Patent Publication Number: US-2018036766-A1

Title: Method for producing a decorative part of a motor vehicle

Description:
TECHNICAL FIELD 
     This invention relates to a method for producing a motor vehicle part made of plastic. 
     BACKGROUND 
     More specifically, the method preferably seeks to produce a decorative part, undergoing a step of spraying a product such as a decorative coating, for example paint. This part is called a decorative part since it is partly visible from outside or inside the vehicle when mounted on the vehicle. The decorative part can be for example an exterior tailgate skin assembled on an inner lining, a spoiler, a side door, a bonnet, etc. 
     The decorative part could have an area to be protected, which must remain as clean as possible and free of sprayed product. According to a first example, the area to be protected is an attachment area for attaching the decorative part to another vehicle part, possibly by bonding. This attachment area must remain as clean as possible before applying glue to it, i.e. it must not be polluted or dirtied before attachment. However, the step of spraying product onto the decorative part generally carried out by spraying the part prior to its attachment to the part to be attached, may generate a deposit of fine paint droplets on the attachment area. The paint particles are in fact dispersed in a mist and may go to the other side of the part, being deposited in the form of a thin film. This may affect the quality of the subsequent attachment, especially the adhesion of the glue or of another material used in particular to create the bond. According to another example, the area to be protected is a section of the decorative surface intended to be free of paint, generally for decorative purposes. In this case also, paint must not be deposited on the area to be protected. 
     One solution to solve this difficulty consists in depositing on the area to be protected, before applying the paint, an adhesive film to mask the area to be protected. This film offers the advantage of not leaving a deposit detrimental to the bonding or appearance once it has been removed. The film is applied manually then removed at the end of the painting step, also manually. These manual steps are long, expensive and may lack repeatability. 
     SUMMARY 
     This invention seeks in particular to simplify the method for producing a motor vehicle part. 
     Thus, this invention relates to a method for producing a motor vehicle part made of plastic, the method comprising a step of spraying product and the part having an area to be protected during spraying, wherein it comprises the following steps:
         applying a fluid material forming a temporary masking layer on the area to be protected, this step being carried out automatically,   spraying the product onto the part, and   removing the temporary masking layer.       

     The temporary mask layer protects the area to be protected, preventing it from being covered with residues from the painting step. Since the layer is deposited by applying fluid material, an automatic method can be used to deposit it, using a robot. The method can therefore be faster, more precise and easily repeatable. 
     Note that the use of a temporary masking layer deposited automatically on the area to be protected is especially advantageous, unlike adhesive masking films. Such adhesive films cannot be applied automatically on a 3D part, since the film generates folds as soon as it must follow curves. 
     Note also that a masking jig could have been used to mask the area to be protected. Such a jig is a wall arranged in front of the area to be protected while spraying product onto the part, forming a protective screen. Nevertheless, the difficulty of such a jig is that it must match the shape of the part precisely to provide a perfect seal against the product sprayed, which is difficult since the parts and the jigs do not always have the same dimensions due to manufacturing tolerances. 
     Note that spraying product onto the part may for example be a step of spraying paint, varnish, chromium-plating and/or salts in solution. This step of spraying product may occur with or without prior surface treatment. 
     Furthermore, a plastic part means a part composed mainly of plastic. Obviously, it may include other particles and composite materials. 
     The method may further comprise one or more of the following characteristics, taken alone or in combination.
         The part has a decorative surface coated with the sprayed product and the area to be protected is a local area of the decorative surface free of sprayed product. In particular, the area to be protected is an area free of paint, so as to retain on a decorative part a visible unpainted section, for a stylish effect.   The part is a decorative part, the area to be protected is an area for attaching another motor vehicle part, called part to be attached, and the step of removing the temporary masking layer is followed by a step of attaching the part to be attached to the attachment area of the decorative part. A “part to be attached” means any part likely to be assembled with the decorative part, in particular any type of functional element. The attachment area is generally arranged on the side opposite the painted side. It is for example a bonding area. It is clear that the method proposed here is particularly interesting when attaching the part by a surface bond at the attachment area. In this case, in fact, it is advantageous to be able to present an extremely clean surface. For example, the part is attached by bonding, by applying a layer of glue on the attachment area. According to another example, the part is attached by additive printing or 3D printing, by applying a bonding layer created by printing. Note also that the step of attaching the part to be attached to the attachment area of the decorative part may take place with or without prior surface treatment.   The part to be attached is attached by applying a glue on the attachment area of the decorative part.   The part is attached by additive printing or “3D printing”, by applying a bonding layer deposited by printing. In this case, attachment can be carried out by mechanical anchoring or adhesion of the materials.   The temporary masking layer comprises a hot melt glue. Note that a hot melt glue is one which becomes fluid under the action of heat and which can therefore be easily be applied in liquid state. Once applied, the glue resolidifies on cooling, adhering to the surface without leaving any gap, and can therefore provide a seal against the paint and resistance to knocks during handling, thereby protecting the area to be protected during the step of spraying product such as paint. In addition, this cooled glue can be removed quickly and easily from the part due to its residual flexibility and good cohesion at ambient temperature, in order to remove the layer, preferably in one go, without fragmenting into many pieces. This hot melt glue is preferably not cross-linkable, i.e. on heating it again, it becomes fluid again and can easily be reused. Hot melt glues generally comprise as basic constituents thermoplastic polymers such as polyethylene, polypropylene, polyamide or EVA type copolymers, to which tackifying resins and additives are added. Hot melt glues are generally liquid above 110° C. and are hard below 90° C. An example of hot melt glue is marketed by Henkel under the name of “TEROSON CF 438”. A former name of this type of glue is “MACROPLAST CF 438”. A particularly interesting example of hot melt glue that can be used is a polyamide type glue, such as that marketed by the company Henkel under the name of “TECHNOMELT PA 2420” and the glue marketed by Henkel under the reference “HENKEL 6208”.   The temporary masking layer comprises a peelable varnish such as that marketed by the company Zuelch under reference 826-W1272-00 or by the company EFTEC under reference EFCOAT PO 380. This type of peelable varnish is generally composed of a PVC-based polymer (polyvinyl chloride). Another type of peelable varnish can be used, similar to peelable solder masks, known under the name of “blue solder masks”, such as that marketed by Atotech under reference “SD 2954”. Such masks can be applied by screen printing.   The temporary masking layer comprises a hot melt glue. The part is hooked to a hooking jig during the step of spraying product and the temporary masking layer is pressed against the jig to be held by bonding on the jig. The part is therefore bonded temporarily against the jig. This holds the decorative part securely during the painting step. Advantageously therefore, the same temporary layer acts as a layer masking the area to be protected and a layer sticking the part to the jig. Preferably, the decorative part is pressed against the jig automatically.   The temporary masking layer comprises a hot melt glue. The temporary masking layer is applied at a temperature greater than 110° C., preferably between 140° C. and 220° C., more preferably between 150° C. and 200° C. and the temporary masking layer is removed at a temperature less than 90° C., preferably between 10° C. and 40° C.   The step of removing the temporary masking layer is followed by a step of reusing the material which formed this temporary masking layer. For example, a temporary masking layer comprising a non-cross-linked hot melt glue can be reused. Reuse may comprise a remelting step at a temperature greater than 140° C. in a melting kettle. A step of cleaning the material to be reused could be planned, for example by filtering the material according to observed physical properties such as density, mechanical effect, particle size, etc.   The temporary masking layer comprises a hot melt glue having a melting point Tc different from, and preferably relatively low compared with, a melting point Tp of the sprayed product. This is particularly advantageous to simplify recycling, or even reuse, of the hot melt glue. With a different melting point, in fact, the glue can be separated from the product, for example by melting the glue while maintaining the product in non-liquid state. A filter can therefore be used to remove the product. For example, the melting point Tc of the glue is less than 150° C., preferably less than 130° C., more preferably about 100° C.; and/or the melting point Tp of the sprayed product is greater than 150° C., preferably greater than 180° C., more preferably about 200° C. According to an example, the hot melt glue is polyamide (PA) of melting point Tc about 100° C., and the sprayed product is a polyurethane type polymer paint of melting point Tp about 200° C.   The step of reusing the temporary masking layer comprises the following steps: recovering the temporary masking layer after removing it, grinding it, drying it, introducing it into an extruder at a temperature T greater than the melting point of the masking layer and less than the melting point of the sprayed product, removing the product by filtering and/or recovering the material to form a new masking layer. Advantageously, during extrusion, an additive can be added, for example one or more stabilizers, to the masking layer so that it can be recycled several times. Note that all the steps can be implemented and in the order given, or some of them can be omitted or replaced by other steps, or the order of implementation can be changed. The solution proposed here allows operational recycling of the masking layer, while ensuring a certain purity, without product residues such as paint residues.   The step of removing the temporary masking layer is followed by a step of degreasing and/or of surface treatment of the area to be protected. The surface treatment can be flaming, treatment by plasma technology and/or by corona effect. Degreasing can be carried out for example using a wipe.   The temporary masking layer is deposited by a configurable process control system, such as a six-axis robot, to predefine the width and thickness of the temporary masking layer. For example, the width is between 10 mm and 30 mm (millimeters), about 20 mm, and the thickness is between 1 mm and 3 mm.   A step of curing the sprayed product layer takes place after the step of applying the temporary masking layer on the area to be protected, preferably at a temperature greater than 80° C. It is for example a paint curing step.       

     The invention further relates to the use of a method as described above for producing a motor vehicle exterior part, such as a tailgate skin, a front or rear bumper skin, a front or rear wing, a window pillar, a rocker box, a side door, a bonnet, a convergent such as a roof spoiler. 
     The invention also relates to an installation for producing a motor vehicle part comprising plastic material, including the following devices arranged one after another on a part production line:
         a device for automatically applying fluid material forming a temporary masking layer on the part,   a device for spraying product onto the part,   a device for removing the temporary masking layer,   and optionally a station for reconditioning the temporary masking layer for reuse.       

     Advantageously, the station for reconditioning the temporary masking layer comprises an extruder at a temperature T, T being greater than the melting point of the masking layer and less than the melting point of the sprayed product, and provided with a filter, for example a rotary filter, to separate the masking layer from the product. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on reading the accompanying figures, which are given solely by way of example and not limiting in any way, in which: 
         FIG. 1  is a diagrammatic exploded perspective view of a tailgate outer skin made according to one embodiment of the method and assembled on an interior tailgate lining, 
         FIG. 2  is an interior view of the outer skin of  FIG. 1 , and 
         FIG. 3  is a diagram illustrating a method for producing the outer skin of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     We refer to  FIG. 1 . A motor vehicle decorative part  10  produced according to the method described above may be a tailgate outer skin. This outer skin  10  is intended to be attached to a tailgate lining  12 , so as to form a tailgate  14 . 
     In this example, the decorative part  10  has two sections  10   a,    10   b:  a lower outer skin  10   a,  for covering the lower section of the lining  12 , and an upper outer skin  10   b,  for covering the upper section of the lining  12 . The decorative part  10  is generally made of injected thermoplastic material, for example polypropylene. It has a decorative surface which is painted, visible from outside the vehicle, corresponding to the outer surfaces of the skins  10   a  and  10   b,  illustrated diagrammatically on  FIG. 1 . It also has an inner surface corresponding to the inner surfaces of the skins  10   a  and  10   b,  illustrated diagrammatically on  FIG. 2 . The inner surface of the skins  10   a,    10   b  is not intended to be visible once the tailgate is assembled and mounted on the motor vehicle. It is a surface which can be qualified as “technical”, in the sense that it is not intended for decoration and may therefore have technical features including means for attaching the decorative part  10  to the tailgate lining  12 . 
     The tailgate lining  12  has the general shape of a frame having an opening  16  for receiving a rear window. This tailgate lining  12  is for example made of a reinforced thermosetting material. 
     In this example, the decorative part  10  is attached to the tailgate lining  12  by bonding. The decorative part  10  therefore has two areas to be protected  18   a  and  18   b,  i.e. attachment areas  18   a  and  18   b,  arranged respectively on the inner surface of the skins  10   a,    10   b.  The attachment areas  18   a  and  18   b  are shown in dotted lines on  FIG. 2 . At the end of the method for producing the decorative part  10 , these attachment areas  18   a  and  18   b  will be covered with bonding tracks, then bonded to the tailgate lining  12  to permanently attach the decorative part  10  to the tailgate lining  12 . 
     The method for producing the decorative part  10  will now be described, with reference to  FIG. 3 . It is understood that the steps for producing the “decorative part  10 ” are described generally for simplification purposes, but these steps are implemented firstly on the lower skin  10   a,  secondly on the upper skin  10   b,  generally independently. 
     The method starts by an injection  20  of material in a mold to form the body of the decorative part  10  by molding, more precisely to form firstly the lower skin  10   a  and secondly the upper skin  10   b.  Before painting the decorative part  10 , a step  22  is carried out in which a temporary masking layer  22   a,    22   b  is applied on the area to be protected  18   a,    18   b.  Step  22  is carried out automatically by a robot. The layer  22   a,    22   b  is produced at the same position as the area to be protected  18   a,    18   b.  Thus, it is shown fictitiously on  FIG. 2 , although it is not present on the final decorative part  10  shown on  FIG. 2 . 
     In this example, the temporary masking layer  22   a,    22   b  comprises a hot melt glue as described above. The step  22  of applying the temporary masking layer  22   a,    22   b  is carried out at a temperature greater than 110° C., preferably between 140° C. and 220° C., more preferably between 150° C. and 200° C. Furthermore, step  22  is carried out by a configurable robot to predefine the width and thickness of the temporary masking layer  22   a,    22   b.  For example, the width of the layer  22   a,    22   b  is about 20 mm and the thickness is between 1 mm and 3 mm. In this case, the robot displacement speed is between 50 mm and 200 mm per second, with a glue flow rate of between 20 and 80 rpm (revolutions per minute). Furthermore, the layer is applied by means of an extended nozzle positioned as close as possible to the part, with a distance of between 2 mm and 8 mm between the part and the end of the nozzle. The glue is therefore deposited more precisely. In addition, so that the layer can be spread over a bonding track of a certain length, for example 25 mm, the viscosity of the glue can be varied, depending on the glue temperature, type and reference. 
     The step  22  of applying the temporary masking layer  22   a,    22   b  on the area to be protected  18   a,    18   b  is followed by a step  24  in which a product, in this case paint, is sprayed onto the decorative part. This step  24  is carried out by spraying paint onto the decorative surface corresponding to the outer surfaces of the skins  10   a  and  10   b.  In this example of spraying paint, step  24  includes a step of curing in an oven for a period between 20 and 30 minutes per part and is carried out at a temperature of about 80° C. The inner surfaces of the skins  10   a  and  10   b  are not painted directly, but indirectly receive paint residues deposited in the form of a thin film, due to the paint mist generated by spraying. Thus, the temporary masking layer  22   a,    22   b,  which is sealed at this temperature, probably receives paint droplets instead of the area to be protected  18   a,    18   b  that it covers. 
     According to a particularly advantageous improvement, during step  24  of applying paint, the decorative part  10   a,    10   b  is hooked to a hooking jig and the temporary masking layer  22   a,    22   b  is pressed against the jig to be held by bonding on the jig. 
     Step  24  of spraying product is followed by a step  26  in which the temporary masking layer  22   a,    22   b  is removed. For example, the end of the layer is raised and pulled. This step of removing the layer  22   a,    22   b  is carried out at a temperature less than 90° C., preferably between 10° C. and 40° C. At this temperature, the hot melt glue forming the layer  22   a,    22   b  is cooled and flexible and can be removed quickly and easily without fragmenting. 
     Step  26  of removing the temporary masking layer is followed by a step  30  of degreasing, and optionally of surface treatment of the attachment area. 
     According to a particularly advantageous embodiment, the step  26  of removing the temporary masking layer is also followed by a step  28  of reusing the material which formed this temporary masking layer. According to an example, the temporary masking layer comprises a hot melt glue having a melting point Tc that is relatively low compared with a melting point Tp of the sprayed product. According to an example, the hot melt glue is polyamide (PA) of melting point Tc about 100° C., and the sprayed product is a polyurethane type polymer paint of melting point Tp about 200° C. The step  28  of reusing the temporary masking layer comprises the following steps: recovering the temporary masking layer after removing it, grinding it, drying it, introducing it into an extruder at a temperature T of about 120° C.-130° C., therefore greater than the melting point Tc of the masking layer and less than the melting point of the sprayed product. Thus, the masking layer melts, but not the sprayed product. The product is then removed at end of extrusion by filtration using a coarse rotary filter to separate the masking layer from the product. The material is then recovered to form a new masking layer. Advantageously, during extrusion, an additive can be added, for example, stabilizers and/or other additives, to the masking layer so that it can be recycled several times. 
     Step  30  is then followed by a step  32  in which the part to be attached is attached to the attachment area of the decorative part, i.e. in this example a step in which the tailgate lining  12  is attached to the attachment area  18   a,    18   b  of the decorative part  10 . The attachment areas  12   a,    12   b  have been shown diagrammatically on  FIG. 1 . 
     At the end of this step  32 , the tailgate  14  is obtained, ready to be assembled on the vehicle. 
     Such a production method is implemented on an installation comprising the following devices arranged one after another on a line for producing the part  10 .
         a device for automatically applying fluid material forming a temporary masking layer  22   a,    22   b  on the part,   a device for spraying product onto the part  10 ,   a device for removing the temporary masking layer  22   a,    22   b.          

     In addition, the installation preferably includes a station for reconditioning the temporary masking layer for reuse. Advantageously, this station comprises an extruder at a temperature T, T being greater than the melting point of the masking layer and less than the melting point of the sprayed product, and provided with a filter, optionally a rotary filter, to separate the masking layer from the product. The filter is preferably coarse, i.e. with filter meshes of between 150 μm and 600 μm (micrometers). 
     According to another embodiment, the part to be produced has a decorative surface coated with the sprayed product and the area to be protected is a local area of the decorative surface free of sprayed product. For example, the outer surfaces of the skins  10   a  and  10   b  of the decorative part  10  have a local unpainted area, free of paint and which is to be protected in the same way as the areas to be protected  18   a,    18   b.  It is understood that the method described above applies similarly to such areas to be protected and that the entire description can be read with such an application. 
     The invention is not limited to the embodiments described and other embodiments will be clearly apparent to those skilled in the art. 
     In particular, the method is used to manufacture a tailgate skin  10  but many other parts can be produced using this method, in particular a front or rear bumper skin, a front or rear wing, a window pillar, a rocker box, a side door, a bonnet, a convergent such as a roof spoiler. 
     In addition, according to another example, the temporary masking layer  22   a,    22   b  comprises a peelable varnish instead of the hot melt glue. For example, a peelable varnish such as that marketed by the company Zuelch under reference 826-W1272-00 is used. This varnish is deposited automatically, with a drying time of about two hours at ambient temperature. The drying time can be reduced by increasing the temperature, for example 15 minutes drying at a temperature of about 140° C. 
     Furthermore, the attachment of the decorative part  10  and of the part to be attached  12  is, in the example described above, by bonding, but other types of attachment are possible, in particular other types of attachment by surface bonding requiring a very clean surface to be attached, with no residues. In particular, attachment by mechanical hooking on a relatively large area can be considered, during which a bonding layer produced by additive printing or 3D printing is applied to the attachment area, in order to hook the part to be attached to the decorative part  10 .