GB2079119A

GB2079119A - Vitreous cooking hob

Info

Publication number: GB2079119A
Application number: GB8019668A
Authority: GB
Original assignee: BFG Glassgroup GIE
Current assignee: BFG Glassgroup GIE
Priority date: 1980-06-17
Filing date: 1980-06-17
Publication date: 1982-01-13

Abstract

A vitreous cooking hob comprises a heat resistant glass sheet 10, preferably of tempered glass, which covers at least one induction in heating element 12 e.g. comprising three coils 14, 15, 16. The sheet 10 may also carry capacitive touch control switch electrodes 0-6, 8 and 9 for controlling operation of the heating element 12. <IMAGE>

Description

SPECIFICATION Vitreous cooking hob The present invention relates to a vitreous cooking hob incorporating at least one heating element.

Vitreous cooking hobs are well known and have the advantage of being very easy to clean when compared with gas hobs or the conventional type of exposed element electric hobs.

Hitherto, such hobs have been made including one or more electrical resistance heaters from which heat is conducted through vitreous sheet to a cooking pan placed on top of the hob. It will therefore be recognised that the thermal conductivity of the vitreous sheet should be as high as possible. Thus such vitreous sheets have hitherto always been made from a vitro-ceramic material. In general, the vitro-ceramics used for this purpose have higher thermal conductivities than other vitreous materials such as glass, but they are also more expensive to produce.

The present invention is based on a recognition that by a suitable choice of heating element, the thermal conductivity of the sheet material becomes less critical and that it is therefore possible to manufacture hobs using a less expensive vitreous material, namely glass.

According to the present invention, there is provided a vitreous cooking hob incorporating at least one heating element characterised in that such hob comprises a heat-resistant glass sheet which covers at least one induction heating element. By virtue of the selection of a heating element which operates on the induction principle, that is, a heating element which directly induces heat in an electrically conductive body which lies within its field of influence, the need for any interposed body, in this case the vitreous sheet, to be thermally conductive is eliminated. Thus a glass sheet can be used and this has cost advantages over the previously used vitro-ceramic sheets.

The glass used for the sheet is of a heatresistant type to increase its resistance to breakage.

In preferred embodiments of the invention, said glass sheet is tempered. This increases the resistance of the sheet to mechanical and thermal shock. In some embodiments of the invention said glass sheet is chemically tempered. High compressive surface stresses and thus great toughening can be achieved by chemical tempering. Chemical tempering of glass is of course well known per se. The most usual chemical tempering treatments involve an exchange of alkali metal ions from a contacting medium with alkali metal ions from the glass while the glass is heated. The ionic interchange takes place in surface layers of the glass which are a few microns or a few tens of microns in thickness and which are thus brought under compressive stress.

In the most preferred embodiments of the invention, however, said glass sheet is thermally tempered. In the thermal tempering of glass, the glass is brought to a temperature close to its softening point (where its viscosity is 107.6 poises) and its surfaces are rapidly cooled so that when the inner regions of the glass sheet cool, the surfaces are forced to contract so that they are brought under compressive stress. These stressed surface layers usually extend up to about one sixth of the thickness of the sheet of glass, and they are accordingly much thicker than the stressed surface layers produced by chemical tempering. Thermal tempering is preferred because the glass sheet is apt to become scratched in use and such scratches are less likely to penetrate the full thickness of thermally stressed surface layers of a tempered glass sheet.

When the glass sheet used in the invention is tempered, it is possible to use soda lime glass for forming the hob. Soda lime glass is inexpensive and widely available, and when thermally or chemically tempered it has adequate resistance to thermal shock.

In some preferred embodiments of the invention, said glass sheet is a borosilicate glass sheet. The borosilicate glass used preferably has a coefficient of linear expansion of not more than 4 X 10-6 per C. Borosilicate glasses generally, and especially those with such a low coefficient of expansion, are resistant to breakage by thermal shock, and this resistance and their resistance to mechanical shock can be augmented by a tempering, especially a thermal tempering treatment.

In other preferred embodiments of the invention, said glass sheet is an alumino-silicate glass sheet. Such an alumino-silicate glass preferebly has a coefficient of linear expansion of not more than 3 x 10 6 per "C.

Alumino-silicate glasses are also resistant to thermal shock.

The hob is preferably opaque for aesthetic reasons. It is especially preferred that such opacity be brought about by providing said glass sheet with an opaque enamel coating.

Of course, such enamel must be able to resist the temperatures to which it will be subjected during use. Generally speaking, it is sufficient if the enamel is resistant to temperatures up to 300"C. If tempered glass is used for the hob, opacifying enamel is preferably applied after tempering.

Such an opaque enamel coating is almost certain to be less wear resistant than the tempered glass sheet, and it is accordingly desirable that it should be located between the glass sheet and the heating elements.

Suitable enamels are available (among other sources) from Rhone Poulenc and from Emerson 8 Cuming Inc. Among suitable enamel forming resins from Rhone Poulenc may be cited silicone-polyester resins (e.g.

catalogue No. 11302) and methyl-phenyl-sili cone resins (e.g. catalogue Nos. 10336 and 1505). Among suitable enamel forming resins from Emerson 8 Cuming inc. is that sold under the trade name SPYCAST resin No.

35DA. These resins are charged with suitable quantities of one or more pigments to form organic-based opacifying enamels. It is gener ally preferred to use an organic enamel rather than a vitreous enamel when the enamel is applied after the sheet has been tempered, since the relatively high temperature which would be required to fuse a vitreous enamel could well allow stress relaxation in the glass sheet thus at least partly defeating the object of tempering it in the first place. It will be appreciated however that it is quite possible to apply a vitreous enamel to an untempered glass sheet, to heat the coated sheet to vitrify the enamel and then to cool the coated sheet under such conditions that the coated sheet is tempered.

If the hob is opaque, it is preferred that said glass sheet is marked to indicate the position of the or each heating element, whereby such position is visible from above the hob. This facilitates the location of a cooking utensil directly in register with a said heating element. Such marking can be provided very easily by etching the upper surface of the sheet.

In some embodiments of the invention, the or each induction heating element rests in bedding material supported by a frame of the hob. This has the advantage of allowing sim ple replacement of one or more heating elements or indeed of the glass sheet should this be desired for any reason.

In other embodiments, the or each said heating element is attached to the glass sheet by a heat resistant glue such as that sold by Rhone Poulenc under the trade mark RHO DERSIL type CAF 538 (thixotropic). This has the advantage of enabling such bedding material to be dispensed with so that the hob frame can be of simple construction.

Preferably said glass sheet has deposited on it one or more capacitive touch control switches for controlling operation of the or each heating element.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a section through a part of a hob according to the invention; and Figure 2 is a plan view of the part of the hob shown in Fig. 1.

In the drawings, a glass sheet 10 is etched with a pattern of markings 11 to indicate a position for a heating element 1 2 (Fig. 1) in the finished hob. The glass sheet 10 is then tempered, for example by immersing it in a bath of molten potassium nitrate at 470"C.

for a sufficient period to allow potassium ions to diffuse into surface layers of the glass sheet. The glass sheet 10 is then provided with an opaque enamel coating 13.

A number of electrode plates each constituted by a conductive coating are applied to the coated and tempered glass sheet. In Fig.

2, seven such plates indicated by reference numerals 0 to 6 are provided on the upper surface of the glass sheet 1 0. These plates constitute touchable capacitor plates of a touch control switching system for controlling the heat output of the heating element 12 The plates 0 to 6 are surrounded by a coating, 7 also on the upper surface of the glass sheet' forming part of a shielding capacitor for the switching capacitors. This coating 7 may be extended to a metal frame for the hob (not shown) so that it is thereby connected to earth. These coatings 0 to 6 and 7 on the upper surface of the glass sheet 10 are suitably formed of SnO2 to which a doping agent has been added. The tin oxide coatings may be applied to the sheet in a single step either before or after deposition of the opaque enamel coating 1 3.

On the underside of the glass sheet, on top of the coating 1 3 there are deposited further electrode plates 17, 8 and 9, suitably formed of conductive (for example silver-containing) enamel. The enamel coating 1 7 on the underside of the glass sheet 10 is in register with the tin oxide coating 7 on the upper surface of the sheet and is also earthed to shield the switching system. A pair 8, 9 of mutually spaced electrodes is deposited in register with each of the electrodes 0 to 6. One of the electrodes 8, 9 of each pair is attached to an AC input source. In fact the AC input electrodes may be constituted by a single continuous bar of conductive enamel. The other electrodes on the underside of the glass sheet 10 are each connected to an output circuit which controls the switching of the heating element 12.

In the embodiment illustrated, the heating element 12 comprises three individually energisable induction coils 14, 15, 16. By way of example only, the innermost coil 14 may be arranged to give a heat output of 200 watts, the intermediate coil 1 5 may be arranged to give a heat output of 400 watts and the outermost coil 1 6 may be arranged to give a heat output of 600 watts.

In such a case, it would be suitable to arrange the electrodes 0 to 6 so that when electrode 0 was touched the heating element 1 2 was totally de-energised, while touching switch electrode 1 would energise the innermost coil 14, switch electrode 2 the intermediate coil 15, switch electrode 3 both the innermost coil 14 and the intermediate coil 15, switch electrode 4 the innermost and outermost coils 14 and 16, switch electrode 5 the intermediate and outermost coils 1 5 and 16, and touching switch electrode 6 would energise all three induction coils.

There now follow various Examples of glass sheet hobs for use in performance of the invention.

Example 1 A sheet of soda lime glass 5 mm in thickness is taken and cut to form a panel measuring 596 mm by 490 mm and its edges are ground to remove sharpness.

The glass has the following composition (% by weight) SiO2 71 to 73 Na2O 12 to 14 CaO 10 to 12 MgO 1 to 4 A1203 0.5 to 1.5 The glass sheet is then etched by serigraphically applying a paste containing HF to provide a pattern of markings such as those shown at 1 in the drawing to provide an indication for the position of each of four separate heating units for the panel. After etching, the glass sheet is thermally tempered. In a variant it is chemically tempered by immersing it in a bath of molten potassium nitrate at 470"C. for a sufficient period to allow sodium-potassium ion exchange between surface layers of the glass sheet and the bath.The glass sheet which has been tempered is then provided with an opaque enamel coating comprising pigment in a matrix of Rhone Poulenc's silicone-polyester resin catalogue No. 11 302. Four separate induction heating units are bonded to the tempered glass panel on its enamel coated side and in register with the four patterns of markings, and the panel is then framed to form a hob having a surface which is very easy to clean and is highly resistant to breakage whether by thermal or mechanical shock.

In a variant of this example, the heating units are not bonded to the glass sheet.

Instead they rest in cradles forming part of the frame.

Example 2 A 6 mm thick sheet of borosilicate glass is cut to form a panel 596 mm by 490 mm, and its edges are ground to remove sharpness. The glass has a coefficient of linear expansion of 2.0 c 10-6 per "C. over the temperature range 0 to 300"C. and is of the following composition (% by weight) SiO2 81.1% Na2O 3.0% B203 13.5% A1203 2.2% + impurities 0.2% The glass sheet is etched in known manner to provide a pattern of markings to indicate the positions of four independent heating units, and an opaque enamel layer is applied to one surface of the sheet. After such opacification the four heating units are bonded to the enamel layer of the panel in register with the marked positions and the unit is then framed to form a hob.

The untempered borosilicate glass sheet provides a surface which is easy to clean and which is resistant to thermal shock by virtue of its low coefficient of expansion and its Young's modulus about (6.8 x 10'0 Pa).

Example 3 A 5 mm thick sheet of borosilicate glass is cut to form a panel 490 mm by 750 mm and its edges are ground to remove sharpness.

The glass has a coefficient of linear expansion of 5.5 X 10-6 per "C. over the temperature range 20"C. to 550"C., Young's modulus 7.4 x 1010 Pa and is of the following composition (%by weight) SiO2 72.6% B2O3 12.38% Na2O 6.60% K20 0.47% A1203 1.08% CaO 4.00% MgO 2.58% + impurities 0.29% The glass is etched to provide a pattern of markings to indicate positions for four independent heating units within an end portion of the panel measuring 490 m by 600 mm, so that the other end portion 490 m by 1 50 mm is left free.

The glass sheet is then thermally tempered.

In a variant the glass is tempered by immersing it for a period of 40 hours in a bath of molten potassium nitrate maintained at 470"C.

A plurality of conductive electrodes e.g. of SnO2 are then applied to the upper surface of the 1 50 mm wide free end portion of the panel in order to constitute touch electrodes of capacitive switching systems for selectively energising each of the heating units to be incorporated within the hob, and conductive enamel coatings are formed on the underside of the glass sheet all as described with reference to the drawings.

The sheet is then coated on one side with an opaque enamel coating which will form the underside of the panel in use. Thus enamel coating is formed from Emerson s Cumings Inc.'s SPYCAST resin No. 35DA (Trade Mark) charged with pigment.

The tempered glass sheet, the heating units and their associated circuitry are then assembled and framed to form the hob.

Example 4 A 6 mm glass sheet is cut to form a panel 490 mm by 596 mm and its edges are ground to remove sharpness. The sheet is of an alumino-silicate glass having a coefficient of linear expansion of 4.6 X 10-6 per "C. and Young's modulus 8.75 X 1010 Pa and has the following composition (%by weight) SiO2 57% A1203 15% B203 5% MgO 7% CaO 10% BaO 6% This glass sheet is then etched in known manner and thermally tempered, and is then given an opacifying enamel layer. Heating units are then bonded to the sheet in register with markings provided by the etching to form the hob.

In a variant, the etching and opacifying steps are dispensed with.

Example 5 A glass sheet of suitable area and thickness is made using glass of the following composition (% by weight) SiO2 55% A1203 21% B203 12.5% MgO 7% CaO 3.5% This alumino-silicate glass has a coefficient of linear expansion of about 2 X 10-6 per "C.

and its Young's modulus is approximately 8.75 X 10'0 Pa.

The glass sheet is then laid in a frame to cover heating units supported by the frame to form a framed hob unit.

In a variant the heating units are bonded to the glass sheet.

In a second variant, the glass sheet is etched to mark positions for the heating units and is opacified.

In a third variant, the heating unit position markings are printed onto the glass sheet serigraphically using an enamel paste and these indications are then covered with an opacifying enamel. The sheet is obviously used enamel side down.

Claims

1. A vitreous cooking hob incorporating at least one heating element, characterised in that such hob comprises a heat resistant glass sheet which covers at least one induction heating element.

2. A hob according to Claim 1, characterised in that said glass sheet is tempered.

3. A hob according to Claim 2, characterised in that said glass sheet is thermally tempered.

4. A hob according to Claim 2 or 3, characterised in that said glass sheet is a soda' lime glass sheet.

5. A hob according to any of Claims 1 to 3, characterised in that said glass sheet is a borosilicate glass sheet.

6. A hob according to Claim 5, characterised in that said borosilicate glass has a coefficient of linear expansion of not more than 4X 10-6 per "C.

7. A hob according to any of Claims 1 to 3, characterised in that said glass sheet is an alumino-silicate glass sheet.

8. A hob according to Claim 7, characterised in that said alumino-silicate glass has a coefficient of linear expansion of not more than 3X 10-6 per "C.

9. A hob according to any preceding claim, characterised in that said hob is opaque.

10. A hob according to Claim 9, characterised in that said glass sheet bears an opaque enamel coating between the glass sheet and the heating elements.

11. A hob according to Claim 9 or 10, characterised in that said glass sheet is marked to indicate the positions of the or each heating element, whereby such position is visible from above the hob.

12. A hob according to Claim 11, characterised in that the upper surface of the sheet is etched to provide such markings.

1 3. A hob according to any preceding claim, characterised in that the or each heating element is attached to the glass sheet by a heat resistant glue.

14. A hob according to any preceding claim, characterised in that said glass sheet has deposited on it one or more capacitive touch control switches for controlling operation of the or each heating element,.

15. A cooking hob incorporating a glass cover sheet and substantially as herein defined.