In the manufacture of large pallets of rigid plastics material and having recesses or spaces for receiving parallel spaced forks of a forklift truck, it is well known to form the pallets with a twin sheet thermoformed upper deck, for example, as disclosed in U.S. Pat. No. 5,197,396, No. 5,413,052, No. 5,676,064 and No. 5,813,355. It is also known to make or form upper decks and other components for a plastic pallet by extruding a plastic parison and directing the parison into blow molding equipment, for example, as disclosed in U.S. Pat. No. 5,845,588, No. 6,962,115 and No. 6,976,437. Blow-molding processes have also been used to form rigid hollow plastic panels, for example, as disclosed in U.S. Pat. No. 6,826,887. Plastic pallets have also been made by using injection molded components, for example, as disclosed in U.S. Pat. No. 6,389,990, No. 6,840,181 and No. 7,197,989. It is further known to make plastic pallets by rotational molding, for example, as disclosed in U.S. Pat. No. 6,807,911.
In the manufacture of any plastic pallet or hollow plastic panel, it is desirable to maximize the strength/weight ratio while minimizing the cost of manufacturing or producing the pallet or panel. Also, for many uses of a plastic pallet or panel, it is desirable for the top surface to be substantially smooth and solid without openings which can trap or collect undesirable debris or material. When the top deck and the bottom deck of a plastic pallet are vacuum-formed by a twin-sheet thermoforming process, it is also known to insert tubular metal reinforcing members between the sheets, for example, as disclosed in above mentioned U.S. Pat. No. 5,197,396 and No. 5,413,052. The top and bottom decks or panels of a plastic pallet formed by a twin-sheet or triple sheet thermoformed process are also commonly reinforced by vacuum-forming integral hollow or open ribs in the top and/or bottom sheets, for example, as disclosed in above U.S. Pat. No. 5,676,064, No. 5,813,355 and No. 6,749,418.
The present invention is directed to an improved twin-sheet thermoformed plastic panel incorporating integral rib reinforcement which significantly increases the strength of the panel and which also helps to minimize the thicknesses of the twin-sheets forming the panel. A panel constructed in accordance with the invention is ideally suited for use as the top deck of a plastic pallet in that the panel has a substantially solid top surface without any recesses or cavities which can collect dirt and debris.
In accordance with the illustrated embodiment of the invention, a reinforced hollow panel is produced by a twin-sheet thermoforming process wherein a lower thermoplastic sheet is vacuum-formed into a lower mold section to form parallel spaced inverted U-shaped elongated open ribs projecting upwardly and having spaced side walls. An upper sheet of thermoplastic material is vacuum-formed into an upper mold section which has a set of extendable and retractable parallel blades to form parallel spaced elongated closed double wall ribs projecting downwardly and having adjacent side walls. When the mold sections close on the heated sheets, top portions of the open ribs in the lower sheet fuse to the upper sheet, and the double walled closed ribs project downwardly between the open ribs and have bottom portions fused to the lower sheet. Air pressure is introduced between the fused together upper and lower sheets, before the sheets solidify, and the blades are retracted so that the adjacent walls of the double walled closed ribs fuse together. The closed double walled ribs extend transversely or perpendicular to the open ribs, and the end portions of the double walled ribs fuse to the side walls of the open ribs, with the result that the hollow plastic panel is reinforced in all directions. When the hollow plastic panel is used for the upper deck panel of a pallet, the supporting structure for the upper deck panel is also preferably formed of plastics material, and a lower deck panel may be formed in the same manner as the upper deck panel.
Other features and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.
The vacuum-forming of the lower sheet also forms a series of peripherally spaced open bottom cavities 46 and 48 (
Referring to
During the twin-sheet thermoforming process, the heated upper sheet 18 is vacuum-formed into a cavity 78 within the plate 59 to provide the sheet with a smooth flat top surface 80 (
The blades 75 are then retracted upwardly (
From the drawings in the above description, it is apparent that a hollow thermoformed plastic panel and a plastic pallet made from the panel, provide desirable features and advantages. As one important advantage, the arrangement of the downwardly projecting closed double wall ribs 90 within the vacuum-formed upper sheet 18 of the panel 15 and the crossing open ribs projecting upwardly from the vacuum-formed lower sheet 20 with the ribs fused to the opposing sheets, along with the fused peripheral flanges, provides the panel 15 with substantial strength in all directions while minimizing the weight of the panel. This feature is especially desirable when the panel 15 is used to form the upper deck of a plastic pallet as shown in
While the form of thermoformed panel herein described and its method of construction constitute a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of panel, and that changes may be made therein without departing from the scope and spirit of the invention as defined in the appended claims.
Number | Name | Date | Kind |
---|---|---|---|
5197396 | Breezer et al. | Mar 1993 | A |
5391251 | Shuert | Feb 1995 | A |
5413052 | Breezer et al. | May 1995 | A |
5676064 | Shuert | Oct 1997 | A |
5755162 | Knight et al. | May 1998 | A |
5769003 | Rose et al. | Jun 1998 | A |
5813355 | Brown et al. | Sep 1998 | A |
5843366 | Shuert | Dec 1998 | A |
5845588 | Gronnevik | Dec 1998 | A |
6018927 | Major | Feb 2000 | A |
6389990 | Apps | May 2002 | B1 |
6749418 | Muirhead | Jun 2004 | B2 |
6807911 | Carson et al. | Oct 2004 | B2 |
6826887 | Skov | Dec 2004 | B2 |
6840181 | Smyers | Jan 2005 | B2 |
6962115 | Markling et al. | Nov 2005 | B2 |
6976437 | Fisch et al. | Dec 2005 | B2 |
7197989 | Apps | Apr 2007 | B2 |
20090183655 | Ogburn | Jul 2009 | A1 |
20100058959 | Endo | Mar 2010 | A1 |
Number | Date | Country | |
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20110174197 A1 | Jul 2011 | US |