Patent Publication Number: US-2023150582-A1

Title: Outer Skin Component for a Motor Vehicle and Method for Producing an Outer Skin Component

Description:
BACKGROUND AND SUMMARY 
     The invention relates to a fiber-reinforced outer skin component for a motor vehicle. 
     Fiber-reinforced components on motor vehicles are known. Usually in such cases a plurality of plies of directed reinforcing fibers are embedded in a plastics matrix. When such a component is heated as part of the painting operation, the different expansion coefficients (also known as the delta-alpha problem) frequently result in fibers showing at the surface. The fiber plies beneath become visible. 
     Fibers showing in this way fall short of the exacting requirements imposed on the surface quality of outer skin components. Particularly in the case of fiber-reinforced plastics components, this results in costly and inconvenient work operations before and after painting, to ensure a class A surface. Fiber-reinforced components have, to date, been painted in stand-alone painting processes outside of the process chain, and installed on the vehicle afterward. 
     Against this background, the object of the invention is to provide a fiber-reinforced plastics component which is improved in terms of paintability and surface quality. The intention, more particularly, is for the plastics component to be producible inexpensively as an outer skin component for a vehicle and for painting together with the vehicle body to be made possible. 
     The object is achieved by an outer skin component and by a method for producing such a component in accordance with the independent claims. Further advantageous embodiments are apparent from the dependent claims and the description hereinafter. 
     A fiber-reinforced outer skin component for a motor vehicle is specified that is configured as a shell component having an outer shell and an inner shell. The inner shell is joined directly or indirectly to the outer shell, preferably by adhesive bonding. Characteristically, the inner shell and the outer shell together model the essential geometry of the outer skin component. In the installed state, the outer shell is facing outward on the vehicle, and the inner shell is facing inward. The outer shell preferably forms the outer skin of the outer skin component. 
     In accordance with the invention, both the outer shell and the inner shell are each configured as fiber-reinforced plastics shells having a fiber reinforcement embedded in a thermoset plastics matrix, the fiber reinforcement of the outer shell consisting of one or more plies of nonwoven material. In other words, the use of directed fibers is deliberately shunned in the outer shell. Nonwoven materials exclusively are used as reinforcing fibers in the outer shell. 
     In a nonwoven, the fibers take the form of random fibers, in other words without direction and with adventitious fiber direction. The distribution of the fibers in the nonwoven is arbitrary, with the result, in macroscopic viewing, that matrix and fibers are distributed more homogeneously in the component than in a component with directed fibers. It has now emerged that an outer shell comprising a fiber reinforcement exclusively of nonwoven materials forms only slight unevennesses, which the human eye still perceives as being homogeneous. Any sink marks and/or showthroughs are permissible, owing to their uniform distribution when the surface of the outer shell is viewed in its entirety, and are not a criterion for rejection. It has surprisingly been found that an outer shell of this kind can also pass through conventional painting operations for a metal body, such as a cathodic electrocoat operation with temperatures of up to 190 degrees, for example, without any impermissible unevennesses or showthroughs occurring on the surface. This is all the more surprising given that the heating in the cathodic electrocoat oven is accompanied by (virtually) 100% curing of the thermoset matrix material. 
     The fiber reinforcement of the outer shell may be formed of a single ply or of two or more plies of nonwoven materials, present in layers one over another. In principle, the nonwoven material is not restricted to particular fibers; instead, all known reinforcing fibers can be used. 
     In one embodiment, the fiber reinforcement of the outer shell is formed of glass fibers—that is the individual nonwoven plies consist of glass fibers. In this way a particularly good surface quality can be achieved, since the coefficient of thermal expansion of the glass fibers is closer to that of the matrix material and therefore the delta-alpha problem is reduced. 
     In an alternative embodiment, the fiber reinforcement of the outer shell is formed of carbon fibers, meaning that the individual nonwoven plies consist of carbon fibers. As a result of this, it is possible to achieve particularly low weight and higher mechanical strength. 
     In a further embodiment, surface optimization with accompanying low weight is achieved in that the fiber reinforcement of the outer shell is formed of carbon fibers and glass fibers. In this case the individual nonwoven plies may comprise only carbon fibers or only glass fibers and may be present in layers one above another. It is also possible to use nonwoven plies in which both carbon fibers and glass fibers are arranged. 
     The requirements relating to the strength of the outer skin component can be realized by way of the inner shell. For this purpose it may be advantageous, in one embodiment, if the fiber reinforcement of the inner shell comprises one or more plies of directed reinforcing fibers, such as carbon fibers, glass fibers or aramid fibers, for example. These fibers may take the form, for example, of a laid scrim, woven fabric and/or braid. Since the outer shell conceals the inner shell from sight, the requirements in terms of the surface quality of the inner shell are much lower. 
     In one preferred embodiment, the inner shell comprises at least one local reinforcement which provides the inner shell with, for example, stiffening that is in keeping with load paths. These one or more reinforcements may be formed, for example, by additional fiber plies, which are arranged in a locally delimited manner in regions of the inner shell. For example, the reinforcements may be visible as striplike thickenings on the inner shell. 
     The matrix material of the outer shell and of the inner shell is a thermoset matrix material; the same matrix system need not necessarily be used for the inner shell and the outer shell. 
     The outer skin component is preferably a vehicle hatch, such as a front/engine hood, a tailgate or a door, or a side wall of a motor vehicle. 
     Additionally specified is a method for producing an outer skin component which is configured as a shell component having an outer shell and an inner shell, more particularly for producing the shell component described above. The outer shell and the inner shell are preferably produced independently of one another and joined to one another in a subsequent step. The method accordingly comprises the steps of: 
     producing an outer shell, by subjecting a thermoset plastics matrix having a fiber reinforcement composed of one or more plies of nonwoven material to compression in a compression mold and curing, 
     producing an inner shell as a fiber-reinforced plastics component having a fiber reinforcement embedded in a thermoset plastics matrix, and 
     durably joining the outer shell to the inner shell to form the shell component. 
     The outer shell may be produced preferably by the RTM process (resin transfer molding) or by liquid resin molding. In the RTM process, the fiber reinforcement is infiltrated with the matrix material only when in the closed compression mold. In the liquid resin process, the matrix material is applied to the fiber reinforcement before the compression mold is closed, and is subsequently compressed with this reinforcement. Both processes are cost-effective processes compatible with long series, enabling the production of large numbers of units at low cost. For the production of the outer shell, advantageously, it is possible to forgo the use of prepregs, whose production and processing is costly and inconvenient. 
     Further to the processes stated above, another option for producing the inner shell is that of hybrid pressing when, for example, there are local reinforcements to be provided which are applied in the form of prepregs, for example, to the rest of the fiber plies of the inner shell. 
     The outer shell and the inner shell are joined durably and inseparably to one another to form the shell component. This is accomplished preferably by adhesively bonding the shell components to one another. 
     By virtue of its specific construction, the outer skin component described above can be painted along with the rest of the vehicle body in the existing process chain and in particular can also be put through the cathodic electrocoating operation. There is no need for separate painting operations. As a result, there are considerable potentials for savings and the possibility for painting of the components in the vehicle color. 
     Features and details described in connection with the shell component are also valid in connection with the method of the invention, and vice versa in each case, and so mutual reference is always made or can always be made with regard to the disclosure concerning the individual aspects of the invention. 
     Further advantages, features and details of the invention are apparent from the description hereinafter, in which exemplary embodiments of the invention are described in detail with reference to the drawings. In this connection, the features referred to in the claims and the description may each individually or in any desired combination be essential to the invention. In so far as the terms “can” or “may” are used in this patent application, they refer both to the technical possibility and to the actual technical realization. 
     Exemplary embodiments are illustrated below by means of the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows an illustrative shell component in an exploded representation; and 
         FIG.  2    shows a sectional view of the shell component from  FIG.  1   . 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG.  1    shows an illustrative shell component  1  in the form of a front hatch of a motor vehicle. The shell component has an outer shell  10  and also an inner shell  20 , which are joined to one another, preferably by adhesive bonding. The shell component  1  may comprise further elements, not represented, such as hinges or pockets, for example, which may be provided additionally or may be integrated, for example, in the inner shell  20 . The outer shell  10  forms an outer face  12  of the shell component  1 , which in the installed state on the vehicle faces outward. The inner shell  20  is arranged on the outer shell  10  side facing away from the outer face  12 . 
     The outer shell  10  is configured as a fiber-reinforced plastics shell, and comprises a fiber reinforcement  13  composed of nonwoven materials, which is embedded in a thermoset plastic matrix  14 .  FIG.  2    shows a sectional view for illustrating the ply construction. Shown purely illustratively therein are four piles  16 ,  17 ,  18 ,  19  of a nonwoven material; the number may of course vary according to the requirements. 
     The inner shell  20  is likewise a fiber-reinforced plastics shell having reinforcing fibers embedded in a thermoset plastics matrix  22 . The inner shell  20  comprises a fiber reinforcement  23  having a ply construction of multiple plies  24 ,  25 ,  26 ,  27 ,  28  (here, illustratively, 5 plies) of directed reinforcing fibers. The inner shell  20  additionally comprises at least one local reinforcement  29 . For this purpose, in a locally confined region, additionally to the above-described construction, a prepreg strip having further reinforcing plies of directed continuous fibers and matrix material is also formed onto the inner shell  20 . 
     For producing the shell component, the outer shell  10  and the inner shell  20  are first produced separately and then bonded adhesively to one another. The outer shell is produced preferably by the RTM process or by the liquid resin process; the inner shell may likewise be produced by these processes or in a hybrid molding process. 
     LIST OF REFERENCE SIGNS 
     
         
           1  Shell component 
           10  Outer shell 
           12  Outer face 
           13  Fiber reinforcement 
           14  Plastics matrix 
           16 ,  17 ,  18 ,  19  Nonwoven plies 
           20  Inner shell 
           22  Plastics matrix 
           23  Fiber reinforcement 
           24 ,  25 ,  26 ,  27 ,  28  Plies of reinforcing fibers 
           29  Local reinforcement