Abstract:
A mold for producing a floor made from a polymer material for a motor vehicle, comprising at least one metal insert in the form of sheet metal that is partially overmolded, i.e. having a portion intended to be covered with polymer material and a portion intended to not be covered in this way, the mold comprising a die and a punch defining a cavity, the cavity comprising a location intended to receive the portion of a metal insert intended to be covered with polymer material, wherein the mold, in this location of the cavity, comprises gadroons on the die and on the punch, the gadroons having a thickness that is greater than a predefined minimum thickness (H). The invention also concerns a floor obtained by this method.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase application of PCT Application No. PCT/FR2013/053269 filed Dec. 27, 2013, which claims priority to the French application 1262949 filed on Dec. 28, 2012, which applications are incorporated herein by reference and made a part hereof. 
     FIELD OF THE INVENTION 
     This invention relates to the technical field of plastic floors for motor vehicles. 
     DESCRIPTION OF THE RELATED ART 
     The manufacture of a motor vehicle starts by a step of metalworking, which consists in producing a body in white, by welding together various metal parts, body which is then immersed in a cataphoresis bath to treat the surfaces of the metal parts and the spot welds against corrosion. 
     After the cataphoresis stage, the body undergoes various assembly and painting steps until a complete vehicle is obtained. 
     It is known that the use of plastic parts in the constitution of a vehicle is a success factor in the search to reduce fuel consumption, thanks to the weight reduction obtained by replacing a metal part by a lighter plastic part. 
     However, the existence of already operational assembly lines organized in the order indicated above, namely first metalworking and cataphoresis, followed by painting and assembly, makes it difficult, even financially impossible, to develop the vehicle manufacturing processes in which this order is not respected. 
     It is therefore to comply with the existing assembly lines that the manufacture of plastic parts with metal inserts to be welded to other metal parts during the metalworking step has already been proposed. In particular, document FR2915129 discloses a motor vehicle floor having inserts partially overmolded in a layer of AMC (Advanced Molding Compound), BMC (Bulk Molding Compound) or SMC (Sheet Molding Compound), a thermosetting material particularly suited to this application due to its mechanical strength and its ability to undergo the cataphoresis step without degrading. The inserts comprise areas not covered by the plastic material, on which welding can be carried out, in order to assemble the floor with other metal parts during the metalworking. 
     One of the difficulties that arise during the manufacture of a plastic floor fitted with overmolded welding inserts is that good mechanical continuity must be guaranteed between the welding inserts and the rest of the floor. 
     The invention aims to overcome this difficulty. 
     SUMMARY OF THE INVENTION 
     To this end, the invention relates to a mold for producing a floor made from a polymer material for a motor vehicle, comprising at least one metal insert in the form of sheet metal that is partially overmolded, i.e. having a portion intended to be covered with polymer material and a portion intended to not be covered in this way, the mold comprising a die and a punch defining a cavity, the cavity comprising a location intended to receive the portion of a metal insert intended to be covered with polymer material, wherein in this location of the cavity, the mold comprises gadroons on the die and on the punch, the gadroons having a thickness that is greater than a predefined minimum thickness (H). 
     Preferably, the gadroons are integral with the mold. They are formed integrally with the inner wall of the die or the punch. 
     The thickness (H) is determined, for a given polymer material, as being the minimum thickness of polymer material with which the insert must be covered so that the floor is robust, i.e. so that it complies with the specifications of the mechanical stresses that have been defined during its design, in view of its intended use. 
     In a preferred embodiment, the thickness (H) is greater than or equal to 1 mm, preferably between 2 mm and 4 mm. 
     Thanks to the invention, the gadroons prevent the sheet metal from being too close to the punch and the die, therefore to the outer faces of the floor. Thus, there is enough plastic material on each side of the sheet metal to guarantee good mechanical continuity between the inserts and the rest of the floor. 
     Optimally, the gadroons have the same height on the punch and on the die, in order to hold the insert in the middle of the plastic material thickness around the insert. 
     Another difficulty lies in the fact that the formation of thin layers of plastic near the gadroons must be avoided. In fact, thin layers of plastic material left on the surface of the inserts may become detached during the cataphoresis, which would lead to pollution of the cataphoresis bath. 
     In order to overcome this additional disadvantage, according to one embodiment of the invention, each gadroon is dimensioned to come into contact with the insert by its free end, after closing the mould, and this free end by which the gadroon comes into contact with the insert is shaped to prevent the plastic material from covering the sheet metal at said end. The quality of this contact depends in particular on the pressure applied by the ends of the gadroons against the inserts, this pressure depending itself directly on the mould closing force and the orientation of the gadroons. 
     In order to overcome this additional disadvantage, according to one embodiment of the invention, each gadroon is dimensioned to come into contact with the insert by its free end, after closing the mold, and this free end by which the gadroon comes into contact with the insert is shaped to prevent the plastic material from covering the sheet metal at the end. The quality of this contact depends in particular on the pressure applied by the ends of the gadroons against the inserts, this pressure depending itself directly on the mold closing force and the orientation of the gadroons. 
     As a variant, the free end by which the gadroon comes into contact with the insert is shaped so that if there is a material infiltration between the end and the insert, the material is not likely to detach in the various situations that the floor will encounter during its life. 
     The insert is therefore clamped between the gadroons, which can be opposite one another or offset. The choice between these two possibilities (gadroons opposite each other or offset) depends in particular on the thickness of the insert and especially on its stiffness, if it is to be prevented from buckling between its support if the gadroons are offset. 
     Thus, no thin layer of plastic material can form on the uncovered parts of the inserts. 
     In a special embodiment, the gadroons are mainly distributed in a repetitive arrangement, for molding under a pressure of 100 bars, with inserts made from sheet steel metal for bodywork of type DC04+ZE according to standard NF EN 10152 (XES quality index according to the former standard NF 36-401), the sheet metal thickness being between 0.7 mm and 2 mm, preferably between 0.5 mm and 0.8 mm, the length of the pattern being comprised between 20 mm and 100 mm, preferably between 20 mm and 55 mm. These various values, although presented together, may be separated from each other to form separate and independent characteristics. 
     The term “mainly” is taken to mean the fact that more than half the gadroons are arranged in the repetitive arrangement. 
     The technical effect of the repetitive arrangement is to hold the sheet metal in position over its entire length. 
     The length of the repetitive arrangement is defined according to the molding pressure, the material and thickness of the sheet metal and the mold closing direction. 
     In another special embodiment, on a portion of the overmolded metal insert, which extends in a direction not perpendicular to the mold closing direction, each arrangement comprises three gadroons, two being close together, and the distance between these two close gadroons is from 10 mm to 15 mm. 
     Advantageously, the gadroons are shaped to facilitate demolding of the floor. 
     For example, for a portion of the overmolded metal insert extending not perpendicularly to the mold closing direction, the contact end of the gadroon has a substantially trapezoidal shape, with a base of 1 mm to 2 mm and a draft angle of 1 degree to 3 degrees. 
     However, for another portion of the overmolded insert extending perpendicular to the mold closing direction, the contact end of the gadroon may have a rectangular shape, due to the fact that demolding is carried out in the direction perpendicular to the contact surface. 
     Finally, the invention relates to a floor made from polymer material for a motor vehicle, comprising at least one metal insert in the form of sheet metal that is partially, overmolded, wherein it comprises, in the thickness of the polymer material, at least one opening leading to the insert. 
     This or these opening(s) correspond to the locations of the gadroons which are in contact with the inserts during overmolding. 
     In one embodiment, the plastic floor has a thickness of 2 mm to 5 mm and comprises at least one partially overmolded insert in the form of sheet steel metal for bodywork of type DC04+ZE according to standard NF EN 10152 (XES quality index according to the former standard NF 36-401), of thickness from 0.7 mm to 2 mm. 
     According to the invention, the term “polymer” designates both homopolymers and copolymers (in particular binary or ternary). Homopolymers are linear or branched polymers of the same monomer. Some non-limiting examples of copolymers are as follows: random copolymers, alternating copolymers, block copolymers and graft copolymers. 
     A mixture of polymers or copolymers may also be used, as well as a mixture of polymer materials with inorganic, organic and/or natural fillers such as, for example, but not limited to: carbon, salts and other inorganic derivatives, natural or polymer fibers. It is also possible to use multilayer structures consisting of stacked layers bonded together comprising at least one of the polymers or copolymers described above. 
     One polymer often used is polyester, especially in a bulk (BMC or AMC) or sheet (SMC) molding compound. 
     These and other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
       It will be easier to understand the invention on reading the description below, given as an example and referring to the drawings, on which: 
         FIGS. 1A-1F  are a set of six drawings showing a mold in an implementation of the method according to the invention; 
         FIG. 2  is a perspective view of a lateral insert of a floor; 
         FIG. 3  is a perspective view of a front insert of a floor; 
         FIG. 4  is a perspective view of a rear insert of the floor of  FIG. 9 ; 
         FIG. 5  is a view along V of a portion of a floor; 
         FIG. 6  is a view along VI of a portion of the floor of  FIG. 7 ; 
         FIG. 7  is a perspective view of a floor produced by molding using the inserts of  FIGS. 2 to 4 ; 
         FIG. 8  is a sectional view along VIII-VIII of  FIG. 7 ; 
         FIG. 9  is a sectional view along IX-IX of  FIG. 7 ; 
         FIG. 10  is a close-up view of portion X of drawing of  FIG. 1 ; 
         FIG. 11  is a perspective view of a rear gadroon of the mold; 
         FIG. 12  is a sectional view along XII-XII of  FIG. 5 ; and 
         FIG. 13  is a sectional view along XIII-XIII of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     We now refer to  FIGS. 1A-1F , which includes six drawings ( FIGS. 1A, 1B, 1C, 1D, 1E and 1F ) representing a two-part mold during six successive steps of a method for manufacturing a part made from polymer material. 
     The mold is mounted on a press (not shown) having a fixed platen  1  and a movable platen  3 . 
     The mold comprises a die  5  which is mounted on the fixed platen  1  of the press, and a punch  7 , mounted on the movable platen  3 . The die is the female (concave) half of the mold, while the punch is its male (convex) half. 
     The mold is of the compression chamber type, which means that during its closing movement from its open position ( FIGS. 1A, 1B, 1C and 1D ), there comes a point where the mold cavity is a closed space  9 , as shown in  FIG. 1D . At this time, the mold is in a closed filling position. The volume of the space  9  is of course greater than that of the mold cavity, which is defined when the mold has reached the molding position, and which corresponds to the finished part. During all the closed filling positions and until the molding position ( FIG. 1E ), the punch  7  slides like a piston along a surface defining a compression chamber. The sealing of the mold with respect to the material flowing in the mold to produce the finished part is in principle provided at the compression chamber. 
     In the case of the mold of  FIGS. 1A, 1B, 1C, 1D, 1E and 1F , the surface delimiting the compression chamber is created by a peripheral movable block  11 , which can take a retracted position ( FIG. 1A ) in the movable part  3  of the press and an advanced position ( FIG. 1D ). This movable block  11 , which, although drawn as a single piece may consist of several pieces, performs two functions. 
     Firstly, by its inner face  11   b , it forms the above-mentioned surface against which the punch  7  slides. To this end, a gap (not shown) is left between the inner surface  11   b  of the movable block  11  and the punch  7 . This gap also acts as a vent, allowing gases trapped in the cavity to escape while the mold is being filled with the polymer material. 
     Secondly, by its lower face  11   a , the block  11  forms, with a blocking area having a face  13  provided on the die  5 , opposite the face  11   a  of the movable block  11 , a means of blocking one or more inserts which will be described later. The movable block  11  is made in a single piece with a frame which is mounted on rams  12  carried by the punch  7  in order to move up to the blocking area  13  before the punch  7  engages by sliding in the compression chamber. In a variant not shown, the block  11  is a part separate from the frame, mounted on the lower side of the frame. 
     The perspective views of  FIGS. 2, 3 and 4  show respectively lateral  15 , front  29  and rear  27  metal inserts, which are adapted to be placed in the mold in open position. Each insert has an elongated shape which is rectilinear for the lateral  15  and rear  29  inserts of  FIGS. 2 and 4 , and non-rectilinear for the front insert  27  of  FIG. 3 . 
     As shown in  FIG. 1B , the lateral insert  15  is positioned on the die  5  so that a portion  15   a  of the insert covers face  13  of the blocking area and so that a portion  15   b  of the insert is housed in the mold cavity. The portion  15   b  housed in the mold cavity is a part of the insert  15  which is to be overmolded. The portion of each insert which is to be overmolded will be designated the “overmolded portion”. The portion  15   a  covering the face  13  is a part which is intended to protrude from the molded part. This latter portion  15   a  of the insert  15  will be designated the “uncovered portion”. As shown on  FIG. 2 , the boundary  15   c , called the overmolding limit, between portion  15   b  intended to be overmolded and uncovered portion  15   a , is an imaginary line schematized by a dashed line on  FIG. 2 . The same overmolding limit  27   c  exists between the uncovered  27   a  and overmolded  27   b  portions of the front insert  27 . The same applies with the overmolding limit  29   c  of the rear insert  29  between its uncovered portion  29   a  and its overmolded portion  29   c , as shown on  FIGS. 3 and 4 . 
     The overmolding limits  15   c ,  27   c  and  29   c  extend from a first end A of each insert to an opposite end B of the insert, without leaving the insert and following approximately its edge  15   d ,  27   d ,  29   d  which is not necessarily rectilinear. 
     Each of the inserts has special contours which are specific to the floor model to be obtained. Similarly, each insert has a transverse cross-section bent at a special angle which may be different from one insert to another or from one location to another over the length of a given insert. These shape characteristics of the inserts will not be described in detail in this description. 
     In the overmolded portions  15   b ,  27   b ,  29   b , each insert  15 ,  27 ,  29  comprises attachment holes  21  intended to be filled by the plastic material to hold the sheet metal securely, as is known by those skilled in the art. In the portion  15   a , the lateral insert  15  comprises centering holes  19  to center it in the direction of its length, i.e. the X direction (see  FIG. 2 ). 
     When an insert  15  is positioned in the mold with its uncovered portion  15   a  between the block  11  and the blocking surface  13 , the overmolded portion  15   b  of the insert  15  is in the mold cavity at a position  8  ( FIG. 1E ) which, in this example, is on the periphery of the cavity. 
     While  FIGS. 1A-1F  represents only the insert  15 , the related explanations are also valid for inserts  27  and  29 . 
     In the position  8 , the mold comprises lateral  10  ( FIG. 10 ), front (not shown) and rear  30  (see  FIG. 11 ) gadroons on the die  5  and on the punch  7  or the block  11 . The gadroons  10 ,  30  are integral shapes of the mold, formed integrally with the inner wall of the die  5  or the punch  7 . The gadroons  10 ,  30  are arranged to come into contact with the inserts  15  on either side, in order to hold them in a predetermined position while molding the floor, away from the walls of the cavity. 
     Each gadroon  10 ,  30 , is dimensioned to come into contact with the insert  15 ,  27 ,  29  by its free end  32 ,  33  ( FIG. 11 ), after closing the mold, and the free end  32 ,  33  coming into contact with the corresponding insert is shaped to prevent the plastic material  11  from covering the sheet metal at the free end  32 ,  33 . 
     The perspective view of  FIG. 11  shows the rear gadroon  30  in detail. The rear gadroon  30  is shaped to facilitate demolding of the floor along the Z direction. In this example, the free end  33  of the gadroon (intended to come into contact with the insert) has a substantially trapezoidal shape whose width increases along the demolding direction Z, with a base e of 1 mm to 2 mm, a draft angle of 1 degree to 3 degrees along the length L of 25 mm and an overmolding thickness H of 2 mm. 
     As shown in  FIG. 6 , the rear gadroons  30  are mainly distributed in a repetitive arrangement  43 , for molding under a pressure of 100 bars, with inserts made from sheet steel metal of type XES (Standard NF 36-401), the thickness of the sheet metal being 0.67 mm. The length P of the pattern is 50 mm. 
     Each arrangement  43  comprises three gadroons  30 , two being close together, the distance E between these two close gadroons being 10 mm. 
     The lateral gadroons  10  are shaped to facilitate demolding of the floor along the Z direction. In this example, the contact end  33  of the gadroon  10  has a substantially rectangular shape whose overmolding length L is 25 mm and the width is 1 mm to 2 mm (preferably 1 mm). Its thickness H is 2 mm. 
     As shown in  FIG. 5 , the lateral gadroons  10  are mainly distributed in a repetitive arrangement  41 , for molding under a pressure of 100 bars, with inserts made from sheet steel metal of type XES (Standard NF 36-401), the thickness of the sheet metal being 0.67 mm. The length P of the pattern is 50 mm. 
     Each arrangement  41  comprises two gadroons  10 , the distance between these two gadroons  10  being from 10 mm to 15 mm. 
     The material used for molding is a thermosetting plastic of type SMC (sheet molding compound). It is deposited in the open mold, on the die  5 , substantially in the middle of the cavity although this is not a necessity, as a blank  17 , as is known. The step of depositing the material in the mold in open position is illustrated by  FIG. 1C . 
     Referring to  FIGS. 1A-1F , we will now describe six successive steps of the molding method according to the invention. 
     In a first step, illustrated by  FIG. 1A , the mold is in open position: the punch  7  is far away from the die  5  and the movable block  11  is retracted in the punch  7 . 
     In a second step, illustrated by  FIG. 1B , the front  17 , lateral  15  and rear  29  inserts are positioned on the blocking area  13  of the die  5 , with their overmolded portions  15   b ,  27   b  and  29   b  housed in position  8  of the cavity and resting on the gadroons  10 ,  30  of the die  5 . 
     In a third step, illustrated by  FIG. 1C , an SMC sheet  17  is deposited in the middle of the cavity, on the die  5 . The second and third steps may be combined or performed in a different order. 
     In a fourth step, not shown, the movable part  3  is brought close to the fixed part  1 , but without closing the mold, in other words without any of the parts of the upper half of the mold, namely the punch  7  and the movable block  11 , touching one of the parts of the lower half of the mold, namely the die  5 . The mold has therefore not yet reached its closed filling positions. 
     In a fifth step, the rams  12  are actuated to lower the movable block  11  towards the blocking area  13  of the die, as illustrated by  FIG. 1D . In so doing, face  11   a  of the movable block  11  moves against the uncovered portions  15   a ,  27   a  and  29   a  of the front  17 , lateral  15  and rear  29  inserts, firmly blocking them in position against face  13  of the blocking area. 
     Thus positioned, the movable block  11  performs its two functions: firstly, it holds the inserts in position ready to fill the mold cavity by flowing of the material  17 , and secondly it forms a compression chamber. The mold cavity, delimited by the punch  7 , the movable block  11  and the die  5 , is therefore closed, but does not yet define the shape of the part to be obtained. The mold is in closed filling position. 
     In a sixth step, illustrated by  FIG. 1E , the movable part  3  of the press continues its path towards the fixed part  1 . In so doing, the punch  7  continues its movement by sliding like a piston in the block  11 . The block  11  remains stationary resting against the inserts  15 , while the rams  12  move back without reducing the pressure holding the inserts. The inner face  11   b  of the movable block  11  forms the lateral wall of the compression chamber against which the punch  7  slides. Due to the reduction in cavity volume and the molten state of the plastic material  17 , the plastic material  17  flows into the mold cavity filling it completely, in particular covering portions  15   b ,  27   b  and  29   b  of the inserts  15 ,  27 ,  29  which are housed in position  8  of the mold cavity. When the punch  7  has stopped moving, the gadroons  10 ,  30  of the punch have come into in contact with the overmolded portions  15   b ,  27   b ,  29   b  of the inserts and the mold is in the molding position. The plastic material  17  has completely filled the mold cavity, but has been prevented by the gadroons  10 ,  30  from coming into contact at their positions with the overmolded portions  15   b ,  27   b ,  29   b  of the inserts. Since the block  11  is separated from the punch  7  by the rams  12 , the pressure of the block  11  on the uncovered portions  15   a ,  27   a ,  29   a  of the inserts  15 ,  27 ,  29  is not disturbed by the pressure of the gadroons  10 ,  30  on the overmolded portions  15   b ,  27   b ,  29   b  of the inserts. In other words, no hyperstatism occurs. 
     The volume of the cavity between the punch  7 , the movable block  11  and the die  5  is that of the finished part. 
     In a seventh step, illustrated by  FIG. 1F , the movable part  3  of the press starts to move in the mold opening direction. This time the movable block  11  remains fastened to the punch  7  and rises up with it, thereby returning the mold to the fully open position. Ejectors  19 , provided in the die  5 , raise the molded floor  20 . The latter is then released and the ejectors  19  retracted, in order to start a molding cycle from the step illustrated by drawing  FIG. 1A . 
     Since the molding direction is vertical, i.e. in the Z direction, we see that that overmolded portions  15   b ,  27   b  of the lateral  15  and front  27  inserts are not perpendicular to the molding direction, while the overmolded portion  29   b  of the rear insert  29  is horizontal, i.e. perpendicular to the molding direction. This can be seen more clearly in  FIGS. 8 and 9 , illustrating respectively a sectional view of the lateral insert  15  and of the rear insert  29 . 
     In another embodiment example not shown, the inserts are inclined relative to the molding direction. 
     The thickness of the sheet metal of the inserts is from 0.7 mm to 2 mm and the thickness H of the plastic material is from 2 mm to 5 mm. 
       FIGS. 5 and 7  show the overmolded portion  15   b  in dotted lines, covered by the plastic material  17 . Notches  51  are formed in the plastic material  17  by the gadroons  10  of the mold. In other words, the floor  20  has openings (notches  51 ) leading to the inserts, in the thickness of the plastic material  17 . 
     We can understand that the shape of a notch is complementary to the shape of a gadroon. 
     Thus, as shown on  FIG. 5 , in each area  53  of the overmolded portion  15   b  which has a notch  51  complementary to a gadroon  10 , the insert  15  is not covered by the plastic material. Each area  53  extends over the entire flat section of the overmolded portion  15   b  of the insert, but stops in the rounded area of the insert which leads to its uncovered portion  15   a . In other words, between the flat section of the overmolded portion  15   b  and up to the overmolding limit  15   c , the rounded area of the insert is covered with plastic material  52 , although the gadroon is located here. 
     To ensure that the plastic material  52  does not detach from the rest of the floor during the lifetime of the floor, the free end  32  of the gadroon  10  may be provided with a recess which, at the boundary between the area  53  and the plastic material  52 , clearly separates the free end  32  from the rounded area of the insert, so that the plastic material layer  52  is sufficiently thick and therefore strong. 
     The diameters of the attachment holes  21  may vary from 4 mm to 10 mm. Preferably, the diameter of the attachment holes  21   a  which are located between the two close gadroons  10   a  and  10   b  is 5 mm. The diameter of the other attachment holes is 6 mm. The distance between each close gadroon  10   a  or  10   b  and its neighboring gadroon is 19 mm. 
       FIG. 6  shows the overmolded portion  29   b  and the non-overmolded portion  29   a  of the rear insert  29 . The overmolded portion  29   b  is covered by the plastic material  17 , except in each area  63  of the overmolded portion  29   b  where there is a notch  61  complementary to a gadroon  30 . The notches  61  are formed by the rear gadroons  30  of mold which are distributed in the repetitive arrangement  43 . 
     The diameters of the attachment holes  21  may vary from 4 mm to 10 mm. Preferably, the diameter of the attachment holes  21   a  which are located between the two gadroons with the same pattern  43  is 5 mm. The diameter of the other attachment holes is 6 mm. 
       FIG. 12  shows a sectional view of a notch  51  on the lateral insert  15 . The edge  55  of the notch  51  is substantially perpendicular to the overmolded portion  15   b  of the insert  15 . We can understand that the wall of the gadroon  10  corresponding to this edge is substantially perpendicular to this overmolded portion  15   b  of the insert  15 . In this embodiment, two gadroons  10  are present opposite each other on the die  5  and the punch  7 , so as to form two notches  51  on each side of the same area  53  of the insert  15 . 
       FIG. 13  shows a sectional view of a notch  61  on the rear insert  29 . The edge  65  of the notch  61  is inclined relative to the overmolded portion  29   b  of the insert  29 , with an angle of 30° to 87° to facilitate demolding in the Z direction. We can understand that the wall of the gadroon  30  corresponding to this edge is also inclined relative to the Z direction. In this embodiment, two gadroons  30  are also present opposite each other on the die  5  and the punch  7 , so as to form two notches  61  on each side of the same area  63  of the insert. 
     In another embodiment not shown, the gadroons  10  or  30  are arranged offset to each other on the die and the punch. 
     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 shape of the gadroons may be different provided that the basic invention remains unchanged. 
     While the process and method herein described constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to this precise process and method, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.