Abstract:
In a ventilation system of a motor vehicle, a film valve can selectively open and close an air path opening defined by a housing with a rib or projection that encircles such opening. The rib can define a point contact between the film valve and the housing when viewed in cross-section. The rib geometry can include substantially straight edges in cross-section and/or can include arcuate edges. Different geometry can define the rib at different cross-sections. The point contact defined between the rib and the film valve substantially reduces noise generated when the film valve moves relative to the rib, especially when the film valve has been subjected to a harsh environment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Provisional Patent Application Ser. No. 60/439,723, for POINT CONTACT VALVE SEATS FOR FILM VALVE APPLICATIONS, filed Jan. 13, 2003. 

   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention generally relates to a film valve assembly and, more particularly, to a film valve assembly for a heating, ventilation and air conditioning system for a vehicle. 
   2. Decsription of the Prior Art 
   Conventional heating, ventilation and air conditioning (HVAC) systems for vehicles generally include a housing, an evaporator, a heater core having a housing, an evaporator assembly coupled to the housing, an evaporator, a heater core, an air inlet, a fan and various doors or mode valves for controlling the volume and direction of air flow and generating outlet air of a desired volume and temperature. Together these components receive, temper and direct the flow of forced air through several outlets or vents in the vehicle. The outlets through which the forced air ultimately exits are determined by the various doors which rotate or swing back and forth to open or close off openings and passages and cause air flow to be forced in one direction or another or a combination of both. Air may be directed to various areas of the car depending on the state of the mode valves. For example, air may be forced through outlets directed at the windshield in a defrost or defog mode, or through outlets directed at mid-height level in an air-conditioning mode, or to lower outlets directed to the floor in a heat mode, or various combinations thereof. Although, air of any temperature may be directed to and through any of them. 
   More recently, designs utilizing a flexible film valve rather than doors have been proposed for controlling volume and direction of air flow. Generally, the flexible film valve is disposed in a frame having rollers. The flexible film valve includes various apertures formed therein, is rolled back and forth over a first roller to cover or uncover various openings and may include an actuator for controlling the roller to wind and unwind the film valve. 
   Typical film valve designs are disclosed in U.S. Pat. No. 5,160,115 to Ito, et al. and U.S. Pat. No. 6,273,811 B1 to Pawlak, III. The &#39;115 patent discloses an HVAC assembly including a film valve assembly having one roller with a biasing member and a motor. The &#39;811 patent discloses an HVAC assembly including a film valve assembly having two rollers, each with a biasing member and a motor, and each operating a separate film valve. Such designs cause excessive wear on the film valve, the roller and the actuator, thereby increasing costs, and introduce noise in the system due to increased operational efforts required which may be displeasing to the driver of the vehicle. 
   In addition, the frame in which the film valve is situated typically comprises a static design specifically manufactured to be fitted onto an HVAC housing. Therefore, a frame must be designed and manufactured to meet each HVAC housing design, thereby increasing engineering and tooling cost. 
     FIG. 7  shows a portion  200  of a prior art ventilation system including a frame  202  defining a plurality of apertures  204 ,  206 ,  208 , and  210 . A fabric member or film valve  212  is wound and unwound relative to drive rollers  214 ,  216  interior of the frame  202 . Idle rollers  218 ,  220 , support the film valve  212  to seal the film valve  212  relative to the apertures  204 ,  206 ,  208 , and  210 . The film valve  212  seals with a flat surface  222  defined by the frame  202 . 
     FIG. 10  shows a frame  202   a  of a prior art ventilation system defining surfaces  222   a ,  222   b . A film valve (not shown) can slide across the surfaces  222   a ,  222   b  during movement relative to the frame  202   a . During movement of the fabric member or film valve relative to the frame  202   a , a noise is produced when debris becomes entrapped between the film valve  212  and surfaces  222   a ,  222   b  when the film valve slides across such surface. 
   The present invention is aimed at solving one or more of the problems described above. 
   SUMMARY OF THE INVENTION 
   The present invention provides a rib extending around an aperture defined by a frame that engages a fabric valve. The rib and fabric valve can seal the aperture of the frame. The rib minimizes the surface area of engagement between the fabric valve and the frame to reduce the noise, friction, and wear resulting from relative movement between the fabric valve and the frame. The rib defines a point of contact with the fabric valve along the cross-section of the rib. 
   Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a heating, ventilation and air conditioning (HVAC) system having a film valve assembly disposed thereon, according to an embodiment of the present invention; 
       FIG. 2  is cross-sectional view of the HVAC system of  FIG. 1 , according to an embodiment of the present invention; 
       FIG. 3A  is a side perspective view of the film valve assembly of  FIG. 1 , according to an embodiment of the present invention; 
       FIG. 3B  is a side perspective view of the film valve assembly of  FIG. 1  illustrating the rotational aspect of a segment thereof, according to an embodiment of the present invention; 
       FIG. 4  is a partially exploded side perspective view of a shaft drive source disposed on the film valve assembly of  FIG. 3 , according to an embodiment of the present invention; 
       FIG. 5A  is a partially exploded perspective view of the take-up roll, according to an embodiment of the present invention; 
       FIG. 5B  is a perspective view of the take-up roll assembly, according to an embodiment of the present invention; 
       FIG. 5C  is a segmented perspective view of the take-up roll installed in the film valve assembly, according to an embodiment of the present invention; 
       FIG. 6  is a fragmented view of a portion of the flexible film valve cartridge across which the flexible film translates during operation; and 
       FIG. 7  is a cross-sectional view of the prior art film valve assembly; 
       FIG. 8  is an enlarged view of a portion of  FIG. 7 ; 
       FIG. 9  is a cross-sectional view of a film valve assembly according to the present invention, illustrating a point contact contour; 
       FIG. 10  is a perspective view of prior art seats for a film valve; 
       FIG. 11  is a cross-sectional view of a first embodiment of a rib according to the present invention; 
       FIG. 12  is a graph of a range of dimensions for the rib shown in  FIG. 11 ; 
       FIG. 13  is a cross-sectional view a second embodiment of a rib according to the present invention; 
       FIG. 14  is a graph of a range of dimensions for the rib shown in  FIG. 13 ; 
       FIG. 15  is a cross-sectional view a third embodiment of a rib according to the present invention; 
       FIG. 16  is a graph of a range of dimensions for the rib shown in  FIG. 15 ; 
       FIG. 17  is a graph of a range of dimensions for the rib shown in  FIG. 15 ; 
       FIG. 18  is a cross-sectional view of a valve seat having a constant rib cross-section; 
       FIG. 19  is a cross-sectional view of a valve seat having a variable rib cross-section; and 
       FIG. 20  is a cross-sectional view of an embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout the several views, a heating, ventilation and air conditioning (HVAC) system for a vehicle according to one embodiment is generally shown at  10 . 
   Referring to  FIGS. 1 and 2  the HVAC system  10  generally includes a housing  12  defining at least one housing aperture  13  therein for directing air to the desired location of a passenger compartment of the vehicle, an evaporator assembly  14  coupled to the housing  12 , a heater core assembly  16  coupled to the evaporator assembly  14  and the housing  12 , an air inlet assembly  18  coupled to the evaporator assembly  14 , a fan assembly  20  disposed on the air inlet assembly  18 , and a film valve assembly  22  disposed on the housing  12  for defining an air mixing chamber  24  therebetween. 
   With continuing reference to  FIGS. 1 and 2 , the evaporator assembly  14  generally includes an evaporator core  26  enclosed in the housing  12 . The heater core assembly  16  generally includes a heater core  28  enclosed in the housing  12 . Typically, the evaporator core  26  is located upstream of the heater core  28 . Air, either pulled-in outside air or recirculated inside air, enters the air inlet assembly  18  having a passageway  32  defining an aperture  34  at an end  36  thereof. The air is pulled into and forced through the HVAC system  10  by the fan assembly  20  having a fan  38  disposed in a fan housing  40  coupled to the air inlet assembly  18 . 
   Typically, air temperature is controlled and determined by the evaporator core  26 , which can be turned on and off along with the rest of the HVAC system  10 , and the heater core  28 , which is generally always activated and hot. The evaporator core  26  generally extends across the entire width of the case housing  12  such that all the forced air passes through it first, regardless of whether the evaporator core  26  is activated and cold or switched off. The heater core  28  traditionally has a constant flow of engine coolant flowing through it whenever the engine is running. Moreover, the degree of heating of air flow is varied not by varying the temperature of the heater core  28 , but by varying the proportion of air flow over and through it. 
   Air flow routed through the heater core  28  is directed up the back face of the heater core  28  and into the air mixing chamber  24 . Within the air mixing chamber  24 , any air that has passed straight through the evaporator core  26  is mixed with any air that has been routed through the heater core  28  to achieve a desired temperature. The tempered air is then forced from the mixing chamber  24  through one or more outlets  30  to an area as selected by an operator, such areas including the windshield in a defrost or defog mode, the mid-height level in an air-conditioning mode, the floor in a heat mode, or any combination thereof. 
   Referring to  FIGS. 3A through 5C , the film valve assembly  22 , according to an embodiment of the present invention, includes a frame  42 , a first roller  44  disposed on the frame  42  having a first biasing device  72  continually urging said first roller  44  to rotate in a first direction under a first constantly applied force, a second roller  46  disposed on the frame  42  having a second biasing device  73  continually urging said second roller  46  to rotate in a second direction under a second constantly applied force, and a film valve or fabric member  48  having a first end  50  disposed on the first roller  44  and a second end  52  disposed on the second roller  46  with the film valve  48  extending therebetween for controlling air flow. The first biasing device  72  and the second biasing device  73  are substantially similar. The film valve  48  defines at least one aperture  54  therein for air flow therethrough. In addition, the frame  42  further may include one or more idle rollers  58  rotatably supported on the frame  42  which supports the film valve  48  and stabilizes the tension of the film valve  48  thereon, as described below. 
   Referring to  FIGS. 5A and 5B , the first roller  44  includes an axle  60 . The axle  60  may be a metal rod, a plastic rod or any other suitable type of rod. A first distal end  62  of the axle  60  is flat so as to interface with the frame  42  and ensure that the axle  60  does not rotate, as described below. 
   With continued reference to  FIGS. 5A and 5B , the first roller  44  further includes a cylinder  66  having a hollow end  68  and a beveled end  70 . The cylinder  66  further includes a locking rib  76  and may be metal, plastic or any other suitable material. The second end  64  of the axle  60  is disposed on the hollow end  68  and extends therefrom. The first biasing device  72  has one end disposed on the beveled end  70  and has an opposite end affixed to axle  60  for continually urging the first roller  44  to rotate in a first direction under a first constantly applied force. The first biasing device  72  may be a torsion spring, a helical torsion spring or any other suitable member which provides torsional force. 
   With continued reference to  FIG. 5B , a shaft  74  of the roller  44  defining a groove  78  is disposed annularly about the first biasing device  72  and the cylinder  66  such that the locking rib  76  slides into the groove  78 . The shaft  74  may be metal, plastic or any other suitable material. Operationally, rotational force applied to the first biasing device  72  is transferred to the shaft  74  to provide rotation thereto. 
   According to an embodiment of the present invention, the second roller  46  includes the axle  60 , cylinder  66 , shaft  74   a , and the second biasing device  73  continually urging the second roller  46  to rotate in a second direction under a second constantly applied force in a configuration identical to that of the first roller  44 . According to one embodiment of the present invention, the first direction is opposite to the second direction. In accordance with yet another embodiment, the first direction is in the same direction as the second direction. The second biasing device  73  may be a torsion spring, a helical torsion spring or any other suitable member which provides torsional force. 
   With reference to  FIGS. 3A ,  3 B,  5 A and  5 C, the rollers  44 ,  46  are disposed on opposite ends  80 ,  82  of the frame  42  with the first and second ends  50 , 52  of the film valve  48  coupled thereto. With reference to  FIG. 5C , a cap  84  is disposed on the frame  42  and an end of the second rollers  46  is disposed thereon with the other end of the roller  46  disposed on the frame. The frame  42  includes a slot  88  which receives the cap  84 . The cap  84  includes an alignment rib  55  which snaps into the slot  88 . The frame  42  may include additional slots and the cap  84  may include additional alignment ribs matching the number of slots. Moreover, the cap  84  may include a fastener  90  for further securing the cap  84  to the frame  42 . The fastener  90  may be a screw, a bolt or any other suitable fastener. 
   With continued reference to  FIG. 5B , the cap  84  further includes a key slot  92  and reinforcement ribs  94 . The first distal end  62  of the axle  60  fits into the key slot  92  and prevents the axle  60  from rotating upon application of torque to the rollers  44 ,  46 . 
   Referring to  FIGS. 1 through 3B , according to another embodiment of the present invention, the frame  42  includes a first segment  96  and a second segment  98  connected to the first segment  96  for movement relative thereto. The first and second segments  96 ,  98  may be connected by a first hinge assembly  100  having a stationary member (not shown) and a rotating member (not shown) which rotatably couples the first and second segments  96 ,  98 . The first hinge assembly  100  enables the first and second segments  96 ,  98  to be repositionable, thereby allowing the frame to change angular shape to be fitted to multiple HVAC assembly configurations and to aid in assembly. The first hinge assembly  100  may be a living hinge or any other suitable hinge. The first and second segments  96 ,  98  may be connected so as be moveable relative to each other by any other suitable means. 
   Referring to  FIGS. 3A and 3B , the frame  42  may include a third segment  102  rotatably coupled to one of the first and second segments  96 ,  98  by a second hinge assembly similar to hinge  100  and positioned proximate to idler roller  58 . Such a three-segmented configuration permits each segment  96 ,  98 ,  102  to be repositionable with respect to each other for improved access or for greater interchangeability between different configurations of HVAC systems. The second hinge assembly may be a living hinge or any other suitable hinge. 
   The frame  42  being repositionable permits features of the housing  12  to be molded that otherwise would be die locked. Thus, the frame  42  may be adapted to many different geometries and many different vehicles, thereby reducing engineering and tooling costs. Moreover, the repositionable frame  42  in an open position allows easier assembly of the frame  42 . As shown in  FIG. 3A , the film valve assembly  22  of the present invention is shown in a closed position. As shown in  FIG. 3B , the film valve assembly  22  is shown in an open position. 
   Referring to  FIG. 4 , an actuator  106  is coupled to HVAC module  10  and to roller  46  thereby providing a non-constant rotational torque coupled to the second roller  46 . The actuator  106  may be a stepper motor, a direct current (DC) motor and gears, or any suitable actuator. The actuator  106  includes a rotatable connector  108  which interfaces the actuator  106  to the second roller  46  in female/male engagement and transfers a positive rotational torque from the actuator  106  to the second roller  46 . The rotatable connector  108  may have a torx head, a phillips head or any other suitable connector. According to yet another embodiment, an actuator  106  may be coupled to each of the first and second rollers  44 ,  46 . 
   With reference to  FIGS. 1 through 4 , the operation of the HVAC assembly  10  according to an embodiment of the present invention will now be described. The HVAC assembly  10  of the present invention utilizes a balanced dual biasing member concept. The first and second biasing devices  72 ,  73 , providing a constant force urging the first and second rollers  44 ,  46  to rotate, are disposed at the ends of the frame  42 . The first biasing device  72  operating at the first roller  44  is pre-tightened and provides the required torque to wind up the film valve  48 . The torque of the first biasing device  72  is designed to operate in extreme conditions of temperature and air flow in the module (typically −40 degrees Fahrenheit and high fan speed). 
   The first biasing device  72  must overcome the frictional forces of the film valve  48  contacting the cartridge  42  and traveling along the frame  42  and also overcome the forces introduced by the bending of the film valve  48  around the rollers  44 ,  46 . The second biasing device  73  on the second roller  46  serves as an assist to the actuator  106  which operates the rotation of the second roller  46 . As the film valve  48  winds on the second roller  46 , thereby tightening the first roller  44 , the actuator  106  must overcome the tension force of the first roller  44  and the frictional forces. With the addition of the second biasing device  73 , torque loads on the actuator  106  may be minimized and better balanced. 
   Upon activation of the actuator  106 , the rotatable connector  108  rotates the second roller  46  in the first direction. Upon rotation by the actuator  106  of the second roller  46  and with the assistance of the tension of the second biasing device  73 , when a torque great enough to overcome the tension force of the first biasing device  72  is generated, the film valve  48  is wound onto the second roller  46  and off the first roller  44 . While winding, the second biasing device  73  in the second roller  46  assists the rotation and reduces the torque required by the actuator  106  to continue winding the film valve  48 . Without the assistance of the second biasing device  73 , the actuator  106  would be required to be larger and more powerful in order to overcome the resistance of the first biasing device  72  so that the film valve may rotate and align the apertures  54  with the outlets  30  as required to accommodate the direction of air flow as selected by the user. 
   The net result is that the torque is positive on each roller  44 ,  46 , thereby making the entire assembly  10  more balanced. In addition, torque loads on the actuator  106  are reduced compared to maximum torque in a single spring system. 
   Referring to  FIG. 6 , the frame  42  further includes ribs  56  disposed thereon over which the film valve  48  travels, as described below. The ribs  56  reinforce the frame  42  around the housing apertures  13  and prevent the film valve  48  from getting trapped on or pushed through the housing apertures  13 . Moreover, the ribs  56  minimize friction created when the film valve  48  travels across the case housing  12  upon rotation, thereby reducing drag and the torque required to rotate the film valve  48 . 
   Referring to  FIGS. 1A ,  1 B,  3 A and  3 B, the film valve assembly  22  of the present invention includes a frame  42 , defining inner  110  and outer  112  surfaces, and at least one housing aperture  13  therewithin for directing air to the desired location of a passenger compartment of the vehicle. The film valve assembly  22  includes a first roller  44  disposed on the frame  42  having a first biasing device  72  continually urging said first roller  44  to rotate in a first direction under a first constantly applied force, a second roller  46  disposed on the frame  42  having a second biasing device  34  continually urging the second roller  46  to rotate in a second direction under a second constantly applied force. The film valve assembly  22  includes a film valve  48  having a first end  50  disposed on the first roller  44  and a second end  52  disposed on the second roller  46  with the film valve  48  extending therebetween for controlling air flow. The film valve  48  includes a fabric. 
   The film valve  48  defines at least one aperture  54  therein for airflow therethrough. In addition, the frame  42  further may include one or more idle rollers  58  rotatably supported on the frame  42  which supports the film valve  48  and stabilizes the tension of the film valve  48  thereon, as described below. The frame  42  being repositionable permits features of the housing  12  to be molded that otherwise would be die locked. 
   As appreciated by those skilled in the art, the frame  42  may be adapted to many different geometries and many different vehicles, thereby reducing engineering and tooling costs. Moreover, the repositionable frame  42  in an open position allows easier assembly of the frame  42 . As shown in  FIG. 3A , the film valve assembly  22  of the present invention is shown in a closed position. As shown in  FIG. 3 , the film valve assembly  22  is shown in an open position. 
   Referring to  FIG. 4 , an actuator  106  is coupled to roller  46  thereby providing a non-constant rotational torque coupled to the second roller  46 . The actuator  106  may be a stepper motor, a direct current (DC) motor and gears, or any suitable actuator. The actuator  106  includes a rotatable connector  108  that interfaces the actuator  106  to the second roller  46  in female/male engagement and transfers a positive rotational torque from the actuator  106  to the second roller  46 . 
   Referring to  FIG. 6  of the present invention, each of the apertures  13  further includes ribs  56 , extending from the inner surface  110  of the frame  42 , positioned around each aperture  13  over which the film valve  48  travels. The ribs  56  prevent the film valve  48  from getting trapped on or pushed through the apertures  13  defined within the frame  42 . Moreover, the ribs  56  minimize friction created when the film valve  48  travels across the housing  12  upon rotation of the rollers  44 ,  46 , thereby reducing drag and the torque required to rotate the rollers  44 ,  46 . 
   Referring now to  FIG. 9 , the ribs  56  define a valve seat, defined by the ribs  56 , wherein the flat portion of the prior art valve seat, as illustrated in  FIGS. 7–8  and  10 , is replaced with a point contact contour, thereby creating a point contact seat for the film valve  48 . The point contact contour  50  of the rib  56  reduces the contact area between the film valve  48  and the frame  42 . One of ordinary skills in the art will appreciate that to provide for areas of “point” contact between the film valve  48  and the inner surface  110  of the frame  42 , an extra plastic is removed from the inner surface  110  to create the rib  56   a  with the point contact contour  50  around each of the apertures  13  defined within the frame  42 . 
   A Module Level Debris Test, in which debris similar to those found airborne in harsh environments was introduced into the HVAC module through the air inlet while the blower fan and fan were at high speed, was conducted between the old design, featuring flat valve seats and the point contact valve seats of the present invention that determined significant differences between the designs. Furthermore, a reduction of a contact between the fabric of the fabric valve  48  and the point contact valve seats defined by the ribs  56  of the present invention, reduces sliding related noises and yields improvement over the prior art design without yielding trade-offs of valve flutter and leakage during the use of the HVAC system  10  of the vehicle. 
   Referring now to  FIG. 20 , the present invention provides a valve for a ventilation system  10   a  of a motor vehicle, the valve including a frame  42   a . The ventilation system  10   a  can include a housing  12   a , an inlet ( 18  shown in  FIG. 1 ), an outlet  114 , and a mixing chamber  24   a  adjacent to the outlet  114 . The frame  42   a  can be connected to the housing  12   a  and enclose the outlet  114 . The frame  42   a  includes an interior surface  110   a  and a first aperture  13   a . The first aperture  13   a  communicates with the outlet  114  of the housing  12   a . The valve also includes a fabric member  48   a  suspended across the first aperture  13   a . The fabric member  48   a  can be suspended and moved across the aperture  13   a  with rollers, pulleys, cords, or gears. By way of example only and not limitation, first and second drive rollers  44   a ,  46   a  can be supported for rotation by the frame  42   a  and be disposed at opposite ends ( 80 ,  82  shown in  FIGS. 3A–3B ) of the frame  42   a . The fabric member  48   a  can be wound and unwound with the first and second rollers  44   a ,  46   a.    
   The valve can include an idle roller  58   b  supported for rotation by the frame  42   a . The fabric member  48   a  can extend between the idler roller  58   b  and the interior surface  110   a  of the frame  42   a . As best shown in  FIG. 18 , the idle roller  58   c  can be offset a distance +/−Ø from the top of the rib  56   f . The distance Ø can be between 0.2 millimeter and 2.0 millimeter. The idle roller  58   b  can be disposed between the first and second drive rollers  44   a ,  46   a.    
   The fabric member  48   a  can include one or more apertures that can be aligned with the aperture  13   a  to define an open position. For example, when an aperture defined by the fabric member  48   a  is at least partially aligned with the aperture  13   a , an air stream can pass from the mixing chamber  24   a  through the aperture  13   a.    
   The valve also includes a rib  56   g  projecting from the interior surface  110   a  of the frame  42   a . The rib  56   g  defines the valve seat of the valve. The fabric member  48   a  is in sealing engagement with the rib  56   g . The rib  56   g  defines a point contact or point contour that minimizes the cross-sectional area of sealing engagement between the fabric member  48   a  and the valve seat. 
   The rib can define several different cross-sections. Referring now to  FIGS. 11–12 , the cross-section of the rib  56   b  can include a pair of arcuate sides  116 ,  116   a  extending outwardly from the interior surface  110   b . The arcuate sides  116 ,  116   a  can be disposed on opposite sides of a planar side  118 . By way of example and not limitation, the radii R 1 , R 2  of the sides  116 ,  116   a  can be between 0.25 mm and 4.50 mm. The planar side  118  does not have to extend parallel to the interior surface  110   b . The planar side  118  can extend a distance F between the arcuate sides  116 ,  116   a . By way of example and not limitation, the distance F can be between 0 mm and 1 mm, as shown in  FIG. 12 . The side  118  defines the valve seat of the film valve. 
   Referring now to  FIG. 13 , the cross-section of the rib  56   c , can include a pair of arcuate sides  116   b ,  116   c  adjacent to one another. The first arcuate side  116   b  can extend outwardly from a point  119  defined on the interior surface  110   c . The first and second arcuate sides  116   b ,  116   c  can define different radii R 3 , R 4 . By way of example and not limitation, the radius R 3  can be between 0.5 mm and 4.50 mm. A planar side  118   a  can extend outwardly from a point  121  defined by the interior surface  110   c  to the arcuate side  116   c . The side  118   a  can extend from the surface  110   c  at an angle Θ relative to the surface  110   c . By way of example and not limitation, the angle Θ can be between thirty degrees and ninety degrees. The side  118   a  can extend from surface  110   c  at any point along a distance D. The distance D can be defined along the surface  110   c  between points  121  and  123 . The second arcuate side  116   c  can extend between the first arcuate side  116   b  and the planar side  118   a . The radius R 4  of the second arcuate side  116   c  can be selected in response to the distance D, the radius R 3 , and the angle Θ. By way of example and not limitation, the radius R 4  can be between 1 mm and 3 mm. 
   Referring now to  FIG. 15 , the rib  56   d  can include a first convex side  116   d  extending outwardly from a point  119   a  defined by the interior surface  110   d . The convex side  116   d  can define a radius R 5  having a center point positioned along a distance D 1  between points  121   a ,  123   a . By way of example and not limitation, the radius R 5  can be between 0.5 mm and 4.5 mm. The rib  56   d  can also include a second convex side  116   e  defining a radius R 6 . By way of example and not limitation, the radius R 6  can be between 1 mm and 3 mmm. The rib  56   d  can also include a concave side  116   f  extending outwardly from a point  125  defined by the interior surface  110   d  to the second convex side  116   e . The side  116   f  can define a radius R 7 . By way of example and not limitation, the radius R 7  can be between 0.5 mm and 4.5 mm. A height H can be defined above the surface  110   d  at the intersection of the surfaces  116   e ,  116   f . A center point of the radius R 7  can be located along the height H or can be offset from the height a distance Y. By way of example and not limitation, the distance Y can be between 1 mm closer to the surface  110   d  than the height H and 4.5 mm greater than the height H. 
   As shown in  FIG. 11 , the cross-section of the rib  56   b  can be symmetrical about a vertical axis  120  of the rib  56   b . Alternatively, as shown in  FIGS. 13 and 15 , the ribs  56   c ,  56   d  can be asymmetrical about a vertical axis. The various ribs  56 ,  56   a ,  56   b ,  56   c ,  56   d ,  56   f  shown in the figures can have a constant cross-section or can have a variable cross-section. 
   The foregoing detailed description shows the preferred embodiments of the present invention are well suited to fulfill the objectives of the invention. It is recognized that those skilled in the art may make various modifications or additions to the preferred embodiments chosen herein to illustrate the present invention, without departing from the spirit of the present invention. It should also be recognized that any feature of the invention can be used in combination with any other feature of the invention. Accordingly, it is to be understood that the subject matter sought to be afforded protection should be deemed to extend to the subject matter defined in the appended claims, including all equivalents thereof. 
   The invention has been described in an illustrative manner, and it is to be understood that the terminology that has been used is intended to be in the nature of words of description rather than limitation. It will be apparent to those skilled in the art that many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that the invention may be practiced otherwise than as specifically described within the scope of the amended claims.