Abstract:
A roof panel for the passenger cabin of a motor vehicle includes a composite or plastic panel-like body and a metal frame that runs continuously along a periphery of the plastic body. The metal frame prevents the thermal expansion of the plastic body at high temperatures, so the expansion of the entire roof panel approximates that of an equivalent metal roof panel. Specifically, the uninterrupted encircling structure of the frame absorbs forces resulting from thermal expansion of the plastic body.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Patent Application No. 102015000866.8, filed Jan. 24, 2015, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure pertains to a roof panel particularly for the passenger cabin of a motor vehicle, and more particularly to a roof panel assembly having a composite panel and a metal frame. 
     BACKGROUND 
     In the continuing pursuit of ways to reduce the weight and also the fuel consumption of motor vehicles, it has already been suggested that plastic be used to produce various components of a vehicle body that are conventionally made from metal. A proposal for a roof panel manufactured mainly from a fiber-reinforced plastic is known for example from DE 10 2008 032 334 A1. This roof panel with a substantially flat cross section, at least in the direction transverse to the vehicle, is supported along the side edges thereof by horizontal flanges of two longitudinal lateral roof support members and fixed to the lateral sides of the longitudinal roof support members with adhesive beads. 
     Since plastics generally undergo considerably greater thermal expansion than metals, the expansion behavior of these conventional roof panels differs significantly from that of the metal longitudinal support members to which it is fastened. Consequently, significant compression and shearing forces occur in the adhesive beads, and over time these can cause the adhesive bond to fail. 
     SUMMARY 
     The present disclosure provides a roof panel for a motor vehicle, in which the concerns for failure of an adhesive bond with a largely metal body is minimized, despite the fact that the roof panel is primarily made of a plastic or composite material. 
     In accordance with one aspect of the present disclosure, the roof panel includes a panel-like composite or plastic body and a metal frame that runs continuously around a border of the plastic body. This border prevents the thermal expansion of the plastic body at high temperatures, so the expansion of the entire roof panel approximates that of an equivalent metal roof panel. Because of its uninterrupted encircling structure, the frame can absorb strong forces from the plastic body. In order to guarantee the high load-bearing capability of the roof panel, one or more of the components may be made from a fiber-reinforced plastic. 
     The frame protects the panel from damage by embedding the frame in plastic body, and also makes it easier to bond the roof panel to surrounding supports of the bodies of the frame not exposed, and the surface of the roof panel that comes into contact with the adhesive is made uniformly of plastic. The frame may include a single wire, which should preferably have a cross section without edges defining a simple closed curve such as a circle or an oval, thereby avoiding local concentration of the stresses arising between the plastic body and the frame, and any damage to the plastic body resulting therefrom. 
     The plastic body may particularly include two components secured one on top of the other, between which the frame is inserted. The components may be for example panel-like blanks; however, one of the components may also include tabs positioned around the periphery along the edge of the plastic body, which have a groove into which the frame can be inserted. 
     In order to prevent the frame from warping due to the effect of thermal tension, connecting wires may cross over an opening in the frame and the two ends thereof may be fastened to the frames. Like the frame itself, the connecting wires themselves are inserted between the components, particularly between two panel-like blanks. The connecting wires may form a braid or a mesh, which preferably fills in the opening in the frame uniformly. If the wires that crossed are fastened to each other at the intersection points of the braid or mesh, a significant fraction of the stress that occurs due to heating of the roof panel can be absorbed directly by the mesh, thereby relieving the frame of this load. 
     A groove for adhesive may extend along at least a part of the periphery of the roof panel to hold the adhesive with which it is fastened to the vehicle body. A groove for adhesive having a suitable cross section, which is open both downwardly and toward the respective periphery of the roof panel may be obtained expediently if the lower of the aforementioned components fastened one on top of the other has a shorter peripheral length than the upper component. 
     In order to improve the load-bearing capacity of the roof panel, it may be reinforced with ribs extending in the transverse direction. The ribs may be joined to the panel-like plastic body as a separate component during production of the roof panel. A plurality of ribs may expediently be connected as a single part in a roof arch having a comb-like cross section. The production of the roof panel may also be rendered more efficient if the ribs are constructed to form rungs of a ladder-like structural element, the side rails of which extend along lateral edges of the roof panel. The side rails may also serve to delimit the groove for adhesive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements. 
         FIG. 1  is a view of the roof panel according to the present disclosure from above; 
         FIG. 2  is a view of the roof panel from below; 
         FIG. 3  is an expanded view of the components used to assemble the roof panel; 
         FIGS. 4 a -4 f    illustrate the stages of a manufacturing process for the roof panel; 
         FIG. 5  is a cross section through a lateral peripheral area of the roof panel; 
         FIG. 6  is a cross section through a front peripheral area of the roof panel and a roof cross member of a vehicle body on which the roof panel is mounted; and 
         FIG. 7  is a section view in the lengthwise direction of the vehicle through a central region of the roof panel. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the present disclosure or the following detailed description. 
       FIG. 1  shows the roof panel  1  according to the present disclosure in a view from above. The periphery of roof panel  1  includes a front edge  2  relative to the installed position of the roof panel in a motor vehicle, which is provided to serve as a border with the top edge of a windscreen, a rear edge  3  that may border the rear window or which includes a small cutaway so that it can accommodate the upper edge of a tailgate, as in the case shown here, and longitudinal edges  4 . The top side of roof panel  1  is constructed with a slight convex curvature in both the longitudinal and transverse directions. This curvature limits the pressure that roof panel  1  can exert on adjacent components of the vehicle along the length thereof when it is heated. Protruding connecting elements  5 ,  6  extend downward along edges  2 ,  3 ,  4  on the underside of roof panel  2  visible in  FIG. 2 . Groups of ribs  7  extend transversely over the underside of roof panel  1 , from one longitudinal edge  4  to the other. 
       FIG. 3  shows components, from which roof panel  1  is constructed in layers. These components include, from top to bottom, an upper blank  8  made from a fiber-reinforced plastic, a perforated metal structure  9 , a lower blank  10  made from fiber-reinforced plastic, and a ladder-like structural element  11  in which two side rails of the ladder structure are formed by the longitudinally aligned connecting elements  6 , and connecting elements  5  and ribs  7  connect connecting elements  6  to each other like the rungs of a ladder. 
     Metal structure  9  includes a frame  12 , in this case formed by a strong wire having a circular cross section and extending without interruption about the periphery, and a braid or mesh  13  made of thinner wires  14 , the ends of which are each attached to frame  12  by spot welding or similar methods. The wires preferably have a cross section without edges which define a simple closed curve such as a circle or an oval. In the braid  13  shown in  FIG. 3 , wires  14  are all straight and cross each other at right angles. Wires  14  may be fastened to each other at the intersection points, by welding, soldering or winding them around each other, for example. 
     Components  8  to  11  are connected to the completed roof panel  1  by placing first blank  8 , followed by the metal structure  9 , blank  10  and structural element  11  in a molding tool lower section  15 , as shown in  FIG. 4 a    to  FIG. 4 f   , and compressing and heating them between lower section  15  and an upper section  16  in order to soften the plastic matrices of blanks  8 ,  10 , so that they melt and fuse with each other in the interspaces between the wires of braid  13 . When a deep bond between the plastic parts of blank  8 ,  10  and the ladder-like structural element  11  has been created in this way, sections  15 ,  16  of the molding tool are separated again, and the completed roof panel  1  can be taken out. 
       FIG. 5  shows the structure of the roof panel  1  obtained in this way in the form of a sectional view through a longitudinal edge  4  in a direction transverse to the vehicle. In  FIG. 4 , boundary lines between the blanks  8 ,  10  and structural element  11  have been highlighted to indicate more clearly where the plastic material of roof panel  1  from these various original components came from; in practice, the fusing of components  8 ,  10 ,  11  should advantageously be so complete that such a boundary line would no longer be clearly visible in a real section. 
     A groove for adhesive  17 , open downwardly and to the side, extends along edge  4  and is provided to hold an adhesive bead for retaining roof panel  1  securely on a longitudinal support member (not shown), which member extends over a door of the vehicle and connects the ends of the A, B and C pillars in a manner known per se. In this context, a peripheral member  18  of roof panel  1  extending above the groove for adhesive  17  is formed solely by the original upper blank  8 . A lateral flank  19  that delimits the groove for adhesive  17  on the side closest to the vehicle middle is created by one of the lateral connecting elements  6  of ladder-like structural element  11 . The strong wire of frame  12  is pressed into connecting element  6  from above; or this purpose, connecting element  6  may be provided with a groove that is upwardly open from the start of the process. 
     The dimension of blank  10  is selected in this context such that the respective edges thereof are touching frame  12  from the inside. It would also be conceivable to select the dimensioning of blank  10  such that it is pressed into the groove of connecting element  6  together with frame  12  and extends as far as flank  19 , as indicated by a dot-dashed line in  FIG. 5 . Under the pressure of the molding tools, the upper sides of ribs  7  are fused with blank  10 , and both blanks  8 ,  10  are individually pressed into interspaces  20  between the wires  14  of braid  13 , where they are thoroughly fused with each other. 
     The construction of roof panel  1  at front and rear edges  2 ,  3  is similar to that of the longitudinal edges  4 , is shown in  FIG. 6  for front edge  2 . Here, frame  12  is pressed in a groove on the upper side of cross member  5 , and cross member  5  in turn is fused together with blanks  8 ,  10  inside and outside of frame  12 . A steel cross member  21  of the vehicle body, which extends between the top ends of two A pillars (not shown), supports the front edge  2  of roof panel  1  and an upper edge of windscreen  23  with an adhesive bead  22 . 
       FIG. 7  is a sectional view in the longitudinal direction of the vehicle body through a central area of roof panel  1 . The sectional plane extends transversely to a group of the ribs  7 . Ribs  7  form the teeth of the comb-like cross-section of a fiber-reinforced plastic roof arch  24 , which in turn forms a rung in ladder-like structural element  11 . A back section  25  of the comb-like cross-section has fused with blank  10  along a dashed line. The neutral fiber in roof panel  1  extends substantially along wires  14 , between the blanks  8 ,  10 . The ribs  7  located at a distance from the neutral fiber therefore help considerably to enhance the stiffness of roof panel  1 . 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.