Patent Publication Number: US-2010109192-A1

Title: Method for sealing a complex shape electronic sensor by low-pressure injection of reactive resin

Description:
The invention relates to a method of sealing an electronic sensor placed in a housing by low-pressure injection of reactive resin. 
     In order to protect the electronic components in an environment with severe ambient conditions (temperature and humidity for example), it is known to incorporate said electronic components and their connections in a watertight and protective assembly. 
     The present invention describes, by way of illustration that is in no way limiting in itself, a proximity sensor incorporated in a door handle of a motor vehicle. Such an electronic sensor is therefore subject to temperature and humidity conditions such that a premature aging and deterioration of the electronic components, even a disabling of them, are often observed in the absence of suitable protection means. 
     Those skilled in the art know that this problem can be resolved by means of a protective reactive resin coating said electronic components. 
     Thus, it is known to produce an element jacketing the electronic components and their associated connection that takes the form of a container, the container then being filled by gravity casting of a reactive resin, such as—for example—polyurethanes, epoxides or silicones. 
     This method has a notable disadvantage, which is its bulk. In practice, gravity casting requires the container to be overdimensioned in order to facilitate the casting and avoid overflows of reactive resin when coating the electronic components. In order to avoid overflows, a “dead volume” is thus systematically provided in the container which generates a certain bulk for the assembly. 
     Furthermore the gravity method cannot be used to fill products of complex shapes without once again generating an overdimensioning of the container in order to be able to suitably fill all the volumes. 
     Now, the current sensors tend to be miniaturized and their shapes are tending to become more and more complex in order to be better incorporated in their immediate environment. Thus, a sensor incorporated in a door handle of a motor vehicle must be able to best hug the shapes of said handle without making the latter too voluminous. It is therefore necessary to combine complex shapes with restricted dimensions, something that overmolding by gravity casting does not allow. 
     In order to resolve this problem, those skilled in the art have several known solutions available. The first consists in not using overmolding and in placing the electronic in a housing that is filled with air and watertight to the outside. Now, the seal-tightness of series-produced parts is difficult to guarantee, if only because the electrical power supply or signal transmission conductors of the sensor need to penetrate inside said housing and therefore, consequently, generate as many potential sources of rupture of the seal-tightness. The second technique consists in overmolding under pressure the electronic part using a thermoplastic material or a reactive two-component material (generically called RIM, an acronym standing for “Reaction Injection Molding”). This requires heavy investment, as much in molds as in machines, which makes this alternative prohibitive when it comes to cost. Furthermore, the injection of thermoplastic materials is generally done in high-pressure and high-temperature conditions that the electronic components do not easily withstand. Moreover, these materials do not have very good qualities of resistance over time given a humid atmosphere. Finally, such an overmolding can imprison air bubbles, detrimental to the seal-tightness of said overmolding. 
     The present invention aims to remedy the above problems, and at a contained cost. 
     To this end, the invention firstly targets a method of sealing, by low-pressure injection of reactive resin, an electronic sensor placed in a housing consisting of at least two attached elements, comprising the following steps:
         i. assembly of the sensor within the elements of the housing,   ii. locking of the elements of the housing, in order to form an injection jacket,   iii. low-pressure injection of a reactive resin through at least one injection jacket filling orifice,   iv. continuing injection until the reactive resin overflows via at least one discharge orifice into at least one overflow container provided to contain the excess reactive resin,
 
said method being characterized in that filling orifice(s) and overflow container(s) are incorporated in the injection jacket.
       

     Thus, the low-pressure injection is done within an already-existing part which forms a casting mold, provided with at least one filling orifice and one orifice for discharging the injected surplus, with no risk of leak other than into the overflow container provided facing the discharge orifice. Furthermore, the low pressure does not impair the electronic components. 
     Advantageously, the method also comprises a subsequent step during which the filling orifice(s) and the overflow container(s) are separated from the injection jacket. 
     Thus, the finished part has an esthetic appearance and a reduced bulk, the two elements (filling orifice and overflow container) required for the sealing according to the invention being eliminated. 
     In one embodiment, the filling orifice(s) and the overflow container(s) have areas of weakness favoring their separation from the injection jacket. 
     It is then easy to separate the two types of element from the injection jacket in an industrial and reliable manner. 
     Preferably, the overflow container(s) comprise means facilitating the flow of the reactive resin, so enabling the low-pressure injection surplus to flow well. 
     Furthermore, the reactive resin used is from the family of polyurethanes. 
     Advantageously, the reactive resin used has a viscosity greater than 7000 mPa·s upon its injection in the step iii and greater than 10 000 mPa·s at the end of the step iv. 
     It is thus possible to ensure easy casting with a pressure that is not very high (between 1 and 1.5 bar overpressure in line with the filling orifice) and not to damage the electronic components with a very high viscosity. 
    
    
     
       Other features, aims and benefits of the invention will emerge from the detailed description that follows, given with reference to the appended drawings which represent a preferred embodiment thereof as a nonlimiting example. In these drawings: 
         FIG. 1  is a three-dimensional representation of a sensor according to the invention, 
         FIG. 2  is an enlargement of the area A of  FIG. 1  of the bottom half-housing of a sensor according to the invention, 
         FIG. 3  is a three-dimensional representation of a sensor according to the invention, assembled, before the low-pressure resin injection step, 
         FIG. 4  is an enlargement of the area B of  FIG. 3 , showing the resin overflow container, 
         FIG. 5  is an enlargement of the area C of  FIG. 3 , showing the filling orifice, 
         FIG. 6  is a three-dimensional view of a sensor according to the invention in its final production step, once the filling orifice and overflow container have been eliminated, 
         FIG. 7  is a side view of a sensor according to the invention. 
     
    
    
     The embodiment presented is that of a proximity sensor intended for incorporation in a door handle of a motor vehicle. 
     It is commonplace for motor vehicles to be equipped with an electronic device making it possible to identify with certainty the people approaching said vehicle in order to allow access inside the vehicle reserved only to duly authorized people. The means implemented to achieve this aim may vary, but the basic principle (reviewed here) is as follows:
         the vehicle is equipped with means of identifying the approach of authorized people (generally wearing an electronic badge capable of securely dialoging with the identification means,   when an authorized person approaches, the identification means nevertheless wait for said person to actually manifest the intention of entering before unlocking the opening or openings of the vehicle. The aim of this twofold check is to enable—for example—an authorized person to pass very close to his vehicle without systematically unlocking access thereto.       

     In this device, one of the means used to determine the actual intention of the authorized person to re-enter into the vehicle consists in placing, in the handle of the openings, a proximity sensor which will enable the identification means to know that the authorized person is extending his hand toward the handle. 
     It is this type of sensor that will be described in more detail in the description. 
     Such a sensor type, incorporated in the vehicle (in this case in the handle of the opening), is, for example, of the capacitive type, and sends a proximity detection signal to the identification means. If the twofold condition stated previously is fulfilled (detection of authorization in the form of a badge for example and detection of a part of the body—generally a hand—close to the handle of the opening), then unlocking takes place and access is authorized. 
     The construction of a sensor  10  according to the invention is detailed with reference to  FIG. 1 . 
     It comprises a bottom half-housing  20  and a top half-housing  30  cooperating together via mechanical locking means  20   a,    30   a  in order to produce an injection jacket in this way. Inside said injection jacket, there is placed the antenna  40  of the sensor  10  which is designed to detect proximity, and the electronic components  50  responsible for processing the information and sending a signal to the identification means (not represented). In order to power the sensor and route the detected signal, electrical conductors  70  also penetrate inside the injection jacket. As illustrated in  FIG. 2 , the bottom half-housing  20  has guides  20   b  allowing for an optimum positioning of the electrical conductors  70  and holding them in position. 
     Said injection jacket can be produced by injecting plastic, thermoplastic or thermosetting material, but not necessarily. It is quite possible to envisage a jacket made from another type of material (metal for example). 
     The top  30  and bottom  20  half-housings incorporate two elements  60   a,    60   b  forming the filling orifice  60 . This filling orifice  60  is used to direct the reactive resin toward the injection jacket in a leaktight manner. For this, all that is needed is to place a resin injection nozzle on the top element  60   a.    
     Once assembled, the sensor appears as represented in  FIG. 3 . It then appears in the form of a monolithic part linked by electrical conductors  70  to the vehicle&#39;s electrical cable loom. 
     As represented in  FIG. 5 , the resin injection is carried out via the filling orifice  60  and more specifically, the injection nozzle cooperates with the resin inlet cone  60   c.  The channel for the introduction of the resin  60   d,  also visible in  FIG. 1 , then guides the reactive resin toward the interior of the injection jacket, in contact with the elements to be overmolded. At this stage of the method, the reactive resin is very fluid (its viscosity is advantageously greater than 7000 mPa·s at the filling orifice  60 ). It will start to harden to reach, at the end of the injection step and once the overflow container  80  is filled, a viscosity greater than 10 000 mPa·s. 
     The next step of the method consists in continuing the injection until the reactive resin overflows through the overflow container  80  provided for this purpose. The resin will therefore advance inside the injection jacket sufficiently slowly (an injection time of between 30 s and 1 min is usual, depending on the shape of the sensor  10  and the volume of resin to be injected) not to damage the electronic components  50 . This is due to the combination of the relatively low viscosity and the low-pressure injection (between 1 and 1.5 bar at the injection orifice  60 ). This wall of resin will, as it advances, drive out the ambient air which will be able to escape via the overflow container  80 . 
     Advantageously, the electronic components are placed close to the injection orifice  60  in order to always be subject to a resin that has a lower viscosity. 
     Also advantageously, the overflow container  80  is placed at the end of the advance of the reactive resin within the injection jacket. 
     Throughout the injection phase, the sensor is placed in a jig which will hold in position the two half-housings  20 ,  30  and ensure that the mechanical locking of the means cooperating to this end  20   a,    30   a  does not yield, which would lead to leaks and render the sensor non-compliant. 
     In the embodiment illustrated, the resin is also used as a glue for the elements of the injection jacket. 
     At the end of the injection phase, as illustrated in  FIGS. 2 and 4 , the resin will leave the injection jacket through the only leak orifices  90  provided for this purpose. These leak orifices  90  open out into the overflow container  80 , which is provided to accommodate the excess material within its reservoir  110 . 
     Advantageously, the overflow container  80  has a pouring cone  100  facing the leak orifices  90 , so that the resin is poured more easily and does not block said leak orifices  90 . The resin will thus fill the reservoir  110  of the overflow container  80  without otherwise soiling the toolage. The injection parameters are optimized in order to fill said reservoir  110  without overflow. 
     It should be noted that the overflow container  80  can include guides  80   a  in order to facilitate the positioning of the electrical connectors  70 , as represented in  FIGS. 1 ,  2  and  4  in particular. 
     The next step is to separate the filling orifice  60  and the overflow container  80  from the rest of the sensor in order to obtain a functional sensor  10  as represented in  FIG. 6 . 
     According to a preferred embodiment, this separation operation is favored by shapes and lessened thicknesses in line with the point of separation  120 . Thus, the force needed to provoke the separation is minimal and the break made systematic by the creation of these areas of lesser rigidity. 
     In order to highlight the benefit of the method according to the invention compared to a gravity casting method,  FIG. 7  represents a side view of a sensor  10  according to the invention. The line L represents the maximum level reached for the given sensor if gravity casting were used to fill the bottom half-housing  20 . It can thus immediately be seen that the electronic components  50  and the antenna  40  could not be totally overmolded and that the seal-tightness sought would not be achieved. 
     The present invention is not limited to the single embodiment described, but covers any adaptation within the scope of those skilled in the art. 
     More complex shapes can easily be imagined, for example, without in any way departing from the present invention. It is also possible to envisage an injection jacket consisting of more than two elements, or indeed one (or more) filling orifice(s)  60  placed in the middle of the injection jacket provided with a plurality of overflow containers  80 .