FIELD OF THE INVENTION 
     The present invention relates generally to an improved leak-proof cup. More particularly, the present invention relates to a cup assembly having a cap bearing a drinking spout at one side and an air vent spaced from the drinking spout, with a flow control element frictionally engaged in the vicinity of the drinking spout and air vent to allow passage of liquid out and air in during use, while preventing significant leakage through the spout and vent when not in use. 
     BACKGROUND OF THE INVENTION 
     Enclosed cups having drinking spouts and separate air vents, which allow the user to drink from the spout without creating excessive vacuum in the cup, are known in the art. However, drinking spouts and air vents are liable to leak liquid stored in the cup between feedings, or if dropped during use. Accordingly, certain cups have been developed that use valving mechanisms at the spout and at the air vent. These valves respond to suction generated during feeding to open and allow liquid to pass through the spout and to allow air to enter the air vent when a vacuum is developed in the interior of the cup. 
     Two patents disclosing such valves are U.S. Pat. No. 5,079,013 to Belanger and U.S. Pat. No. 5,542,670 to Morano, both commonly assigned or licensed to the assignee of the present application. Applicant hereby incorporates the disclosure of those two patents herein by reference. Applicant has on the market a cup that employs a valve assembly similar to that shown in U.S. Pat. No. 5,079,013 that is secured to sleeves in the underside of the cup&#39;s top, but in which the valves are mounted on a single base element. Applicant is also aware of a competitive product having a flow control element of the configuration depicted in FIG. 1, sold as part of the Tumble Mates Spill Proof Cup by the First Years. 
     Despite the effectiveness of these cup mechanisms, applicant has discovered an improved flow control element and corresponding valve configuration that provides improved fluid flow rates without sacrificing the valve&#39;s resistance to spills or the valve&#39;s durability. 
     SUMMARY OF THE INVENTION 
     Thus, it is an object of the present invention to provide an improved valve mechanism for a cup assembly that is substantially leak-proof even when upended, dropped or shaken. 
     It is a further object of the present invention to provide an improved valve mechanism for a leak-proof cup that gives higher fluid flow rates at normal suction forces without sacrificing durability or resistance to spills. 
     It is a further object of the present invention to provide an improved valve mechanism, cap and cup that are easy to clean and easy to assemble. 
     Accordingly, the present invention provides a drinking cup assembly including a cup having an open end; a cap adapted to enclose the open end, the cap including a drinking spout and an air vent and mating surfaces adjacent or incorporated into the drinking spout and the air vent; and a flow control valve including two stacks adapted to engage the mating surfaces, each of the two stacks having a concave valve face at a top portion thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a prior art valve mechanism; 
     FIG. 2 is a perspective view of a cup, cap and valve assembly according to the present invention; 
     FIG. 3 is a section diagram taken along the lines 3-3&#39; in FIG. 2; 
     FIG. 4 is a side view of the valve of FIG. 3; and 
     FIG. 5 is a top view of the valve of FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the figures and, in particular, FIGS. 2 through 4, the cup, cap and valve assembly of the present invention is generally referred to by reference numeral 10. The assembly 10 includes a cup 12, a cap 14 and a flow control valve 20. Cap 14 is adapted to seal cup 12, with the exception of the apertures in the spout 16 and air vent 18 formed in its surface. Flow control valve 20 is adapted to communicate with spout 16 and air vent 18, to form the substantially spill-proof assembly 10. 
     Cap 14 is formed with mating surfaces, preferably adjacent to or incorporated into spout 16 and air vent 18, to frictionally engage flow control valve 20 and place the flow control valve in fluid communication with spout 16 and air vent 18. In the embodiment depicted in FIG. 2, cap 14 is formed with cylindrical recesses 17 within spout 16 and below air vent 18. These recesses 17 are configured to accept flow control valve 20. 
     In the embodiment shown in FIGS. 3 through 5, flow control valve 20 includes two stacks 24. Stacks 24 include lower portions 26, upper portions 28 and valve faces 30 bearing slits 32. These stacks 24 are adapted to be pressed into recesses 17 to friction fit flow control valve 20 into cap 14. Accordingly, when recesses 17 have a lower cylindrical portion, as preferred, lower portions 26 of stacks 24 are also preferably substantially cylindrical in shape. 
     As also shown in FIGS. 3 and 4, each stack 24 is elongated. In addition, each stack 24 is of significant diameter and of substantially equal height. The elongated shape of stacks 24 enables them to place valve faces 30 and slits 32 (see FIG. 5) in close proximity to the apertures in spout 16 and air vent 18. The diameter of stacks 24 permits significant, relatively unconstrained fluid flow to the area of slits 32. It has been found that this arrangement provides optimal balancing of suction needed to open slits 32 and the fluid flow through the slits. Similarly, its substantial cylindrical diameter and resulting inner contour presents a simple, wide opening and tube to enable thorough cleaning of the stacks 24 after use and to minimize the number of corners and niches in which dried or congealed liquid can be deposited. It is preferred that the outer contour of stacks 24 be stepped, as shown in FIGS. 3 and 4, but that the inner contour of the stacks be a constant diameter or of constantly diminishing diameter, thus presenting a smooth, unstepped inner face. Thus, the smooth inner face is preferably either cylindrical, frustoconical, or a combination of the two. This smooth inner face further enhances free fluid flow and promotes easy cleaning of stacks 24. The fact that this preferred flow control valve 20 is easy to clean is very important both to the proper and sanitary functioning of the assembly 10, and also to consumer acceptance of the valve. 
     It has also been discovered that the preferred concave shape of valve faces 30, in conjunction with the attendant curved shape of slits 32, provides superior fluid flow rate through slits 32 than existing valve configurations. This makes the assembly 10 easier to drink from and less frustrating and tiring to use. Furthermore, it has been found that elongated single slits 32 are preferred to cross-cuts or other types of apertures through valve faces 30. It is also preferred that slits 32 extend substantially from edge to edge of concave valve faces 30. 
     Most preferably, the radius of curvature of the valve face 30 that is aligned with spout 16 is about 0.267 inches. The most preferred radius of curvature of the valve face 30 that is aligned with air vent 18 is also about 0.267 inches. The most preferred length of slit 32 that is aligned with spout 16 is about 0.235 inches. The most preferred length of slit 32 that is aligned with air vent 18 is about 0.170 inches. The most preferred inner diameter of the stack 24 that is aligned with spout 16 is from about 0.301 inches to about 0.368 inches, ideally a frustoconical shape having the foregoing as minimum and maximum diameters. The most preferred inner diameter of the stack 24 that is aligned with air vent 18 is from about 0.252 inches to about 0.368 inches, ideally a frustoconical shape having the foregoing as minimum and maximum diameters. The most preferred height of the stack 24 that is aligned with spout 16 is about 0.803 inches from top to bottom, and about 0.521 inches from indentation to bottom. The most preferred height of the stack 24 that is aligned with air vent 18 is about 0.730 from top to bottom, and about 0.550 from indentation to bottom. The two stacks 24 are preferably 1.60 inches on center. The preferred outer diameter of the lower portion 26 of the stack 24 that is aligned with spout 16 is about 0.522 inches. The preferred outer diameter of the lower portion 26 of the stack 24 that is aligned with air vent 18 is about 0.457 inches. These dimensions provide an interference fit with a cup lid having cylindrical recesses 17 having preferred inner diameters of about 0.499 inches and about 0.439 inches, respectively. All of the foregoing measurements are subject to a preferred tolerance of plus or minus about 0.005 inches. In addition, a further dimension that is most preferred is the thickness of valve face 30. It is most preferably about 0.023 inches thick, with a preferred tolerance of only about plus or minus 0.002 inches. This dimension has been found to be very important in providing proper flexion of the valve faces 30 and opening of slits 32 under suction during use. 
     It is preferred that the flow control valve 20 be formed from a single piece of elastomeric material to facilitate easy insertion into and removal from recesses 17. However, flow control valve 20 can be formed of two separate valving elements, each adapted to be inserted into recesses 17 or otherwise engage cap 14. The elastomeric material used is most preferably silicone, but TPE (thermoplastic elastomer), natural rubber, and synthetic rubber (e.g., isoprene) are also preferred. 
     The following data demonstrate the improved flow rates of a flow control valve 20 according to the present invention. Six samples of a flow control valve as depicted in FIG. 3 (Valve A) were tested against six samples of a flow control valve as depicted in FIG. 1 (Valve B) and against ten samples of a flow control valve as disclosed in U.S. Pat. No. 5,079,013 to Belanger (Valve C). 
     
         ______________________________________   Suction to Suction forSample     Start Flow                  Continuous                            Time to EvacuateNumber             (psi)                      Flow (psi)                        100 ml water (sec.)______________________________________Valve A1       1.23         2.21      492               1.47                        2.21                                  373               1.47                        2.46                                  514               1.47                        2.33                                  445               1.23                        2.33                                  566               1.23                        2.21                                  50Avg.         1.35            2.29                                  48Valve B1        0.98       2.82       582                0.98                       2.95                                  413                1.72                       2.46                                  444                1.72                       2.70                                  575                1.47                       2.70                                  636                1.23                       2.46                                  75Avg.          1.35          2.68                                  56Valve C1       2.46        4.42       362                2.95                       4.54                                  273                2.95                       4.42                                  764                2.46                       3.93                                  245                2.95                       4.42                                  386                3.19                       4.17                                  337                2.46                       3.93                                  788                3.19                       4.42                                  299                2.46                       3.93                                  4010              2.95                       3.93                                   26Avg.            2.80                       4.21                                 40.7______________________________________ 
    
     These data show that the Valve A, a valve according to the present invention, requires lower suction to generate a continuous flow than the prior art valves, and requires less time to evacuate 100 ml of water than the prior art elastomeric valve, Valve B. Moreover, this Valve A is more consistent from sample to sample than the controls. This provides a more acceptable product. 
     Various modifications may be made to the foregoing disclosure as will be apparent to those skilled in the art. Thus, it will be obvious to one of ordinary skill in the art that the foregoing description and drawings are merely illustrative of certain preferred embodiments of the present invention, and that various obvious modifications can be made to these embodiments in accordance with the spirit and scope of the appended claims.
A drinking cup assembly including a cup having an open end; a cap adapted to enclose the open end, the cap including a drinking spout and an air vent and mating surfaces adjacent or incorporated into the drinking spout and the air vent; and a flow control valve including two stacks adapted to engage the mating surfaces, each of the two stacks having a concave valve face at a top portion thereof.