Abstract:
A process is provided for bending a board in synthetic material filled with more than 20% by weight of alumina trihydrate in a methyl methacrylate matrix: a section of the board is heated so that it can be bent; the section to be bent is bent while holding the convex upper surface of the board such that it does not become elongated. Also provided are a device for implementing such a process, and a bent board obtained by such a process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims priority from prior French Patent Application No. 04 11565, filed Oct. 29, 2004, the entire disclosure of which is herein incorporated by reference.  
       FIELD OF THE INVENTION  
       [0002]     The invention concerns a process and device for bending a sheet of synthetic material filled with alumina trihydrate in a methyl methacrylate matrix.  
       BACKGROUND OF THE INVENTION  
       [0003]     A material sold under the name Corian (registered trademark) is sometimes used in the manufacture of bathroom and kitchen furnishings. This synthetic material reproduces the appearance of stones such as marble or granite. It is formed from a methyl methacrylate polymer filled with pigmented alumina trihydrate. The proportion of filling can vary from 20 to 85% of the weight, but preferably exceeds 60%. Document U.S. Pat. No. 3,847,865 describes the manufacture and the properties of such a material.  
         [0004]     This material can be worked using conventional joinery methods. It can also be thermoformed. The possibilities of hot-bending the material are however limited due to the risks of crazing and discoloration of the material in the event of excessive bending. This is why the supplier recommends not exceeding a bend radius of 4 to 6 times the thickness of the sheet of material.  
         [0005]     Document U.S. Pat. No. 5,486,324 proposes producing a countertop in Corian with a downturned edge at the front and an upturned edge at the rear of the countertop, with bend radii relative to the thickness of the board exceeding the limits mentioned above. To achieve this, it proposes cutting a groove to reduce the thickness of the board along the length of the bends between the main body of the countertop and the edges, and then to hot-bend the material.  
         [0006]     However, although a board with a tight bend radius is obtained in this manner, the countertop is weakened due to the local reduction in thickness.  
       SUMMARY OF THE INVENTION  
       [0007]     The invention aims at providing a process for bending a Corian board with a tight bend radius without the risk of discoloration, crazing or weakening the board.  
         [0008]     With these aims in view, an object of the invention is a process for bending a board in synthetic material with a filling of more than 20% alumina trihydrate in a methyl methacrylate matrix, a process whereby the board section that is to be bent is heated and then bent. During bending, the top convex surface of the board is held such that its elongation is prevented.  
         [0009]     By proceeding in the manner indicated, that is to say such that the material has no areas that become elongated during bending, no discoloration or crazing occurs, even if a bend radius of less than four times the thickness of the board is produced.  
         [0010]     To achieve this, a flexible guide is placed along and held firmly against the upper surface without sliding during the bending stage. The guide provides a bearing surface for the upper surface and prevents elongation during bending.  
         [0011]     The board is preferably heated from the underside. A temperature gradient is obtained through the thickness of the board. The underside area, which is hotter and therefore more malleable, flows more easily than the upper area, which reduces the bending stresses, particularly along the upper surface.  
         [0012]     Another object of the invention is also a device for implementing the abovementioned bending process, this device comprises means of retaining the board such that it can be bent while holding the upper surface such that it does not become elongated.  
         [0013]     The means of retention includes, for example, a flexible guide and means of holding the guide against the upper surface of the board.  
         [0014]     In a particular arrangement, the holding means includes at least one pair of jaws to clamp the board along the length of a section to bend, the flexible guide being fixed to one of the upper jaws. When the jaws grip the board, the board is secured between them and can no longer slide. As the guide is secured to the upper jaw, a first joint is made between the board and the guide. If two pairs of jaws are used and positioned on either side of the section to bend, the two upper jaws are interconnected by the guide and the board is blocked between the jaws. This arrangement prevents the top surface of the board from becoming elongated during bending.  
         [0015]     In another variant, the holding means comprises a flange against which one edge of the board abuts before the bending step. The flange is thus another means of blocking the guide with respect to the top surface of the board.  
         [0016]     In a particular arrangement, the device comprises a bending beam mounted such that it can move between an initial position in which the bending beam is above the guide, and a final position in which the beam has driven the guide in a bending movement by applying pressure to the flange.  
         [0017]     In a particular application, the upper jaw comprises a bearing surface provided to apply against the board, the flexible guide being offset from the bearing surface of thereby allowing a boss to form along the length of the upper jaw during the bending step. One can thus obtain a board on which the boss retains any liquid that might have spilled onto the horizontally positioned top surface of the board. The boss forms in the space left free beneath the guide and into which the board material can creep during the bending operation.  
         [0018]     Another object of the invention is also a bent board obtained by the previously described process.  
         [0019]     Such a board is of particularly interest when the bend radius of the underside is between one and four times the thickness of the board. These bend radius values are smaller than those that could be obtained until now without the appearance of whitening or crazing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The invention will be better understood, and particularities and advantages will become apparent on reading the following description, which makes reference to the appended drawings, in which:  
         [0021]      FIG. 1  is a cross-sectional schematic view of a device in conformity with the invention, in a position before bending;  
         [0022]      FIG. 2  is a view similar to  FIG. 1  after bending;  
         [0023]      FIG. 3  is a view similar to  1  of a device in conformity with a second method of production of the invention, in a position before bending;  
         [0024]      FIG. 4  is a similar view to  FIG. 3  after bending; and  
         [0025]      FIG. 5  is a cross-sectional view of a board bent using the device illustrated in  FIGS. 3 and 4 .  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0026]     Preferred embodiments of the present invention will be described in detail hereinbelow with reference to the attached drawings.  
         [0027]     A first method of producing a device in accordance with the invention is shown in  FIGS. 1 and 2 . This device comprises an upper jaw  10  and a lower jaw  11  between which is clamped a board  2  that is to be bent. Jaws  10 ,  11  are actuated by clamping means, not illustrated, for example by hydraulic jacks in a conventional manner. A guide  13  is secured to the upper jaw  10  and extends along a bearing surface  101  of the upper jaw  10 . The guide  12  is for example a steel sheet of thickness between 0.2 and 0.4 mm.  
         [0028]     The guide  13  is extended by a flange  16  running parallel to the upper jaw  10 , perpendicularly to the face of the guide  13  and, and towards the lower jaw  11 .  
         [0029]     The bending device comprises, among other things, a beam  12  that pivots around a shaft B parallel to the jaws  10 ,  11 , between an initial position, illustrated in  FIG. 1 , and an advanced position, illustrated on  FIG. 2 . The beam  12  comprises a working face  121  which is in contact with guide  13  at the flange  16  when the beam  12  is in the initial position. Beam  12  moreover comprises a second flange  120  that extends parallel to the pivoting axis B of the beam  12 , perpendicular to the working face  121 , and directed towards jaws  10 ,  11 . When the beam  12  is in the advanced position, the flange  16  of guide  12  bears against the flange  120  of the beam, and guide  12  is partially in contact with the working face  121 . Guide  12  then displays a bend between the upper jaw  10  and the working face  121 , through an angle of about 90°.  
         [0030]     The device moreover comprises retractable means of heating  14 , to heat a surface in the continuation of a bearing surface  111  of the lower jaw  11 .  
         [0031]     When this device is used, a Corian board  2  is placed between jaws  10 ,  11  of the bending device such that an edge  26  of board  2  abuts against the flange of the guide. After tightening the jaws  10 ,  11  around board  2 , for example with a pressure of 20 to 30 kN/m, the heating means  14  are switched on to heat the Corian board from the underside  25  opposing the upper side  20  which is in contact with the guide  13 . Once the temperature of the upper side  20  has reached a predetermined threshold, between 135 and 155° C. for example, typically 145° C., the means of heating is retracted and the beam is made to move from the initial position to the advanced position. During the bending phase, the upper side  20  of the board remains in contact with the guide  13  while the edge  26  of the board is blocked against the flange  16  of guide  13 . As a result of this, the upper side of the board does not suffer any elongation. Guide  13  bends elastically roughly through an arc of a circle.  
         [0032]     By proceeding in this manner, it has been observed that the upper side  20  of Corian boards of 12.5 mm thickness, after cooling, displayed no discoloration or crazing in spite of having a bend radius of 25 mm in the upper surface.  
         [0033]     In a second method of producing the invention, shown in  FIGS. 3 and 4 , the upper jaw  10  is modified with respect to that of the first method of production in that a shim  15  is added to the guide  13  on the bearing surface  101  of upper jaw  10 . Thus, the contact with the top surface  20  of board  2  in the initial position is made through a clamping surface  151  of shim  15 . A space  30  is left free along the edge of the upper jaw  10 , between guide  13  and the top surface  20  of board  2 .  
         [0034]     When board  2  is bent by the device in accordance with the second method of production, the board creeps into space  30  and thereby creates a boss  23  on board  20 . The shape of shim  15  along boss  23  can be adapted to the desired shape of the boss  23 .  
         [0035]     Corian boards of 12.5 mm thickness were bent with a 3-mm thick shim resulting in a top surface bend radius of 25 mm. No signs of discoloration or crazing were observed on these boards.  
         [0036]      FIG. 5  shows an example of a product obtained by the process according to the invention using the second method of producing the device. This product is, for example, a countertop  2  for bathroom facilities. This countertop  2  comprises a main surface  24  designed to be installed horizontally, and a downturned edge  22 , obtained by bending the board, creating a fascia at the front of the countertop. The main surface  24  ends with the boss  23  that limits the flow of any liquid spilled on the main surface  24  towards the downturned edge  22 .  
         [0037]     With both methods of producing the devices, it is possible to heat the board before it is placed between the jaws. The means of heating can be radiating elements or a base heated by electrical heating elements placed in contact with the area to heat.  
         [0038]     The invention is not limited to the methods of production given purely as examples. The bending angle is not necessarily 90°, but can vary from 100 to 180°. The thickness of the boards can be smaller or larger.  
         [0039]     While there has been illustrated and described what are presently considered to be the preferred embodiments of the present invention, it will be understood by those skilled in the art that various other modifications may be made, and equivalents may be substituted, without departing from the true scope of the present invention. Additionally, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from the central inventive concept described herein. Furthermore, an embodiment of the present invention may not include all of the features described above. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the invention include all embodiments falling within the scope of the appended claims.