Patent Publication Number: US-6986309-B2

Title: Synthetic resin pallet

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to plastic pallets used in situations such as transportation and storage of various articles and the like. 
   2. Description of the Related Art 
   Conventionally, when transporting and storing articles, wooden pallets have been used, but recently, plastic pallets are becoming more popular because they have the advantages that they are lighter than wood, do not degrade easily, and do not generate splinters which are characteristic of woods. In particular, the integrally molded pallet where both the upper and lower surfaces are injection molded as one, is used in place of wooden pallets in a wide range of fields because of its high production efficiency. 
   This type of plastic pallet, as shown for example in  FIG. 23 , has a rectangular upper plate part  200  and a lower plate part  202 , and a plurality of columns  204  connecting the upper plate part  200  and the lower plate part  202 , and a pair of fork insertion apertures  206  are formed parallel to each other between these columns  204 . In the example of  FIG. 23 , the fork insertion apertures  206  are formed only on two sides, but there are also models where pairs of fork insertion apertures are formed on all four sides. 
   The upper plate part  200  is called a deck board, and as shown in  FIG. 24 , has many beams  208  extending horizontally. Similarly, the lower plate part  202  has many beams  212  extending horizontally. Because the beams  208  that form the upper plate part  200  and the beams  212  that form the lower plate part  202  are arranged shifted alternately, many holes are respectively formed between the beams  208  and between the beams  212 . For the beams  208  and  212 , the cross-section is generally constructed in the form of a ‘U’ or a ‘T’, and these are devised in order to increase the strength of the upper plate part  200  and the lower plate part  202 . 
   The columns  204  are formed with many vertical walls  210  combined length and breadthwise, and connect the beams  208  and  212 . To prevent bending of the vertical walls  210 , reinforcing ribs  214  are formed at some places along the vertical walls  210  and connected to the beam  208  and  212 . 
     FIG. 25 , illustrates a method disclosed in Japanese Examined Patent Application, Second Publication number Sho 49-582, as one example of a molding method for this kind of integrally molded pallet. As shown in this figure, because the beam  208  and  212  pairs are displaced horizontally, even if complicated slide cores are not multiply used, this can be formed relatively easily between an upper mold  216  and a lower mold  218 . In addition, compared to the case where the upper and lower parts are formed separately and the pallet then formed by joining the two together, the time-consuming joining process is unnecessary. Therefore, the production efficiency of the integrally molded pallet is high. 
   However, in the case where the pallet is loaded with opposite ends of the pallet supported, in the above-mentioned plastic pallet, there is a problem in that because the four corners of the fork insertion aperture  206  are subjected to a large bending stress, the fork insertion aperture  206  are easily deformed into a parallelogram, and the entire pallet bends easily compared to the wooden pallet. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a plastic pallet where the strength of the whole pallet can be increased by increasing the rigidity of the surroundings of the fork insertion apertures in particular, and moreover for which production efficiency is high. 
   To achieve the above object, the plastic pallet of the present invention comprises an upper plate part and a lower plate part arranged parallel to each other, and a plurality of columns connecting the upper plate part and lower plate part, and fork insertion apertures being formed between the columns. Corresponding to the fork insertion apertures, there is formed a reinforcing frame surrounding a cross-section perpendicular to a fork insertion direction of the fork insertion apertures, and a hollow portion is formed in an interior of at least one portion of the reinforcing frame. 
   According to the present invention, because each of the reinforcing frame of which at least one part has a hollow construction surrounds the fork insertion aperture, it is possible to suppress any distortion of the cross-section shape of the fork insertion apertures when a heavy load is applied, and as a result, it is possible to increase the rigidity of the plastic pallet. 
   The reinforcing frame may have an upper horizontal beam formed along the upper plate part, a lower horizontal beam formed along the lower plate parts and a pair of vertical beams connecting the upper horizontal beam and the lower horizontal beam, and at least three of the upper horizontal beam, the lower horizontal beam and the pair of vertical beams may have hollow portions communicating with each other. 
   Of the upper horizontal beam, the lower horizontal beam and the pair of vertical beams, a section which does not have a hollow portion in the interior may have a thickness smaller than that of a section which does have a hollow portion in the interior. More specifically, it is desirable that the width of the section that does not have a hollow portion in the interior is 10 to 50% of the width of the section that does have a hollow portion in the interior. 
   The upper horizontal beam, the lower horizontal beam and the pair of vertical beams may all have hollow portions communicating with each other. 
   A gas inlet communicating with the hollow portion may be formed in an outer face of the reinforcing frame, and a non-slip member to close the gas inlet may be attached to the upper plate part or the lower plate part. In this case, it is possible to prevent the infiltration and the like of rain water from the gas inlet. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view showing a plastic pallet of a first embodiment of the present invention. 
       FIG. 2  is a bottom view of the plastic pallet of the first embodiment. 
       FIG. 3  is a front elevation of the plastic pallet of the first embodiment. 
       FIG. 4  is a side elevation of the plastic pallet of the first embodiment. 
       FIG. 5  is a cross-section along the line A—A in  FIG. 1 . 
       FIG. 6  is a cross-section along the line B—B in  FIG. 1 . 
       FIG. 7  is a cross-section illustrating an example of a manufacturing method of a plastic pallet of the first embodiment. 
       FIG. 8  is a cross-section showing a securing construction for a non-slip seal in the plastic pallet of the first embodiment. 
       FIG. 9  is a plan view showing a second embodiment of a plastic pallet of the present invention. 
       FIG. 10  is a cross-section of the plastic pallet of the second embodiment cut parallel to at a fork insertion aperture. 
       FIG. 11  is a cross-section of a reinforcing frame in the plastic pallet of the second embodiment. 
       FIG. 12  is a bottom view of the reinforcing frame in the plastic pallet of the second embodiment. 
       FIG. 13  is a bottom view of a reinforcing frame of a modified example. 
       FIG. 14  is a bottom view of the reinforcing frame of the modified example. 
       FIG. 15  is a plan view showing a plastic pallet of a third embodiment of the present invention. 
       FIG. 16  is an perspective view showing an arrangement of a reinforcing frame in the second embodiment of the present invention. 
       FIG. 17  is an perspective view showing an arrangement of the reinforcing fame in the third embodiment of the present invention. 
       FIG. 18  and  FIG. 19  are perspective views showing modified examples of the present invention. 
       FIG. 20  through  FIG. 22  are perspective views showing other embodiments of the reinforcing frame. 
       FIG. 23  is an perspective view showing the outward appearance of the plastic pallet. 
       FIG. 24  is a perspective view showing the main parts of a conventional plastic pallet (a cross-section along the line D—D in  FIG. 23 ). 
       FIG. 25  is a cross-section illustrating a construction method of the conventional plastic pallet. 
       FIG. 26  through  FIG. 31  are cross-sections of test pieces used for strength tests. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereunder is a description of suitable embodiments of plastic pallets according to the present invention, with reference to the drawings. However, the present invention is not limited to the following embodiments, and may involve for example appropriate combinations of constituent elements of these embodiments. 
   First Embodiment 
     FIG. 1  through  FIG. 6  show a first embodiment of a plastic pallet of the present invention.  FIG. 1  is a plan view,  FIG. 2  is a bottom view,  FIG. 3  is a front elevation,  FIG. 4  is a side elevation,  FIG. 5  is a cross-section along the line A—A in  FIG. 1 , and  FIG. 6  is a cross-section along the line B—B. 
   The plastic pallet of this embodiment is of box shape overall, and has a square upper plate part  1  and a square lower plate part  2  arranged parallel to each other, and columns  3 ,  4  and  5  connecting the upper plate part  1  and the lower plate part  2 . The columns  3  are formed at the four corners of the upper plate part  1  and the lower plate part  2 , the columns  4  are formed at the midsections of the four sides of the pallet, and the column  5  is formed at the center section of the pallet. All of the columns are constructed by many vertical walls. 
   Respective pairs of rectangular shaped fork insertion apertures  6  are formed between the columns  3  and  4  in the side faces of the four sides of the pallet, and these fork insertion apertures  6  pass horizontally through the whole pallet between the columns  4  and  5 . Accordingly, in the pallet of this embodiment, it is possible to insert two forks from the direction of any of the four sides. However, the plastic pallet of the present invention is not limited to this construction. The fork insertion aperture may also be formed only on the two opposite sides (refer  FIG. 18 ), and the columns and the position and number of the fork insertion apertures may also be changed appropriately. 
   A feature of the present invention, as shown in  FIG. 6 , is that the rectangular reinforcing frames  70  are multiply formed surrounding the cross-section of the fork insertion apertures  6  orthogonal to the fork insertion direction, for each of the four fork insertion apertures  6 , and a hollow portion  70 ′ is formed inside each of these reinforcing frames  70 , around the whole circumference of the reinforcing frame  70 . 
   In this embodiment, in the vicinity of the openings of both ends of each fork insertion aperture  6 , namely between the columns  3  at the four corners and the columns  4  at the side midsections, is respectively formed two reinforcing frames  70 . In the case where the reinforcing frames  70  are formed in the vicinity of both ends of the fork insertion apertures  6  in this way, when the forks are inserted into the fork insertion apertures  6 , it is possible to support the load particularly from the front ends and the rots (rear ends) of the fork with the reinforcing frame  70 . Therefore, this is more desirable than the construction where the reinforcing frame  70  is formed only in the midsection of the fork insertion aperture  6 . However, the present invention is not limited to this construction. The formation position of the reinforcing frame  70  may be changed optionally, if necessary. The number of reinforcing frames  70  for one fork insertion aperture  6  is not particularly limited, and it is acceptable to provide for example 1 to 6 according to the total length of the fork insertion aperture  6 . 
   The reinforcing frame  70  of this embodiment is consisted of an upper horizontal beam  71  formed in the upper plate part  1 , a lower horizontal beam  72  formed in the lower plate part  2 , a vertical beam  73  extending vertically along the columns  3  at the four corners, and a vertical beam  74  extending vertically along the columns  4  at the midsections of the sides. The hollow portions  70 ′ respectively formed inside the upper horizontal beam  71 , the lower horizontal beam  72  and the vertical beams  73  and  74 , are communicated with each other. 
   The hollow portions  70 ′ are formed as a cavity equivalent to the volume decrease caused by the process of solidifying the resin at the inner section of the beams  71  to  74 , by for example the infusion of high pressure gas from the middle section of the upper horizontal beam  71 , immediately after the injection molding of the plastic pallet. Accordingly, it is possible to also prevent a roughening of the surface caused by shrinkage, and a drop in molding accuracy. Moreover, an effect is also obtained of shortening the solidifying time of the beams  71  to  74 , by making the cross-section midsection that is hard to solidify hollow. 
   By forming the beams  71  to  74  as a rectangular cylinder having the hollow portion  70 ′ inside, the production efficiency is not decreased, and the weight of the pallet is also not increased by a significant amount, in comparison to the conventional beams of a U shape or a T shape in cross-section. However, it is possible to considerably increase the strength of the periphery of the fork insertion apertures  6 . Consequently, in the case where the pallet bears a heavy load, it is difficult for the fork insertion apertures  6  to change into a parallelogram, and it is possible to suppress the bending of the pallet. 
   As for the upper horizontal beam  71  and the lower horizontal beam  72  of this embodiment, the cross-section is rectangular, and while the dimensions are not limited, in general it is desirable that the width in the horizontal direction is 15 to 50 mm and the thickness in the vertical direction is 15 to 40 mm. 
   The vertical beam  73  and the vertical beam  74  also have rectangular cross-sections, and it is desirable that the width in the fork insertion direction is equal to the width of the upper horizontal beam  71  and the lower horizontal beam  72 , and that the thickness in the perpendicular direction to the fork insertion direction is 10 to 30 mm. 
   As shown in  FIG. 1 , on the upper plate part  1 , between the columns  3  at the four corners and the columns  4  at the midsections of the sides, a beam  11  is formed so as to extend parallel to the upper horizontal beam  71 , and a beam  12  is formed so as to extend in the fork insertion direction, and the upper horizontal beam  71  and the beam  11  and the beam  12  intersect as a grid. Many openings are formed between each grid. 
   As shown in  FIG. 5 , on the lower plate part  2 , between the columns  3  at the four corners and the columns  4  at the midsections of the sides, a horizontal plate  21  is formed so that the insertion of the fork or the pallet truck is performed smoothly along the upper edge of the lower horizontal beam  72 . Moreover, on each side of the horizontal plate  21 , there is formed, a downward sloping inclined face  22 , and a downward sloping inclined edge  24 ′ formed with ribs  24  provided on a bottom plate  23 . By forming this kind of horizontal plate  21 , inclined face  22  and inclined edge  24 ′ on the lower plate part  2 , it is possible to improve the operability at the time of inserting the fork of a pallet truck, in and out of the fork insertion aperture  6 . 
   On the rear face of the horizontal plate  21 , as shown in  FIG. 5 , beams  25  are formed so as to extend parallel to the lower horizontal beams  72 , and beams  26  are formed so as to extend in the fork insertion direction, and the lower horizontal beam  72  and the beams  25  and  26  intersect as a grid. 
   Inside the columns  3  formed at the four corners, as shown in  FIG. 6 , beams  31  that connect to the upper horizontal beam  71  are formed on the upper plate part  1 , and on the lower plate part  2 , beams  32  that extend in the same direction as the beam  31  are formed so as not to vertically overlap with the beams  31 . 
   Inside the columns  4  at the side midsections, on the upper plate part  1  as shown in  FIG. 1 , beams  41  that connect to the upper horizontal beams  71  are formed. As shown in  FIG. 2 , on the lower plate part  2 , beams  42  are formed in the same direction as the beams  41 , so as not to vertically overlap with the beams  41 . 
   Inside the column  5  formed at the center of the pallet, beams  51   a  are formed between the column walls facing the upper plate part  1 , and on the lower plate part  2 , beams  52   a  are formed so as to extend in the same direction as the beams  51   a , so as not to vertically overlap with the beams  51   a . In addition, beams  51   b  are formed between the column walls and partition walls  53 , and also, on the lower plate part  2 , beams  52   b  are formed so as to extend in the same direction as the beams  51   b  so as not to vertically overlap with the beams  51   b.    
   Between the columns  4  formed at the side midsection and the column  5  formed at the center of the pallet, on the upper plate part  1 , beams  17  are formed for connecting the columns  4  formed at the side midsection and the column  5  formed at the center of the pallet. There are also beams  27  formed on the lower plate part  2 , for connecting the columns  4  formed at the side midsection and the column  5  formed at the center of the pallet, at a position so as not to vertically overlap with the beams  17 . 
   Moreover, ribs  28  are formed at the left and right edges of the beam  27  which connect the columns  4  at the side midsections and the column  5  at the center of the pallet, and by forming inclined sides  28 ′ by these ribs  28 , it is possible to improve operability at the time of inserting the forks of a hand-lifter, in and out of the fork insertion apertures  6 . 
   The beams  17  connecting the columns  4  at the side midsection and the column  5  at the center of the pallet on the upper plate part, are connected to the beams  51   a  or  51   b  that are formed inside the column  5  at the center of the pallet. In addition, the beams  27  that connect the columns  4  at the side midsection and the column  5  at the center of the pallet on the lower plate part  2 , are connected to the beams  52   a  or  52   b  that are formed in the inside of the column  5  at the center of the pallet. 
   In the section where the fork insertion apertures  6  intersect, on the upper plate part  1 , a rectangular peripheral portion  18  that surrounds the periphery of the intersecting section is formed and on the inner side of this rectangular peripheral portion  18 , a grid beam  19  comprising many intersecting beams is formed. On the lower plate part  2  at a position opposite to the grid beam  19 , rectangular openings of approximately the same dimensions as the grid beam  19  are respectively formed. 
   On the upper plate part  1  of the plastic pallet, a plastic non-slip tape  81  is attached to the top of each upper horizontal beam  71 . By providing this type of non-slip tape  81 , when articles are placed and transported on the plastic pallet, it becomes difficult for the articles to slip on the plastic pallet. Non-slip tape  81  may also be respectively provided on the upper horizontal beams  71 , the beams  31  and the beams  41 . 
     FIG. 8  shows one example of an attachment configuration for the non-slip tapes  81 . On the upper horizontal beam  71 , a shallow hollow  81   a  with a width slightly larger than the width of the non-slip tape  81  is formed, and the non-slip tape  81  is bonded to the middle of this hollow  81   a . For the method of bonding, a method where the bonding surface is heated and melted and the surfaces then clamped together is suitable. 
   The depth of the hollow  81   a  is made smaller (for example 1 mm) than the thickness of the non-slip tape  81  (for example 2 mm), so that an effect of slippage prevention by the non-slip tape  81  is obtained, while preventing displacement of the non-slip tape  81 . In addition, the non-slip tape  81  also achieves the effect of blocking the gas inlet  70   a  of the hollow portion  70 ′. There is the possibility that rain water may infiltrate into to the hollow portion  70 ′ if the gas inlet  70   a  is left open. 
   Non-slip tapes  82  are attached to the bottom surface of the lower horizontal beam  72  on the bottom surface of the lower plate part  2  of the plastic pallet, in the same way as for the non-slip tapes  81 . Accordingly, in the case where articles are loaded on the plastic pallet and the pallets stacked, the event of the upper plastic pallet placed on the articles on the lower pallet, slipping on the good on the lower pallet can be suppressed. 
   For the material of the non-slip tape  81  and  82 , for example ethylene-vinyl acetate copolymer, polyolefin type elastomer and the like can be given. 
   The plastic pallet of the present invention can be formed by plastics such as polyethylene, polypropylene and the like, and additives such as coloring and fillers may be appropriately added into the plastic. The reinforcing frame that is the feature of the present invention, is also constructed of the same material as that of the plastic pallet body. Of course, for the materials of the pallet, plastics of higher strength than polyethylene or polypropylene may be used. 
   The plastic pallet of the present invention can be formed by injection molding involving injecting a melted plastic into a metal mold. When injecting the melted plastic into the metal mold, by filling gas into the middle of the melted plastic that has been injected into the metal mold, it is possible to form the hollow portion  70 ′ in the interior of the upper and lower horizontal beams  71  and  72 , and the vertical beams  73  and  74 . Accordingly, while the thickness is large, distortion due to shrinkage is small, and the smooth orthogonal horizontal beams  71  and  72  and vertical beams  73  and  74  having smooth surfaces can be formed. 
   In order to form the hollow portion  70 ′ that extends around the entire periphery of the reinforcing frame  70 , a resin receiver  72   a  as shown in  FIG. 7 , is formed on the inner section of the injection molding mold on the opposite side to the gas inlet  70   a,  so that the resin may flow out into this resin receiver  72   a  as a waste tab  72   b.  According to this procedure, it is easy to form the hollow section  70 ′ along the entire periphery of the reinforcing frame  70 . 
   The plastic pallet of this embodiment, is a single side use type which can only carry articles on the upper plate part  1 , having large openings provided in the lower plate part  2  below the intersecting sections of the fork insertion apertures  6 , so that it can be transported by a hand-lifter. However, the present invention can also be applied to a both side use type plastic pallet where articles can be placed on either of the upper plate part  1  or the lower plate part  2 . In addition, in this embodiment the pallet is a four-way type where the forks can be inserted from any of the sides, however the present invention can also be applied to two-way types where the forks are inserted from two sides. 
   The reinforcing frame  70  may have a shape corresponding to the fork insertion apertures  6 , but is not particularly limited to this shape. However, it is desirable that the cross-section of the reinforcing frame  70  is rectangular from the point of ease of manufacture and the degree of reinforcing effect. 
   As described above, regarding the plastic pallet this embodiment, by forming a hollow reinforcing frame  70  surrounding the fork insertion apertures  6 , the weight of the plastic pallet is not increased, but it is possible to increase the strength near the fork insertion apertures  6 . 
   In addition, in the plastic pallet of the present embodiment, the non-slip tapes  81  and  82  are provided on the upper and lower horizontal beams  71  and  72  of the reinforcing frame  70  tit are not easily deformed under load and are very strong. Therefore, in the case where articles are placed on and transported on the plastic pallet, and articles and the plastic pallets are stacked in multistages, a superior non-slip effect is obtained. 
   Furthermore, in the plastic pallet of the present embodiment, by forming the hollow section  70 ′ in the interior of the upper and lower horizontal beams  71  and  72 , then regardless of the large width (thickness), it is possible to reduce distortion due to resin shrinkage, and make the surface of the upper and lower horizontal beams  71  and  72  smooth. Accordingly, the surface of the non-slip tapes  81  and  82  that are provided on the upper and lower horizontal beams  71  and  72 , becomes smooth, and the non-slip effect can be made even better. 
   Second Embodiment 
   Next,  FIG. 9  to  FIG. 12  show a second embodiment of the present invention. The same reference symbols are used for places corresponding to respective parts of the first embodiment, and description is omitted. 
   The main difference is that, in the first embodiment, as shown in  FIG. 6 , the hollow section  70 ′ is formed along the whole periphery of the reinforcing frame  70 , while in this embodiment, as shown in  FIG. 11 , the hollow section  70 ′ is formed only in one part of the reinforcing frame  70 . 
   The reinforcing frame  70  of the second embodiment, as shown in both  FIG. 10  and  FIG. 11 , has an upper horizontal beam  71  that surrounds the four sides of the fork insertion apertures  6 , vertical beams  73  and  74  and a lower horizontal beam  72   c . In this embodiment, only the opposite side portions of the lower horizontal beam  72   c  are made hollow, and are connected to each hollow part of the vertical beams  73  and  74 . Accordingly, the four corners of the reinforcing frame  70  are all manufactured hollow with a high degree of strength. 
   On the other hand, the section not including the opposite side portions of the lower horizontal beam  72   c  is of a solid construction. Moreover, in comparison to the upper horizontal beam  71  and the vertical beams  73  and  74 , as shown in  FIG. 10  and  FIG. 12 , the width in the fork insertion direction is made smaller. In this embodiment, as shown in  FIG. 12 , the lower horizontal beam  72   c  is formed along one side in the width direction of the reinforcing frame  70 . However, it is not limited to this construction, and as shown in  FIG. 13 , the lower horizontal beam  72   c  may be formed along the center line of the reinforcing frame  70 . 
   In addition, in the example shown in  FIG. 12  and  FIG. 13 , the opposite ends of the lower horizontal beam  72   c  that intersect with the vertical beams  73  and  74  are made thick. However, as shown in  FIG. 14 , the opposite ends of the lower horizontal beam  72   c  may be made thin. 
   Preferably a ratio W 2 /W 1  of a width W 2  of the middle section of the lower horizontal beam  72   c  to a width W 1  of the upper horizontal beam  71 , is from 0.3 to 0.8, and more preferably from 0.4 to 0.6. If within this range, the balance of strength and ease of molding is good 
   According to this second embodiment, because there no need to form a hollow section  70 ′ along the whole periphery of the reinforcing frame  70 , it is possible to injection mold the reinforcing frame  70  easily, without using high pressure gas, or a resin receiver as shown in  FIG. 7 . 
   In addition, because the four corners of the reinforcing frame are a hollow construction with high rigidity, even if the middle section of the lower horizontal beam  72   c  is a solid construction, it compares favorably for strength. 
   Moreover, because the wide upper horizontal beam  71  becomes a articles bearing surface, as well as being easy to attach the non-slip tape  81  and  82 , the strength of the hollow construction can be effectively used. 
   In the second embodiment, the upper horizontal beam  71  and the vertical beams  73  and  74  have hollow structures. Instead, the lower horizontal beam  72   c  and the vertical beams  73  and  74  may have hollow structures, and the width of the upper horizontal beam  71  may be made smaller. Moreover, by providing a gas inlet in either of the vertical beams  73  and  74 , then the upper horizontal beam  71 , either one of the vertical beams  73  and  74 , and the lower horizontal beam  72   c  may have hollow structures. 
   In addition, in the second embodiment, the hollow section  70 ′ reaches to both ends of the lower horizontal beam  72   c . However, it is acceptable if the hollow section  70 ′ reaches as far as the middle of the vertical beams  73  and  74 . In this case, preferably the solid structured section that is not formed hollow has a smaller width, as mentioned above. 
   Third Embodiment 
   Next,  FIG. 15  shows a third embodiment of the present invention. The same reference symbols are used for places corresponding to respective parts of the first embodiment, and description is omitted. 
   In the first embodiment and the second embodiment, the reinforcing frame  70  is formed only between the columns  3  at the four corners and the columns  4  at the side midsection. However, the third embodiment is characterized in that reinforcing frames  70  are also respectively formed between the columns  4  at the side midsection and the column  5  at the center of the pallet. The construction of these added reinforcing frames  70  has the same construction as either of the reinforcing frame  70  of the first embodiment or the reinforcing frame  70  of the second embodiment. Some of the reinforcing frames  70  may have the same construction as the first embodiment, and the other of the reinforcing frames  70  may have the same construction as the second embodiment. 
   In the first embodiment and the second embodiment, the reinforcing frames  70  are arranged as shown in  FIG. 16 . In contrast to this, in third embodiment, as shown in  FIG. 17  the number of reinforcing frames  70  is doubled. Therefore, it is possible to prevent deformation of the section surrounding the fork insertion apertures  6  and to increase the strength of the plastic pallet. 
     FIG. 16  and  FIG. 17  are four-was type pallets. In the case of a two-way type pallet, the reinforcing frames  70  may be arranged as shown in  FIG. 18  and  FIG. 19 . 
   Fourth Embodiment 
     FIG. 20  is a partially cut away perspective view showing the reinforcing frame  70  of the fourth embodiment. In the first embodiment and second embodiment, the reinforcing frame  70  comprises the upper horizontal beam  71 , the vertical beams  73  and  74 , and the lower horizontal beam  72  (or  72   c ) arranged on the same plane. However, this type of construction creates the necessity of using a slide core when injection molding. 
   Therefore, in this fourth embodiment, by arranging the upper horizontal beam  71  and the lower horizontal beam  72   c  alternately with each other when viewed in the vertical direction, it is possible to form the reinforcing frame  70  without using the slide core. As a result, molding efficiency can be further increased. 
   That is, the gas that is blown in from the center section of the upper horizontal beam  71 , passes though the vertical beams  73  and  74  that form one section of the vertical wall, and flows to the lower horizontal beam  72  (or  72   c ) arranged alternately with the upper horizontal beam  71  when viewed in the vertical direction, so that at least both end sections of the lower horizontal beam  72  (or  72   c ) are formed hollow. 
   In the embodiment of  FIG. 20 , the thickness of the vertical beams  73  and  74  is the same as the vertical wall. However, as shown in  FIG. 21  and  FIG. 22  (a perspective view on line C—C in  FIG. 21 ), it is possible to make these slightly thicker than the vertical wall. If the thickness is made greater in this way, the hollow section  70 ′ inside the reinforcing frame  70  is easier to form, and the strength of the vertical beams  73  and  74  is also increased. 
   The constructions such as in  FIG. 20  through  FIG. 22  are beneficial, particularly in the third embodiment, when forming the reinforcing frame  70  between the columns  4  at the side midsection and the column  5  at the center of the pallet. This is because it becomes difficult to use a slide core the farther the position from the opening end of the fork insertion aperture  6 . 
   EXPERIMENTAL EXAMPLES 
   Experiment 1 
   Beams (bars) having sections shown in  FIG. 26  through  FIG. 31  were formed from polypropylene, the geometrical moments of inertia were measured, and the bending strengths were compared. The results are shown in Table 1. 
   
     
       
         
             
             
             
             
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               TEST PIECE 
               FIG. 26 
               FIG. 27 
               FIG. 28 
               FIG. 29 
               FIG. 30 
               FIG. 31 
             
             
                 
             
           
          
             
               RELATIVE 
               105 
               100 
               40 
               62 
               40 
               93 
             
             
               STRENGTH 
             
             
                 
             
          
         
       
     
   
   From the results of Table 1, it can be seen that in the case of the hollow construction shown in  FIG. 27 , the strength is hardly decreased compared to the case of  FIG. 26  which is not hollow, while compared to the conventional constructions shown in  FIG. 28  and  FIG. 29 , the strength can be significantly improved. 
   Experiment 2 
   A plastic pallet having a reinforcing frame  70  was actually molded, and two test pieces  1  having a length of 200 mm were cut from the hollow upper horizontal beam  71 . 
   On the other hand, two U shaped beams were cut from a conventional plastic pallet having beams of U-shaped cross-section and having the same dimensions as the former plastic pallet, to obtain test pieces  2 . 
   The dimensions of the test piece  1  were, length 200 mm, transverse width 36.0 mm, vertical width 35.0 mm, and wall thickness 4.0 mm. The dimensions of test piece  2  were also: length 200 mm, transverse width 36.0 mm, vertical width 35.0 mm and wall thickness 4.0 mm. 
   Opposite ends of the two test pieces  1  and  2  were supported, and the amount of bending (mm) for the case with a 200 kgf applied to the middle, and the load (kgf) necessary to displace the middle section by 5 mm, were measured. The results are shown in Table 2. The upper number is the result for the first test piece, and the lower number is the result for the second test piece. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
                 
               Bending 
               Load to 
             
             
                 
                 
               amount 
               displace by 5 mm 
             
             
                 
                 
               (mm) 
               (kgf) 
             
             
                 
                 
             
           
          
             
                 
               Test piece 1 (from the 
               3.5 
               269 
             
             
                 
               present invention) 
               3.8 
               255 
             
             
                 
               Test piece 2 (from the 
               5.2 
               193 
             
             
                 
               conventional model) 
               5.7 
               180 
             
             
                 
                 
             
          
         
       
     
   
   As shown in Table 2, with test piece  1  the bending strength was improved by 35 to 40% compared to test piece  2 . Consequently, a significant improvement in strength for the whole pallet can be expected. 
   INDUSTRIAL APPLICABILITY 
   The present invention, in a plastic pallet used for the transportation and storage of articles, enables an increase in the rigidity of the surroundings of the fork insertion aperture in particular, by forming a hollow structure reinforcing frame in at least one part so as to surround the fork insertion aperture, and enables an increase in the strength of the whole pallet. Moreover, by making at least one part of the reinforcing frame a hollow structure, the weight of the pallet increases only slightly compared to the strength improvement, and production efficiency is also increased.