GB2240884A - Microwave seal - Google Patents

Abstract

A microwave seal (10) for an oven comprises an outer coating of conductive silicone elastomer (18) positioned around a resilient stainless steel wire mesh central core (12). The seal is thus substantially opaque to microwave radiation, is resilient and is resistant to heat. Copper wires (14) and a knitted glass fibre yarn sheath (16) are interposed between the core (12) and the coating (18). <IMAGE>

Description

MICROWAVE SEAL This invention relates to a seal for use with electrical equipment to reduce or eliminate the passage of high frequency 300MHz to 18 GHz.

Microwave ovens operate by radiating food to be cooked with radio frequency energy in the microwave band referred to above. Egress of energy is prevented by metal shielding of the oven and door, e.g. by use of a Faraday cage.

However it is necessary to seal around the door opening so that microwave energy does not escape between the door and the opening. This is currently done employing flat strips of ferrite material. This is incompressable and therefore relys on accurate alignment to perform effectively, and also is only suitable for use at low temperatures.

Increasingly "combination" microwave ovens are becoming popular. The latter include some form of heating, for example infra-red heating, as well as microwave irradiation to give a desirable "browned" effect to the food. Such ovens get much hotter than microwave ovens and thus standard microwave seals cannot be employed. It has been proposed to use flat energy absorbant strips e.g.

conducttive high temperature materials such as silicone elastomer (which is also relatively incompressible) plus an additional seal for sealing in cooking vapours similar to a standard cooker seal.

The invention seeks to provide a seal which is substantially opaque to microwave radiation, is resilient, and is resistant to heat.

According to the present invention there is provided a seal for preventing the passage of microwave radiation which comprises a resilient mesh central core covered with a conductive silicone elastomer.

In order to make a silicone elastomer conductive it is necessary to load it with graphite and this adversely affects other physical properties, for example making it weak. The use of a resilient mesh acts as a support for the conductive polymer and enables it to withstand the repeated flexing of a door opening and closing.

Preferably the resilient mesh is a wire mesh, such as a stainless steel wire mesh.

The seal of the invention also preferebly incorporates a cover of heat resistant yarn, for example glass fibre yarn, ceramic yarn, or the like. The latter improves the mechanical strength of the silicone polymer and is preferably located immediately below it over the resilient mesh. The polymer is not chemically bonded to the heat resistant fibre but is physically keyed thereto. The heat resistant yarn layer may be braided or knitted, preferably the latter.

Additionally, the seal of the invention may contain extra conductors, for example a conductive wire or wires located above the mesh and below the silicone elastomer. Copper wires have been found to be particularly useful. These alter the characteristics of the seal in relation to microwave radiation, giving more absorption and less reflection of radiation. The latter is particularly important when used with microwave catering ovens of high power, ie greater than 750 watts.

While the seal of the invention is particularly useful in connection with microwave ovens, especially combination microwave ovens, it is also useful in a wide variety of other end-uses desired to block the passage of microwave radiation. For example it may be used in connection with communication equipment whre its primary use is to stop radiation getting in rather than getting out.

The invention will be described further, by way of example, with reference to the accompanying drawing, the sole figure of which is a partly cut away diagramatic view of a seal in accordance with the invention.

Referring to the drawing, a seal generally designated 10 in generally tubular form comprises a central stainless steel wire mesh core 12 having supported on it, successively, a plurality of copper wires 14, a knitted glass fibre yarn sheath 16 and finally a coating of conductive silicone elastomer 18.

The stainless steel wire mesh 12 supports the other layers and provides the necessary resilience to stand repeated compressions without deterioration. The copper wire increases the absorption of microwaves as opposed to reflection. The glass fibre sheath 16 proveds strength for the outer coating of silicone elastomer 18 which is physically "keyed" thereto. The silicone elastomer is loaded with graphite to render it conductive.

The seal, besides it high frequency characteristics, is capable of withstanding repeated compressions without deterioration and can be operated at temperatures up to 2500C.

A typical application of the seal is in a microwave combination oven to prevent excessive leakage of energy, particularly at the fundamental, third and fifth harmonic frequencies, and to act as a vapour seal. The performance of the seal at the third and fifth harmonic frequencies is consider to be of particular importance because of the growing concern in telecommunication circles of the interference effects of these frequencies generated by the use of microwave ovens.

The durability of the seal of the invention in a typical combination appliance is in excess of 200,000 door operations.

Claims (9)

1. A seal for preventing the passage of microwave radiation which comprises a resilient mesh central core covered with a conducting silicone elastomer.

2. A seal according to Claim 1 further comprises a heat resistant yarn.

3. A seal according to Claim 2 wherein the heat resistant yarn comprises ceramic yarn.

4. A seal according to Claim 2 or 3 wherein the yarn is located immediately below the silicone elastomer and is physically keyed thereto.

5. A seal according to any one of Claims 2 to 4 wherein the yarn is knitted.

6. A seal according to any one of the preceding claims further comprising extra conductors located above the mesh and below the silicone elastomer.

7. A seal according to Claim 6 wherein the extra conductors comprise conducting wires.

8. A seal according to Claim 7 wherein the wires are formed from copper.

9. A seal substantially as hereinbefore described with reference to the accompanying drawings.