Abstract:
A heat generating element for an electrical heating device of a motor vehicle, including at least one PTC element and two contact plates which are in contact with opposite sides of the PTC element and are provided on their outer side facing away from the PTC element with an insulating layer. Shielding is provided, comprising a first shell-like shielding plate and a second shell-like shielding plate which are each provided on the outer side of the insulating layer and are connected thereto in a thermally conductive manner. The longitudinal wall sections, directed towards one another, of the first and second shielding plate form an overlapping region. In order to reduce EMC problems, two shielding plates are sealed by way of an adhesive which is received in one of the two shielding plates and into which the end of a rim of the other shielding plate dips.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a heat generating element, in particular for an electric heating device of a motor vehicle, preferably with a positioning frame forming a holding fixture in which at least one PTC element is received, and two contact plates which are in contact with opposite sides of the PTC element. 
         [0003]    2. Discussion of the Related Art 
         [0004]    Such heat generating elements are generally known as a component of an electric heating device, in particular for heating air in a motor vehicle, which is also to be further developed with the present invention. The heat generating element forms a layer of a layered structure which commonly comprises corrugated fm elements that are in contact with both sides of the heat generating element. This layered structure commonly has at least a plurality of layers of corrugated fin elements and heat generating elements stacked in a plane. 
         [0005]    A generic heat generating element is known from EP 2 190 256 A1. In this prior art, corrugated fins that are on the outside in contact with the heat generating element are adhesively bonded with the heat generating element. For this, curing insulating adhesive, such as silicone, is introduced into free spaces within the positioning frame and fills the remaining free spaces, so that the interior of the positioning frame is to be sealed hermetically by the insulating cured adhesive against the environment. The curing adhesive also reaches the phase boundary between the heat generating element and the corrugated fin elements, so that these corrugated fin elements are after curing of the adhesive connected to the heat generating element. The result is a self-contained heating rod that can as such be handled and placed or inserted into an electrical heating device. 
         [0006]    The proposed solution according to EP 2 190 256 A1 has certain drawbacks. For example, heat expansion occurs during operation of the heat generating element. Due to the different nature of the components constituting the heat generating element, different thermal expansions arise. The cured insulating adhesive can not reliably compensate the different expansion values Permanent cracking occurs, so that the desired sealing effect by the cured adhesive for the heat generating element is lost. 
         [0007]    Electrical heating devices comprising a heat generating element of the generic kind sometimes cause electromagnetic interference which are undesirable in particular in a motor vehicle as they are noticeable acoustically, for example, when operating a radio in the vehicle interior. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is based in particular on a heat generating element as is known from EP 2 395 295 A1. This prior art with its embodiment according to  FIGS. 12 to 14  shows a heat generating element for an electrical heating device of a motor vehicle with two shell-like shielding plates, where the shell-like shielding plates in this prior art have a substantially U-shaped cross section. The shell-like shielding plates, however, form only an enclosure on the rim side like a tube that is open at the face sides. Total shielding is only achieved by end pieces that are at the end sides mounted onto the free ends of the tube. The two half shells can according to the disclosure of EP 2 395 295 A1 be formed of metal and after joining be connected by adhesive bonding. 
         [0009]    The present invention is based on the problem of providing a heat generating element, in particular for an electrical heating device for a motor vehicle, which takes into account the EMC problems in an improved manner and securely insulates against the environment and protects from the ingress of environmental influences. Furthermore, the present invention is to provide a method for producing such a heat generating element. 
         [0010]    To solve the problem according to the device, the present invention proposes a heat generating element including at least one PTC element and two contact plates which are in contact with opposite sides of said PTC element and are provided on their outer side facing away from said PTC element with an insulating layer. Shielding is provided comprising a first shell-like shielding plate and a second shell-like shielding plate which are each provided on the outer side of said insulating layer and are connected thereto in a thermally conductive manner. Longitudinal wall sections, directed towards one another, of said first and second shielding plate form an overlapping region. The two shielding plates are sealed by way of an adhesive which is received in one of said two shielding plates and into which the end of a rim of said other shielding plate dips. This heat generating element thus comprises shielding which shields against the electromagnetic radiation caused by the heat generating element and its operation. Accordingly, the electrically conductive contact plates as well as the PTC element of the heat generating element are encased by the shield. The shielding is not in direct electrical contact with the current-carrying elements of the heat generating element, i.e. the contact plates and the least one PTC element. The plates forming the shielding are accordingly commonly electrically insulated from the current-carrying elements. 
         [0011]    Though more than two shielding plates can be provided, which are commonly overall connected electrically to each other, i.e. are connected to each other in an electrically conductive manner, commonly only two shielding plates are used for reasons of manufacture. 
         [0012]    In view of good dissipation and hence a high degree of efficiency of the PTC element, the first and the second shielding plates are in a thermally conductive manner connected to the insulating layer, which in turn is commonly in direct contact with the associated contact plate. This therefore results in a preferably layered structure with the PTC elements located on the inside, contact plates which are in contact with opposite sides thereof, and insulating layers that are in turn in direct contact with their outer side. These insulating layers can be formed from ceramic and/or plastic film. Particularly preferred is a film made of polyimide or silicone. The layered structure in turn has the first and the second shielding plates on its outer side. The first and the second shielding plates there have an at least shell-like configuration, so that the joined shielding plates form a tube possibly still open at the end sides which can be closed by end caps further completing the shielding at the face sides. The shielding plates, however, preferably have a pan-shaped configuration with a substantially circumferentially encircling rim which also on the edge side overlaps the commonly elongated heat generating element comprising several PTC elements arranged in the longitudinal direction one behind the other. The bent rim of the two pan-shaped shielding plates accordingly reaches over part of the positioning frame. 
         [0013]    The two shielding plates are preferably configured such that they have an overlapping region in which the shielding plates are each provided on the rim immediately adjacent to each other, contacting each other, and in two layers. Pan-shaped shielding plates result in an encircling overlapping region in the circumferential direction. With shell-like configurations, the overlapping region arises on the opposite longitudinal sides. A pan-shaped configuration is a shell-like configuration with rims provided on the face side, i.e. at oppositely disposed ends of the tube. In the overlapping region, the shielding plates commonly electrically contact each other, so that total shielding results for the PTC element and the two contact plates in contact therewith. Only a few contact points for contacting the two shielding plate can be provided in the overlapping region. In the case of a joint area, they preferably directly adjoin each other along a large portion of their face-side length. In an overlapping region, the shielding plates reach across each other in the vertical direction. Due to the pan-shaped configuration, both shielding plates comprise longitudinal wall sections extending substantially in the vertical direction of the PTC element. The longitudinal wall sections are formed circumferentially encircling and preferably only experience a through hole where contact pins are from the outside led up to the contact plates to electrically connect the heat generating element. In the overlapping region, the longitudinal wall section of one of the shielding plates, which is hereinafter also referred to as a lower shielding plate, extends outwardly and accordingly forms a holding fixture for the longitudinal wall section of the other shielding plate. In this overlapping region, the shielding plates commonly contact each other. The free end of the outwardly extending longitudinal wall section preferably has an outwardly formed funnel-like configuration, so that the positioning frame and also the other regularly second shielding plate can be easily introduced into the lower first shielding plate. 
         [0014]    Furthermore according to the invention, the shielding plates are sealed by adhesive in the overlapping region. This curing adhesive seals at least the first shielding plate against the second shielding plate. Due to the desired contact of the two shielding plates and in view of a secure seal, a major portion, preferably all curing adhesive is located in the interior of the shielding and seals the free end or ends of the shielding plates. The adhesive is preferably silicone adhesive exhibiting certain elasticity and, moreover, good electrically insulating properties. The adhesive is due to its elasticity preferably introduced for filling a particular chamber volume such that the adhesive does not cure as a thin film, but that a bead-like sealing strip is formed around in the circumferential direction by the adhesive. 
         [0015]    This bead-like sealing strip of adhesive, which can also be referred to as a bead of adhesive, provides a certain volume into which one rim of the shielding plate commonly provided on the inside dips and thereby seals the volume provided within the shielding against the atmosphere. The rim of the shielding plate dipping into the adhesive is there commonly located also on the inner wall of the outer shell-like shielding plate. In view of a very good and reliable seal, however, the front end of the rim of the inner shielding plate can be bent inwardly and thereby both bear against the inner circumference of the encircling rim in an electrically conductive manner and on the outer circumference be dipped securely into the bead of adhesive. 
         [0016]    For forming a suitably large chamber volume, the circumferential outer rim is adapted to a preferably provided positioning frame with at least one holding fixture for the at least one PTC element. Typically, the positioning frame is fitted at the free space defined between the positioning frame and the shield. The positioning frame is preferably configured such that it projects backwardly in part or totally at the outer rim, thereby forming a chamber in which the adhesive can be introduced with the required volume. 
         [0017]    In view to the best possible creepage and air distances, which are of significance in particular during operation of the electrical heating device under high-voltage conditions, for example, in an electric vehicle, it is to be ensured that the electrified parts of the heat generating element are adequate spaced from the non-electrified and insulating parts. The cured adhesive preferably there contacts no exposed surfaces of the contact plates and the PTC element. Exposed surfaces of the PTC elements are, in particular, the overlapping rims of the contact plates and the face sides of the PTC element which extend between the two contact plates. Also the face-side rim surfaces of the contact plates are usually exposed. For this, the positioning frame regularly has a stepped structure which is notched such that the contact plates with their free rim extending over the PTC element or elements, respectively, indeed cover part of the positioning frame, but do not contact the positioning frame or do not bear thereagainst at the face side, respectively. One or both insulating layers can be connected to the positioning frame with adhesive, in particular with curing adhesive. One insulating layer can also, for example, by insert-molding the plastic forming the positioning frame, be directly connected thereto. The contact plates can be either simply placed onto the associated insulating layers or additionally adhesively bonded thereto. Also by solely placing the layers of the beat generating element, sufficient positioning of the contact plates and the PTC elements relative to the positioning frames can be achieved when applied compression stress is maintained by the connection of the two shielding plates upon the layered structure located within the shielding plates. 
         [0018]    For optimum arrangement of the adhesive in a chamber between the face-side rim of the positioning frame and the first and the second shielding plate such that the adhesive bonds at least these three elements and possibly another one or two insulating layers with each other, the outer rim of the positioning frame is preferably specially configured and adapted to the shielding plate or plates, respectively. According to a preferred development, the chamber is at an underside defined substantially only by a rim web formed integrally at the positioning frame. In width direction, the positioning frame is dimensioned such that rim webs being in this width direction oppositely disposed are fitted into the shell element of the lower, i.e. the first shielding plate. The clearance width in the interior of the shielding plate at the level of the rim webs accordingly corresponds substantially to the outer dimension of the rim web. This leaves at most a small gap between the face side of the rim web and the inner surface formed by the longitudinal wall section of the first shielding plate. However, the gap is dimensioned such that this adhesive can not or only to a small extent exit the chamber through the gap during the expected compression of the curing adhesive when the shielding is closed. The rim section is there usually located at the bottom rim of the positioning frame. On the surface of the rim section, commonly the insulating layer associated with the underside also bears on the positioning frame and the rim web. This configuration results in a chamber located within the face side of the rim web and defined by the remaining height of the positioning frame. The rim web can have a height corresponding to one quarter to one sixth of the height of the positioning frame. Above the rim web, the positioning frame may form an inclined surface extending inwardly defining the rim web with an upper end of the positioning frame. 
         [0019]    The above description was given in the width direction, i.e. in the perspective of a cross-sectional view through the elongated heat generating element. A corresponding configuration should preferably also be seen in a longitudinal sectional view. Accordingly, the circumferentially encircling rim web of the positioning frame is circumferentially fitted into the pan-shaped first shielding plate in the manner described above, whereby the chamber is above the rim web defined circumferentially towards the bottom. 
         [0020]    If the first longitudinal wall section encompasses a second longitudinal wall section of the second shielding plate in the overlapping region on the outside, the second shielding plate defines the chamber on the upper side, possibly an insulating layer bearing thereagainst on the inside, so that curing adhesive located in the chamber there bonds the two shielding plates to each other and seals the second shielding plate encompassed by the first shielding plate. If the shielding plates are each formed as being substantially circumferentially enclosed shells, this results in secure shielding of all electrically conductive and energized elements within the shield. 
         [0021]    A method for producing a heat generating element of the aforementioned kind is also proposed with the present invention. A shielding plate is during production first formed having a shell-like configuration. This shielding plate has a circumferentially encircling longitudinal wall. A first lower insulating layer and a lower contact plate commonly in direct contact therewith are first introduced into this shell. Above the contact plate, the PTC element or elements, respectively, comes to bear against the lower contact plate, where a positioning frame surrounds the PTC elements on the outside, where commonly at least one positioning frame forming a holding fixture for at least one PTC element is likewise introduced into the shell. A second contact plate is placed on the opposite side of the PTC element. 
         [0022]    The positioning frame is commonly at least together with the contact plates and the PTC element prepared as a preassembled unit, i.e. is introduced as such into the shell-like first shielding plate. The components can there be adhesively bonded to the shielding element and/or the first insulating layer in order to position it. Liquid adhesive is subsequently introduced between a rim of the first shielding plate and the positioning frame. A second shielding plate is in the method according to the invention then, with interposition of a second insulating layer, placed onto the further contact plate in such a manner that the rim of the second shielding plate enters into the liquid adhesive. The latter solidifies, so that the rim of the second contact plate is sealed by the solidified liquid adhesive. 
         [0023]    When the rim enters the liquid adhesive, the latter is usually applied pressure and is displaced. Any voids within the bead of adhesive applied are filled in line with this compression. The liquid adhesive is commonly filled in as a circumferential encircling bead of adhesive into the space between the positioning frame and the rim of the first shielding plate. After the liquid adhesive has cured, the region provided within the shielding is hermetically sealed. 
         [0024]    The amount of liquid adhesive is preferably adjusted and filled in between the positioning frame and the first shielding plate such that the positioning frame is entirely sealed, without the liquid adhesive contacting any of the contact plates and/or the PTC element. These elements can therefore compensate thermal expansions during heating or cooling, respectively, without directly contacting the shielding or directly contacting parts that are in contact therewith. The adhesive is preferably filled in having a volume which is less than the chamber volume. It is thereby prevented that, when the adhesive is displaced by the rim of the further contact plate, [sic] is displaced to the extent that the adhesive reaches the PTC element and/or the contact plates. 
         [0025]    The positioning frame commonly forms connection pieces integrally formed thereon, which extend beyond the shielding on the outside and form channels into which contact pins can be inserted in a sealing manner. It is with this measure possible to contact the heat generating element in the interior while simultaneously maintaining the electrically conductive parts of the heat generating element sealed. Said connecting pieces can at their outer periphery also comprise a seal that allows the heat generating element to be inserted into a corresponding connection housing in a sealed manner, for example, a control housing of an electrical heating device. 
         [0026]    Because according to a preferred embodiment of the present invention, the PTC element and/or the contact plates that are in contact therewith are positioned relative to the positioning frames such that at least a respective air gap remains between the exposed surfaces of the contact plates and the PTC element on the one hand, and the positioning frame on the other hand This air gap is interrupted only at certain points at the phase boundary between the PTC element and the positioning frame by support points which are commonly formed by the positioning frame, which basically extend linearly, i.e. at right angles and project beyond the walls of the positioning frame defining the holding fixture and support the at least one PTC element at the outer circumference. The support points act in a punctiform manner against the face side of the PTC element or elements, respectively. 
         [0027]    The present invention further proposes a method for producing a heat generating element that is simplified in terms of production engineering. This heat generating element has a PTC element and two contact plates that are in contact therewith on opposite sides. The heat generating element is received in an encasing. This encasing can be the shielding as previously discussed. The encasing, however, need not necessarily be provided electrically insulating against the contact plates and/or have shielding properties. The encasing is in any case sealed by adhesive. This sealing can be done, for example, by connecting shell elements. An area is in the method according to the invention sealed which serves to introduce and/or close a housing of the encasing. The housing can also be a tube in which the PTC element and one of the contact plates are supported. In this case, the other contact plate can be formed by a wall section of the circumferentially closed tube. It is according to the invention proposed to form the adhesive as being thermosetting and to cure it with heat that is generated by energizing the heat generating element. Accordingly, the present method dispenses with passing the mounted heat generating element provided with the adhesive through a heating furnace. It is merely necessary to energize the prepared heat generating element. An operational test can here simultaneously be performed, thereby reducing and simplifying the production steps necessary for producing the heat generating element. 
         [0028]    In order to ensure good through heating, the shielding plates have a plate thickness of commonly 0.3 mm and less, thereby improving adhesive curing under heat. 
         [0029]    The adhesive is preferably cured by heat prior to the heat generating element being installed in a heating device. The electrical heating device there commonly comprises a frame which usually circumferentially encloses several heat generating elements contained in a layered structure as well as corrugated fin layers bearing thereagainst on both sides and forms through holes for the passage of the medium to be heated. At least the corrugated fin layers are in this through hole usually exposed. 
     
    
     
       BRIEF DESCRIPTION OF IRE DRAWINGS 
         [0030]    Further details and advantages of the present invention shall become apparent from the following description of an embodiment in combination with the drawing. Therein: 
           [0031]      FIG. 1  shows a perspective side view in an exploded representation of a heat generating element; 
           [0032]      FIG. 2  shows a cross-sectional view of the heat generating element shown in  FIG. 1 ; 
           [0033]      FIG. 3  shows a longitudinal sectional view of the heat generating element shown in  FIG. 1 ; 
           [0034]      FIG. 4  shows detail IV according to  FIG. 3  in an enlarged representation; 
           [0035]      FIG. 5  shows detail V according to  FIG. 3  in an enlarged representation; 
           [0036]      FIG. 6  shows a sectional view taken along line VI-VI according to the representation in  FIG. 3 ; and 
           [0037]      FIG. 7  shows an enlarged perspective plan view of a face-side end of the heat generating element prior to placing the further shielding plate. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    As illustrated in particular in  FIG. 1 , an embodiment of a heat generating element  300  shown there is composed of several superimposed layers. The heat generating element  300  is designed substantially symmetrically, where a positioning frame marked with reference numeral  302  is provided at the center and made of electrically insulating material, in particular plastic. The positioning frame  302  forms holding fixtures  304  for PTC elements  306 . Two PTC elements  306  are in the width direction received in one holding fixture  304  (cf.  FIG. 2 ). Contact plates  308  bear against opposite sides of the PTC elements  306 . These two contact plates  308  are configured identically and formed from punched electrically conductive sheet metal. The contact plates  308  are placed as separate elements onto the PTC elements  306 . They can additionally be provided with a vapor-deposited electrode layer as is generally common. The electrode layer, however, is no contact plate  308  within the meaning of the invention. 
         [0039]    The contact plates  308  are sized such that, though being received within the positioning frame  302 , they are arranged circumferentially spaced from the positioning frame  302 . The circumferential gap thus formed is indicated in  FIG. 2  by reference numeral  310 . This circumferential gap  310  is formed as an air gap between the inner circumference of the holding fixture  304  and the PTC elements  306  provided therein and is crossed solely by support points  312  tapering in the vertical and width direction and being integrally formed on the positioning frame  302 . The support points  312  are substantially pyramid-shaped and respectively spaced from each other distributed on the inner circumference of the holding fixture  304 . The surface of the support points  312  is curved—presently concave—for enlarging the creepage distance. As  FIG. 2  additionally illustrates, the contact plates  308  are also via an air gap  314  each spaced from the positioning frame  302 . The latter has a stepped configuration such that an inner region of the positioning frame  302  defining the holding fixture  304  is notched and inwardly offset. The free rim of the contact plates  308  extending over the PTC elements  306  is located in this notched region. The upper and lower side of the positioning frame, with interpositioning of an insulating layer  316 , commonly bear against contact surfaces of a first shielding plate  318  or a second shielding plate  320 , respectively. But since care must always be taken that the shielding plates  318 ,  320  in the vertical direction, with interposition of the insulating layers  316 , the contact plates  308 , and the PTC elements  306 , contact each other directly and with good thermal conductivity, it can be that the positioning frame  302  has a lower height and accordingly is also spaced from one or both insulating layers  316 . 
         [0040]    The positioning frame  302  at its outer rim forms a circumferential rim web  322  which has a height of about one quarter of the positioning frame  302 . At the level of the rim web  322 , the shell-like first shielding plate  318  is sized such that the positioning frame  302  with its rim web  322  just fits between oppositely disposed longitudinal wall sections  324  that are presently formed by the first shielding plate  318 . The sheet material of the shielding plate  318  is above the rim web  322  slightly bent outwardly, but extends in vertical direction. The free [sic] of the longitudinal wall section  324  is bent outwardly in a funnel-like manner. 
         [0041]    The second shielding plate  320  also has oppositely disposed longitudinal wall sections  326  extending in the vertical direction. After joining, they bear against the inner surface of the longitudinal wall sections  324 . The free space first formed between the two shielding plates  318 ,  320  and the positioning frame  302  forms a chamber  328  in which adhesive  330  is received 
         [0042]    The adhesive  330  is in the circumferential direction formed encircling the positioning frame  302  as bead of adhesive and by solidifying connected, firstly, to the positioning frame  302  and, secondly, to the two shielding plates  318 ,  320 . The same applies to the insulating layer  316 ,  328  placed onto the second shielding plate  320  and extending outwardly over the chamber. As can be seen from  FIG. 2 , however, the adhesive  330  is filled in such that it does not reach the interior of the positioning frame  302 . As can be seen, the volume of the adhesive  330  is less than the volume of the chamber  328 . The adhesive  330  is in the embodiment shown connected only with the positioning frame  302  and the two shielding plates  318 ,  320 . Free surfaces of the contact plates  308  and the PTC elements  306  are therefore free of adhesive  330 , so that the adhesive  330  can not contribute to possible creepage paths. The shielding formed by the two shielding plates  318 ,  320  is sealed by the adhesive  330  in an overlapping region  332  of the two shielding plates  318 ,  320 . Specifically the rim, i.e. in the cross-sectional view according to  FIG. 2 , the longitudinal wall section  326  of the second shielding plate  320 , dips into the bead of adhesive and is therefore after the adhesive is cured sealed in a tight manner in the circumferential direction, so that the inner region of the shielding is sealed by adhesively bonding the two sheet metal shells  318 ,  320 . 
         [0043]    Access to the interior of the positioning frame  302  is provided only by connection pieces  334 , formed at one face side of the positioning frame  302  and by its material integrally formed thereon, and entirely circumferentially surrounding a channel  336  for receiving the pin-shaped contact elements  338  (cf.  FIG. 1 ). At their free end, the connection pieces  334  support sealing elements formed from thermoplastic elastomer or PTFE with a labyrinthine sealing structure  339 , which by insert molding or plugging on can be connected to the associated connection piece  334 . On the face side of each positioning frame  302 , two connection pieces  334  with an identical configuration and seal are provided for receiving two contact pins  338  for electrically contacting the contact plates  308 . 
         [0044]    As  FIGS. 1 and 4  further illustrate, the contact plates  308  comprise female clip element holding fixtures  340  being produced by punching and bending which are formed on laterally offset protrusions  342  of the contact plate  308 , said protrusions  342  terminating within the boundary defined by the adhesive  330  and each covering associated clip openings  344  shaped by the positioning frame  302 . The contact pins  338  are exposed in the clip openings  344  and are connected to the female clip element holding fixtures  340  of the contact plates  308 . At the connection side of the heat generating element  300  comprising the connection pieces  334 , the clip connections described can be realized either by positioning the contact plates  308  in their installed position and subsequently introducing the contact pins  338  through the channels  336  or by locking the female clip element holding fixtures  340  with the contact pins  338  that are already in position  338 . 
         [0045]    The positioning frame  302  at its face side end and also at cross webs  345 , spacing the individual holding fixtures  304  from each other, comprises locking openings  346  with which locking tongues  38  formed on the contact plate  308  by punching and bending engage to connect the contact plate  308  across its entire length to the positioning frame  302  (cf. also  FIGS. 5 and 6 ). Furthermore, multiple flexible tongues  350  are formed on the contact plate  308  by punching and bending and are in preloaded contact with the PTC element  306  for improving contact (cf.  FIG. 1 ). 
         [0046]    When producing the embodiment discussed, the positioning frame  302  made of injection molded plastic is first covered on one side with a contact plate  308 . The contact plate  308  is by use of the locking tongues  348  clipped to the positioning frame  302 . The contact plate  308  on the connection side also locks to the corresponding inner end of the contact pin  338 . The PTC elements  306  are no introduced into the holding fixture  304  thus closed at the underside. Thereafter, the other contact plate  308  is in the manner described placed onto the positioning frame  302  and locked thereto. 
         [0047]    The heat generating cell of the heat generating element is therewith completed as a preassembled unit. It is now introduced into the first pan-shaped shielding plate  318 . Its longitudinal wall section  324  is modified at the connection side such that the contact pins  338  can pass contactlessly through the longitudinal wall section  324 . 
         [0048]    The adhesive  330  is then filled into the chamber  328  by a circumferentially applied bead of adhesive. The further insulating layer  316  is placed on. Furthermore, the second shielding plate  320  is placed on and with its circumferential longitudinal wall section  326  pressed into the first shell-like shielding plate  318 . The adhesive  330  is there slightly displaced and compressed. The adhesive  330  surrounds at least the free end of the second shielding plate  320  and seals this second shielding plate  320 . The shielding formed by the shielding plates  318 ,  320  is thereby hermetically sealed against the atmosphere. Humidity or dirt can not enter the interior of the heat generating element  300 . 
         [0049]    As  FIG. 7  illustrates, the shielding plate  320  as well as the associated positioning frame  302  are provided with mutually associated positioning devices that are in the illustrated embodiment formed by a beveled corner  352  of the first shielding plate  318  and an associated beveled corner  354  of positioning frame  302 . Unique positioning of the fitted contact plate  308  in the first shielding plate  318  is determined by these positioning devices. In other words, the positioning frame  302  [can] be mounted in the shell-like first shielding plate  318  only in the direction shown in  FIG. 6 . It goes without saying that the second shielding plate  320  is formed correspondingly and also engages with the beveled corner  352  of the first shielding plate  318  in the manner previously described and overlaps therewith.