The present invention concerns a tubular heating element, for example, a cartridge or armoured resistor, and a method for its production.

In more detail, such a heating element includes a hollow casing, within which there is at least one resistive member, in particular a resistive wire, and an insulation and filling material, and from which at least one terminal for the electrical supply of the heating element projects through a seal.

The materials used to form the seal must exhibit high electrical and thermal resistance, be highly adhesive to the metals from which the casing and terminals are usually formed, and highly impermeable. Epoxy, polyurethane and silicone resins are currently used for this purpose. These materials are able to resist temperatures of the order of 200°C, rising to approximately 250°C for silicone resins with, however, a significant reduction in the capacity to adhere to the metals. These temperatures are therefore the maximum temperatures to which the seals of known heating elements can be taken, in particular where they are of large dimensions, which significantly limits their range of use and detrimentally imposes constraints on their production process.

Ceramic materials, in particular magnesium oxide, which have the desired characteristics of electrical insulation at high temperatures, thermal conductivity and low cost, but which are at ^' the same time very hygroscopic, are usually used for the insulation and filling materials. During the production process it is necessary to dry the heating element completely by bringing it to a temperature of approximately 300°C to remove any traces of moisture in the filling material which would compromise its insulating properties.

However, the materials which have until now been used for making the seals cannot withstand such high temperatures. The known heating elements are therefore dried before the resins intended to form the seals are introduced, which takes place at a temperature of approximately 200°C or less at which the resins polymerise. In fact, the resins would carbonise if these temperatures were exceeded, causing short circuits and, in any case, polymerisation would occur too rapidly, which could lead to the premature sealing of the heating element with, consequently, the incomplete removal of moisture therefrom.

In the known production methods, there is therefore a time interval between the stages of drying and sealing, during which traces of moisture are able tc re-enter the heating element, thereby compromising the insulation

property of the filling material, and leading to leakage of curren .

The object of the present invention is therefore that of providing a heating element in which the material intended to form the aforesaid seal is able to resist temperatures significantly higher than those mentioned above.

This object is achieved by a heating element of the type indicated at the start of the present description and characterised in that the material from which the said seal iε made is of the vitreous-ceramic type and includes the following components in the percentages by weight indicated:

Si0 ₂ from 5 to 40%

PbO from 30 to 85%, and

B ₂ 0 ₃ from 4 to 25% .

The seals of the heating element according to the invention are able to withstand temperatures of approximately 380°C with an end point at 400°C, while at the time maintaining their good adhesion to the metals and their stability regardless of the dimensions of the heating element. By contrast, the resins used until now to form this type of seal tend, after a certain period of use or storage, to become detached from the contiguous metal walls, allowing moisture to enter the respective

heating elements.

The good characteristics of the seals of the invention remain unchanged even following thermal shock and repeated and rapid heating/cooling cycles, and in the presence of considerable expansion, provided that the aforesaid temperatures are not exceeded, which temperatures, in any case, correspond to significantly greater temperatures of other parts of the heating element, in particular the resistive wire.

Even if the intended operating temperatures were exceeded, the seals of the invention, made from inorganic material, would always simply melt and return to the solid state when the cause of the increase in temperature ceases, without becoming carbonised and causing short circuits, as happens with the known resins.

Preferably, the material from which the seal of the element of the invention is formed also includes, as well as the components mentioned above, Al ₂ 0 ₃ in a percentage by weight of between 0 and 5% and/or ZnO in a percentage by weight of between 0 and 10%.

The total of the percentages by weight cf the various components mentioned above may be less than 100%, in particular it may be as low as 90%, being made up to 100% by various impurities such as, for example, oxides of

iron, sodium and potassium.

A further object of the present invention is a method for the production of a tubular heating element of the type indicated at the start of the present description, characterised in that it includes the steps of:

- disposing vitreous-ceramic material intended to form the seal, in the form of a solution, a powder, granules or pellets, in the region where the at least one terminal projects from the casing into which the insulation and filling material and the resistive wire have already been introduced;

- subjecting the heating element to a first heating stage at a temperature of between 50°C and 350°C whereby to dry it substantially completely;

- subjecting the heating element, without a break after the first stage, to a second heating stage at a temperature of between 400°C and 750°C to cause the fusion of the said vitreous-ceramic material; and

- allowing the heating element to cool such that the said vitreous-ceramic material solidifies, forming the seal .

Preferably, the first heating stage is effected at a temperature of between 200°C and 300°C, and the second heating stage is effected at a temperature of between 500°C and 750°C.

An essential characteristic of the method of the invention is the fact that the first and second heating stages are consecutive without discontinuity, and with a rising temperature profile. Therefore, the moisture, which is eliminated completely during the first stage, is unable to enter the heating element which is immediately sealed in the second stage.

The heating elements of the invention can therefore be used immediately without first having to be: preheated to eliminate traces of moisture which could otherwise cause damaging leakage currents .

Further advantages and characteristics of the present invention will become clearer from the following detailed description, given purely by way of non-limitative example and with reference to the accompanying drawing, in which: the single figure is a schematic representation of an end of a heating element according to the invention.

A tubular heating element 10, in particular a cartridge resistor, comprises a hollow casing 12, within which there is a resistive member (in particular a wire, not visible in the drawing) , and an insulation and filling material 14, for example, magnesium oxide. First and second terminals 18 for the electrical supply of the resistive wire and/or sensors which may be present in the

element 10 project from one end of the casing 12 through a seal 16.

The material from which the seal 16 is formed is of the vitreous-ceramic type, and one example of its composition is as follows:

Si0 ₂ 21.2%

PbO 66.2%

B ₂ 0 ₃ 8.8%

A1 ₂ 0 ₃ 0.9%

ZnO 2.05% impurities 0.85%.

A method for the production of the tubular heating element 10 comprises an initial stage, in which the vitreous-ceramic material intended to form the seal 16 and in the form of a solution, a powder, granules or pellets, is disposed in the form of a plug in the region where the terminals 18 project from the casing 12. The insulation and filling material 14 and the resistive wire have already been introduced into the casing 12.

The heating element 10 is then subjected to a first heating stage in an oven at a temperature of between 50°C and 350°C, so as to cause it to be substantially completely dried.

The heating element 10 is then immediately subjected,

without a break after the first stage, to a second heating stage in the oven at a temperature of between 400°C and 750 °C, and preferably of between 500°C and 750°C, so as to cause the fusion of the vitreous-ceramic material .

The duration of the two heating stages depends on the dimensions of the heating element 10. For example, for a diameter of 20 mm, the first stage can last from between 0.5 and 72 hours and the second stage from 3 minutes to 60 minutes, with a transition period of between 30 seconds and 60 minutes necessary for the transition between the operating temperatures of the two stages.

Finally, the heating element 10 is allowed to cool, such that the vitreous-ceramic material solidifies completely in the temperature interval between 510 ^C 'C and 450°C, thereby forming the seal 16.

The solidified material adheres perfectly to the adjacent metal surfaces of the terminals 18 and the casing 12, rising up them by some millimetres. The seal 16 is grey in colour, with a smooth and homogenous appearance and a hard and glassy consistency.

Moisture is therefore irreversibly excluded from the heating element 10, which is able to withstand operating

temperatures of approximately 380°C, with peaks of 400°C, without any of its constituents, in particular the seal 16, being affected.

Naturally, the principle of the invention remaining the same, the details of construction and the embodiments may be fully varied with respect to that described and illustrated by way of example, without departing from the ambit of the invention.

In particular, a heating element according to the invention may be an armoured resistor, in which the first and second terminals project from opposite ends of the casing, at each of which there is formed a respective seal using methods and materials similar to those described above.