Abstract:
A one piece integrally formed moulded plastic chair ( 11 ) is disclosed having a seat ( 2 ), backrest ( 3 ) and legs ( 4 - 7 ). A U-shaped rib ( 19 ) extends between the upper regions of the rear legs ( 6, 7 ) to cross-brace same. A gap ( 24 ) is proved between the cross piece ( 20 ) of the rib ( 19 ) and the underside of the seat ( 2 ). In use the seat ( 2 ) deforms to reduce the size of the gap ( 24 ) under the weight of a sitter. This deformation thereby provides a more comfortable seat.

Description:
BACKGROUND 
       [0001]    The present invention relates to chairs and, in particular, to integrally moulded, one piece plastic chairs. 
         [0002]    When plastic chairs were originally fabricated several decades ago, a plastic shell was moulded and this was screwed or otherwise connected to a steel frame which formed at least the legs of the chair. Such plastic shells did not include substantial reinforcing ribs and were thus relatively flexible. The metal legs were rigid and the shell could therefore flex relative to the frame and legs. 
         [0003]    In more recent times, one piece chairs where the seat, backrest and legs of the chair are all simultaneously formed have also been produced. These are the chairs with which the present invention is concerned. 
         [0004]    Such chairs are made by injection moulding and although the moulds for such chairs are very expensive, the individual chairs produced by such moulds are inexpensive and, if able to be sold in sufficiently large numbers, more than pays for the cost of the mould or tool. Educational institutions including schools and universities are a large market for such chairs. A prior art chair of this type is illustrated in  FIGS. 1 and 2  and is known from Australian Design Registration No. 316,934 (to which US Design Patent No. D607,654 corresponds). As seen in  FIG. 1 , the prior art chair  1  has a seat  2  and a backrest  3  and four legs  4 - 7  which are respectively a left front leg  4 , a right front leg  5 , a left rear leg  6  and a right rear leg  7 . 
         [0005]    As seen in  FIG. 2 , the upper portions of the front legs  4 ,  5  are joined by a front rib  8  and the upper portions of the rear legs  6 ,  7  are joined by a rear rib  9 . This provides a strong cross brace for each pair of legs. Both the front rib  8  and rear rib  9  extend across the underside of the seat  2  and are integrally formed with it. This has the consequence that the seat  2  is relatively rigid and so provides a firm support for the sitter. 
       SUMMARY 
       [0006]    The genesis of the present invention is a desire to provide an essentially similar chair to that of the chair  1  but in which the seat  2  exhibits some “give” or flexibility and so is able to deform under the weight of a sitter. This has the consequence that the seat  2  to some extent moulds itself to the shape of the sitter. This has the result that the chair is more comfortable for the sitter. 
         [0007]    In accordance with a first aspect of the present invention there is disclosed an injection moulded one piece plastic chair comprising:
   a seat having a front region and a rear region, a pair of front legs, and a pair of rear legs;   wherein said seat and legs are integrally connected to each other, the upper portions of said front legs are interconnected by a first rib which projects from the underside of said seat across substantially the width of said seat front region and provides a support for said seat front region and a cross-brace for said front legs; and   the upper portions of said rear legs are connected by a second substantially U-shaped rib which is located below said seat rear region, which forms a cross-brace for said rear legs, which extends across substantially the width of said seat rear region, and which is connected with said seat only adjacent said rear legs, so that a first opening is formed between the underside of said seat and said second rib, whereby said rear region of said seat can flex toward said second rib under the load of the weight of a sitter   
 
         [0011]    According to another aspect of the present invention there is provided a method of deforming the seat of a one piece integrally formed moulded plastics chair under load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    A preferred embodiment of the invention will now be described, with reference to the accompanying drawings in which: 
           [0013]      FIG. 1  is a front perspective view of a prior art chair; 
           [0014]      FIG. 2  is an inverted plan view of the chair of  FIG. 1 ; 
           [0015]      FIG. 3  is a perspective view from the underside looking from the rear of the chair of the preferred embodiment; 
           [0016]      FIG. 4  is a perspective view from the underside and looking from the front of the chair of  FIG. 3 ; 
           [0017]      FIG. 5  is a side elevational view of the chair of  FIGS. 3 and 4  with part of the view being a vertical cross-section; 
           [0018]      FIG. 5A  is an enlarged view the rib  19  of  FIG. 5 ; 
           [0019]      FIG. 6  is a side-elevational view similar to  FIG. 5  but illustrating the deformation of the seat under the load imposed by a sitter; and 
           [0020]      FIG. 7A-71  are each cross-sectional views similar to  FIG. 5  but each illustrating a rib profile arrangement. In  FIGS. 7A -71  the cross hatching utilized in  FIGS. 5 and 5A  has not been used so as to not overburden the drawings. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In  FIGS. 3-6 , the chair  11  of the preferred embodiment is illustrated with those portions of the chair  11  which are the same as the chair  1  being allocated like designation numbers. Thus the seat  2 , backrest  3  and legs  4 - 7  are as before. In addition, the front rib  8  which projects from the underside of the east  2  and interconnects the front legs  4 ,  5  is also unchanged. 
         [0022]    However, as seen in  FIGS. 3 and 4 , the rear rib  19  which interconnects the rear legs  6 ,  7  is changed so as to be generally arcuate or U-shaped having a cross piece  20  and a right upright  21  and a left upright  22 . As a consequence of this construction, there is a gap  24  between the upper edge of the cross piece  20  and the lower surface of the seat  2 . 
         [0023]    As seen in  FIG. 4 , the lower forward edge of the rear rib  19  is provided with a thickened portion  25  to increase the rigidity of the rear rib  19 . Preferably the thickened portion  25  is hollow and is formed by injecting gas into the thickened portion  25  thereby reducing the amount of plastic contained in the rear rib  19  and allowing the rib to cool rapidly after moulding. The thickened portion  25  (which adds to the rigidity of the cross-brace between the rear legs  6 , 7 ) compensates both for the material removed from the rib  19  (relative to the rib  9  of  FIG. 2 ) so as to create the gap  24  and for the fact that the rib  19  is only connected to the seat  2  by means of the uprights  21  and  22 . 
         [0024]    Turning now to  FIGS. 5 and 6 , it will be seen from  FIG. 5  that with the chair  11  unloaded, the gap  24  between the seat  2  and cross piece  20  of rib  19 , is appreciable. However, as seen in  FIG. 6 , with the chair  11  loaded by the weight of the sitter, the seat  2  has moved downwardly thereby reducing the size of the gap  24 . This downward movement of the seat  2  allows the seat  2  to flex and to conform to some extent to the shape of the body of the sitter. This makes the seat  2  more comfortable and is thought to prolong sitter concentration. By comparison the seat of the prior art chair  1  of  FIGS. 1 and 2  is not “flat” or smooth because the presence of the rib  9  creates a corresponding inflexible ridge like undulation on the upper surface of the prior art seat which can be uncomfortable. 
         [0025]    An unexpected benefit of the above described construction is that since most of the rear rib  19  does not contact the under surface of the seat  2 , there is less likelihood for the upper surface of the seat  2  to be blemished. Such blemishes are sometimes caused by the existence of an integrally formed rib under the surface of the seat  2  which can create differential rates of cooling of the moulded plastic. 
         [0026]    Turning now to  FIGS. 7A-71 , in each drawing is illustrated one of a corresponding series of rib profiles  29 - 109  each of which is a replacement for rib  19 . Thus each of the ribs  29 - 109  can operate in conjunction with the air gap  24  and provide a sufficiently rigid cross-brace for the rear legs  6 , 7  because each rib&#39;s stiffness is greater than a simple planar rib (such as rib  9 ). Each rib is illustrated in cross-sectional fashion in the same manner as  FIG. 5  save that the cross-hatching is omitted. 
         [0027]    For rib  19 , the increase in stiffness is achieved by a single thickened region  25  which is preferably hollow. For rib profile  29  of  FIG. 7A , the increase in stiffness is achieved by a kink or bend. For rib profile  39  of  FIG. 7B , the increase in stiffness is achieved by both a kink and a thickened region  35  in the middle of the rib  39 , the thickened region  35  again preferably being hollow. 
         [0028]    In  FIG. 7C , the rib  47  is provided with a J-shaped profile or cross-sectional shape to provide the increase in stiffness. In both  FIGS. 7D and 7E  the rib profiles  59  and  69  respectively are provided with a thickened region  55  and  65  which are respectively of circular and approximately tear drop cross-sectional shapes. Again, preferably both thickened regions  55  and  65  are hollow. 
         [0029]    In  FIGS. 7F and 7G , the corresponding rib profiles  79  and  89  are each provided with two thickened regions  75 A,  75 B and  85 A,  85 B respectively. These thickened regions extend in the same direction in  FIG. 7F  and in opposite directions in  FIG. 7G . In  FIG. 7H  the increase in stiffness is achieved by a double J (or S-shaped) profile for the rib  99 . In  FIG. 71  the rib profile  109  is provided with three thickened regions  125 A,  125 B and  125 C. 
         [0030]    It will be appreciated by those skilled in the plastic moulding arts that the tool required to mould the chair of  FIGS. 3-6  is more complicated than the tool required to mould the prior art chair of  FIGS. 1 and 2  with a conventional planar rib  9 . This is because retractable parts are required to form the gap  24 . Alternatively, conventional ejector mechanisms can be shaped to, and thereby used to, form the gap  24 . 
         [0031]    The foregoing describes only some embodiments of the present invention and modifications, obvious to those skilled in the plastic moulding arts, can be made thereto without departing from the scope of the present invention. 
         [0032]    The term “comprising” (and its grammatical variations) as used herein is used in the inclusive sense of “including” or “having” and not in the exclusive sense of “consisting only of”.