METHOD FOR PRODUCING THIN-WALLED TRAYS, APPLICABLE AS PACKING MATERIAL

Abstract: Method of producing thin-walled plates, applicable as packing material, from polystyrene whereby prepared po­ lystyrene granules are supplied to a mould in which the gra­ nules are heated to make them expand, the plate-form thus obtained being cooled and removed from the mould after stabilization.

Description: Method for producing thin-walled plates, applicable as packing material, mould for executing the method and device for de-stacking the plates. The invention relates to a method for producing thin-walled plates, applicable as packing material, from polystyrene. The invention further rela#tes to a mould for executing such a method, to the thin-walled plates produced according to such a method and to a device for de-stacking the plates. In the packaging industry many plates are used for pre-packing meat, poultry, vegetables, fruit and fish. For over 25 years butchers have been using plates for weighing sliced meat in them. At first cardboard plates were used for this purpose, but their disadvantage was that the moisture from the meat soon softened them and thus they lost their formstability, whilst the wet cardboard plates were a breeding ground for undesired growth of bacteria, fungi and the like. An improvement in respect of such packing materials was found in producing plates from polystyrene, which plates, as a result of the minimum absorption of moisture, retained their form better and besides polystyrene is not a good breeding ground for fungi and bacteria. These polystyrene plates were manufactured from extruded polystyrene film by using a moulding system under sub-atmDspheric pressure. Such plates arefr-mknowri ±##O# tre Dutch patent a?plication 6904977 and 7309281. These two patent applications are specifically aimed at the structure of the bottom in which elevations have been provided so that the moisture from the meat to be stored is no longer in direct contact with the meat during storage and the plates can be stacked better. The known plates are based on a smooth film, so that the moulding of the plates has its restrictions as regards the various thicknesses of the material with a certain product. Although these plates have been used frequently in replacement of cardboard plates, these polystyrene plates manufactured by extrusion of smooth film might be improved, because the known plates slightly deviate from each other with regard to their form and dimensions and the form-stability can be further improved by providing the plate e.g. with an edge thicker than the centre part of the plate. If, however, a thicker foil is used, so that the minimum required edge thickness is obtained, these plates become too expensive and too heavy, because the centre part also becomes proportionally thicker in that case. The deviations in the form of the known plates are especially a problem when the plates are handled automatically in automated packing equipment, e.g. for packing meat, poultry and fish, because de-stacking or de-nesting the plates is difficult when these plates are not stacked in a reproducable manner. With mechanical packing, moreover, higher demands are made on the form-stability of the plates. The known polystyrene plates have now been improved by using a method for producing such plates according to the invention and said method for producing thin-walled plates, applicable as packing material, from polystyrene, is characterised in that prepared polystyrene granules are supplied to a mould, in which the granules are heated to make them expand, the plate form thus arisen is cooled and removed from the mould after stabilization. Plate-shaped objects of the kind referred to above are usually stored in stacks and have to be taken off such a stack one by one before use. For the further displacement of the objects taken off the stack it is thereby usually advantageous if said taking off the stack always takes place on at least substantially the same level. within the scope of the invention the aim is now to obtain a device of a simple construction by means of which such taking plates off a stack one by one can take place quickly and efficiently. This can be achieved by means of the device according the invention which has been provided with at least two pairs of at least two arms pivotable about at least substantially horizontally extending pivot axes, said arms being provided with protruding fingers, whereby the fingers of the arms of the one pair are located higher than the fingers of the other pair, whilst drive means have been provided for pivoting the arms, in such a manner that alternatingly the arms of the one pair and the arms of the other pair are pivoted from the position in which the fingers of the relevant pair are located closest together to another position, whilst during the pivoting of the arms of the one pair the arms of the other pair are kept at least substantially in the position in which the fingers of this other pair are located closest together. By using this construction it is possible, when a stack of objects is supported by the lower fingers, to place the upper fingers under the edge of the lowermost object but one, after which the fingers located under the lowermost object can be swivelled back, so that the lowermost object can then fall downwards in order to be discharged. Then the lower fingers can be slewed towards each other again after which the upper fingers can be slewed from each other, so that the stack will descend on the lower fingers, after which the cycle described hereinabove can be repeated again. Thus the invention on the one hand relates to the production of such plates and on the other hand to the preparation of the basic material for the mould, as well as to the mould construction and to the plates thus produced. The invention will be further explained hereinafter with reference to the following description, whereby reference is made to the appended drawing, in which: Fig 1 is a process diagram for preparing the polystyrene granules and the mould processing. Fig. 2 is a section of a mould. Fig. 3 is a perspective view of a plate according to the invention, stacked, and Fig. 4 is a diagrammatic illustration of a device for de-stacking plates as illustrated in fig. 3. The production of objects from plastic material by using a mould to which the plastic granules are supplied, and whereby the plastic granules are heated in the mould so that the plastic material takes on the form of the mould, is known by itself. Thus a method for preparing an anti-static material and cups and holders made from said anti-static material are known from the European patent application 0.114.503. According to this method the thermoplastic material is also pre-expanded by preparing the thermo-plastic granules and then heating them to make them expand. A liquid anti-static agent is thereby provided on the thermoplastic granules, whereby the means-to be used, the objective and the objects produced are different, however, from the objectives and objects under consideration. Also from the American patent specification 4.302.549 a method is known for expanding polystyrene granules by heating said granules by means of steam. A lubricant is thereby used with the granular material, which lubricant can be decomposed under the influence of steam, so that freely flowable expanded polystyrene in granulated form is obtained from which certain objects to be used as packing material can be produced. The British patent specification 1.158.181 describes a device and method for producing thin-walled cups from such a material. For producing thinwalled plates such a method has always been too problematic, however, so that the forming of thin-walled plates from polystyrene by means of a mould has not been applied in practice. One of the problems that have resented themselves is that with a less accurate preparation of the basic material air inclusions arose in the mould, in which place no polystyrene couid be present, which resulted in an opening in the thin-walled plate thus produced, which is not acceptable with a large-scale production. Such faults were eliminated by preparing the polystyrene granules in-line w#th the production of the plates in the moulds. As a result of the in-line rearation no detectable differences occurred between the prepared polystyrene granules, so that equally divided material was present in the mould which material flowed evenly as a result of heating, so that no more air inclusions occurred. The preparation of the polystyrene granules takes place in the first part of the device according to fig. 1 and consists of the application of an upper layer, the enlargement of the diameter of the granule by pre-expansion and the sifting of the pre-expanded granules. These processes will be further explained hereinafter. Said prepared material is directly supplied to the moulds, so that the prepared material supplied is as homogeneous as possible. Part 1 of fig. 1 illustrates the preparative part of the inline preparation of polystyrene, from which in part 2 the plates are produced. Polystyrene granules, prepared by means of the polymerization of styrene, known by itself, are supplied to funnel 3. During polymerization pentane is added to said polymer granules, so that a quantity of about 5 per cent by weight of pentane is included in the granules, which serves as a driving or blowing agent, whereby, however, also other agents with a relatively low boiling point can Me applied. Said pentane containing polystyrene granules 40 are supplied to a weighing device 5 by means of a screw conveyor 4, so that a weighed quantity of pentane containing polystyrene granules 41 is supplied to container 6. By means of a pneumatically operated shut-off valve 7 the weighed quantity of polystyrene is supplied to mixer 8. By means of dosaging device 9 an agent forming an upper layer, such as a stearate, specifically zinc stearate, is supplied to mixer 8 via intake 42. The upper layer agent is supplied in a quantity of about 0.5 %, calculated on the polystyrene granules. As soon as an equal upper layer of the stearate round the polystyrene granules has been obtained, shut-off valve 10 is opened and the material 43 provided with an upper layer is supplied to supply container 11. Via pipe 12 the homogeneous polystyrene, provided with an upper layer, is sucked via suction device 13 into the weighing device 14, so that a certain weighed quantity of polystyrene provided with an upper layer is present there. By means of shut-off valve 15 said weighed quantity of material 44 is supplied to pre-expander 16, whereby steam 46 is supplied at the bottom side, so that a pre-expansion of the polystyrene granules 45 takes place. As a result of this the polystyrene granules are enlarged from 0.3-0.4 mm diameter to about 1 mm and said enlarged granules are supplied, by means of fan 17 via pipe 18, to silo 19, in which the pre-expanded polystyrene material 47, provided with an upper layer, is present. by means of shut-off valve 20 this material 47 is supplied to a vibrating sieve 21, so that a too fine-grained fraction 48 is discharged into container 22 and the too coarse-grained material is supplied to silo 19 by means of fan 23 via suction pipe 24. On passing the fan 23 and in suction pipe 24 the coarse-grained material can be made smaller sot that with a second circulation said coarse-grained material can be brought within the desired granule size and be supplied, together with the larger part of the granular material, via pipe 25, to collecting room 26 from where various plate-producing machines 29 can be supplied via pipe 27. In distribution element 28 the desired quantity of polystyrene is supplied to the separate moulds 30 in the plate-producing device 29, which moulds are illustrated in more detail in fig. 2. Steam and cooling Water are supplied to the Flate-produc;::S machine 29 via lines 49 and 50 resp. and from the moulds 30 the plates 31 are obtained. Said plates are more specifically illustrated in fig. 3. In fig. 2 mould 30, diagrammatically illustrated in fig. 1, part 2, is drawn in section, whereby it is apparent that the mould consists of two parts, viz. a fixed part 60 and a movable part 70. Both parts 60 and 70 consist of a casing 61 and 71 resp. ca#e ofal:lminium provided with an insulating covering 67,77, whilst the die mould 62, 72 respectively is made of a relatively hard material at the sides facing each other. Further both parts 60 and 70 have been provided with cooling water inlets 63 and 73 resp., with supply andexhatst pipes for steam indicated by 64 and 74 and with supply and exhaust pipes for steam, indicated by 65 and 75. The mould surfaces 62 and 72 are provided with very small perforations 66 and 76, equally divided across the mould surface, for letting through steam. Further illustrated in the fixed part 60 is a supply inlet 68 for supplying the granular polystyrene, provided with an upper layer, which is supplied to the mould via pipe 27 and is injected into the mould by way of supply inlet 68 via opening 69. The novabie part of the mould is thereby moved towards the fixed part, so hat only a thin slot remains between the two mould surfaces in which ehe granular polystyrene can be divided equally. Between the mould arts an air slot remains open for discharging the air taken along. After the mould has been filled with the polystyrene granules nder an excess pressure of 0.8-2.0 atmosphere, steam is supplied via steam inlet 75 which is exhausted via the perforations 76 and 66 by way af sheam exhaust 65. By leading through said steam for a few seconds, e.g. two seconds, the polystyrene granules are further expanded, so that they form a closed surface in the mould, whereby the form of the mould is exactly assumed. Because in the first instance steam is supplied via steam inlet 75 and exhausted via steam exhaust 65, in a second processing steam is supplied via pipe 65, said steam remaining only in part 60, however, so that the polystyrene present at the side of the fixed part is subjected to an identical temperature as the polystyrene at the side of the movable part 70. After a few seconds, e.g. 3 seconds, the steam is exhausted via pipe 65, whereby the plate in the mould has been shaped definitely, although it cannot yet be released because the temperature is about 100C C and the plate is still in a condition too soft for releasing. In order to cool down the still plastic plate a reduced pressure is created in the casings 61 and 71 via the pipes 64 and 74, whereby the steam pipes 65 and 75 are shut off. By applying a reduced pressure remaining water in the formed plate is evaporated and because of the heat of evaporatidn heat is drawn from the polystyrene, as a result of which the temperature is lowered. On the other hand cooling water is supplied via the pipes 63 and 73, which cooling water does not come into direct contact with the polystyrene, however, because the cooling water is sprayed against the mould surface. Within ten seconds the temperature has been lowered sufficiently, viz. to below 400 C, so that the mould can be opened by displacing the movable part 70. The plate is now form retaining and remains behind on the fixed part 60 and can be removed therefrom by means of suction sups. It is also possible to cool the moulded plate without supplying cooling water, viz. by only applying a reduced pressure, in which case the cooling only takes place under the influence of the heat of evaportation. In that case, however, the reduced pressure must be applied for 20-30 seconds instead of for 10 seconds as indicated in the first processing. In practice the combination of applying reduced pressure and atomizing cooling water is preferred. In order to ensure that the moulded plate remains behind on the fixed side when the mould is opened, air can be supplied to the movable part via pipe 74, so that the release of the plate relative to the movable part is stimulated. Fig. 3 illustrates a stack of 8 plates. These plates have an inner side 81, an upright edge 82 and a de-stacking slot 83. The upright edge 82 has been moulded thicker than the inner part 81 of the thin-walled plate 80, so that the thickened edge 82 provides the plate with a greater solidity. The de-stacking slot 83 makes it possible that with the mechanical packing of meat the packing machine can easily take a plate off a stack of several plates, which was problematic so far, because the plates obtained were not completely identical and not provided with such a de-stacking slot 83. The inner surface 81 of the plate is provided with upwardly protruding unevennesses, so that any moisture coming from the meat to be stored remains between the projections and does not come into direct contact with the meat or the product in the plate, provided the quantity of moisture separated is not too large. In the centre of the plate a bulge 84 is detectable, which has come about as a result of the air bump which is applied on the cooled down plate in order to remove it from the movable part 70 of the mould as has been discussed hereinabove. The plates can have various forms such as rectangular forms or circular forms or polygonal forms as illustrated in fig 3. With rect angularly shaped plates it is possible to provide the de-stacking slots along four sides, so that the plates can have two or four de-stacking slots. As it is undesirable that juices coming from the material to be packed such as meat or fish remain in the plate, it has become customary to provide the plate, before use, with a sheet of paper or a moisture absorbing film, as known from the Dutch patent application 6904977. Putting in this sheet of paper is labour intensive because when .he meat fish or fruit are packed a sheet of paper must be put in the platen at the packing machine, which sheet is also easily displaced during the packing process. Thereby it has been suggested with the improvement of these tes to provide such a sheet in the plate and fix it with glue which t an be used in the foodstuffs industry. Thus, as a final stage of processing, sheet of paper may be glued on plates which are used for tacking @@@@@ia@ that may separate moisture, said gluing being no longer Labour intensive because it can be done mechanically as a final operation more the stack of plates is packed and dispatched to the customer. Thus he Invention also relates to the plates thus formed provided with a xe-stacking slot and possibly provided with a sheet of paper applied by olueing. The device illustrated in fig 4 comprises a frame 101, in which a horizontal shaft 102 is accommodated rotatable. The shaft 102 can be driven by means of a chain or belt drive 103 from an electromotor 104. Cam discs 105 have been mounted on both ends of the shaft 102 protruding from the frame. On the facing side faces of the cam discs 105 cams 106 have been provided. By means of pivot axes 107 extending horizontally and perpendicularly to shaft 102 a pair of arms 108, pivotable around the iV'Dt axes 107, have been coupled with the frame near the cam discs 105. Fingers 109 have been mounted on the ends of the arms 108 located above the pivot axes 107 and extending towards each other from the arms 108 located oppostite each other. Rolls 110 have been provided to the ends of the arms 108 located under the pivot axes, said rolls being freely rotatable, relative to the arms 108, around axes 1 11 extendinn parallel to the axes 107. In a similar manner a second pair of arms 112 has been coupled pivotable with the frame. The arms 112 are thereby Divotable relative to the frame around pivot axes located in the extension of the pivot axes 107. On the upper ends of the arms 112 fingers 113 have been mounted, which also extend from the arms 112 into a direction directed towards each other. The construction is thereby such that the fingers 113 are located a little higher than the fingers 109. Rolls 114, corresponding with the rolls 110 have been provided at the bottom ends of the arms 112. In the centre of the shaft 102 an eccentric disc 115 has been mounted on the shaft 102. Bearing against said eccentric disc 115 is a roll 116 which is connected, freely rotatablearound an axis of rotation extending parallel to the shaft 102, with the bottom end of a driving rod 117 extending vertically upwards. Mounted on the upper end of the driving rod 117 is a funnel-shaped suction cup 118, widening upwards, which is open at its upper end and in which ansub-atmospheric pressure can be generated by means of means not further illustrated. In the position illustrated in fig 4 the suction cup 118 is located under a table 119, which is provided with a passage 120 through which the suction cup 118 can be moved upwards from the position illustrated in fig. 4. As is further indicated in fig 4 draw springs 121 have been provided between the arms 108 and 112 and the frame, which attempt to pivot the arms such that the rolls 110 and 114 remain in contact with the side faces of the cam discs 105 that face each other. In the illustrated position of the arms 108 the fingers 109 of said arms 108 are located as closely together as possible and a stack of plates or plate-shaped objects 122 is supported by said finger 109. Said objects are provided with protruding edges 123 and thereby formed such that when the objects are stacked into each other, as illustrated, there is a certain gap between the edges 123 of the objects 122 located above each other. The edge 123 of the lowermost surface rests on the fingers 109. In the position illustrated in fig 4 the rolls 114 , which are connected withthe arms 112 bear against the cams 106, so thatconsequently the fingers 113 are pivoted, from the position which they assume when the rolls 114 are located beside the cams 106 on the side faces of the discs 105 facing each other and in which the fingers 113 are located as closely together as possible, into a direction turned away from each other such that the free ends of the fingers are located beside the objects 122 stacked on each other. When the shaft 102 is now rotated the rolls 114 bearing against the arms 112 will be disengaged from the cams 106 so that the fingers 113 can move towards each other and will engage under the edge of the lowermost object 122 but one of the stack. Furthermore the suction cup 108 will be moved upwards. When the suction cup 108 has reached its highest position asub-atmospiSeric pressure can be generated in the suction cup by means of means not further illustrated, so that the lowermost object is sucked against the suction cup. At the same time the rolls 110 bearing against the arms 108 will strike the cams 106 mounted on the discs 105, so that the upper ends of the arms 108 and with them the fingers 109 are pivoted into a direction away from each other, so that the lowermost object 122 is released by the fingers 109. The suction cup 118 will thereby start moving downwards as well and take along the lowermost object and place it on the table 119. As soon as the object comes into contact with said table 119 the sub-atmospheric pressure in thesuctior cup Is released by means of means not further illustrated. The rolls of the lever 110 now become disengaged from the cams 106 so that the fingers 109 can again move towards each other until they have reached a position in which they are located under the protruding edges 123 of the objects. Once the fingers 109 have reached this position the rolls 114 will again strike the cams 106 mounted on the discs 105, so that the arms 112 are again pivoted towards the position illustrated in the figure, so that the stack of objects is released by the fingers 113 and can move downwards until the stack is again stopped by the fingers 109. Then the arrangement illustrated in the figure has been reached and the cycle described hereinabove will repeat itself again. In the meantime the object placed on the table 119 may have been removed by means of means not further illustrated or by hand. Possibly the table 119 can also take the form of a conveyor belt for discharging the objects.

Claims: CLAIMS 1. Method for producing thin-walled plates, applicable as packing material, from polvst@rene characterised in that prepared polystyrene granules are supplied to a mould, in which the granules are heated to make them expand, the plate-torm thus arisen is cooled and removed from the mould after stabilization. 2. Method according to claim 1, characterised in that the preparation of the polystyrene granules takes place in-line with the production of the plates in the moulds. 3. Method according to claim 2, whereby polystyrene granules containing a blowing agent, such as pentane, are supplied to preparatory part placed in-line, char acterised in that the granules, having a diameter of 0.4-0.6 mm are provided with an upper layer at an increased temperature, are to pre-framed of steam, fractioned by fil- trating after which @@@ @@@@@@@@ fractioned to size are supplied to the mould. 4. Method according to claim 1, characterised in that the prepared polystyrene granules are blown into the mould which is open via the inlet and an air clearance, the mould is shut-off by closing the inlet, the mould is cooled by cooling water and/or the application of sub-atmospneric pressure andthe former @@ate is removed from the mould. 5. Method according to claim 4, characterised in that the cooling of the mould is effected by applying a sb-atmospheric pressure and the spraying of @@@ling water into the mould chamber. 6. Mould for performing a method according to claim @, oharacterisedin that the mould is built up of two mould parts, the inner walls of which fit into each other whereby each mould Dart has been provided with a chamber for the supply and discharge of steam and cooling water one mould part has been provided with an intake and both inner walls are provided with perforations for letting through steam. 7. Mould according to claim 6, characterised in that recesses have been provided in the mould, so that the moulded plate is provided with a de-stacking slot. 8. Thin-walled plate for packing purposes, made from polystyrene, and using a method according to claims 1-5. 9. Plate according to claim 8, characterised in that the plate is provided with a de-stacking slot. 10. Plate according to claim 9, characterised in that a rectangular plate is provided with two or four de-stacking slots. 11. Plate according to claims 8-10, characterised in that a moisture absorbing paper sheet has been glued in the centre part of- the inner surface, covering 20-50 % of the surface. 12. Device for taking plate-shaped objects one by one off a stack of such objects, which are each provided with a protruding edge, whereby the objects are formed such that the protruding edges of objects located above each other are located at least locally at some distance of each other, characterised in that the device is provided with at least two arms pivotable about at least substantially horizontally extending pivot axes, said arms being provided with protruding fingers, whereby the fingers of the arms of the one pair are located higher than the fingers of the other pair, whilst drive means have been provided for pivoting the arms, in such a manner that alternatingly the arms of the one pair and the arms of the other pair are pivoted from the position in which the fingers of the relevant pair are located closest together to another position, whilst during the pivoting of the arms of the one pair the arms of the other pair are kepU at least substantially in the position in which the fingers of this other pair are located closest together. 13. Device according to claim 12, characterised in that the device is provided with a horizontally driven shaft on which discs have been mounted, the facing side faces of which are provided with cams, which co-operate with cam-followers mounted on the ends of the arms. 14. Device according to claim 13, characterised in that an eccentric disc has been mounted on the shaft, by means of which a suction cup is movable up-and-down, such that said suction cup is located of the level of the lowermost object at the moment that the fingers supporting said lowermost object are pulled back from under the object and can then move downwards from this position for delivering the object to discharge means.

Inventors: VAN OOSTERHOUT, A.
Applicants: INDUSTRIELE PRODUCTEN BRABANT B.V.
Application year: 1986
Main classification: B29C67
IPC codes: B29C44, B65D1