Abstract:
The invention proposes a method of manufacturing a hollow optical reflector of the type which comprises a rear body overmolded around the convex rear external face of a front insert, the concave front internal face of which is covered with a reflective layer, the method comprising:  
     a step of producing the insert by forming;  
     a step of overmolding the body on the insert; and  
     a step of depositing the reflective layer on the concave front face of the insert;  
     wherein the step of depositing the reflective layer is prior to the forming step.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention proposes a method for the manufacture of an optical reflector.  
         BACKGROUND OF THE INVENTION  
         [0002]    The invention proposes more particularly a method of manufacturing a hollow optical reflector of the type which comprises a rear body overmoulded around the convex rear external face of a front insert, the concave front internal face of which is covered (in particular at least partially) with a reflective coating, the method comprising:  
           [0003]    a step of producing the insert by forming;  
           [0004]    a step of overmoulding the body on the insert; and  
           [0005]    a step of depositing the reflective layer on the concave front face of the insert (or on at least part of said face).  
           [0006]    Such optical reflectors are generally used in the motor industry for manufacturing lights or headlights for equipping motor vehicles.  
           [0007]    The reflective face of an optical reflector is generally concave and its generator is designed so as to obtain a light beam of given conformation.  
           [0008]    Production of optical reflectors by moulding of a thermoplastic material, and then coating of the functional internal concave face with a reflective layer which is for example aluminium-based, are known.  
           [0009]    However, when the surface of the mould is degraded, or else when it is not at an optimum temperature, the surface of the component obtained has irregularities and rough patches which, even after coating with the reflective layer, are detrimental to the quality of the light beam.  
           [0010]    In order to remedy this problem, a proposal has been made to use a surfacer which fills in the various rough patches of the surface of the moulded component in order to form a smooth surface and which is next coated with the reflective layer.  
           [0011]    However, when the reflective face has a complex shape, and it has sharp edges, the surfacer generally has a tendency to make these edges smoother, so that it also is detrimental to the quality of the light beam.  
           [0012]    The document EP-A-0 727 677 proposes to produce the reflective face on an insert produced from a sheet of thermoformed plastic material, to overmould the body of the optical reflector on faces of the insert which are not intended to form the light beam, and then to coat the functional concave surface with the reflective layer.  
           [0013]    The thermoforming of the plastic sheet makes it possible to have reflecting surfaces of very high quality and which are not impaired during moulding of the body. However, in the case of a complex reflecting surface, certain areas are sometimes difficult to access, so that the whole reflecting surface is not sufficiently fully covered by the reflective layer.  
         SUMMARY OF THE INVENTION  
         [0014]    In order to remedy these drawbacks, the invention proposes a method of the type described previously, characterised in that the step of depositing the reflective layer is prior to the forming step.  
           [0015]    According to other characteristics of the invention:  
           [0016]    the step of forming the insert is a step of thermoforming a sheet;  
           [0017]    the moulding step consists of injecting into a mould a mixture which is composed of the material constituting the body and a gas in liquid form and which is capable of forming a foam in the mould;  
           [0018]    during the moulding step, the front face of the insert rests on a protuberance of a block of the mould which has a shape complementary to that of the front face.  
           [0019]    Other characteristics and advantages of the invention will emerge from a reading of the following detailed description, for the understanding of which reference should be made to the accompanying figures, amongst which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    FIGS.  1  to  3  are schematic views in perspective which depict the successive stages of the step of thermoforming the insert from a thermoplastic sheet, in accordance with the invention;  
         [0021]    [0021]FIGS. 4 and 5 are longitudinal and transverse sections of the insert and the mould depicting the successive stages of the step of overmoulding the body in accordance with the invention;  
         [0022]    [0022]FIG. 6 is a longitudinal section on a larger scale of the optical reflector obtained by the method in accordance with the invention; and  
         [0023]    [0023]FIG. 7 is a perspective view from below of the optical reflector obtained after the overmoulding step depicted in FIGS. 4 and 5. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    For the description of the invention, the vertical, longitudinal and transverse orientations in accordance with the indicators V 1 , L 1 , T 1  and V 2 , L 2  and T 2  shown in the figures will be adopted non-limitatively.  
         [0025]    The direction from back to front as being the longitudinal direction and from right to left referring to FIG. 4 will also be adopted.  
         [0026]    In the description which follows, identical, similar or analogous elements will be designated by the same reference numbers.  
         [0027]    [0027]FIG. 7 depicts an optical reflector  10  which is a hollow element produced in two parts. It has a concave insert  12  which is produced from a sheet of thermoformed plastic material  14 , and which has a concave front internal face  18  which is covered with a reflective layer  20  in particular an aluminium-based one, and a convex rear external face  22 .  
         [0028]    The optical reflector  10  also has a body  16 , which gives the reflector  10  its structural rigidity, and which is overmoulded around the rear face  22  of the insert  12 .  
         [0029]    FIGS.  1  to  5  depict the successive steps of a method of producing the optical reflector  10 , in accordance with the invention.  
         [0030]    According to a first step of the method, not depicted, a first front face  14   a  of the sheet  14  is coated with a reflective layer  20  based on aluminium or another reflective material, by any known method, for example by vacuum deposition or by another method such as Physical Vapour Deposition (PVD) or Plasma Enhanced Chemical Vapour Deposition (PECVD).  
         [0031]    A second step of the method depicted in FIGS.  1  to  3  consists of producing the insert  12  by forming the sheet  14 .  
         [0032]    For this, a die  24  is used, the shape of which is determined so that the first face  14   a  of the sheet  14  takes the shape of the die  24  in order to form the first faces  18  of the insert  12 .  
         [0033]    Here, the die  24  is in relief, and the sheet  14  is therefore positioned with its first face  14   a  facing the die  24  so that the front face  18  is concave.  
         [0034]    Conversely, and according to a variant, not depicted, the die  24  is hollow and the sheet  14  is then positioned with its first face  14   a  opposite to the die  24 .  
         [0035]    The sheet  14  is made from a thermoplastic material, for example polycarbonate. Thus, for forming of the sheet  14 , the latter is heated and then applied onto the die  24 . So that the sheet  14  takes the exact shape of the die  24 , a vacuum is produced between the sheet  14  and the die  24 , so as to flatten the sheet against the die  24 .  
         [0036]    In a variant, without departing from the scope of the invention, the thermoforming can be achieved by other technologies well known in the art.  
         [0037]    The insert  12  is then obtained which, as depicted in FIG. 3, is removed from the mould after cooling, and its edges  26  are cut.  
         [0038]    According to a third step of the method, depicted in FIGS. 4 and 5, the insert  12  is placed inside a mould  28  for overmoulding of the body  16  around the rear face  22  of the insert  12 .  
         [0039]    The mould  28  is formed of a fixed block  30  which has a protuberance  32  which has a shape complementary to that of the front face  18  of the insert  12 , which makes it possible on the one hand to position the insert  12  in the mould  28  and on the other hand to prevent the insert  12  becoming deformed during moulding.  
         [0040]    The mould  28  is also formed of a movable block  34  having an impression which defines a volume  36 , with the rear face  22  of the insert  12 , when the mould  28  is closed.  
         [0041]    The volume  36  is next filled with a thermoplastic substance in order to form the body  16  of the reflector  10 . The injected substance is a mixture of a thermoplastic material and a gas in liquid form. When the substance is inside the volume  36 , the gas expands, to form a foam. Such a substance and its use are for example described in the document EP-A-1 040 158.  
         [0042]    The formation of such a foam makes it possible to reduce the amount of material used.  
         [0043]    When the foam forming the body  16  has cooled, that is to say it has rigidified, the assembly is then removed from the mould and forms the reflector  10 .  
         [0044]    It will also be understood that simple mechanical inversions can constitute variant embodiments of the invention. For example, the material injected into the mould does not have to be in the form of a foam.  
         [0045]    Furthermore, the coating of the sheet  14  can be carried out before the cutting thereof, that is to say when it is still in a roll. Deposition of the reflective layer  20  can then be performed continuously, hence a considerable time saving compared with the prior art where each optical reflector was coated one after another with the reflective layer.  
         [0046]    One very important advantage of the invention is that it makes it possible to deposit reflective layers on materials whose surface condition is not excellent: even with initial surface conditions of average quality, reflectors of entirely satisfactory optical quality are obtained after deposition of the reflective layer. The appearance constraints which it was possible to have with the standard layer deposition methods can thus be avoided. Consequently, polymer materials with a significant filler level can be used for the material constituting the body of the reflector. These fillers can be introduced either for cost reasons, or as a filler for strengthening the polymer material. With the invention, the proportion of filler in the polymer can be at least 10% by weight, and possibly up to 60% to 70% by weight of the total weight of the body of the reflector, while obtaining a satisfactory level of optical surface quality after deposition of the reflective coating. For constituting the polymer of the reflector, the choice can be made for example from amongst one or more of the following polymers: polycarbonate, polyamide or polyethylene, either thermoplastic or thermosetting, preferably containing filler as mentioned above. The fillers can be organic or mineral, for example based on carbon or calcium carbonates, and can be in the form of particles, fibres, etc.