Abstract:
Method and installation for vacuum foaming refrigerator cabinets ( 11 ), freezers and the like; the installation comprises a rotary drum ( 21 ) supporting a first and at least a second foaming jig ( 35 ), selectively actuated to move each foaming jig ( 35 ) along a circular path from a bottom position towards an underlying table ( 26 ) supporting the cabinets ( 11 ) to be closed into foaming jigs ( 35 ). A vacuum foaming chamber ( 41 ) comprises a bell ( 42 ) secured to the rotary drum ( 21 ) in correspondence with each foaming jig ( 35 ), and a closure cap ( 43 ) on a vertically movable table ( 26 ) to perform in a vacuum foaming chambers ( 41 ), in the bottom position of a bells ( 42 ) and the foaming jigs ( 35 ).

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention refers to the foaming of refrigerators, freezers and the like, and in particular is directed to a method and an installation for foaming refrigerators, whereby it is possible to cyclically operate under vacuum conditions with two or more foaming jigs, and considerably accelerate the rising step of the foam, to ensure a complete filling or packing of hollow walls of refrigerator cabinets, in a comparatively short length of time.  
       STATE OF THE ART  
       [0002]     The foaming of refrigerator cabinets is normally carried out by enclosing the cabinet in a foaming jig capable to oppose the strong internal thrusts created by the expansion of the polyurethane foam in the hollow walls of the cabinet.  
         [0003]     Usually, a foaming jig comprises a bottom platen for supporting a refrigerator cabinet, four peripheral platens movably supported to be shifted close to and away from corresponding side walls of the cabinet, and a shaped plug to penetrate into a compartment or compartments of the cabinet, to oppose the internal thrusts of the polyurethane foam during expansion.  
         [0004]     The thrust plug is usually secured to a supporting structure, while the bottom and peripheral platens of the jig are supported by a movable table to be raised towards the overlying thrust plug.  
         [0005]     Conventional foaming jigs for foaming refrigerator cabinets are disclosed for example in IT-A-1 168 059, U.S. Pat. No. 4,370,759 and U.S. Pat. No. 4,411,413.  
         [0006]     The conventional foaming jig installations, in use for many years, have several limits and drawbacks, in particular:  
         [0007]     each foaming jig is substantially fixed and associated with a processing line which requires specific auxiliary equipments. In complex installations, having two or more processing lines, all this involves an enormous waste of space, as well as extremely high investment and management costs, in that each processing line must be served by respective equipments;  
         [0008]     since, after the foaming step, the refrigerator cabinet must remain closed in the jig for a relatively long enough period of time to allow the polymerisation and sufficient curing of the foam, the working cycle requires long times and the productivity of the installation is comparatively low;  
         [0009]     moreover, whenever it is necessary to change the model of cabinet to be foamed, the installation must be shut down for a prolonged period of time necessary for removing and replacing the previous thrust plug with a new one of a different type, and for carrying out the necessary adjustments of the entire foaming jig, with a consequent loss of productivity.  
         [0010]     In order to partially obviate these limits and drawbacks in installations which make use of conventional foaming jigs, U.S. Pat. No. 4,664,614 suggests the use of a foaming apparatus provided with two or more thrust plugs supported by a rotary drum, selectively positionable towards an underlying foaming jig.  
         [0011]     Even though an apparatus of this kind, due to its versatility of use, has permitted a certain improvement, in point of fact part of the problems typical of a conventional foaming installation have remained substantially unsolved.  
         [0012]     In fact, since the length of time that the cabinet remains in the foaming jig has remained substantially unchanged, in that it depends upon the reactivity of the chemical system of the polyurethane mixture, it has consequently not been possible to reduce the length of the entire working cycle.  
         [0013]     Moreover, a specific processing line must still be dedicated to each individual foaming apparatus, with consequent requirements in terms of space, auxiliary equipments, and higher investment and management costs.  
         [0014]     In all the cases, the foaming of refrigerator cabinets takes place at atmospheric pressure, maintaining the jig with the cabinet in a working area open towards the external environment.  
         [0015]     It is also known that the foaming of a polyurethane mixture is closely related to the reactivity of the chemical system; therefore the time required for rising of the foam and for filling the walls of a refrigerator cabinet is a conditioning factor in determining the length of the working cycle of a conventional foaming installation.  
         [0016]     In order to reduce the cycle times and improve the productivity of these installations, in recent years attempts have been made to find chemical solutions; in particular, new chemical systems for so-called “fast” polyurethane mixtures, having extremely short reaction times, have been developed and tested.  
         [0017]     Although the use of highly reactive or fast polyurethane mixtures makes it possible to reduce the cycle times in the foaming of refrigerator cabinets, and in general in the production of moulded articles, in practice this solution also has some drawbacks; in general, it is not very suitable for foaming refrigerator cabinets, or for foaming in moulds of large dimensions, or having a particularly complex design.  
         [0018]     In fact, due to the excessive reactivity and polymerising speed of the polyurethane mixture, the latter tends to rapidly increase its viscosity, and then to solidify before the foam has completely filled the walls of the cabinet or the cavity of the mould, resulting in the production of faulty cabinets or moulded articles to be discarded.  
         [0019]     For this reason, the chemical solution has proved to be not much suitable in the foaming of refrigerator cabinets, using conventional methods and equipment.  
         [0020]     The use of reduced pressure or suction has also been proposed in order to remove gases from a mould; the simple use of reduced pressure, in apparatuses for moulding plastic articles, is described for example in U.S. Pat. No. 3,970,732, DE-A-43 27 832 and EP-A-954 025.  
         [0021]     While on the one hand the simple reduction of atmospheric pressure, or the simple suction of the air, serves to remove the gases that develop inside a mould, also facilitating a certain distribution of the foam, when accomplished in this way it is not suitable for foaming refrigerator cabinets and freezers for domestic use having a complex design, by means of highly reactive polyurethane mixtures. In fact, creating a relatively reduced vacuum, between the two half shells of the side walls of a refrigerator cabinet, would tend to deform them, causing them to bent inwards; the use of spacers or thrust elements inside the walls of the cabinet, in addition to not completely solving the problem, would further complicate the manufacturing process, involving additional time and costs.  
         [0022]     There is consequently a need to reduce the cycle times and increase the productivity of installations for foaming refrigerator cabinets, and at the same time find new technical solutions which allow the use of polyurethane formulations characterised by a high reactivity.  
       OBJECTS OF THE INVENTION  
       [0023]     The main object of this invention is to provide a method for foaming refrigerator cabinets and the like, capable of improving the manufacturing process and of reducing the cycle times for the entire installation, allowing at the same time a satisfactory foaming and a complete packing of the cabinet walls, in a comparatively short length of time, while maintaining pressure balanced conditions during the foaming.  
         [0024]     A further object of the invention is to provide a method and an installation for foaming refrigerator cabinets, whereby use is made of a single foaming station equipped with several foaming jigs interlocked to a single processing line, thereby drastically reducing the space consumption, the number of auxiliary equipments, investment and management costs.  
         [0025]     A still further object of the invention is to provide a method for foaming refrigerator cabinets, which is particularly suitable for use with polyurethane mixtures having high reactivity characteristics.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0026]     The above can be achieved by a method for foaming refrigerator cabinets according to claim  1 , and by an installation according to claim  10 .  
         [0027]     According to the invention, the aforementioned scopes are achieved by supporting two or more foaming jigs by a rotary drum, and cyclically enclosing each foaming jig with the refrigerator cabinet in vacuum chamber, in which it is possible to generate a pre-established vacuum degree, to accelerate the distribution and rising of the polyurethane foam, until totally filling the cavities of the cabinet walls.  
         [0028]     In particular, according to the invention, a method has been provided for vacuum foaming of refrigerator and/or freezer cabinets having hollow walls, within a foaming jig, characterised by the steps of:  
         [0029]     providing first and at least second foaming jigs and respective jig-housing bells, on a rotary support drum, to sequentially move each jig and bell from a bottom position along a circular path;  
         [0030]     feeding a chemically reactive foaming mixture into hollow walls of a cabinet, in the bottom position of the foaming jig;  
         [0031]     closing the housing bell by a bottom cap to perform a vacuum chamber;  
         [0032]     generating vacuum conditions within the vacuum chamber and the foaming jig in the bottom position of the circular path; and  
         [0033]     sequentially positioning each foaming jig and housing bell in the bottom position to vacuum foam a new cabinet, while moving the foamed cabinets along said circular path.  
         [0034]     The best possible foaming conditions and vacuum degree must be found by means of suitable preliminary tests, in relation to the polyurethane formulation used and the type of refrigerator cabinet or article to be foamed; for example, it may be found convenient to work with a vacuum degree ranging from 600 to 900 millibars (6000+9000 Pa) and with formulations having a 20÷25% lower degree of polymerisation compared to formulations currently in use, under same conditions for the other process parameters, such as for example the final density of the foam, the thermal conductivity, type and quantity of foaming agent and/or of reaction water.  
         [0035]     For the purposes of present invention, the term highly reactive polyurethane mixture is understood to mean a mixture having a gel time equivalent to or less than 40-45 seconds, and a polymerisation time less than 300 seconds, for foam thicknesses equivalent to or more than 80 mm.  
         [0036]     According to a further feature of the invention, an installation has been provided for vacuum foaming of hollow walls of refrigerator and/or freezer cabinets into foaming jigs characterised by comprising:  
         [0037]     a rotatably supported jig-holder drum;  
         [0038]     first and least second foaming jigs angularly spaced apart on the rotary drum, to move along a circular path;  
         [0039]     jig-housing bells secured to the jig-holder drum to enclose respective foaming jigs;  
         [0040]     a removable closure cap, to provide a vacuum chamber in the bottom position of each jig-housing bell;  
         [0041]     a vertically movable support table beneath the rotary drum, for supporting a cabinet and the closure cap;  
         [0042]     first control means being provided for step rotation of the drum, to sequentially dispose each housing bell and a foaming jig with a downwardly facing disposition, above the support table and the closure cap;  
         [0043]     second control means to vertically move the support table and the closure cap between a lower and an upper position to open respectively to close the vacuum chamber enclosing a foaming jig; and  
         [0044]     control means to selectively connect the vacuum chamber to a vacuum source.  
         [0045]     The use of vacuum foaming and a rotary drum installation for supporting several foaming jigs, according to the invention, not only makes it possible to use highly reactive chemical mixtures, but also helps to considerably reduce the length of the work cycle, and to improve rising, distribution and packing of the foam. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0046]     These and further features of the method and the installation according to the invention, will be more clearly evident from the following description, with reference to the drawings, in which:  
         [0047]      FIG. 1  is a side view of a foaming installation according to the invention;  
         [0048]      FIG. 2  is an enlarged detail of a wall of a refrigerator cabinet;  
         [0049]      FIG. 3  is an enlarged cross-sectional view along the line  3 - 3  of  FIG. 1 , with the movable support table, in a completely lowered position, in an open condition of the vacuum chamber;  
         [0050]      FIG. 4  is a cross section similar to that of the preceding figure, with the movable table in an intermediate position;  
         [0051]      FIG. 5  is a cross section similar to that of the preceding figures, with the movable table in a completely raised position in which closes the vacuum chamber;  
         [0052]      FIG. 6  is a flow diagram of the method according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0053]      FIG. 1  shows an installation for foaming refrigerator cabinets, according to one possible embodiment of the invention.  
         [0054]     As shown, a foaming installation comprises a preheating station  10  for the refrigerator cabinets  11 , on a side of a foaming station  12 , and an unloading station  13 , on the opposite side from the foamed cabinets, can be moved towards a resting area for the final curing of the foam.  
         [0055]     The refrigerator cabinets  11  can be transferred from the preheating station  10  to the foaming station  12 , and from the latter to the unloading station  13  in any suitable way. For example, it is possible to use one or more motor-driven trolleys  14 A,  14 B provided with gripping means  15 , for example a plurality of suction caps for grasping the cabinet  13  on both sides. The trolleys  14 A and  14 B are appropriately controlled to run along guide rails which extend longitudinally between the work stations.  
         [0056]     The preheating stations  10  and unloading stations  13  can be of conventional type; for example, the preheating station  10  can be provided with roll tables  17  for supporting the individual cabinets  11  to be heated by one more heating units  18 .  
         [0057]     In turn, the unloading station  13  can comprise one or more roll tables  19  onto which the foamed cabinets from time to time may be transferred towards a resting area for the final curing of the foam, not shown.  
         [0058]     The foaming station  12  comprises a frame  20  for supporting a rotary drum  21  onto which are secured two or more angularly spaced apart foaming jigs  22 , two in the example shown.  
         [0059]     In the case of  FIG. 1 , the jig-holder drum  21  is supported by the structure  20  to rotate according to a horizontal axis  23  which extends in the longitudinal direction of the foaming station  12 ; however, other conformations and/or dispositions of the drum  21  and its rotational axis are not excluded.  
         [0060]     First control means comprising a ratiomotor  24  which controls the selective step rotation of the drum  21 , stopping it from time to time with one of the foaming jigs  22  facing downwards; a locking bolt operated by a hydraulic or pneumatic cylinder engages the drum  21  to lock it in place.  
         [0061]     In a position beneath the drum  21  is a movable table  26  upon which rests a support base element  27  designed to provide a thrust force against the rear wall of a refrigerator cabinet is shown.  
         [0062]     The support table  26  can be moved between a completely lowered position in an open condition of the foaming jig, shown in  FIGS. 1 and 3 , and a completely raised position in which it closes the foaming jig, shown in  FIG. 5 , passing through an intermediate position for transferring the refrigerator cabinets  11 , shown in  FIG. 4 , in which the trolleys  14 A,  14 B can lay down and respectively grip a refrigerator cabinet  11  from the base  27  on the support table  26 .  
         [0063]     The raising and lowering movements of the table  26  can be achieved in any way whatsoever, by means of suitable mechanical, electrical, hydraulic, pneumatic control means or their combination; for example, as shown, the table  26  can be moved vertically by means of a rack and pinion system  28  operated by an electric motor  29 ; lastly, reference  30  in the various figures has been used to indicate a mixing head for injecting a metered quantity of a reactive polyurethane mixture into the hollow walls of the refrigerator cabinets.  
         [0064]     In the example shown, the jig-holder drum  21  has two opposite planar side faces for supporting a corresponding number of foaming jigs  22 .  
         [0065]     Each foaming jig  22  comprises a shaped plug  31 A,  31 B, conformed to penetrate into a compartment or compartments of a refrigerator cabinet  11 , and to come into contact with the internal surfaces of the cabinet walls.  
         [0066]     The plug  31 A,  31 B is interchangeably secured to a plate  32  in turn fastened to a side face of the rotary drum  21 .  
         [0067]     Extending from the plate  32  are columns  33  which, by means of connecting links  34 , support peripheral platens  35  designed to come into contact, during the closure of the jig, with the external surfaces of the side walls of the refrigerator cabinet, to counteract the internal thrust of the foam undergoing expansion.  
         [0068]     The jig-holder drum  21  is provided with a plurality of foaming jigs  22 , two in the example under consideration; each jig  22  can be moved, by step rotation of the drum  21 , to several angular positions, starting from a pre-established angular position, also referred to as starting or foaming position, in which the foaming jig is facing downwards, above the table  26  for loading and removing the cabinets, towards one or more subsequent angular positions for the curing of the foam, in which the already foamed refrigerator cabinets remain in their respective foaming jigs, and are cyclically moved to return to the starting position, where each cabinet which has completed the curing of the foam is removed from the support table and replaced with another cabinet to be foamed, which in the meantime has been pre-heated in the heating station  10 .  
         [0069]     A refrigerator cabinet, as schematically shown in the detail of  FIG. 2 , is substantially composed of an external shell  36  and an internal shell  37  having folded edges  36 ′ e  37 ′ between which a hollow space  38  is formed into which a chemically reactive mixture is injected, for example a mixture based on a polyol and an isocyanate to form a polyurethane foam  39  capable of completely packing the hollow space  22  of the cabinet walls.  
         [0070]     A gap  40  is provided between the two shells  36  and  37  in correspondence with the opposite folded edges  20 ′ and  21 ′, to vent the air during the expansion of the polyurethane foam, which make it impossible to generate a vacuum directly in the walls of the cabinet, by conventional foaming methods and jigs.  
         [0071]     According to this invention, the vacuum foaming of a refrigerator cabinet can be carried out by cyclically enclosing each of the foaming jigs supported by the rotary drum, in a vacuum chamber  41  designed to be opened and tightly closed, in such a way as to allow the introduction and removal a refrigerator cabinet  13 , in a given angular position corresponding to said foaming position in which each jig is facing downwards.  
         [0072]     The vacuum chamber  41  can be made in any way whatsoever; in the form shown comprises an upper housing bell  42  for each foaming jig, and of a bottom closure cap  43 , common to all the housing bells; each bell  42  is secured to the rotary drum  21 , while the cap  43  is arranged on vertically movable table  26  to be is moved between a lowered opening position and a raised closing position defining the vacuum chamber, as explained further on.  
         [0073]     As previously mentioned, and as will be illustrated in greater detail with reference to the flow diagram of  FIG. 6 , the injection and foaming steps of the polyurethane mixture take place under vacuum, by generating and maintaining a high vacuum degree in the vacuum chamber and consequently in the hollow space  38  between the external shell  36  and the internal shell  37  of a refrigerator cabinet.  
         [0074]     In this connection, each foaming jig with the refrigerator cabinet  13  is enclosed in the vacuum chamber  41  formed by a housing bell  42  secured to the rotary drum  21 , and the cap  43  secured to the movable table  26 .  
         [0075]     The peripheral walls of the bell  42  and of the cap  43  have peripheral edges appropriately shaped and/or provided with gaskets, to ensure the necessary sealing of the vacuum chamber  41 ; lastly, reference  44  in the various figures has been used to indicate a pump or vacuum source, selectively connectable to each bell  42  by means of a control valve  45  and a rotary distributor  46 .  
         [0076]     As an alternative, the connection to the vacuum source may be made through the cap  43 .  
         [0077]     The method for vacuum foaming of refrigerator cabinets, and the working operation of the installation, will be illustrated in greater detail hereunder with reference to the preceding figures and the flow diagram of  FIG. 6 .  
         [0078]     The refrigerator cabinets  11  to be foamed are initial introduced into the preheating oven  10 , where they remain for a length of time necessary to bring them up to a given temperature (step S 1 ), necessary for the subsequent foaming of the polyurethane mixture.  
         [0079]     After a preheating step of the cabinet in the oven  10  (step S 1 ), the cabinet  11  is removed and then transferred to the foaming station  12  by means of the trolley  14 A (step S 2 ).  
         [0080]     The foaming jig  22  which at that time is downwardly oriented, is opened by disengaging the locking bolts which hold the base  27  secured to the peripheral walls  35  of the foaming jig (step S 3 ).  
         [0081]     In the meantime, the support table  26  has been raised to an intermediate position ( FIG. 4 ) to receive the base member  27  with a foamed cabinet which has first completed the foam hardening or running cycle, and which can therefore be transferred to the unloading station  13  by the trolley  14 B (step S 11 ), after the table  26  with the base member  27  has been completely lowered ( FIG. 3 ).  
         [0082]     After the removal of an already foamed cabinet, the trolley  14 A picking another preheated cabinet  11  from the roll table  17 , depositing it on the base member  27  supported by the table  26 , which in the meantime has been raised again to the intermediate cabinet unloading and receiving position of  FIG. 4  (step S 4 ).  
         [0083]     The jig  22  and the vacuum chamber  41  are then simultaneously closed (step S 5 ), by completely raising the table  26 ; in fact, after the trolley  14 A has moved backwards again into the oven  10 , an electronic control unit which governs the operation of the entire installation, actuates the raising of the table  36  with the base element  37  and the new cabinet  11 , to the upper position of  FIG. 5 .  
         [0084]     In these conditions, the plugs  31 A,  31 B are inside the cabinet  11 , while the four peripheral platens  35  of the foaming jig, in a per se known way, are thrust against the external surfaces of the side walls of the cabinet  11 .  
         [0085]     Simultaneously, the base element  27  of the foaming jig which at that time is downwardly directed, is locked to the peripheral platens  35 ; the vacuum chamber  41  is in turn hermetically sealed in that the peripheral edges of the cap  43  have come into contact with the peripheral edges of the overlying bell  42 .  
         [0086]     At this point the chamber  41  can be connected to the vacuum source  44  (step S 7 ′) before feeding the polyurethane mixture into the hollow space  38  of the refrigerator cabinet walls (step S 6 ); as an alternative, the vacuum in the chamber  41  can be generated simultaneously to the feeding of the polyurethane mixture, or immediately after (step S 7 ); the choice of the moment the vacuum is created will depend upon specific processing requirements and may be determined each time by means of appropriate tests.  
         [0087]     Upon completion of the feeding step of the polyurethane mixture, and after having reached the desired vacuum degree, the vacuum is maintained for a pre-established interval of time sufficient to allow the complete expansion of the foam and the correct filling or packing of the walls of the cabinet  11 .  
         [0088]     After the foaming of the new cabinet has been completed, and after the table  26  has been completely lowered (step S 8 ), to open the vacuum chamber (step S 8 ′) the drum  21  is made to rotate by one step (step S 9 ).  
         [0089]     The rotation by one step of the drum  21  causes all the foaming jigs  22  to move forward, bringing a new jig, which has completed the hardening cycle and/or partial curing of the foam (step S 10 ), into the foaming station  12  in which it is directed downwards, above the table  26  for supporting the bases  27  and the cabinets  11 .  
         [0090]     The cycle can then started again from step S 3 , as described previously.  
         [0091]     The proposed solution is consequently extremely advantageous in that, it makes possible to cyclically operate with a single processing line, under vacuum condition, with two or more foaming jigs of the same type, or of different types; in this way, not only is it possible to reduce the time length of the working cycle, but the curing step of the foam can be at least partially carried out in real time, inside the same foaming station, without having to stop the plant for the curing time of each cabinet on the rotating drum.  
         [0092]     The use of vacuum, according to the method previously described, also permits the use of more highly reactive polyurethane mixtures, which helps to further reduce the processing times, and to find an appropriate balance between foaming speed and satisfactory distribution of the foam, which proves to be difficult to achieve with the conventional technologies.  
         [0093]     Moreover, the productivity of the installation is considerably increased, maintaining investment and management costs comparatively lower compared to a conventional foaming installation.  
         [0094]     It is understood however that what has been described and shown with reference to the drawings, has been given purely by way of example in order to illustrate the general features of the invention, and one of its preferential embodiments.  
         [0095]     Therefore, other modifications or variations may be made to the entire installation, or part thereof, and to its operating mode, without thereby deviating from the scope of the accompanying claims.