GB2267421A

GB2267421A - A glass hot plate having a resistive heating coating

Info

Publication number: GB2267421A
Application number: GB9311011A
Authority: GB
Inventors: Wictor Chichlowski; Gerald Lipton
Original assignee: CHINACRAFT Ltd
Current assignee: CHINACRAFT Ltd
Priority date: 1992-05-28
Filing date: 1993-05-28
Publication date: 1993-12-01
Anticipated expiration: 2013-05-28
Also published as: GB9311011D0; GB2267421B; GB9211331D0

Abstract

There is provided a hot plate for food, having a heating element assembly comprising a glass sheet 2 coated with an electrically conductive material 5 and adapted to be connected to an electricity supply, via electrical contact areas 8, 9. The contacts are applied to the resistive layer 5 as a paste of silver, glass particles and a carrier medium. The resistive layer may be made of tin doped with fluoride. The glass is toughened by heat treatment which also fuses the contact areas to the resistive material. The element may have a further layer 11 of toughened glass attached thereto to form the hot plate surface or may be part of a laminated structure (Fig. 3). <IMAGE>

Description

HOT PLATE This invention relates to a hot plate. Such a hot plate has applicability both for keeping food, etc. hot or for heating food, etc. and as a cooking surface or hob for food, etc. The invention also relates to an article incorporating such a hot plate. Examples of such articles are tables, trolleys, ovens, hobs, etc.

Existing hot plates for keeping food warm rely upon a support surface and spaced therebeneath a heat source such as a wax candle or 'night light', a mains or bottled gas supply, or one or more conventional electrical heating elements or coils. Existing hot plates for cooking food rely upon a support surface and, therebeneath, a heat source such as a plurality of gas jets or one or more electric heating coils.

It is an object of the present invention to provide a hot plate which possesses advantages over known hot plates.

According to a first aspect of the present invention there is provided a hot plate for food, etc which comprises glass coated with an electrically conductive material and adapted to be connected to an electricity supply, the material having an electrical resistance such that when an electric current is passed therethrough the glass will heat up.

The glass may be a 'hard-coat' or 'soft-coat' glass, e.g. a glass coated with a conductive metal oxide or with a metal. 'Hard -coat ' glass, or K-glass as it is sometimes called, is preferred, preferably glass coated with a layer of tin oxide, which may be doped with fluorine.

Such glass is available e.g. from Pilkington and its subsidiaries and associated companies, and from other sources such as the Glaverbel Company in France.

In such a glass the coating is not merely physically adhered to the glass but is intimately bonded or fused thereto. Such a coated glass may have an electrical resistivity of 16 ohms/square or more.

It is generally necessary subsequently to treat the coated glass to enhance its performance. Such treatments may include steps to reduce electrical arcing which will otherwise result in localised heating and fragmentation of the glass, e.g. edge treatments of the glass and/or the coating, steps to increase the electrical resistivity of the coating and steps to toughen the glass to enable it better to withstand the effect of heat and stress.

The hot plates of this invention may include a second layer bonded or otherwise to overlie the coated glass, and adapted to be the actual food, etc.-heating/cooking surface. Such a second layer may be of any suitable material, such as glass, ceramic material, metal or enamelled metal. The provision of this second layer may increase the strength of the hot plate, increase its flexibility and resistance to impact damage, and improve the aesthetic appearance of the hot plate.

In order to connect the electrically conductive coating of the coated glass to an electricity supply it is necessary to provide the coating with some form of electrical input and electrical output, hereinafter referred to as "contacts". The contacts should be such that electricity passes through/across the conductive coating in a more or less uniform manner. In the case of a rectangular sheet of coated glass, it has been found by experiment that such contacts, fused to the coating, and arranged along opposite edges of the coating, are satisfactory. In preferred embodiments the contacts are formed from a paste consisting of finely divided precious metal and finely divided glass in a viscous matrix applied e.g. by screen-printing, along said opposite edges. A subsequent heating step fuses the paste to the coating to provide the contacts in the form of contacts on said opposite edges.Such contacts allow very uniform passage of electrical current into, across and out of the electrically conductive coating, with little or no heating effect in the contacts themselves (i.e. the resistance of the contacts is preferably very low).

The hot plate of this invention may be provided with one or more regions which become hot on passage of electric current and with one or more other regions which do not become hot. The said region or regions which do not become hot may be created by selective removal of the conductive coating, e.g. by sand-blasting, to create one or more non-conductive areas on said coated glass surface. Alternatively, or as well, an article may incorporate more than one hot plate according to the present invention, the hot plates being separated from each other by regions which do not become hot.

More than one set of the contacts may be provided, whereby the hot plate is provided with at least two regions defined between respective pairs of the contacts and which become heated to different temperatures and/or at different rates from each other.

Embodiments of hot plates according to the present invention and methods for their preparation will now be described, by way of example only, by reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of a first embodiment of a hot plate according to the present invention; Figure 2 is a diagrammatic cross section through the hot plate of Figure 1; and Figure 3 is a diagrammatic plan view of a second embodiment of a hot plate according to the present invention.

Referring firstly to Figures 1 and 2 of the drawings, a hot plate 1 comprises a rectangular sheet 2 of K-glass manufactured by Pilkington of St. Helens, Lancashire, England. This glass has on its under-surface 3 a fusedon, electrically conductive coating of fluorine-doped tin oxide. The glass may be 4mm or 6mm thick; the coating is very thin, e.g. approximately 3000 A.

Because the sheet 2 of glass is preferably made by a float-glass procedure the upper surface 10 may be very flat and smooth.

The sheet of glass is first of all cut to the desired shape and size. The edges of the glass are then rendered very smooth, e.g. by grinding or lapping (i.e. polishing using an abrasive) to reduce the possibility of electrical arcing, unevenness or discontinuity.

To provide the glass 2 of the hot plate 1 with a first region 4 which does not become heated and a second region 5 which does become heated, a mask (not shown) is temporarily applied to the under-surface 3 of the glass to mask off the region 5. The coating in region 4 is then removed by a careful sand-blasting operation, particular care being taken to ensure that smooth edges are defined for region 5, once again to reduce electrical arcing, unevenness and discontinuity.

Thereafter, a paste consisting of finely divided silver and finely divided glass dispersed in a suitable carrier medium (the silver being present e.g. in an amount of about 80% by weight of the paste) and having a viscosity of 25-35 poise is screen-printed onto two opposed edges 6, 7 of region 5 of the sheet of glass 2. A suitable paste is available from Englehard Ltd of Cinderford, Monmouthshire under the designation T190 30 X.

The screen-printed paste is then dried. After this drying step, there results a layer of dried paste having a resistance of about 3.5 milliohms/sq at 20 microns dry thickness.

The glass sheet 2 is then subjected to a heat-toughening and tempering process. This involves heating the glass sheet 2 to a temperature of e.g. 610-6150C for a period of, say, 5.5 minutes, and then rapidly cooling the glass to a temperature of less than 4750C over a period of 5 to 5.5 seconds. This process not only toughens the glass and renders it better able to withstand thermal stress but also a) increases the electrical resistivity of the electrically conductive coating and b) fuses the screenprinted strips to said opposed edges of region 5 to create electrical contacts 8, 9.

Heating of the glass to a temperature below 6100C does not normally produce sufficient toughening of the glass.

Heating of the glass to a temperature above 6150C may result in disintegration of the coating.

After the glass has cooled, metal electrical terminals (not shown) are soldered to the electrical contacts 8, 9. This soldering is preferably effected using a bismuth-based solder and an acid-free flux such as is obtainable from Fry's Metals of Birmingham, England. The advantage of such a solder and flux combination is that very little disturbance of the electrical contacts 8, 9 and their conductive connection to coating 2 occurs when the terminals are soldered to the contacts. It has been found in practice that it is preferred that the contacts/terminals connections have a breaking strain of approximately 35 lbs.

To upper surface of the sheet 2 a second sheet of glass 11 is bonded, using a suitable adhesive, preferably a flexible adhesive, such as aircraft grade silicone adhesive or mastic 12. The lower surface 13 of said second glass sheet 11 may be painted, decorated, printed, inked or coated to enhance the visual appearance of the hot plate 1, to emphasize that region or those regions of the hot plate 1 which become hot, to hide from view the conductive region 5, and/or to hide any ancillary devices which may be visible through the glass.

The painting etc on the lower surface 13 of the second sheet of glass 11 is preferably applied electrostatically as a powder coating to surface 13 and then adhered thereto by heating. Said second sheet of glass 11 is itself preferably toughened and tempered and, again, is preferably flat, smooth float glass. The juxtaposition of the respective very smooth, flat surfaces improves the performance of the resultant laminate 14.

The resultant glass sandwich or laminate 14 is connected through the terminals (not shown) of contacts 8, 9 to an electricity supply. This electricity supply may be, for example an A-C mains supply of single phase, 240 volt, 50 cycles per second frequency or 110 volts, 60 cycles per second.

A typical hot plate used for keeping food, etc. hot, may draw approximately 4 amperes of current and heat up to a temperature of 1800C or higher. Where the hot plate of this invention is used as cooking surface or hob, , the hot plate may be arranged to heat up to a temperature of 300or or greater. A temperature regulator or thermostat 15 may be affixed to surface 3, connected into the electricity supply to maintain a steady and predetermined temperature. Likewise, control means may be provided for varying the temperature achieved by the hot plate.

An overtemperature regulator 16 is preferably also present to shut off the electricity supply if the temperature of the glass exceeds a predetermined maximum. It has been found in practice that if the temperature of the glass exceeds approximately 3500C, the subsequent performance of the hot plate may be greatly impaired.

Referring now to Figure 3 of the accompanying drawings, there is shown a second embodiment of a hot plate according to the present invention.

It will be seen that this embodiment incorporates two sheets 2a, 2b of K-glass in accordance with the present invention. In other words, the hot plate of Fig. 3 has two separate ones of the hot plates 1 as described with reference to Figs 1 and 2 of the accompanying drawings.

To either side of the respective hot plates la, 1b and between them, are arranged glass sheets 20a, 20b, and 21. These glass sheets 20a, 20b, 21 are not coated with a conductive coating and are thus not capable of being directly heated. The assembly of glass sheet 20a, hot plate la, glass sheet 21, hot plate lb, and glass sheet 20b may be mounted within a frame, or may be bonded to a suitable subsurface. The arrangement of Fig. 3 is able to reduce the amount of distortion of the assembly which occurs when the hot plates are operational. It also creates less physical stress in the glass heating panels and can lead to increased efficiency of the selected heating areas. It is of course a simple matter to create an alternative configuration, for example, one with three or more separate hot plate surfaces.It may also dispense with the need to sand-blast away regions of the conductive coating.

According to a second aspect of the present invention there is provided an article which includes a hot plate as hereinbefore defined.

The hot plate of this invention may be incorporated into another article or may be provided with appendages such as legs, handles, etc to increase its practical utility.

Examples of articles into which the hot plate of the invention may be incorporated include tables, trolleys, ovens, cookers and hobs.

A table incorporating a hot plate according to the present invention may be of conventional height or may be of a height suitable to be used by one or more people sitting or lying on a floor surface. A trolley incorporating a hot plate according to the present invention may be provided with wheels or castors to increase its mobility and usefulness.

A table incorporating a hot plate according to the present invention may be arranged either to be a complete table in its own right, or to act as a table unit to co-operate with further table units to create a single larger table assembly having a larger hot plate area. The table units may be electrically connected together so that only a single source of electric current need be supplied to the table assembly.

A cooker or hob incorporating a hot plate according to the present invention may, as has been stated previously, may have an uppermost load-bearing or 'working' surface of glass, ceramic material, stainless steel, enamelled metal, etc.

Each table, trolley or other article may be provided with temperature indicator means, e.g. an LED or LCD temperature read-out (as indeed may be the hot plate itself of this invention).

It will be appreciated that the region 4 of the hot plate of regions 1 and 2 of Figs 1 and 2 is optional. It can be arranged that the entire surface 3 of the sheet of glass 2 be subjected to resistance heating. It has been found however that it is preferred to have a relatively cool peripheral region of the hot plate, particularly when it is incorporated into a table, so that persons utilizing the table do not burn themselves or become uncomfortably hot by contact with or adjacency to the heated surface of the hot plate.

It is preferred in practice that a hot plate or article according to the present invention be provided with a fuse or with a circuit breaker to disconnect the electricity supply in the event of a fault. A residual current circuit breaker may be used as well as, or instead of, a mains circuit breaker.

The present invention provides a hot plate which is efficient, which may be very thin in cross-section, and which may, when incorporated into an article, be of a most pleasing and attractive appearance. It offers a convenient and practical alternative to existing hot plates.

The present invention may be performed otherwise than as has been particularly described and the invention includes within its scope all modifications and improvements which would be apparent to one skilled in the art.

Claims

1 A hot plate for food, etc. which comprises glass coated with an electrically conductive material and adapted to be connected to an electricity supply, the material having an electrical resistance such that when an electric current is passed therethrough the glass will heat up.

2 A hot plate according to Claim 1, wherein said glass coated with an electrically conductive material comprises glass coated with a layer of tin oxide.

3 A hot plate according to Claim 2, wherein said layer of tin oxide is doped with fluoride.

4 A hot plate according to any one of the preceding Claims, wherein said glass coated with an electrically conductive material has an electrical resistivity of 16 ohm/square or more.

5 A hot plate according to any one of the preceding Claims, wherein electrical input means and electrical output means are fused to the electrically conductive material.

6 A hot plate according to Claim 5, wherein said electrical input and output means are each formed by applying a paste of finely divided precious metal and finely divided glass in a viscous matrix to the electrically conductive material, and fusing said paste thereto.

7. A hot plate according to Claim 6, wherein said paste applied to form the electrical input and output means includes finely divided silver.

8. A hot plate according to Claim 7, wherein the finely divided silver is present in said paste in an amount of approximately 80% by weight of the paste and in which the paste has a viscosity of from 25 to 35 poise.

9. A hot plate according to any one of Claims 6 to 8, wherein the paste is applied to the electrically conductive material by screen-printing.

10. A hot plate according to any one of the preceding Claims, wherein edges of said glass coated with an electrically conductive material are rendered very smooth, thereby to reduce the possibility of electrical arcing or unevenness or discontinuity of electrical supply.

11. A hot plate according to any one of the preceding Claims, wherein said glass coated with an electrically conductive material is treated to create one or more non-conductive areas on said coated glass surface.

12. A hot plate according to any one of the preceding Claims, wherein said glass coated with an electrically conductive material is provided with three or more electrical contacts and so arranged that different regions of said coated glass surface become heated to different temperatures and/or at different rates from each other when electricity is supplied thereto.

13. A hot plate according to any one of the preceding Claims, wherein there is bonded to an upper surface of said glass coated with an electrically conductive material a sheet of glass.

14. A hot plate, substantially as hereinbefore described with reference to Figures 1 and 2, or to Figure 3 of the accompanying drawings.

15. A table which incorporates a hot plate as claimed in any one of the preceding Claims.

16. A trolley which incorporates a hot plate as claimed in any one of Claims 1 to 15.

17. A cooker which incorporates a hot plate as claimed in any one of claims 1 to 15.

18. An oven which incorporates a hot plate as claimed in any one of claims 1 to 15.

19. A hob which incorporates a hot plate as claimed in any one of claims 1 to 15.

20. A method of making a hot plate which includes the steps smoothing the edges of glass coated with an electrically conductive material, applying to regions of the glass by screen-printing a paste comprising a finely divided precious metal and finely divided glass in a viscous matrix, heating the glass to a temperature such that the paste fuses to the electrically conductive material, and cooling the glass rapidly to toughen the glass.

21. A method according to Claim 20, wherein said glass has an electrically conductive coating of fluorine-doped tin oxide and wherein the glass is heated to a temperature of from 6100 to 6150.

22. A method according to Claim 21, wherein the glass is cooled from the temperature of from 6100 to 6150 to a temperature of less than 4750 in a time of from 5 to 5.5 seconds.

23. A method of making a hot plate substantially as hereinbefore described.

24. A hot plate produced by a method as claimed in any one of Claims 20 to 22.