You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a cover for spas, hot tubs, and the like and more particularly to an improved core for such covers.  
         [0002]     In order to provide insulation, limit pollution, and prevent the risk of accidents, it is desirable to cover the open top of a spa or hot tub. A conventional cover typically includes a foam material that is covered with fabric. These conventional covers are generally designed to cover the entire open surface of a hot tub, often having two separate halves capable of folding together to aid in removal of the cover.  
         [0003]     Conventional hot tub covers have several disadvantages. First, the cover can become filled with water by a leak or other means, causing the cover to be excessively heavy. When this happens, the weight of the cover is greatly increased and removal becomes extremely burdensome. Second, in extreme cases, the foam core can become waterlogged, leading to interior bacterial growth and or weakening of the foam. Third, conventional covers support limited weight, especially if the core has become weakened. Children playing on the cover can break the cover and fall into the water. This risk is elevated if the children are relatively large or if they jump on the cover. Fourth, conventional covers tend to sag over time, especially if waterlogged, allowing standing water on the upper surface of the cover permitting undesirable bacterial growth.  
       SUMMARY OF THE INVENTION  
       [0004]     The aforementioned problems are overcome by the present invention wherein a cover for a hot tub or spa includes a structural, non-water absorbing plastic core.  
         [0005]     In a preferred embodiment, the upper surface of the core slopes from the center of the cover to the perimeter of the cover to provide water run off.  
         [0006]     In another preferred embodiment, the plastic core is ribbed to provide increased load bearing capabilities. The ribs improve the static load capabilities of the cover, and resist sagging. A foam or similar material may be injected between the ribs to enhance the thermal properties of the cover.  
         [0007]     The present invention provides several advantages. First, the cover resists filling with water, avoiding the growth and weight problems associated with such water. Second, the core cannot become waterlogged. Third, the cover supports significantly increased weight, and therefore reduces the likelihood that a child with break the cover. Fourth, the cover provides water run off from its sloped upper surface; and the cover does not sag.  
         [0008]     These and other objects, advantages, and features of the invention will be more fully understood and appreciated by reference to the detailed description of the preferred embodiment and the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a perspective view of the hot tub cover of the present invention installed on a hot tub.  
         [0010]      FIG. 2  is a sectional view of a central portion of the cover taken along line  2  in  FIG. 1 .  
         [0011]      FIG. 3  is a sectional view similar to  FIG. 2  of an alternative embodiment of the cover.  
         [0012]      FIG. 4  is a side view of the cover with portions of the vinyl jacket removed. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]     A hot tub in accordance with the preferred embodiment is shown in  FIG. 1  and generally designated  10 . It should be understood that the hot tub  10  may be rectangular (as shown) or any other desired shape. The cover  12  fits over the entire opening  11  of the hot tub  10 . The cover  12  generally includes a vinyl jacket  14  and a plastic core insert  16 . The vinyl jacket  14  encloses the core  16  and is conventionally sealed around the core  16 . The core insert  16  includes a hinge  22  that allows the cover  12  to fold onto itself. The core insert  16  tapers from the hinge towards the periphery and includes a plurality of strengthening ribs  18 .  
         [0014]     The jacket  14  is well known and generally made of a marine grade vinyl, or another high-strength waterproof fabric. The jacket  14  is large enough to fit over a rigid core  16 , and includes a front edge  24 , rear edge  26 , and an outer edge  28 . In a preferred embodiment, the jacket  14  is capable of being sealed to completely enclose the core  16  by conventional means such as a zipper, sewing, or similar method (not shown) after the core  16  is inserted. The jacket  14  includes a section of extra material  15  approximately in the center capable of fitting within the hinge portion  22  of the core insert  16 . The extra material  15  allows the cover  12  to fold in half. The jacket  14  may also include attachments (not shown) such as snaps that attach the cover  12  to the hot tub  10  when the cover  12  is installed.  
         [0015]     The core  16  is preferably made from a thermoformable polymer such as ABS. The core is shaped to fit inside a corresponding vinyl jacket  12 , such that the combination of jacket  14  and core  16  fit over the opening  11  of a hot tub  10 . The core  16  is a single piece of plastic, including opposing lateral portions  34  and  35  joined by a central hinge portion  22 . Each lateral portion  34 ,  35  extends from an inner edge  20  to an outer edge  21  (shown in  FIG. 4  on lateral portion  35  only). The outer edge  21  is in communication with the outer edge  28  of the jacket  14  when inserted in the jacket  14 . The hinge  22  extends parallel to edges  20  and  21  from a front edge  36  to a rear edge  37  (not shown), so that the core  16  is foldable approximately in half. The front edge  36  and rear edge  37  are in communication with the front  24  and rear  26  edges of the jacket  14  respectively when the core  16  is inserted in the jacket  14 .  
         [0016]     In another preferred embodiment, the core  16  includes an upper surface  30 , a lower surface  32 , and a plurality of ribs  18  extending therebetween. As shown in  FIGS. 2 and 4 , the ribs  18  generally run parallel to the hinge  22  and extend from the front edge  36  to the rear edge  37  of the core  16 . In a preferred embodiment, the ribs  18  are evenly distributed between the hinge  22  and outer edges  34  and  36 . In another preferred embodiment, each rib  18  has a height  38  that is uniform from the front edge  36  to the rear edge  37 . However, the height  38  of each sequential rib  18  decreases from the hinge  22  toward the outer edge  20 , such that the upper surface  30  and lower surface  32  converge approaching the outer edge  20  of the lateral portions  34  and  35 .  
         [0017]     In the preferred embodiment, the open spaces  40  between individual ribs are filled with air so the cover  12  retains heat in the hot tub  10  when installed. However, as shown in  FIG. 3  the spaces  40  may alternatively be filled with an insulating foam  42  or other insulating material to further enhance the thermal properties of the cover  12 .  
         [0018]     In operation, the core  16  is inserted into the jacket  14 . The core  16  may be a replacement for a used core insert, such that it is inserted into an existing used jacket, or the core  16  may be inserted into a brand new jacket  14  of the desired dimensions. The jacket  14  is sealed about the insert  16  and placed over the opening of a hot tub  10 . Removal of the cover  12  consists of folding the cover  12  back about the hinge  22  and pulling the cover off the hot tub  10 .  
         [0019]     The above description is that of a preferred embodiment of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. Any reference to claim elements in the singular, for example, using the articles “a,” “an,” “the” or “said,” is not to be construed as limiting the element to the singular.

Summary:
A hot tub cover having a structural, non-water absorbing, plastic core and a jacket enclosing said core. The core includes two identical core halves hinged to one another. Each core half includes an upper wall, a lower wall, and a plurality of ribs extending therebetween. The upper wall of each half slopes downwardly from the center to the peripheral edge.