NO960910L - plastic pallet - Google Patents

Description

TECHNICAL AREA

The invention relates to a method for producing a plastic pallet, more specifically a pallet with integrated plate-shaped support legs along the sides of the pallet.

PRIOR ART

Pallets are part of the goods to be transported and/or stored on them, and it is therefore important that the pallet is light, but at the same time strong and rigid. Plastic pallets have the advantage over wooden pallets that they are light, that they do not absorb moisture, and that they can be cleaned/disinfected, which is decisive in areas where hygiene is important, e.g. in the food industry. Such pallets must have a smooth surface without hard-to-reach parts where dirt can accumulate.

In EP patent document no. 0 400 640, a plastic pallet is described which is assembled from an upper deck and a lower deck with screed, or of a deck and screed. The parts are designed with legs that are inserted into each other when joined. To ensure that the parts fit together, mechanical securing can be used in the form of metal plates with punched notches, staples or the like. Both the upper deck and lower deck are constructed with detached, stiffening reinforcement ribs.

CH patent document no. 685549 A5 describes a plastic pallet which is injection-molded in two parts, namely an upper deck and a lower deck comprising blocks and a screed. The upper deck and lower deck have reinforcement ribs both in the transverse and longitudinal directions in a checkered pattern. The upper deck and lower deck are mirror welded together so that the side edges and ribs are welded against each other. Immediately inside all four side edges and in the middle in the longitudinal direction, there are semi-circular recesses in the ribs where steel pipes or flat bars can be clamped down for mechanical reinforcement of the pallet. The plastic pallet is preferably made of polyethylene, and it is intended for use in, among other things. a. food and pharmaceutical industry.

Known plastic pallets are produced by injection molding parts which are then assembled. The parts will typically have reinforcement ribs in desired locations to achieve a light and rigid construction. Injection molding of such large objects requires expensive injection molding tools and injection molding machines with great holding power. It is therefore desirable to be able to produce plastic pallets using a simpler and more economical method. A solution could be to make the pallet by hot forming an extruded plastic sheet, or from such a hose that is used when blowing large objects such as barrels, petrol tanks, etc. To get a light but at the same time rigid construction, a co-extruded, layered structure where one of the layers is at least partially foamed. US Patent No. 4,874,649 thus describes a method for blow molding a co-extruded hose consisting of at least two layers, one of which, possibly several, is at least partially foamed.

The applicant's concurrent Norwegian patent application, NO 960128, concerns a pallet produced by hot forming a blow molding hose as described in US 4,874,649. The hose is formed into a pallet deck and into cutting parts, and these are joined to form a finished pallet in a subsequent separate operation. This results in a light but at the same time rigid and strong construction. The pallet can optionally be reinforced with profiles laid in cavities formed between the pallet deck and the screed.

For many purposes, light pallets are desirable, preferably of a smaller size than standard pallets. It has been found that this can be achieved by producing a pallet of the same material as described in the aforementioned patent application belonging to the applicant, but by a simplified method and with a simpler design of the pallet legs.

SUMMARY OF THE INVENTION

The present invention thus provides a method for producing a plastic pallet comprising a deck and legs on the underside of the deck, where the pallet deck and integrated support legs along the longitudinal and/or transverse sides of the pallet deck are formed in one operation from an extruded blow molding hose in a hot state in a forming tool which one substantially flat piece, after which the shaped article is removed from the forming tool and the shaped support legs are bent perpendicular to the pallet deck and fixed in this position.

In one embodiment of the pallet produced by the method according to the invention, a support leg is formed along each of the pallet deck's longitudinal sides, alternatively transverse sides, and in the lower part of the pallet deck, mainly in the middle and parallel to the support legs, two or more load-bearing block-shaped pallet legs are additionally deep drawn.

In another embodiment of the pallet produced by the method according to the invention, instead of the block-shaped pallet legs, a continuous U-shaped profile is formed, and to the underside of this is attached a separately produced leg construction comprising an upper and lower cutting part.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a perspective sketch of a ready-made pallet with support legs and a central leg structure in the form of brick-shaped pallet legs. Fig. 2 shows the principle of providing the support legs. Fig. 3 and 4 show alternative designs of the plate-shaped support legs. Fig. 5 shows a section through a pallet with legs in the form of deep-drawn blocks, where the section is laid in the longitudinal direction of the pallet along the center line. Fig. 6 shows an alternative design of the brick-shaped pallet legs. Fig. 7 is a perspective sketch of the pallet deck and screed parts before joining. Fig. 8 shows a partial section through the pallet deck and screed parts with associated blocks for the embodiment with a pallet which has screed parts which are fixed under the middle of the pallet.

DETAILED DESCRIPTION OF THE INVENTION

The present method for producing a pallet is carried out in several steps. First, a blow molding tube is extruded, preferably as described in US 4,874,649 discussed above. The extrudate can be given varying wall thicknesses around the periphery by appropriate design of the extrusion tool, and different wall thicknesses can thereby be achieved for parts of the pallet. The extruded blow molding tubing is flattened and hot formed into a substantially planar piece in a forming tool. By using combinations of compressed air and vacuum, parts of the plate can also be given the desired shape, such as e.g. profiles, deep-drawn blocks, etc. When the hose is laid flat, the parts of the hose's inner walls that are pressed against each other will fuse together and thereby form a double plate. The principles of extrusion and forming are known to those skilled in the art.

In its simplest form, the blow molding hose consists of one homogeneous (solid) material. To make the extruded structure lighter, the hose can be co-extruded with several layers where at least one layer is at least partially foamed. Modern co-extrusion technology allows a number of alternative designs and constructions of the hose. Thus, in one embodiment of the invention, the extruded structure can consist of a foam plastic layer with a solid skin on only one side. The foam should preferably have a closed cell structure, and in practice it can have a density down to approx. 100 kg/m<3>. The skin will form the outside of the pallet so that a durable and easy-to-clean surface is achieved.

In another embodiment, the extruded structure consists of a foam plastic layer with solid skin on both sides, or possibly multiples of such a layered structure. As is well known, such a sandwich construction will have great torsional stiffness and provide a light construction. Typically, the foam layer makes up approx. 70% of the plate structure's thickness, and each skin layer approx. 15%, more preferably approx. 80% and approx. 10%. Since the foam has a closed cell structure, it can in some cases also make up the entire wall thickness. The foam can have a density in the range from the density of solid material downwards as far as it is practically possible to produce a stable polymer foam. To achieve an optimal, rigid structure, there must be very good adhesion between the foam and the skin layer. The foam and the skin must therefore be co-extruded in such a way that an integrated, layered structure is formed. Current materials for the production of the extruded hose with a layered structure include in principle all extrudable thermoplastics with sufficient stiffness. Particularly suitable thermoplastics are polyolefins, polyvinyl chloride (PVC), acrylonitrile-butadiene (ABS), styrene plastics, polyamides (PA) and polycarbonates (PC). Of these, the polyolefin materials are preferred. However, there is nothing to prevent the foam material and the skin layer, or layers, from being made of different materials. Thus, a suitable construction will comprise an inner and outer layer of high-density polyethylene with ultra-high molecular weight (HDPE-HMW) and an intermediate layer of foamed HDPE. Another construction has an inner layer of PP homopolymer, an outer layer of PP block copolymer, and an intermediate layer of foamed PP. Other constructions are of course also possible and fall within the scope of the invention.

The materials that make up the solid part of the extruded layered structure may optionally be reinforced with chopped glass fibers and/or long glass fibers in the direction of extrusion.

During the extrusion and during the forming of the hose, production waste will be formed which, for economic reasons, should be brought back into production. This can be done by incorporating the scrap material into one or more of the layers in the co-extruded layered structure. In the practical implementation of the invention, it will therefore be advantageous to use materials which are miscible with each other. In this respect, polyolefin materials are highly suitable, in particular certain types of polypropylene (PP) and high density polyethylene (HDPE). Smaller amounts of glass fibers in the skin layers will be able to be included in the skin layers, and even in the foam layer or layers, without any problems during grinding and new extrusion.

It is also possible to add thermoplastic cross-linking agents to cross-link the material in the manufactured pallets. The disadvantage is that such production waste cannot be recycled. Another disadvantage is that the pallets can usually only be joined mechanically, even though there are silane-based cross-linking agents that can be cross-linked after the pallet has been manufactured.

The plastic materials can of course be added with the additives and auxiliaries that are common for thermoplastics, such as antistatic agents, heat and UV stabilizers, dyes, etc. In the layered structure, different layers can have different colors as desired. It is of course also possible to have different colors on pallets designed for different purposes.

In the method according to the invention, the pallet deck and pallet legs along the side edges of the pallet are produced as one, mainly flat piece in one operation. This is achieved by the forming tool squeezing V-grooves in the plate at given distances from the plate edges along the entire length of the plate, in the direction of extrusion and/or in the transverse direction. The grooves are pressed into shape with an angle of approx. 90°, as can be seen from fig. 2, to the appropriate depth, but the plate must not break during the subsequent handling. After the plate has cooled and been shaped, the outermost parts of the plate are bent along the V-grooves until the side plates are perpendicular to the main plate, and they are then fixed in this position. The attached side plates will thereby form a set of integrated pallet legs, here called support legs, made in the same plate structure as the rest of the plate, i.e. the pallet deck.

It will be obvious that any combination of plate-shaped support legs in longitudinal and/or transverse direction is possible and lies within the scope of the invention. In order for the pallet to withstand a larger load, legs must be arranged under the center of the pallet which can take up the majority of the load. If the form is executed with support legs under both the longitudinal and transverse sides of the pallet, the areas of application will normally be such that additional legs under the middle of the pallet are not required. In the embodiment where the pallet has support legs only along two opposite sides of the pallet deck, it will normally be necessary to have load-bearing legs under the pallet midway between and mainly parallel to the support legs. Such legs can have different structural designs.

Preferred embodiments of the pallet will now be explained in detail

more with reference to the attached drawings.

Fig. 1 shows one embodiment of the finished pallet 1 with pallet deck 2, support legs 3 and a leg construction in the form of several brick-shaped pallet legs, of which only one pallet leg is seen in the figure. Fig. 2 shows the principle for shaping the support legs. During the hot forming of the blow molding hose, a profiled V-groove 5 is pressed into the plate along its entire length at a certain distance from the side edges of the plate. After the plate has been removed from the forming tool, the side plates 3 are bent until they are perpendicular to the main plate 2 and fixed in this position. This is indicated by the dashed lines in fig. 2. The main plate 2 will thus form the pallet deck and the side plate 3 support legs. The main plate 2 can preferably be made somewhat thicker than the side plate 3. This is achieved by regulating the thickness of the blow molding hose, as mentioned above, and then providing final dimensions when forming in the forming tool. Differences in thickness will help to provide a rounded transition between the pallet deck 2 and the support legs 3.

In order for the pallet to be easily lifted and moved, there must normally be openings in the support legs so that the forks of a truck can be easily passed through. Fig. 3 shows one embodiment of the support legs 3 with approximately rectangular openings with rounded corners, but the openings can of course have any suitable shape. Another embodiment of the support legs is shown in fig. 4 where the openings go all the way down so that three separate support legs are formed, or alternatively one partial support leg at each outer edge. Desired openings in the side plates are marked during the forming in the forming tool in the form of impressed profiled grooves, and after the pallet has been shaped, the desired plate pieces are pressed out.

In the embodiment shown in fig. 1, it is in the lower part of the pallet deck in the middle between the support legs and parallel to these deep-drawn blocks which then form a set of legs. In fig. 1 only shows one block at the end of the pallet. Normally, three blocks will be drawn evenly spaced along the center line of the pallet, with one block 4 at each end and one in the middle. This is evident from fig. 5 which shows a section through the pallet taken in position A-A in fig. 1. The blocks 4 can be deep-drawn to full depth and thereby form a set of block-shaped pallet legs. The disadvantage of drawing so deeply is that the wall thickness can be small in the corners at the bottom of the bricks. The properties of the material from which the pallet is made are decisive for deep drawing conditions, wall angles, design of corners, etc. The problems associated with deep drawing of thermoplastics are well known by experts in the field.

An alternative embodiment of the brick-shaped pallet legs is shown in fig. 6. Here, the blocks 4 are not deep drawn all the way down, but for example to half the size of the final pallet legs. In a separate operation, separately produced blocks 6 are attached to the underside of these half-high block-shaped pallet legs 4. These can in principle have any design and be made of any material, provided that they can be permanently attached. The blocks 6 must be of such a height that together with the block-shaped pallet legs 4 they form finished pallet legs with the correct height. The advantage of this embodiment is that the pallet legs become stronger and with a smaller deep drawing ratio for the brick-shaped pallet legs.

An alternative embodiment to that shown in fig. 6, is to attach a separately produced lower cutting part 9, shown in fig. 7, to the blocks 4. The cutting part is designed as a lap bank (10) with two or more upwardly extending blocks (11), and these are attached with the upper side to the lower side of the block-shaped pallet legs (4) deep drawn in the tire (2).

In fig. 7 shows an embodiment of the pallet where, on the underside of the pallet cover, a continuous U-profile 7 is hot-formed along the entire length of the cover and midway between and parallel to the support legs 3. Two cutting parts 8, 9 are attached to this U-shaped profile. Each cutting part is produced separately in a separate operation. This is done by shaping a blow molding hose into a lap bank 10 with blocks 11. The upper cutting part is attached with its lap bank 10 to the U-shaped profile 7. The lower cutting part 9 is attached with the upper side of the blocks 11 to the underside of the matching blocks 11 on the upper cutting part 8.

In fig. 8 shows a section through a finished pallet 1 in position B-B indicated in fig. 7. The section is taken through the pallet deck 2 with the U-shaped profile 7, and through blocks 11 on the upper cutting part 8 and lower cutting part 9. The U-profile 7 is designed with a channel 12 and two closed cavities 14, 14. The upper cutting part 8 is designed so that a channel with the same width as the channel 12 on the underside of the U-profile is formed on the upper side of the lower bank in the longitudinal direction. By joining the pallet deck and upper cutting part, a closed channel 12 is thus formed in the longitudinal direction of the pallet, the length, width and depth of which can be adjusted as required. The U-profile 7 and the upper cutting part 8 are usually designed so that the channel 12 is sealed at each end, and a closed cavity is thereby formed. Alternatively, the design can be such that the channel is open at both ends. The lower cutting part 9 is in principle designed in the same way as the upper cutting part 8, but so that the lower bank 10 forms the contact surface against the floor. Each block on the upper and lower cutting parts 8, 9 is designed as a symmetrical profile so that two equally shaped cavities 15 and a recess 13 are formed which, when the blocks are joined, will also form a closed cavity. The constructional design of the pallet blocks can deviate from what is outlined in Figure 8 by varying the dimensions of the channel 12 and the cavities 13 and 15. However, the supporting walls inside the blocks must be designed so that a sufficiently rigid construction is achieved.

In the simplest version of the pallet, the leg structure under the middle is omitted and instead the pallet is equipped with plate-shaped support legs on both the short and long sides. The edges in the corners where the plate-shaped support legs on the long sides and short sides meet are joined to achieve a rigid construction.

In order to stabilize the support legs, it may be appropriate for certain applications to attach screeds under them in the cross direction of the pallet. In particular, this can be appropriate when the pallet's support legs consist of side plates with openings all the way down, i.e. for example three separate support legs on each side. A corresponding number of screeds can then be fixed across under the pallet, one screed under each pair of opposite support legs. Any leg structure below the middle must then be adapted in terms of height to such an embodiment. Each of these coulters can have a design like coulter part 9 in fig. 7. In a preferred embodiment, half-height block-shaped pallet legs 4 are deep-drawn under the center of the pallet in the lower part of the cover, as shown in fig. 6. The coulters transversely under the pallet are then designed with a block in the middle that corresponds to the block-shaped pallet leg 4, while the blocks on the ends of the labank are made low and so that they fit under the support legs 3. The underside of the cobbler's labank, 10, will then form contact surface against the floor.

The individual parts of the pallet can most appropriately be joined by welding, preferably mirror welding, but other joining methods, such as, for example, extruder welding, gluing and gluing, mechanical joining etc., can also be used.

In order to ensure that the pallet will not sag in the middle under great stress, the channel 12 in fig. 8, a suitable bracing profile is inserted. To keep the pallet's own weight down, a profile of light metal, plywood or other suitable plastic materials is preferably used.

Since objects made of plastic materials have a smooth surface, and especially polyolefins such as HDPE and PP, it will often be desirable to apply a friction-increasing material to the pallet deck. This can be done by co-extruding an extra layer of friction material on the outside of the blow molding hose, in a width from narrow strips to a continuous coating around all or parts of the hose's periphery. Friction coating on the upper side of the pallet deck will prevent goods from sliding off the pallet, while on the underside it will prevent the pallet from sliding off the truck forks so easily when handling the pallet. An alternative is to apply a friction coating to the pallet deck on ready-made pallets. This is easily done by extrusion application, for example by passing the pallets under a stationary extruder in an application line.

Preferably, the friction material is applied to the pallet in the form of parallel, raised, longitudinal profiles with a height of approx. 1 mm and width from narrow strips to a continuous coating. When the pallet is manufactured in a polyolefin material, a suitable friction material is selected from among polyolefin-containing materials such as EVA, EBA and similar.

The smooth and even surface of the pallet means that it is very easy and simple to clean and disinfect, it is therefore particularly well suited for use in the food industry and related businesses. The pallet's construction and design provide great strength in relation to its weight. It will therefore be very well suited where low weight is required, e.g. in the case of air freight, and where there is a lot of manual handling.

In the foregoing, preferred embodiments of the pallet are explained with references to pallets of rectangular shape. The pallet can of course also be square or have another suitable geometric shape. Similarly, references to the longitudinal and transverse sides of the pallet can be changed without deviating from the idea behind the invention. After the pallet has reached its end of life, it can easily be recycled in line with other thermoplastic items.

Claims (17)

1. Method for manufacturing a plastic pallet comprising a deck (2) and legs (3) on the underside of the deck, characterized in that the pallet deck (2) and integrated support legs (3) along the longitudinal and/or transverse sides of the pallet deck are formed in an operation from an extruded blow molding hose in a hot state in a molding tool as one substantially flat piece, after which the molded article is removed from the molding tool and the molded support legs (3) are bent perpendicular to the pallet deck (2) and fixed in this position. 2. The method according to claim 1, characterized in that a support leg (3) is formed along each of the pallet deck's longitudinal sides, alternatively transverse sides, and that in the lower part of the pallet deck, mainly in the middle between and parallel to the support legs (3), in In addition, two or more load-bearing brick-shaped pallet legs (4) are drawn deep. 3. Method according to claim 2, characterized in that the brick-shaped pallet legs (4) are deep-drawn which are lower than the height of the finished pallet, and that a separately produced leg construction is attached to the underside of these with such a height that this together with the brick-shaped pallet legs ( 4) form final pallet legs with the correct height. 4. Method according to claim 3, characterized in that appropriately designed, separately manufactured blocks (6) are used as separately produced leg construction, which are attached under the half-high block-shaped pallet legs (4). 5. Method according to claim 3, characterized in that as a separately manufactured leg construction a separately manufactured cutting part (9) designed as a lap bank (10) with two or more upwardly extending blocks (11) is used, and these blocks are attached with the upper side to the lower side of the block-shaped pallet legs (4). 6. Method according to claim 1, characterized in that a pair of support legs (3) is formed and that in addition a continuous U-shaped profile (7) is formed in the lower part of the pallet deck, mainly in the middle between and parallel to the support legs, and that a separately produced leg structure is attached to the underside of this U-shaped profile (7) comprising an upper and a lower cutting part, where the upper cutting part (8) is designed as a lap bank (10) with downward-extending blocks (11) and the lower cutting part (9) is designed as a flat bank (10) with upwardly extending blocks (11), and the upper cutting part is fixed with the flat bank to the U-shaped profile (7), and the lower cutting part is fixed with the upper sides of the blocks to the underside of the corresponding blocks (11) on the upper cutting part. 7. Method according to claim 6, characterized in that the upper side of the upper cutting part (8) and/or the U-shaped profile (7) against which the cutting part rests is designed with a longitudinal recess so that a straight line is formed between the cutting part and the tire , open channel (12) where a stiffening profile can possibly be inserted. 8. Method according to claims 1-7, characterized in that the support legs (3) are provided with sufficiently large openings so that the forks of a truck can be inserted through them. 9. Method according to claim 8, characterized in that the openings go all the way down through the bottom edge of the support legs so that these take the form of at least two separate plate-shaped support legs (3). 10. Method according to claim 9, characterized in that under each opposite pair of support legs (3) on opposite sides of the pallet, a screed is attached in the form of separately manufactured screed parts (9) with blocks (11) of varying height adapted to the support legs (3) and a possible leg construction under the center of the pallet. 11. Method according to claims 1 - 10, characterized in that the blow molding hose is extruded from a thermoplastic selected from among polyolefins, PVC, ABS, styrene-plaster, PA and PC. 12. Method according to claim 11, characterized in that the blow molding hose is co-extruded as an integrated layered structure consisting of foam with a solid skin on both sides, or multiples of such a structure. 13. Method according to claim 11 or 12, characterized in that one or more polyolefin materials are used as plastic materials. 14. Method according to claims 11-13, characterized in that plastic materials are used which are reinforced with chopped and/or long glass fibres. 15. Method according to claims 1 - 14, characterized in that strips of a polyolefin-based material with a high coefficient of friction are co-extruded on at least part of the hose's circumference. 16. Method according to claims 1-14, characterized in that friction strips are applied to the pallet cover of ready-made pallets by extrusion application. 17. Method according to claim 15 or 16, characterized in that the friction material is applied in the form of parallel, raised, longitudinal profiles with a height of approx. 1 mm and width from narrow strips to a continuous coating.