Abstract:
A pallet formed of a pair of thermoplastic sheets molded and fused together. The sheets have configurations that impart high stiffness and high strength, and the ability to be stacked on full pallets or nested on empty pallets. In particular, the molded configurations enable the sheets to be fused at a plurality of planes including mutually perpendicular planes for high strength union of the sheets. The pallet is especially suited for use with generally cubic milk crates.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to pallets used for material handling and storage and, in particular, to improvements in molded plastic pallets. 
     PRIOR ART 
     It is known to mold pallets of plastic and/or other materials thus departing from traditional wood slat construction. More specifically, it is known from U.S. Pat. No. 4,428,306 to Dresen et al. and U.S. Pat. No. 5,046,434 to Breezer et al., for example, to mold pallets of two sheets fused or otherwise joined together. The sheets are typically molded with shallow grooves to increase their individual stiffness. These patents teach that the main areas of the two sheets be arranged relatively close together and adjacent the top plane of the pallet. Commonly, pallets of this type have molded feet that depend downwardly from the main planes of the double sheets at locations spaced throughout the plan area of the pallet. These double sheet pallets with depending feet are limited in their rigidity and strength as a result of the relatively close proximity of the sheets. Moreover, a pallet of this style often cannot be practically stacked onto another loaded pallet because the legs or feet may not properly register with the goods or packages on the loaded pallet. Still further, the relatively small surface areas represented by the feet can produce relatively high unit pressure on the goods or containers on an underlying loaded pallet. 
     SUMMARY OF THE INVENTION 
     The invention provides an improved molded plastic pallet that achieves a high rigidity and strength for its material content or weight and that is readily stackable onto either a loaded pallet or onto an empty pallet. The disclosed pallet is comprised of a pair of sheets that are molded into unique configurations. The sheets provide upper and lower platforms with relatively large surface areas for adequately supporting a load and for stabilizing the pallet when it is stacked on a loaded pallet. 
     The sheets are molded with interfitting formations that support the upper platform or deck above the lower platform or base a vertical distance adequate to receive the carrying forks of a lift truck or a floor jack. The formations, likewise, are configured to provide generous horizontal clearance for reception of the forks while preventing undue distortion or structural failure under load. 
     More particularly, the formations are configured to adequately transfer the load on the upper deck to the carrying forks or to the base without buckling or other gross distortion. 
     The disclosed pallet is especially useful for transporting and storing conventional milk crates which typically are molded plastic and are generally cubic in shape. The pallet is arranged to carry such crates in 3×3 layers (9 crates per layer) and with a stack height of 3 layers on each pallet. In this milk crate application, loaded pallets constructed in accordance with the invention can be stacked 3 pallets high with the base of successive loaded pallets safely and stably resting on the tops of the crates of an upper layer of an underlying loaded pallet. The pallet of the invention includes on the top deck a peripheral flange that horizontally locates and stabilizes the milk crates. Moreover, this upper flange is configured to receive a lower base flange of another identical pallet in a nesting fashion thereby facilitating stacking of empty pallets. 
     The disclosed pallet is preferably produced by forming the upper and lower sheets, made of thermoplastic material, into their configurations on shaped aluminum tools through application of heat, vacuum and/or pressure. Once molded to form, the sheets are pressed together using their forming tools as platens. This pressing operation is done while the sheets are at an elevated temperature thereby causing the sheets to fuse together at local surface areas of contact. The surface areas where fusion occurs, ideally, are provided with relief zones, typically in the nature of grooves in one of the sheets. The relief zones allow a small amount of sheet material to squish out from the contact areas to ensure that contact and fusion occurs at all intended fusion areas regardless of slight variations in material thickness, molded form, and/or relative positions of the tools when pressing the sheets together. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a pallet constructed in accordance with the invention, only a portion of the configuration of a top sheet thereof being illustrated for simplification; 
     FIG. 2 is an isometric view of the pallet loaded with milk crates arranged in 3×3 layers, three layers high; 
     FIG. 3 is a plan view of the pallet with half of the top sheet being broken away to reveal areas in which the top sheet is fused to the bottom sheet; 
     FIG. 4 is an elevational view of the pallet partially in section taken along the planes indicated by the line  4 — 4  in FIG.  3  and viewing the pallet along the line of fork entry; 
     FIG. 5 is a side elevational view of the pallet partially in section taken along the planes indicated by the line  5 — 5  in FIG. 3; 
     FIG. 6 is a fragmentary cross-sectional view of the pallet on an enlarged scale taken in the plane indicated by the line  6 — 6  in FIG. 3; 
     FIG. 7 is a fragmentary plan and cross-sectional view on an enlarged scale of fused wall areas of the top and bottom sheets taken in the plane indicated by the line  7 — 7  in FIG. 6; 
     FIG. 8 is a fragmentary cross-sectional view of the pallet taken through fused areas of the top and bottom sheets along the plane indicated by the line  8 — 8  in FIG. 6; 
     FIG. 9 is a fragmentary cross-sectional view of the pallet taken in the plane indicated by the line  9 — 9  indicated in FIG. 6; 
     FIG. 10 is a fragmentary cross-sectional view of the pallet taken in the planes indicated by the line  10 — 10  in FIG. 3; and 
     FIG. 11 is a fragmentary exploded isometric view of a typical comer portion of the pallet. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A pallet  10 , constructed in accordance with the invention, comprises a pair of plastic sheets  11  and  12  first formed into mating configurations and then pressed and fused together. A separate aluminum tool (not shown) is used to form each of the sheets  11  and  12 . It will be understood that the tool forming the upper or top sheet  11  is disposed on an upper face  13  of the top sheet and the tool forming the bottom sheet  12  is disposed on a lower face  14  of the bottom sheet. The tools have shapes complementary to the desired finished shapes of the respective sheets  11  and  12 . These shapes are discussed in some detail hereinbelow. The sheets  11 ,  12 , which typically are of the same material such as high molecular weight polyethylene (HMWPE) are heated to a temperature sufficient to permit them to be molded into the shape of the tools with a vacuum (less than atmospheric pressure) applied between a tool and a sheet and/or a fluid pressure (greater than atmospheric pressure) applied to the exterior of a sheet. By way of example, the sheets  11 ,  12  can have a starting gauge of 0.300 inch; the disclosed pallet made with this material will have a weight of approximately 34.5 pounds. With the upper and lower sheets  11 ,  12  properly formed on their respective tools and with the sheets having a temperature of about between 300 and 350° F. the tools are pressed towards one another and the sheets become fused together at certain locations discussed more fully below. 
     As suggested in FIG. 2, a pallet  10  is proportioned to carry 3×3 layers of milk crates. The nominal size of a pallet  10  in plan view is 40″×40″. The pallet has a fork entry direction indicated by arrows  16  and determined by channels or passages  17  disposed between the upper and lower sheets  11  and  12  for receiving the forks of a lift truck or floor jack. 
     For reference, the nominal plane of the top sheet  11  is taken as the horizontal plane in which a large collective surface area on the upper face or deck  13  is adapted to support a load. Similarly, for reference, the nominal plane of the bottom sheet  12  is taken as the plane in which the lowermost collective surface area on the lower face or base  14  of the bottom sheet exists. 
     The bottom or lower sheet  12  is molded with three upstanding embossments or legs  21 ,  22 , all parallel to the fork entry direction  16 . Two of the legs  21  are adjacent opposite edges of the pallet  10  and a third leg is disposed in the middle of the pallet. The legs  21 ,  22  extend substantially the full length of the pallet and include opposed sidewalls  23 ,  24  and end walls  25 . The sidewalls  23 ,  24  are stiffened against buckling under vertical loads by vertical ribs  28  spaced along their horizontal extent. Each leg  21 ,  22  has a pair of pinch points  29  spaced from the end walls  25  and from each other. The pinch points  29  have wall portions  30  that are transverse to the sidewalls  23 ,  24  and that strengthen the sidewalls to avoid in and out flexing of such walls when carrying a load on the pallet. 
     As shown in FIG. 4, the fork receiving channels  17  exist on each side of the center leg  22  between the center leg and the outer legs  21 . Below the fork receiving channels or openings  17 , the lower sheet  12  is molded with cruciform embossments  32  to stiffen the lower sheet; gaps  33  between the cruciform embossments  32  work as pinch points to strengthen the lower sheet  12  in both the fork entry direction  16  and in the horizontal direction perpendicular to this entry direction. Four pad-like areas  34  (a typical one is illustrated in FIG. 11) are raised slightly out of the plane of the bottom sheet lower face  14 . The pads  34  serve as a template for optionally cutting out the pad area for the operation of the wheels of a floor jack. Along its full periphery, the lower sheet  12  is molded with a right angle flange  36  having a vertical section  37  and a horizontal section  38 , the latter being spaced above the plane of the bottom sheet lower face  14 . 
     A plurality of generally rectangular pockets  39  ( 12  in the illustrated embodiment) are formed in the upper regions of the legs  21 ,  22 . The pockets  39  are grouped in aligned pairs formed on an outer leg  21  and an adjacent side of the center leg  22 . The pockets  39  each have generally vertical side faces  41  and generally vertical opposed end faces  42 . The pockets  39  are open at a side associated with the respective fork opening  17 . The vertical faces  41 ,  42  are molded with grooves  43 . 
     The upper sheet  11  has the majority of its plan area (i.e. as viewed from above) embossed with serpentine grooves  46  that provide strength in all directions in the plane of the upper face  13 . 
     Arched ribs  47  are molded into the top sheet  11  at a plurality of locations where they extend across the fork receiving channels  17 . In the illustrated construction, the arched ribs are at three locations distributed along the length of a fork receiving channel  17 . The arched ribs  47 , which are formed in adjacent pairs at each location, have their ends merged with plug-like formations  48  that are adapted to be received in respective ones of the lower sheet pockets  39  with a tight fit as discussed below. The plug formations  48  at each end of the paired arched ribs  47  include a vertical end wall  44  and oppositely facing generally vertical sidewalls  45 . 
     The fall periphery of the top sheet  11  is molded with an inverted U-shaped flange  51 . The flange  51  has an inner vertical section  52 , a horizontal section  53  and an outer vertical section  54 . The inner flange  52  extends above the plane of the upper face  13  of the top sheet  11  thereby enabling it to laterally restrain milk crates or other material from slipping horizontally off the top sheet. The peripheral flange  51  formed in the top sheet  11  is dimensioned to receive the vertical section  37  of the peripheral flange  36  formed in the bottom sheet  12  of an identical pallet so that empty pallets can be stably stacked and nested one upon the other. 
     After the top and bottom sheets  11 ,  12  are molded on their respective tools, the tools are forced towards one another to press the sheets into solid contact at certain locations. This pressing operation is done while the sheets are at an elevated temperature sufficiently high to enable the sheets to fuse together. The sheets  11 ,  12  contact on several planes including parallel planes and generally mutually perpendicular planes. The primary planes of contact are in horizontal orientations and bear the pressing force applied to the tools during the fusion step. These horizontal planes exist at the top of the legs designated  21 ,  22  and at the bottom surfaces, designated  40 , of the pockets  39 . The underside of the serpentine grooves  46  are fused onto the tops, designated  26 ,  27 , of the legs  21 ,  22 , and the bottoms, designated  49 , of the plugs  48  contact the pocket bottoms  40 . 
     The top and bottom sheets  11  and  12  are, additionally, fused in secondary planes generally parallel to the vertical pressing direction imposed by the tools. The secondary planes of fusion are created between the generally vertical side faces  41 ,  42  of the pockets  39  and the generally vertical walls  44 ,  45  of the plugs  48 . The tools used to mold and fuse the sheets  11 ,  12  together, are proportioned to ensure that the vertical surfaces  44 ,  45  of the plugs  48  are tightly squeezed against the respective vertical surfaces  42 ,  41  of the pockets  39  when the sheets are pressed together. The grooves  43  in the side faces  41 ,  42  allow any excess material in the area of a pocket  39  and plug  48  due to variations in the manufacturing process to squish out and be received into these grooves  43 . This assures that all of the design fusion areas are reliably brought into contact. 
     The multi-plane fusion between the top and bottom sheets  11 ,  12  assures a strong interconnection between these members. The arched ribs  47  serve to distribute a load when the pallet is being carried on a pair of forks uniformly to the legs  21 ,  22 . The arched ribs  47  also stabilize the legs  21 ,  22  from horizontal distortion in directions perpendicular to the fork entry direction. The functions of the arched ribs  47  are enabled by the strong joints formed between the pockets  39  and plugs  48 . 
     The coplanar load carrying surface areas of the upper face or deck  13  collectively represent an area that is a substantial fraction of the horizontal or plan area spanned by the upper sheet  11  which, in the illustrated embodiment, is essentially the same as that spanned by the pallet  10 . Similarly, the lower sheet  12  has coplanar load supporting surface areas on its lower face  14  that collectively represent a substantial portion of the plan area spanned by the lower sheet  12  which in the illustrated embodiment is essentially the same as that spanned by the pallet  10 . Additionally, it will be seen that the load carrying and load supporting surface areas of the top and bottom sheets  11 ,  12  each have outer surface portions adjacent their perimeters and inner surface portions distributed at a plurality of locations inward of their outer surface portions including locations adjacent the center of the pallet  10 . The outer surface portions of the load carrying and load supporting surface areas of the top and bottom sheets  11 ,  12  are effectively continuous in their respective planes. 
     It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.