Abstract:
A constructional unit has a frame and a cover. The frame and the cover are formed of materials with different coefficients of thermal expansion. A plurality of spacers is arranged between the frame and the cover and defines a gap therebetween. An adhesive is provided in the gap to attach the frame to the cover and simultaneously seal the gap.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a constructional unit, such as a housing, having a frame and a cover with different coeffiecents of thermal expansion wherein the frame and the cover are simultaneoulsy connected and sealed with an adhesive. The invention further relates to a method for producing the same.  
         BACKGROUND OF THE INVENTION  
         [0002]    In many sectors of technology, particularly in the automobile sector, constructional units, such as housings, are hermetically sealed to protect components housed therein. In the automobile sector there is especially a need for housings that are resistant to chemicals and are sealed to fluids, such as water, oil, fuel, etc., even when the housings are exposed to large variations in temperature. It is also necessary to dissipate heat from inside the housing, if any of the components housed within the housing are heat-generating components, such as electronic circuits. To remove heat from inside the housing, one face of the housing is generally constructed as a metal plate. The metal plate is thermally coupled to the heat-generating components and acts as a heat sink. The remainder of the housing is generally made of a plastic material, because forming the entire housing out of metal is expensive and complicated. Because plastic and metal have different coefficients of thermal expansion, however, the seal between the plastic and the metal deteriorates when the housing is exposed to large variations in temperature.  
           [0003]    To address the deterioration of the seal between the metal plate and the plastic portion of the housing, it is known to provide strips of an elastic material, such as silicone, between the plastic portion of the housing and the metal plate. The elastic material is positioned between the plastic portion of the housing and the metal plate, and then the metal plate is fastened to the plastic portion of the housing by a screw or other clamping means, which presses the metal plate into the elastic material. Positive-locking means, such as groove and tongue connections, are also used to improve the quality of the seal between the plastic portion of the housing and the metal plate. This method of fastening and sealing, however, is complicated, requires additional production steps, and is expensive.  
         SUMMARY OF THE INVENTION  
         [0004]    It is therefore an object of the invention to develop a constructional unit, particularly a housing, with the aforementioned properties that can be more cost efficiently produced.  
           [0005]    This and other objects are achieved by a constructional unit having a frame and a cover. The frame and the cover are formed of materials with different coefficients of thermal expansion. A plurality of spacers is arranged between the frame and the cover and defines a gap therebetween. An adhesive is provided in the gap to attach the frame to the cover and simultaneously seal the gap.  
           [0006]    This and other objects are further achieved by a constructional unit having a frame and a cover. The frame and the cover are formed of materials with different coefficients of thermal expansion. An adhesive is provided between the frame and the cover that attaches the frame to the cover and seals a region between the frame and the cover. The region has indentations that provide repositories for the adhesive.  
           [0007]    This and other objects are still further achieved by a method for sealing a constructional unit. The method includes providing a frame and a cover formed of materials with different coefficients of thermal expansion. A pre-determined amount of an adhesive is applied to the frame or the cover. The frame is attached to the cover via the adhesive so that a seal is formed therebetween. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic perspective view of a first embodiment of a constructional unit according to the invention;  
         [0009]    [0009]FIG. 2 is a schematic side view of the constructional unit shown in FIG. 1; and  
         [0010]    [0010]FIG. 3 is a schematic side view of an alternate embodiment of the constructional unit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    [0011]FIG. 1 shows a first embodiment of a constructional unit  10 . The constructional unit  10  comprises a frame  14  and a cover  16 . The term “frame” as used herein is to be understood as a structure that has at least one face that defines an opening wherein the cover  16  is attached to the face. The frame further has side walls  24  that can be connected to a housing member (not shown) so that a self-contained housing is produced. Alternatively, the side walls can themselves form a housing member  22 . The term “cover” as used herein is to be understood as a structure that covers the opening defined by the face.  
         [0012]    As shown in FIG. 1, the constructional unit  10  illustrated herein is formed as a housing  20 , which can accommodate electronic components, such as a gear control device. The frame  14  of the housing  20  is formed as the essentially pot-shaped rectangular housing member  22 . The housing member  22  has a base (not shown) and the side walls  24 . The side walls  24  and the base (not shown) form an opening  28  that extends from a face  26  of the frame  14  into an interior of the frame  14 . Spacers  34  are formed on the face  26 . The spacers  34  are evenly distributed over the face  26  and extend upward and perpendicular thereto. The housing member  22  is made from a plastic material by injection moulding. The frame  14  could also be, for example, constructed as a component of the housing member  22 . Additionally, the frame  14  could be any desired shape, and the opening  28  could be any variety of shapes and sizes. Heat-generating components (not shown) may be accommodated in the opening  28  of the housing member  22 .  
         [0013]    As shown in FIG. 1, the cover  16  is formed as a lid  30 . The lid  30  has a flat bottom surface  32 . The bottom surface  32  is formed to extend essentially parallel to the face  26  and has a dimension such that it covers the opening  28  when the lid  30  is attached to the housing member  22  at the face  26 . The lid  30  is made from a metal material. The heat-generating components (not shown) that are accommodated in the housing member  22  may be thermally coupled to the lid  30 . The lid  30  thereby conveys heat from the interior of the housing  20  to the exterior of the housing  20  by acting as a heat sink.  
         [0014]    As shown in FIG. 1, an adhesive  40  is arranged between the lid  30  and the housing member  22 . The adhesive  40  is a hot-melt adhesive. The hot-melt adhesive is elastic and has optimal sealing characteristics so that the adhesive  40  is capable of sealing the housing  20  and absorbing stress from shearing forces occurring during thermal expansion of the housing member  22  and the lid  30 . The hot-melt adhesive, depending on the case of application, should be oil-resistant and capable of withstanding temperatures as low as minus 40° C. Adhesive properties of the hot-melt adhesive do not play a dominant role with respect to the sealing quality and elasticity. The hot-melt adhesive may be a one-component material or a two-component material. The two-component material enables the temperature of use to be above the processing temperature of the hot-melt adhesive. Additionally, reactively cross-linking two-component hot-melt adhesives or polyamide based hot-melt adhesives can be used. The polyamide based hot-melt adhesives are particularly favourable for their chemical resistance properties, in particular to oil, flexibility even at high minus temperatures, processing ability, and elasticity. Other adhesives can also be used, which after hardening have the aforementioned properties. These adhesives could, for example, be silicone based or epoxy based adhesives.  
         [0015]    As shown in FIG. 2, the lid  30  is attached to the housing member  22  by applying the adhesive  40  to the face  26  or, alternatively, to corresponding regions on the bottom surface  32  of the lid  30 . The adhesive  40  is applied by known application methods and, as such, will not be described in greater detail. The lid  30  and the housing member  22  are pressed together until the adhesive  40  has bonded. Because the spacers  34  create a gap between the lid  30  and the face  26 , the amount of adhesive  40  applied to the face  26  is pre-determined by the height of the spacers  34 . The height of the spacers  34  is preferably such that the least amount of adhesive  40  is used while still maintaining optimal elastic and sealing characteristics. The adhesive  40  thereby simultaneously attaches the lid  30  to the housing member  22  and seals the housing  20 .  
         [0016]    Mechanical elements (not shown) may additionally be used to further attach the lid  30  to the housing member  22 . The use of mechanical elements (not shown) can help to relieve the adhesive  40  from stress caused by the shearing forces. The mechanical elements may be, for example, plastic rivets, clamps, etc.  
         [0017]    [0017]FIG. 3 shows an alternate embodiment of the constructional element  10 . In the alternate embodiment, instead of the spacers  34 , the amount of adhesive  40  applied to the face  26  is pre-determined by indentations  36  that receive the adhesive  40 . The indentations  36  may be distributed evenly over the face  26  and are formed such that the least amount of adhesive  40  is used while still maintaining optimal elastic and sealing characteristics. Additionally, the face may be formed with both the spacers  34  and the indentations  36 .  
         [0018]    The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. For example, the face  26  may be formed to have step portions, and the lid  30  may be formed to rest directly or indirectly via the spacers  34  on one step of the face  26  and flush with an upper step of the face  26 . Additionally, the face  26  may be formed to have step portions drawn inward such that the lid  30  is formed parallel to the base (not shown) of the housing member  22 . Thus, the opening  28  defined by the face  26  and the lid  30  is smaller than a bottom face of the housing  20 .