AU708707B2 - Hotplate with a casting and method for its manufacture - Google Patents

Abstract

The electric cooking plate for a cooker appliance is a casting, with the cooking surface (12) wholly coated with an enamel layer (26). Also claimed is a mfg. process, where, after the heating elements (17) have been fitted, the enamel coating material is sprayed onto the cooking surface. The enamel is fired at 1000-1100 K, pref. 1050 K.

Description

P:\OPER\KAT\42165-96.125 7/5/99 -1A- This invention relates to an electric hotplate.

Electric hotplates with a casting and an upper, usually circular cooking surface surrounding a central zone may be coated with a graphite-like appearing coloured coating, which loses at the time of being put into operation for the first time its not heat-resistant binder fractions.

It gives the surface a uniform appearance and to a limited extent has a corrosion-preventing action.

According to the present invention there is provided an electric hotplate with a hotplate body of cast material, which comprises an upper cooking surface, an underside, which is heated by electric heating elements and a enamel coating characterized, in that the cooking surface, and downwardly directed outer rim are coated with the enamel coating whereas an underside of the hotplate body, which comprises slots, which receive electric heating elements in an electrically insulating embedding surface, does not comprise an enamel coating.

Hotplates constructed in accordance with the invention may be cast, and may be significantly S'protected against corrosion and can be well adapted to the kitchen design.

a S*o* Generally the enamelling of hotplates has not been carried out, because the hotplates operate with contact heat transfer and the enamel coating was considered to be prejudicial and also *::subject to breakage and heat cracks. However, it has been found that the feared disadvantages do not occur to a significant extent and that the advantages preponderate. A hotplate may be obtained with a corrosion-resistant, acid-proof, hard and scratch-resistant surface with a very good long-term behaviour. The coating may be physiologically 25 unobjectionable. Unlike in the case of the hitherto known hotplate surfaces when put into operation for the first time formation of odours or smoke generation may be reduced. In addition, unlike in the case of the previously known exclusively anthracite-coloured coatings, an almost unlimited choice of colours can be made, so that there is a wide spectrum of uses for this very reliable, user-friendly hotplate, also where special kitchen designs are involved.

P:\OPER\KAT42165 96.125 7/5/99 -2- In order to achieve a very uniform and complete enamelling, in the enamelled area the hotplate body may have only rounded edges. The cooking surface and outer rim can be enamel coated, whereas the underside of the hotplate, where the heating body embedding takes place and surrounded by the outer rim may undergo enamelling action.

The enamel coating thickness, may be below 250 Itm, preferably approximately 100 Am. A part is played by two factors, on the one hand the different expansion coefficients of the materials of the casting and the coating, as well as the heat conduction of the coating, in order to ensure an adequate contact heat transfer. Due to the high thermal shock loading in the value range of the material expansions of the casting and the enamel coating, the latter should always remain sufficiently elastic to follow the expansions of the material without splitting.

This may be ensured by an adequately thin skin. This also has the positive effect that the coating does not impede heat emission and consequently has less negative influence on the cooking system efficiency.

.i The use of a hotplate is also aided by the fact that cooking vessels, which cannot always be oprecisely adapted to the cooking surface shape, stand so reliably on the hotplate that they do e:eoo S not slip or turn e.g. when stirring within such a vessel. It is therefore appropriate to give the surface of the enamel coating roughness values Rz (according to DIn 4768) between 10 and 40 Im, preferably between 15 and 25 Itm. This roughness range may ensure that the enamel °..*.coating is not too smooth and offers a certain frictional resistance, but is not too rough, so as to impede heat transfer and easy cleaning.

In one method for the manufacture of hotplates, in accordance with the invention, enamel 25 coating-forming material is applied by spraying following the fitting of the heating elements and is then baked at temperatures between 1000 and 1100 K and preferably at approximately 1050 K. In particular, baking may take place together with a drying or sintering of the embedding substance for the heating elements of the hotplate. This offers the advantage that the in any case necessary heat treatment of the hotplate is used in order to attain TF--0 corresponding baking temperatures for the enamel coating. Whereas hitherto hotplates have i .4 s ;3 P \OPER\KAT\4216.5-96.125 7/5/99 -3been heat treated for a much longer time, it may be now possible for this heat treatment process, which also treats the embedding substance, to be reduced to between 10 and minutes, e.g. 15 minutes.

A conventional spillage rim in the form of a sheet metal ring surrounding the hotplate body can be fitted with press fit, preferably following baking. This may ensure that the enamel coating below it is closed, so that freedom from corrosion may also be ensured in this area.

These and further features can be gathered from the claims, description and drawings and the individual features, both singly and in the form of sub-combinations, can be implemented in an embodiment of the invention and in other fields and represent advantageous, independently protectable constructions for which protection is hereby claimed.

BRIEF DESCRIPTION OF THE DRAWING The single drawing is a vertical section through a hotplate body.

o DESCRIPTION OF THE EMBODIMENT 20 The drawing shows a hotplate body 11 of an electric hotplate. It is made from iron-based cast material and has the basic shape of a substantially disk-like, usually circular plate with an :upper cooking surface 12, which can be constructed as a planar, circular area around a somewhat lowered central zone 13. It ends at the outside in a rounded, projecting marginal flange 14, to which is connected at the bottom with a step a substantially cylindrical, downwardly projecting outer rim The outer rim 15 surrounds the underside 16 of the hotplate, which in the circular, heated area, which at the top forms the cooking surface 12, carries an electric heating means 17 in the form of helical heating resistors inserted in spiral slots 18 on the underside 16 of the 30 hotplate body. They are surrounded by a compressed or compacted embedding substance P:\OPER\KAT\42165-96.125 7/5/99 -3Awhich ensures the electrical insulation and simultaneously provides a good heat transfer to the hotplate body.

The slots 18 interrupted by relatively thin ribs 19 terminate at a smaller inner rim 20, which surrounds the downwardly curved central zone 13. In the middle is provided a pin 21 with a reception bore for the fastening of the hotplate.

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4 Whereas most surfaces remain rough from casting on the underside, the top of the hotplate is surfaced in order to give it a planar form. The same applies regarding the central zone 13, the flange 14 and the upper part 22 of the outer rim 14, which is constructed for receiving a spillage rim 23. The latter is supported on the underside of the marginal flange 14, is located in press fit on the substantially cylindrical surface 22 and has a sloping area, e.g. bevelled by 45 to the outside. With the downwardly directed outer edge it is normally located on the rim of an installation opening of a not shown hob.

The entire outer surface, i.e. the area of the cooking surface 12, the central zone 13, the marginal flange 14 and the outer rim 15 is coated with an enamel coating 26, which can have a coating thickness of 250 um, preferably about 100 jm. Through a corresponding treatment and/or choice of material, it is constructed in such a way that it has roughness values RZ according to DIN 4768 between 10 and 40 pm, preferably between 15 and 25 pm. As a function of the different expansion values of the enamel and cast metal, whilst also taking account of the different temperatures occurring therein, it is possible for said values to vary. Preference is given to an enamel, whose expansion behaviour is as close as possible to that of the casting. The enamel can be manufactured in different colours, optionally also with different shading in different areas of the hotplate, e.g. with a differently coloured, lowered central zone.

MANUFACTURE

In its basic form the hotplate body is made from an iron cast material. The underside with the spirally directed slots and very narrow intermediate ribs 19 is manufactured in very precise manner and requires no reworking. Follow- •go ing the surfacing of the cooking surface 13 and the turning of the remaining surfaces 13, 14 and 22, the hotplate is provided with the heating means. For eoe• this purpose in the slots 18 is placed a mineral bulk material embedding substance 25 and the heater coils 17 are introduced and then the material is Scompacted somewhat. The projecting connecting pins 24 are used for electrical S connection purposes.

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go e P:\OPER\KAT\42165-96.125 7/5199 The outer surface (cooking surface 12, central zone 13, flange area 14 and outside of the outer rim 15) are then coated by spraying with a basic enamel material. The hotplate then undergoes a heat treatment at a temperature of 1000 to 1100 K (preferably 1050 The heat treatment, which lasts between 10 and 20 minutes, preferably 15 minutes, can either be in the form of an individual baking process, or preferably takes place simultaneously for drying and/or sintering the embedding substance 25 in the slots 18. Therefore the enamel coating of the surface can be readily integrated into the manufacturing process without additional method steps.

Following baking, the stainless steel sheet metal spillage rim 23 with its substantially cylinder ring-shaped inner edge is pressed onto the outside of the outer rim 25. Its top side then engages on the lower edge of the outer flange 14.

The hotplate is then assembled, by fitting the electrical terminals and a lower, not shown cover plate, which covers the underside of the hotplate.

Throughout this specification and the claims which follow, unless the context requires •otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps I 20 but not the exclusion of any other integer or step or group of integers or steps.

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P.\OPER\KAT\42165-96,125 7/5/99 -6- THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:- 1. Electric hotplate with a hotplate body of cast material, which comprises an upper cooking surface, an underside, which is heated by electric heating elements and a enamel coating characterized, in that the cooking surface, and downwardly directed outer rim are coated with the enamel coating whereas an underside of the hotplate body, which comprises slots, which receive electric heating elements in an electrically insulating embedding surface, does not comprise an enamel coating.

2. Hotplate according to claim 1, wherein the cooking surface has a central zone, which is depressed compared with the cooking surface.

3. Hotplate according to claim 1 or claim 2, characterized in that, in the enamelled area, the hotplate body only has rounded edges.

4. Hotplate according to claim 1, claim 2, or claim 3, characterized in that the enamel coating thickness is below 250 Alm and is preferably approximately 100 Itm.

5. Hotplate according to one of the preceding claims, characterized in that the surface of the enamel coating has roughness values of Rz (according to DIN 4768) of 10 to 40 /xm, preferably 15 to 25 /m.

6. Method for the manufacture of a hotplate according to one of the preceding claims, characterized in that the material forming the enamel coating is applied by spraying, 25 preferably after the fitting of the heating element, and is then baked at 1000 to 1100 K, preferably at approximately 1050 K.

7. Method according to claim 6, characterized in that the baking takes place together with a drying and sintering of the embedding surface for the hotplate heating elements.

Claims (2)

1. 8. Method according to claim 6 or 7, characterized in that, following baking, a spillage rim in the form of a sheet metal ring surrounding the hotplate body is applied over the enamel coating, preferably with press fit. DATED this 7th day of May, 1999 E.G.O. ELEKTRO-GERATE BLANC UND FISCHER GMBH CO., KG. By its Patent Attorneys Davies Collison Cave a 9* a a. a a. S S
2. 9. SS C a S. S S S. 9 a a S a S. a *SSSSa a ABSTRACT The hotplate body of a cast iron material, electric hotplate is coated in the vicinity of its upper cooking surface (12) and the central zone (13) therein, as well as in the outer area of the outer rim with an enamel coating The enamel is relatively thin and has on its surface roughnesses in the range 10 to 40 um, in order to ensure an adequate frictional contact with the cooking vessel. Manufacture takes place by spraying the basic enamel material after introd- ucing the heating elements (17) into the embedding substance This is followed by a baking process and the subsequent fitting of a spillage rim (23). (cf. single drawing) t 4 B 0 e f* f* 0* OBO B~ 0* rB