 18 which is operable associated therewith.

The element 1 according to the invention can be manufactured as follows.

The plate 2 may comprise a stainless steel plate with a sufficient chromium content to form a chromium oxide surface layer when fired at a temperature of 8SO'C to 900'C. After suitable pre-treatment such as grit blasting and the formation of a chromium oxi.de surface layer, the dielectric layer 3 is deposited over the surface 4, fnr example using a silk-screen printing method, permitted to dry and then fired to cause it to adhere to the oxidised steel substrate. The layer 3 may comprise a single dielectric layer or a plurality of layers, typically three layers, and the plate 2 can be repeatedly fired after each 11 of the layers 3 is applied or subject to a single firing to co-fire the layers 3 together. If protection of the outer surface of the element 1 by a dielectric layer is also required, then both sides of the plate 2 can be covered with the dielectric material.

Any suitable dielectric material, typically a glass or ceramic material, can be used for the layer 3 but it is preferably selected to have a coefficient of thermal expansion approximately equal to that of the steel plate 2. In addition, it is important to ensure that the layer 3 will ensure a minimum voltage withstand of 1250 volts rms.

A thick film circuit layout is then applied over the layer 3 by silkscreen printing. The layout will comprise one or more conductive tracks constituting the heating resistive track 5 and any required resistive track constituting a thermistor or temperature sensor. The length, width and thickness of the printed tracks and the resistivity of the track material will determines its resistance and therefore the desired wattage of the heating element 1. Preferably, the track material comprises a resistive ink made from palladium silver but may alternatively be made of other conducting materials such as nickel, platinum, silver, or carbon, for example. Thick film resistive heating tracks such as the track 5 are usually deposited at a constant thickness such that after firing a thickness of between approximately 10 to 14 microns is achieved.

Preferably, the track 5 or tracks 5 follow a tortuous path of predetermined length over the majority of the area of the substrate surface 4 to maximize the heated area of the element 1.

12 After the track or tracks 5 have been printed, the plate 2 is again fired to set the track 5 in place.

The contact pads 6 are then deposited on the plate 6 at the ends of the track or tracks 5. The material used for the pads 6 will have a significantly higher conductivity than that used for the track or tracks 5 in order to prevent over-heating of the pads 6 when they are connected to a mains supply.

The encapsulating dielectric layer 8 is then applied over the whole or part of the preceding layers to completely cover the track or tracks 5 apart from the pads 6.

The second plate 9 is prepared in the same way as the described. above for the first plate 2. However, it may only be provided with a dielectric layer 11, as shown in Fig. 3, but it can also be printed with one or more resistive tracks 5 as previously described and as required in the finished element 1. The dielectric material can also be used to cover both sides of the plate 11 if required for protection purposes.

After the dielectric layer 8 has been applied to the plate 2 and the uppermost layer, for example layer 11, applied to the plate 9, the two Plates 2 and 9 are aligned with the substrates 8 and 11 apposed and then placed in contact with one another. The adjoining plates 2 and 9 are then co-fired at a temperature of 8SO'C to 900'C to bond them together and produce the element 1.

The exterior surfaces 1.2 and 13 can then be coated a.-, described above with non-stick or other protective materials. However, It is important that any such coating does not require curing at tempe-atures in excess of 60CC 13 in order to ensure that the bond between the plates 2 and 9 remains unaffected. Conventional non-stick, wipe-clean coatings require curing at a temperature around 4SO'C and are therefore suitable for the element 1.

14

Claims

1. An electrical heating element comprising a first plate defining a substrate which is covered by at least one dielectric layer on which is formed a thick film resistive track, and at least one encapsulating dielectric layer which covers the track, and characterised in that a second plate is bonded to said encapsulating dielectric layer to seal the resistive track between the first and second plates.

2. An element as claimed in Claim 1, characterised in that the second plate is bonded to the encapsulating layer by a third dielectric layer which covers the surface of the second plate apposed to the encapsulating layer.

3. An element as claimed in Claim 1 or Claim 2, characterised in that the outer surfaces of the first and the second plates are covered by a protective or non-stick coating.

4. An element as claimed in any one of Claims 1 to 3, characterised in that the second plate defines a substrate which is covered by at least one dielectric layer and on which is formed a second thick film resistive track that is apposed by the encapsulating dielectric layer and that is sealed between the first and second plates.

5. An element as claimed in Claim 4, characterised in that the second thick film resistive track comprises a thermistor whereby the temperature of the element can be measured.

6. An element as claimed in Claim 4, characterised that the serond. thick fl-1m resistive track compri-,e.-; a heating track.

7. An element as claimed in any one of Claims 1 to 6, characterised in that a plurality of thick film resistive trac'.,s are formed on the first plate, at least one of which tracks comprises a thermi.stnr.

8. An element as claimed in any one of Claims 1 to 7, characterised in that electrical contact pads are formed at the ends of the track which are located on a protruding portion of the first plate that is not covered by the encapsulating dielectric layer and the second plate.

9. An element as claimed in Claim 8, characterised. in that the protruding portion of the first plate comprises a male electrical connector whereby the heating element can be plugged into a complementary female electrical connector.

10. A combination of an electrical heating element as claimed in Claim 1 with an complementary electrical connector, the heating element comprising electrical contact pads located. on a protruding male portion of the first plate which can be plugged into a recess defined by a complementary female connector, and characterised in that the female connector comprises a releasable retaining means, which when engaged operates to retain the protruding portion in the recess with the electrical contact pads in electrical connection with corresponding electrical contacts located within the recess to supply current to the resistive track and when released frees an associated mechanism which isolates the electrical contacts from the respective electric contact pads and permits the heating element to be removed from the connector.

16

11. A combination as claimed in Claim 10, characterised in that the contacts within the recess are spring-loaded into connection with the electrical contact pads of the heating element.

12. A combination as claimed in Claim. 11, characterised in that the mechanism associated with the retaining means comprises a carriage into which the protruding portion of the first plate locates and which is movable between a first position, wherein the retaining means is engaged and the protruding portion is retained within the recess, and a second position, wherein the retaining means is released and the heating element can be removed from the connector, movement of the carriage between the first and second positions acting against the spring-loading of the contacts in order to move the contacts out of electrical connection with the contact pads of the heating element.

13. A combination as claimed in Claim 12, characterised in that the carriage is spring-loaded and operably connected to the retaining means whereby release of the retaining means frees the carriage for movement under the influence of its spring-loading from the first position into the second position.

14. A combination as claimed in any one of Claims 10 to 13, characterised in that in its second position the carriage covers the entrance to the recess.

15. A method of manufacturing an electrical heating element comprising the steps of applying one or more dielectric layers to a surface of a first plate defining a substrate:

forming, a thick film resistive track on the exposed surface of the iielectric layer.:

17 applying an encapsulating dielectric layer over the resistive track; and characterised in that a second plate is bonded to the encapsulating dielectric layer to seal the resistive track between the first and second plates.

16. A method as claimed in Claim 15, characterised in that it comprises the additional step of applying one or more dielectric layers to a surface of a second plate and using these layers to bond the second plate to the encapsulating layer applied to the first plate.

17. A method as claimed in Claim 16, characterised in that the second plate is bonded to the first plate by cofiring the adjoining plates at a temperature of at least 8500C.

18. A method as claimed in any one of Claims 15 to 17, characterised in that it comprises the additional step of applying an electrically conducting layer with a lower resistivity than that of the resistive track to end portions of the resistive track to form electrical contact pads for the track.

19. A method as claimed in any one of Claims 15 to 18, characterised in that it comprises the additional step nf coating the outer surfaces of the first and second plates with a protective or non-stick layer.

20. A method as claimed in any one of Claims 15 to 19.. characterised in that the plates comprises stainless steel plates.

21. An electrical heating element substantially as described herein with reference to any one of Figs. 1 to 3.

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22. A combination of an electrical heating element as claimed in Claim 10, with an complementary electrical connector substantially as described herein with reference to any one of Figs. 1 to 6.

23. A method of manufacturing an electrical heating element substantially as described. herein with reference to any one of Figs. 1 to 3.