EP1296539A1 - Method of producing heating elements - Google Patents

Abstract

Fabrication of heating element (1) from metal tube (2) consists of assembling resistance wire (4) and two connection terminals (6) inside tube, filling tube with magnesia (8), followed by compacting and high temperature firing operation. During cooling, the end (18) of the tube is plugged by resin (10) in contact with the magnesia. The resin is then partially polymerized leaving resin temporarily in intermediate gel form. Fabrication of a heating element (1) from a metal tube (2) consists of putting together an assembly of a resistance wire (4) and two connection terminals (6) inside the tube, filling the tube with magnesia (8), followed by a compacting operation and a high temperature firing operation. After firing and during cooling, the end (18) of the tube is plugged by a resin (10) in contact with the magnesia. The resin is then polymerized prior to the shaping of the tube, The polymerization is left incomplete and so leaves the resin temporarily in an intermediate gel form.

Description

The present invention relates to the field of heating elements able to equip household appliances and more particularly to their manufacturing.

The heating elements targeted by the present invention are commonly called "shielded" heating elements, their structure having a metal tube inside which is housed a resistant wire surrounded by magnesia. The load of these elements is quite significant, since it can exceed 30 W / cm ² . Such elements are frequent in household appliances such as kettles, electric ovens, grills, ...

The present invention relates in particular to heating elements, the tube is metallic. Although many manufacturing processes exist for the realization of such elements, they all have in common the realization of the metal tube, filling said tube with a resistant wire and magnesia, then passing through an annealing oven. They are then shaped before capping operations.

• fabrication du tube,
• introduction de l'ensemble composé du fil résistant et des bornes de connexion, puis remplissage de magnésie,
• compression de l'ensemble,
• recuit de l'ensemble pour enlever l'humidité de la magnésie,
• si nécessaire : mise à longueur exacte du tube et redressage,
• formage du tube selon l'appareil destinataire (forme arquée,...)
• étuvage dans un four (car la magnésie a repris de l'humidité), pendant 60 à 90 min,
• imprégnation de résine aux extrémités de la résistance,
• pose manuelle des perles céramiques de bouchage aux extrémités,
• polymérisation de la résine, pendant 90 min,
• opération de formage final à la presse.

Un des inconvénients majeurs d'un tel procédé est, entre autres, la longueur du cycle de fabrication, notamment par les étapes thermiques et le refroidissement consécutif. Par exemple, les résistances, après leur passage dans le four de recuit, présentent une température élevée. Elles doivent donc être stockées dans une zone de refroidissement avant les étapes suivantes.Such a process can thus be broken down into the following steps:

• tube manufacturing,
• introduction of the assembly consisting of the resistant wire and the connection terminals, then filling with magnesia,
• compression of the assembly,
• annealing the assembly to remove the moisture from the magnesia,
• if necessary: exact length of the tube and straightening,
• tube forming according to the recipient device (arched shape, ...)
• steaming in an oven (because magnesia has regained moisture), for 60 to 90 min,
• resin impregnation at the ends of the resistor,
• manual placement of ceramic plug beads at the ends,
• polymerization of the resin, for 90 min,
• final press forming operation.

One of the major drawbacks of such a process is, inter alia, the length of the manufacturing cycle, in particular by the thermal steps and the subsequent cooling. For example, the resistors, after passing through the annealing furnace, have a high temperature. They must therefore be stored in a cooling zone before the following steps.

Furthermore, magnesia being very hydrophilic, as long as it is not dehydrated and then isolated from the outside environment, stages of desiccation are required.

Furthermore, in such a process, the resin, introduced after the setting resistance form, seals the pearl and seals the resistance. It must therefore fill all the space between magnesia and the pearl ceramic, which translates, in practice, into a necessary overdose of resin which overflows when the ceramic bead is put in place, so to ensure that the resin is in good contact with said pearl. Such overflows can lead to manufacturing rejects or even heating element malfunctions completed.

The present invention aims to reduce the time required for manufacturing heating elements as previously described. This reduction in manufacturing time results in reduced production costs.

A simplification of the stages and especially of their number must be able reduce handling of elements and hidden time work which requires space and generates large amounts outstanding.

The present invention is achieved using a manufacturing process heating element from a straight metal tube, said method including the steps of setting up an assembly made up of a wire resistant and two connection terminals inside said tube, both terminals opening at each end of said tube, the latter being filled with magnesia, these steps being followed by a compacting operation followed by a annealing at high temperature in an oven, characterized in that, following this annealing step, during the tube cooling phase, is carried out a step of plugging at least one of the ends of the tube using a resin in contact with magnesia, said resin being subsequently polymerized, and this prior to any shaping of said tube, the polymerization of the resin being incomplete and temporarily leaving the resin in one step gel intermediate.

By carrying out, immediately after the annealing phase of the tubes, the plugging of the elements with an appropriate resin, magnesia, after this drying step, is isolated from the outside environment and cannot regain moisture during cooling. No other cycles thermal to eliminate the recovery of moisture by magnesia is then necessary, which greatly reduces the manufacturing time of heating elements.

Furthermore, by plugging fairly early in the manufacturing process, and this definitely, the ends of the tubes, we avoid any loss of magnesia, which, in addition to causing unnecessary losses of material, dirty the various production lines on which tubes pass during their transformation.

In addition, leaving the resin in an intermediate state between the phase liquid or semi-liquid and the solid phase after complete polymerization, we allows the heating elements to be shaped according to the configuration desired without risk of breaking the polymerized resin.

This state of freezing for a sufficiently long time after their exit from the oven is not a natural state of resins. Most of them, in fact, rapidly polymerize and further form the resistance, that whether forming, stretching or sizing results in breakage of the end of said resistance. It may therefore seem prohibitive to want plug the heating elements with a resin before any shaping for obvious reasons of breakage in said resin as soon as a shaping step: bending, stretching, U-shaped, ... is implemented.

However, after numerous laboratory tests, it turned out possible to select, essentially by experience, a resin which, brought to a certain temperature for a certain time, has a state gel, allowing to combine, both its tube plugging properties, since the resin does not flow out of the tube, while temporarily keeping a flexibility allowing it to follow mechanical stresses during different tube shapes.

Admittedly, the polymerization being initiated, it will irretrievably continue, but by choosing correctly, not only the resin, but also the polymerization parameters it is possible to obtain rapidly a state of gel persisting at least during the subsequent stages to transform the tube into a heating element ready to be mounted on a sub-assembly heating.

Another advantage in the course of this process compared to art prior resides in the fact that the deposited resin is no longer used to ensure, at the both plugging and sealing the end bead but is only used for its function of impregnating and sealing magnesia. from then, a volumetric deposition of resin is quite possible, which makes the easily automated resin deposition and polymerization operation.

According to the invention, the resin used is a resin of the type methylphenylsiloxane, which has, in addition to good sealing properties, a fairly simple implementation, the hardening curve can be adjusted by a catalyst. Furthermore, it can polymerize in layers thick without bubble formation during polymerization.

According to a preferred implementation of the invention, the temperature of polymerization is between 160 and 220 ° C.

Preferably, the polymerization time is between 5 and 15 min.

Furthermore, the present invention is also characterized in that the manufacturing process comprises, after plugging at least one of the ends of the tube, a step of closing said end with a flexible silicone sleeve self-tightening on the terminal linked to the resistant wire, said end of the tube then being constricted on the flexible sleeve to ensure sealing against projections of said closure.

This sealing step is necessary for optimum operation heating elements and has the advantage of a purely maintenance mechanical of the sleeve in the tube. Its automated implementation is thus easier. A relatively easily automated solution is all the more interesting since the blockage phase of magnesia can be also, as previously described.

The main difference with the processes of the prior art is therefore plug, by depositing a resin, the straight resistances taken out of the annealing, and this, before their shaping. Keeping the resin in the "semi" state polymerized ", the resistance can be transformed without deterioration clogging. In addition, the removal of a sleeve, blocked by narrowing of the tube on said sleeve, allows the automation of all stages of the proceeded with an excellent mastery of the different stages and therefore of the final product quality.

The present invention also relates to a heating element comprising a metal tube inside which is housed an assembly composed of a wire resistant, surrounded by magnesia, and two connection terminals leading to each end of the element, the tube being blocked, at least at one of its ends, by a polymerized sealing resin, in contact with the magnesia, characterized in that it comprises, at said end, a sleeve flexible sealing silicone, threaded onto the terminals, said end of the tube being mechanically constricted on said sleeve.

The combination of sealing resin and flexible sleeve shutter held mechanically by the tube guarantees sealing and passage normative of the heating elements thus constituted, while presenting a significant longevity and solidity.

Advantageously, the flexible silicone sleeve is self-tightening on the connection terminal, which simplifies the manufacturing procedure by facilitating the automation of its removal, while making the final product more reliable by ensuring watertightness at the connection between the terminal and the sleeve flexible.

Advantageously, a space is provided between the sleeve and the resin. This material-free zone between the sleeve and the resin overcomes dimensional differences linked in particular to the tolerances of the different components as well as mechanical rolling and straightening of the heating elements, either during their manufacturing cycle, or later when using them. This space is variable between 0 and 2 mm and will preferably be at least 1 mm.

Another aspect of the present invention also relates to an element heater with a stainless steel tube inside which is housed a set composed of a resistant wire, surrounded by magnesia, and two terminals of connection opening at both ends of the tube, at least one of ends of said tube being closed, vis-à-vis the outside, by a sleeve silicone, characterized in that it comprises a single sealing resin and impregnating magnesia, said resin being of the type methylphenylsiloxane.

Impregnation and sealing of magnesia with a single resin reduces manufacturing costs while ensuring quality increased clogging by avoiding several components.

Other features and advantages of the present invention will become apparent and will emerge in more detail on reading the following description, detailing a non-limiting example of a heating element manufacturing process according to the present invention, with reference to Figure 1 attached having, in section, a completed heating element, ready to be mounted in a sub-assembly heating.

1 ○ fabrication du tube inox 2, droit, coupé à longueur,

2 ○ introduction de l'ensemble composé du fil résistant 4 et des bornes 6, soudées au fil 4, puis remplissage du tube de magnésie 8,

3 ○ recuit de l'ensemble dans un four à passage atteignant 1000°C afin de déshydrater la magnésie. Le temps de cycle comprenant la préchauffe, le recuit et un refroidissement contrôlé des pièces est de l'ordre d'une heure.

4 ○ à la sortie du four, dépôt des tubes sur un support vertical où les extrémités des tubes sont maintenues à 180°C pour leur bouchage par une résine. Cette opération, réalisée pour chaque extrémité, comprend :

• l'étape de dépose calibrée de résine 10 à l'aide d'un pistolet volumétrique délivrant 0,05 g ± 10% de produit. La quantité de produit est déterminée pour que la résine imprègne la magnésie 8 sur une distance d, tout en présentant un surplus de résine 14 entre la magnésie 8 et l'extrémité du tube 2 qui agit tel un bouchon. Toutefois, ce surplus n'atteint pas l'extrémité du tube. La zone d'imprégnation 12 est, dans le cas de figure présenté, voisine de 10 mm.
• l'étape de polymérisation partielle de la résine à une température de 180 °C pendant 10 min. La résine se présente alors sous la forme d'un gel avec une croûte superficielle 11. Cette première phase de transformation de la résine est suffisante pour garantir le bouchage des tubes.

5 ○ évacuation des tubes sur un support et refroidissement jusqu'à sensiblement la température ambiante. Ils sont alors prêts à recevoir, après éventuellement une phase d'étirage, des manchons silicone sur une machine automatisée.

6 ○ pose des manchons silicone 20 qui sont de petits tubes souples venant en coulissement serré sur les bornes rigides 6.

7 ○ sertissage de l'extrémité du tube à l'aide d'un outil conique venant pincer l'extrémité 18 du tube 2 pour la rabattre sur le manchon souple afin de l'immobiliser. Par ailleurs, la souplesse du silicone permet une grande latitude de serrage, ledit manchon se déformant plus ou moins selon la contrainte mécanique générée. La machine de sertissage automatique permet de positionner précisément le manchon 20 par rapport à l'extrémité 18 du tube 2 permettant de laisser, entre la résine 10 et le manchon 20, un espace 16 qui évitera d'éventuelles interactions entre la résine et le manchon lors des opérations suivantes de formage.Il est à noter que le sertissage sur le manchon souple permet également d'assurer une étanchéité aux projections (de poussières, de fluides,..) de la partie terminale de l'élément chauffant ainsi constitué.

8 ○ après un éventuel redressage des éléments chauffants afin de les rendre rectiligne, les éléments sont mis en forme ou arqués. Par l'expression "élément arqué", on entend tout élément présentant au moins un rayon de formage, par opposition aux éléments linéaires. Il peut donc s'agir d'élément de forme simple en U, présentant un seul rayon de formage, ou plus complexe, en double U, en spirales, en torsades,... Durant toutes les étapes de mise en forme du matériau où le tube métallique est sollicité mécaniquement, que ce soit par étirage, redressage ou formage, la résine, toujours sous forme de gel, peut suivre les mouvements imposés au tube sans désagrégation de sa structure.

9 ○ après contrôle électrique, les éléments chauffants sont soudés sur les sous-ensemble chauffants qui leur sont destinés, et testés.

Thus, an example of a method for manufacturing a heating element 1 according to the present invention can be described by the following steps:

1 ○ manufacture of stainless steel tube 2, straight, cut to length,

2 ○ introduction of the assembly composed of the resistant wire 4 and the terminals 6, soldered to the wire 4, then filling of the magnesia tube 8,

3 ○ annealing of the assembly in a passage oven reaching 1000 ° C. in order to dehydrate the magnesia. The cycle time including preheating, annealing and controlled cooling of the parts is of the order of one hour.

4 ○ at the exit of the oven, deposit the tubes on a vertical support where the ends of the tubes are maintained at 180 ° C for their plugging with a resin. This operation, performed for each end, includes:

• the calibrated resin removal step 10 using a volumetric gun delivering 0.05 g ± 10% of product. The quantity of product is determined so that the resin impregnates the magnesia 8 over a distance d, while presenting a surplus of resin 14 between the magnesia 8 and the end of the tube 2 which acts as a plug. However, this surplus does not reach the end of the tube. The impregnation area 12 is, in the case shown, close to 10 mm.
• the step of partial polymerization of the resin at a temperature of 180 ° C for 10 min. The resin is then in the form of a gel with a surface crust 11. This first phase of transformation of the resin is sufficient to guarantee plugging of the tubes.

5 ○ evacuation of the tubes on a support and cooling to appreciably ambient temperature. They are then ready to receive, after possibly a stretching phase, silicone sleeves on an automated machine.

6 ○ install silicone sleeves 20 which are small flexible tubes coming in tight sliding on the rigid terminals 6.

7 ○ crimping the end of the tube using a conical tool pinching the end 18 of the tube 2 to fold it over the flexible sleeve in order to immobilize it. Furthermore, the flexibility of the silicone allows great latitude for tightening, said sleeve deforming more or less depending on the mechanical stress generated. The automatic crimping machine makes it possible to precisely position the sleeve 20 relative to the end 18 of the tube 2 making it possible to leave, between the resin 10 and the sleeve 20, a space 16 which will avoid possible interactions between the resin and the sleeve during the following forming operations. It should be noted that crimping on the flexible sleeve also makes it possible to seal the projections (of dust, fluids, etc.) of the terminal part of the heating element thus formed.

8 ○ after a possible straightening of the heating elements in order to make them rectilinear, the elements are shaped or arched. The expression "arched element" means any element having at least one forming radius, as opposed to linear elements. It can therefore be a simple U-shaped element, having a single forming radius, or more complex, a double U, in spirals, in twists, ... During all the stages of shaping of the material where the metal tube is mechanically stressed, whether by stretching, straightening or forming, the resin, always in the form of a gel, can follow the movements imposed on the tube without disintegrating its structure.

9 ○ after electrical control, the heating elements are welded to the heating sub-assembly intended for them, and tested.

Overall, the process takes about an hour compared to more than 6 hours according to the prior art.

It should be noted that the step of setting up the resistant wire and the terminals may have variants, the two terminals may or may not already be soldered on the wire before introduction into the tube. The present invention covers also the cases where the resistant wire is introduced into the tube and where at least one of the terminals is soldered after this operation.

The polymerization time, very short compared to more processes conventional, is due to the incomplete transformation of the resin. This transformation is completed, either over time (approximately one year) if the element heater is not used, either very quickly (a few tens of minutes) as soon as the heating element is used for the first time, since it is then brought to high temperature.

Thus, by placing the resin as soon as it leaves the annealing furnace, and by not only partially polymerizing said resin, it is possible to simplify notably the stages of the manufacturing process of heating elements in stainless steel, saving time which immediately results in a reduction significant costs and stocks.

Furthermore, by proposing steps that can be automated, we also reduces handling costs while improving the quality of heating elements produced.

The present invention is not limited to stainless steel tubes, but relates to also all metal tubes.

Claims (10)

Method for manufacturing a heating element (1) from a straight metal tube (2), said method comprising the steps of placing an assembly composed of a resistant wire (4) and two connection terminals (6) inside said tube (2), the two terminals (6) opening at each end of said tube (2), the latter being filled with magnesia (8), these steps being followed by a compacting operation followed high temperature annealing in an oven, characterized in that , following this annealing step, during the tube cooling phase, a step of plugging at least one of the ends (18) is carried out ) of the tube using a resin (10) in contact with magnesia (8), said resin (10) being subsequently polymerized, and this prior to any shaping of said tube (2), the polymerization of the resin (10) being incomplete and temporarily leaving the resin (10) in an intermediate stage of g el. A method of manufacturing a heating element (1) according to one of the preceding claims, characterized in that the resin (10) used is a resin of the methylphenylsiloxane type. Method for manufacturing a heating element (1) according to one of the preceding claims, characterized in that the polymerization temperature of the resin (10) is between 160 and 220 ° C. Method for manufacturing a heating element (1) according to one of the preceding claims, characterized in that the polymerization time of the resin (10) is between 5 and 15 min. Method for manufacturing a heating element (1) according to one of the preceding claims, characterized in that , after plugging at least one of the ends (18) of the tube (2), a step d closing said end (18) with a flexible silicone sleeve (20) self-tightening on the terminal (6) linked to the resistant wire (4), said end (18) of the tube (2) then being constricted on the flexible sleeve (20) in order to seal the projections of said closure. Heating element (1) for a household appliance, characterized in that it is produced using a manufacturing process according to one of claims 1 to 5. Heating element (1) comprising a metal tube (2) inside which is housed an assembly composed of a resistant wire (4), surrounded by magnesia (8), and two connection terminals (6) opening at each end (18) of the element (1), the tube (2) being blocked, at least at one of its ends (18), by a polymerized sealing resin (10), in contact with magnesia ( 8), characterized in that it comprises, at said end (18), a flexible silicone sleeve (20) for sealing, fitted over the terminals (6), said end (18) of the tube (2) being mechanically constricted on said sleeve (20), and in that a space is provided between the sleeve (20) and the resin (10). Heating element (1) according to the preceding claim, characterized in that the flexible silicone sleeve (20) is self-tightening on the connection terminal (6). Heating element (1) according to claim 7, characterized in that the space between the sleeve (20) and the resin (10) is at least 1 mm. Heating element (1) comprising a stainless steel tube (2) inside which is housed an assembly composed of a resistant wire (4), surrounded by magnesia (8), and two connection terminals (6) opening out to the two ends (18) of the tube (2), at least one of the ends (18) of said tube (2) being closed, vis-à-vis the outside, by a silicone sleeve (20), characterized in that that it comprises a single resin (10) for plugging and impregnating magnesia (8), said resin (10) being of methylphenylsiloxane type.

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