Abstract:
A method is provided for reinforcing a plastic pallet having a plurality of relatively weak structural portions each including a sheet portion with a plurality of vertical ribs extending therefrom. The method contemplates affixing a plurality of sheet strips to the vertical ribs in the plurality of relatively weak structural portions, respectively, to form a plurality of substantially rectangular hollow vertical cross-sections along the length of the relatively weak structural portions for improved stiffness.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 09/108,532 filed Jul. 1, 1998, pending and is related to commonly owned Ser. No. 09/004,389, filed Jan. 8, 1998. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a method of reinforcing a plastic pallet, and more particularly to a method of reinforcing a plastic pallet by applying a plurality of sheet strips along relatively weak structural portions of the pallet to form a plurality of substantially rectangular hollow vertical cross-sections along the length of the relatively weak structural portions for improved stiffness. 
     BACKGROUND ART 
     Replacing wood pallets with plastic pallets has been a goal for many years. The advantages of the plastic pallets are many as compared to wood, including greater durability, lighter weight, more consistent dimensions, improved cleanliness, water resistance, higher residual value for recycling, and no nails which may damage products being supported thereon. 
     One major hurdle to overcome with plastic is the cost. Plastic pallets are more expensive than wood by three to five times. This cost can be offset by the number of trips or shipments that can be achieved with plastic versus wood pallets. Another major hurdle is the stiffness of plastic pallets. Racking loaded pallets in warehouses for up to 30 days is common, and the combination of low tensile strength and creep limit the use of plastic. 
     There are three conventional methods of overcoming these weaknesses. The first is to add reinforcement such as steel or a composite to the pallet. This generally adds significant cost and weight and complicates recycling of the pallet. The second is to make the pallet taller. This limits the height of product to be stacked on the pallet. The third is to use reinforced or engineered resins. Again, this adds significant cost and weight. All three obviously limit the acceptance of plastic pallets. 
     U.S. Pat. No. 3,580,190 provides a partial solution to the stiffness problem by attaching top and bottom sheets  22 , 24  to the structural network  23 , as shown in FIG. 1 thereof. However, this solution does not resolve the bending stiffness problem because large lateral and longitudinal unsupported areas still exist, such as in areas  26 ,  37 ,  38 ,  49  and  50 . In other words, this design merely further stiffens the support column areas  67 ,  68 ,  69 ,  97 ,  98 ,  99 ,  28 ,  30 ,  32 , which already provide substantial stiffness merely as a result of their height. The weakness of this design is apparent in column 6, lines 60-71, where Fowler recommends the use of a material having a flexural modulus (or Young&#39;s modulus) greater than about 200,000 psi. Such a high modulus material is apparently required because the structure described does not provide significant resistance to deflection along the length and width of the pallet. High modulus materials add substantial cost to the pallet. 
     Further complicating the problem, modern pallets typically require large openings for receipt of pallet jacks. For example, the pallet shown in FIGS. 1-3 includes a top deck portion  16  supported on a plurality of support columns  18 , which are attached to support rails  20 , which form the bottom deck  19 . Such structure cooperates to form two large openings  11 , 13  on each side of the pallet  10 , as well as four bottom openings  15  formed in the lower deck  19 . In this configuration, the rails  20  of the lower deck  19  are typically structurally weak, resulting in poor deflection stiffness. Such problems have proven very difficult to overcome because of the very thin nature of the lower deck  19 . Similarly, the thin design of the top deck  16  results in the same deflection problem between columns  18 . 
     Because pallets are exposed to significant abuse, any solution to the stiffness problem must not adversely effect the impact strength of the pallet. 
     Accordingly, a need exists for improving the stiffness of modern plastic pallets configured to receive a pallet jack, without reducing impact strength of the pallet. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of reinforcing a modern plastic pallet by affixing sheet strips along relatively weak structural portions of the pallet to form a plurality of substantially rectangular hollow vertical cross-sections along the length of the relatively weak structural portions for improved stiffness without loss of impact strength. 
     More specifically, the present invention provides a method of reinforcing a plastic pallet having a thin top deck portion, a plurality of support columns extending from the top deck portion and a plurality of support rails connected to the support columns to form a thin bottom deck portion, wherein the support rails each include a sheet portion with a plurality of vertical ribs extending therefrom. The method includes the step of welding a plurality of plastic sheets to the vertical ribs between the support columns to form a plurality of substantially rectangular hollow vertical cross-sections along the length of the support rails for improved stiffness. It is contemplated that the substantially rectangular hollow vertical cross-sectional areas may be filled with a secondary material, such as structural foam for improved structural integrity. 
     Accordingly, an object of the present invention is to provide a method of structurally reinforcing a modern plastic pallet configured to receive a pallet jack, in a manner which improves stiffness without loss of impact strength. 
     The above object and other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a perspective view of a modern plastic pallet in accordance with the present invention; 
     FIG. 2 shows a bottom exploded perspective view of the pallet of FIG. 1; 
     FIG. 3 shows a top exploded perspective view of the pallet of FIG. 1; 
     FIG. 4 shows a cut-away perspective sectional view of a pallet in accordance with an alternative embodiment of the invention; 
     FIG. 5 shows a top exploded perspective view of a pallet in accordance with a second alternative embodiment of the invention; and 
     FIG. 6 shows a bottom exploded perspective view of the pallet of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In a racking scenario, the modern plastic pallet  10  (which is configured to receive a pallet jack from any side), shown in FIG. 1, is supported from below on two opposing edges  12 , 14 , and loaded on the top deck portion  16 . The pallet must support this load with a minimum of deflection. The top deck portion  16  is supported by a plurality of support columns  18  extending from the top deck portion  16  and attached to the support rails  20 , which form the bottom deck  19 . The support rails  20  are generally relatively weak structural portions of the pallet because they are thin in vertical cross-section and are supported only at opposing ends by the columns  18 . Accordingly, the support rails  20  tend to deflect when the pallet is loaded. 
     Using simple beam formulas it is known that the deflection increases as the load or the span distance increases and decreases as the material modulus (E) or section moment of inertia (I) increases. Since the load and the span are defined for a given application, the variables used to minimize deflection are the material and the section design. 
     A good rule of thumb with plastics is that as the E value increases for a given material, the impact strength decreases. There are engineered materials that can solve these problems but they are too expensive for wide spread use. The most common method is to use a commodity resin such as polyethylene or polypropylene and add a filler to stiffen the resin. Fillers add weight and reduce impact strength for a given material as well as impact the recyclability in some cases. They also add cost, which can be the biggest problem to overcome. So, the ideal pallet would use a commodity resin because of cost, weight, and impact strength. Accordingly, optimizing the design of the pallet is the preferred method to achieve improved pallet performance. 
     As mentioned above, deflection decreases as the section moment of inertia (I) increases. For a pallet, the easiest method to increase stiffness is to increase height. However, in practice there is a maximum allowable height for pallets, and existing pallets are generally designed at this maximum value. Accordingly, the only alternative is to maximize the moment of inertia for each pallet component, namely the top deck  16  and bottom deck support rails  20 . 
     For a given section geometry, the highest I value is for a solid section. For instance, the stiffest top deck design is one that is a solid plastic. Obviously, this is impractical because of weight and cost. Most designs attempt to overcome this by using ribbed sections instead of solid sections to minimize the loss of I and reduce the weight to an acceptable level. Another method is to use foaming agents along with the ribs to minimize the weight of the ribs and improve the I value. Both methods have a limit to their effectiveness. Namely, ribs are not the ideal geometry to maximize the I value, and while foaming improves this slightly, it also reduces the impact strength of the material. 
     It is known that for a given section the material closest to the neutral axis has the least effect on the I value, and the material farthest away has the greatest effect. In other words, a hollow or I-beam section is stiffer than a rectangular section of equal height and area. Therefore, the object of the design is to create hollow or boxed sections everywhere possible. Conventional injection molding techniques make this almost impossible to create, but by using a simple secondary operation, we are able to make a boxed top deck  16  and bottom deck  19 . 
     A pallet in accordance with the present invention includes the top deck portion  16 , which is injection molded conventionally and consists of a flat upper surface  22  with a series of ribs  24  protruding from the upper surface  22  as shown in FIG.  2 . The top deck portion  16  includes a plurality of pockets  26  for receiving the support columns  18 . A plurality of plastic sheet strips  28  are sonically welded to the ribs  24  to form a plurality of substantially rectangular hollow boxed sections between the pockets  26  within the top deck  16  (as described later with reference to FIG.  4 ). Alternatively, other attachment methods such as vibratory welding, hot plate welding, adhesive etc. may be used for attachment of the plastic sheet strips  28 . 
     The bottom deck  19  is constructed similarly but has the support columns  18  integrally molded therewith. Ribs  30  protrude downwardly from the sheet portions  32  of the support rails  20 , and a plurality of plastic sheet strips  34  are welded to the ends of the ribs  30  to form a plurality of boxed cross-sections along the length of the rails  20  between the columns  18 . The top and bottom decks  16 , 19  may be joined permanently by welding, or can be snapped together as commonly known in the art. 
     The method described above is preferably used to stiffen conventional ribbed pallet designs. The small sheets of plastic  34  are welded into critical deflection areas of the existing pallets for stiffening. Also, new pallets could be designed to accept the sheets for applications that require racking, and would eliminate the sheets for lighter, lower cost applications. For example, the ribs  30  may be recessed in order to receive the sheets  34  in a position flush with the bottom surface of the support rails  20 . 
     The method described above is particularly applicable for use in pallets such as that shown in FIG. 1 which has a very thin top deck  16  and bottom deck  19  to allow four-way entry of pallet jacks. The method described may be used to maximize the moment of inertia of each deck member. 
     Referring to FIG. 4, an alternative embodiment of the invention is shown. Similar to the embodiment shown in FIG. 1, the bottom deck rails  40  include a sheet portion  42  with a plurality of vertical ribs  44  extending therefrom. The plastic sheet strips  46  are welded to the ribs  44  to form the plurality of substantially rectangular hollow vertical cross-sections  48  along the length of the support rails  40 . Of course, numerous ribs  44  could be added to create numerous rectangular cross-sections for further improved structural integrity. 
     The pallet shown in FIG. 4 differs from the earlier embodiment described with reference to the FIGS. 1-3 in that a large sheet  50  is welded to the ribs  52  across the breadth of the upper deck  54  for improved structural integrity of the upper deck  54 . 
     It is contemplated that good results could be achieved even by only welding the peripheral ribs to the plastic sheet strips. It is further contemplated that the plastic sheet strips need not be welded, but could be affixed in any manner, such as adhesive, etc. It is also contemplated that the sheet strips need not be plastic. 
     Referring to FIGS. 5 and 6, a second alternative embodiment of the invention is shown. In this embodiment, the pallet  110  includes a thin top deck  116  connected to a thin bottom deck  119  by nine support columns  118 . The bottom deck  119  is comprised of a plurality of support rails  120  which extend between the columns  118 . Each support rail  120  includes a sheet portion  122 . Because each support rail  120  forms a relatively weak structural portion of the pallet  110 , an extruded plastic rectangular tube  127  is welded against each respective sheet portion  122  to add stiffness to each support rail by forming substantially rectangular vertical cross-sections along the length of each support rail  120 . Similarly, the top deck  116  includes open channels  128  adjacent the top sheet  130 , and an extruded plastic rectangular tube  131  is welded within each channel  128  against the top sheet  130  between the columns  118  to form substantially rectangular vertical cross-sections along the length of each channel  128  between the columns  118  for improved stiffness. In this configuration, the rectangular tubes  127 , 131  may be inexpensively extruded, and add substantial structural integrity to the pallet  110  without limiting the pallet&#39;s ability to receive pallet jacks from any side thereof. 
     While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.