Abstract:
A thermal insulation panel exhibits little or no warping due to temperature differences between different portions of the panel. The panel comprises insulation material and an envelope for encapsulating the insulation material. The envelope comprises a base member and a cover member. The cover member is supported by the base member in a floating and sealing engagement with the base member. The cover member can slide relative to the base member to compensate for differential thermal expansion effects on the respective base member and cover member. In an alternative embodiment the cover member is adapted to be exposed to a temperature significantly below the temperature to which the base member is exposed. The cover member is formed of a metal having a low coefficient of thermal expansion to compensate for differential thermal expansion effects in the respective base member and cover member. The cover member and the base member are joined to provide the envelope with a hermetic seal.

Description:
FIELD OF THE INVENTION 
     The present invention relates to insulation panel systems. More particularly, it relates to an insulated panel design having improved resistance to warpage due to the thermal strain imparted by exposure of the panel to large temperature differences. 
     PRIOR ART 
     Insulated panel systems for use in freezing or for containing materials at very low temperatures have been practiced in many forms in the art. Such systems can be easily fitted to odd shapes, can be made lightweight, provide easy maintenance, and can be mass produced at low cost. When subjected to large temperature differences, however, thermal strains are developed in the panel envelope and in the internal insulation. These can result in undesired warpage and/or breakage of the panel due to the temperature difference between the top and bottom surfaces thereof. 
     The problem of insulation panel warpage is significant due to its potential for causing leaks from a freezer or container, damaging or deforming the insulation panels, and damaging or deforming components in the freezer or container that connect to the insulation panels. A good seal of the insulation panel when cold is critical to the proper operation of a freezer. Leaks can allow infiltration of air into the freezer which can affect its efficiency and lead to icing problems. The panels can be permanently deformed or cracked if not designed properly. Typically in a liquid nitrogen food freezer, for example, the insulation panel should have a fully welded construction to prevent oxygen from the air from condensing and concentrating inside the panel and posing a safety problem. Also, damage and cracks on the outside of a panel can allow moisture to enter the panel and cause degradation of the thermal insulation over time. The following patents illustrate various prior approaches to solving such problems. 
     In U.S. Pat. Nos. 4,116,150 and 4,170,952 to McCown there are disclosed cryogenic insulation systems for the storage of cryogenic liquefied gases which employ a metal membrane formed of a high nickel steel such as Invar. 
     In U.S. Pat. No. 4,155,482 to Swaney, there is disclosed a modular, bolt on, insulation system for use in the inner cargo hold of liquid natural gas (LNG) tanker ships. The panels are stacked in multiple sealed layers and are made of composites. There is a complicated system of joints and seams to completely cover the inner cargo hold. The modularity allows for replacement of damaged panels and inner vessel maintenance. The panels are not sealing surfaces but are simply applied to the inner vessel to provide thermal insulation between the cold LNG and the ambient environment. 
     In U.S. Pat. No. 4,527,844 to Klee, there is disclosed a thermally insulated chamber with an insulated door. The design of the chamber walls and door use flexible expansion joints, flexible support members, and limit the inner liner linear dimensions to control buckling and warping. 
     In U.S. Pat. No. 5,032,439 to Glicksman et. al. there are disclosed thermal insulation panels using evacuated powders enclosed in a ceramic glass envelope which is evacuated to a low pressure. This is a complicated design that was generated to form a panel without using CFC producing closed cell foams. 
     In U.S. Pat. No. 5,445,857 to Nowobilski, (assigned to the assignee of the present invention) there are described vacuum insulation panels that are provided with textured surfaces that localize thermal expansion. The large thermal strains that can produce cracking and warping are thereby avoided. 
     In U.S. Pat. No. 5,502,982 to Venetucci, there is disclosed the use of a tie pin to limit the deformation of the panel due to ice build up in the inner metal surfaces. The tie pins hold a plywood inner support next to the inner metal skin and are composed of a low heat leak composite material. Several of the pins are required for each panel. They are said to prevent the deformation of the inner surface of the insulation panel and thereby alleviate problems in a tunnel freezer operation caused by the deformation. 
     The above patents are specifically incorporated by reference herein. 
     The designs presented in accordance with this invention are significantly easier and cheaper to fabricate and construct than the prior designs and provide an essentially equivalent degree of thermal insulation. The prior art has tended to solve the problems of buckling and warpage by complicated mechanical means (tie pins or flexible expansion joints). The designs of this invention solve the problems of thermally generated warpage without significant mechanical complexity. The embodiments of this invention which employ Invar, preferably use it only on the portion of the panel which will be exposed to significant temperature variations from ambient temperature, in order to minimize cost. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention new thermal insulation panel designs are provided having very low warpage when the inside surface of the panel is exposed to temperatures as low as −320° F. while the outside of the panel is at ambient temperature. The insulation panels of this invention have a simple modular design that is easy and inexpensive to fabricate. 
     The thermal insulation panel in accordance with a preferred embodiment of the invention exhibits little or no warping due to temperature differences between different portions of the panel. The panel comprises insulation material and an envelope encapsulating the insulation material. The envelope comprises a base member and a cover member which cooperate to envelope the insulation material. The cover member is supported by the base member in a floating and sealing engagement with the base member. The cover member can slide relative to the base member to compensate for differential thermal expansion effects when the respective base member and cover member are exposed to different temperatures. 
     Preferably in accordance with one embodiment the base member includes a peripheral slot directed inwardly of the panel and the cover member is captured within the slot and is free to slide in the slot within a given range of motion. In accordance with another embodiment the cover member includes a peripheral slot directed inwardly of the panel and the base member is captured within the slot so that the cover member is free to slide relative to the base member within a given range of motion. 
     A first sealing member is arranged about a peripheral edge of the cover member or the base member to provide a seal between the respective cover member or base member and the slot. Preferably the cover member and the base member are formed of metal and the first sealing member comprises a polymer. 
     In accordance with further preferred embodiments of this invention there is provided a thermal insulation panel which exhibits little or no warping due to temperature differences between different portions of the panel. The panel comprises insulation material and an envelope for encapsulating the insulation material. The envelope comprises a base member and a cover member. The cover member is adapted to be exposed to a temperature significantly below the temperature to which the base member is exposed. The cover member is formed of a metal having a low coefficient of thermal expansion to compensate for differential thermal expansion effects in the respective base member and cover member. The cover member and the base member are joined in a manner which provides the envelope with a hermetic seal. 
     Preferably the cover member and the base member are welded together along a peripheral seam of the panel. Preferably the seam is arranged away from the low temperature exposed portion of the panel. 
     It is therefore an object of the present invention to provide an improved insulation panel having resistance to warping due to differential thermal expansion effects (i.e. due to the effect upon different portions of the panel being exposed to different temperatures). 
     It is another object of the present invention to provide an insulation panel as above which is easy to fabricate. 
     It is another object of one alternative of the present invention to provide an insulation panel as above which includes a sliding seal to inhibit moisture infiltration into the panel. 
     It is another object of another alternative of the present invention to provide an insulation panel as above which includes a hermetically sealed envelope to prevent moisture infiltration into the panel. 
     The above and further objects and advantages of this invention will become apparent upon consideration of the following description thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of an insulation panel in accordance with a preferred embodiment of this invention. 
     FIG. 2 is a side view of the insulation panel of FIG.  1 . 
     FIG. 3 is a partial cross-sectional view along the line A—A in FIG. 1 when a desired face of the panel is exposed to a very cold temperature. 
     FIG. 4 is a partial cross-sectional view along the line A—A in FIG. 1 when a desired face of the panel is exposed to ambient temperature. 
     FIG. 5 is a partial cross-sectional view showing the mounting of a panel in accordance with this invention to a frame of a cryogenic freezer or container. 
     FIG. 6 is a top view of an insulation panel in accordance with an alternative preferred embodiment of this invention. 
     FIG. 7 is a side view of the insulation panel of FIG.  6 . 
     FIG. 8 is a partial cross-sectional view along the line B—B in FIG. 6 when a desired face of the panel is exposed to a very cold temperature. 
     FIG. 9 is a partial cross-sectional view along the line B—B in FIG. 6 when a desired face of the panel is exposed to ambient temperature 
     FIG. 10 is a top view of an insulation panel in accordance with a further alternative preferred embodiment of this invention. 
     FIG. 11 is a side view of the insulation panel of FIG.  10 . 
     FIG. 12 is a partial cross-sectional view along the line C—C in FIG.  10 . 
     FIG. 13 is a top view of an insulation panel in accordance with a further alternative preferred embodiment of this invention. 
     FIG. 14 is a side view of the insulation panel of FIG.  13 . 
     FIG. 15 is a partial cross-sectional view along the line D—D in FIG.  13 . 
     FIG. 16 is a top view of the box portion of a panel before folding and welding. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although the present invention will be described with reference to the embodiments shown in the drawings it should be understood that the present invention may be embodied in many forms of alternative embodiments. In addition, any suitable size, shape or type of materials or elements could be used. 
     The design of one preferred embodiment of the insulation panel  10  of this invention will be described by reference to FIGS. 1 through 5. The panel  10 , which is shown by way of example, comprises a rectangular box or pan portion  12 , which is preferably filled with polyurethane foam. 
     The bottom portion  12  of the box, as best shown in FIG. 16, is made by bending a flat sheet  14  of (e.g.  18  gauge) stainless steel. The bottom  16  of the box portion  12  has a rectangular shape in this exemplary embodiment. Four side wall flaps  18  extend out from the bottom  16  and are adapted to be bent upwardly from the bottom  16  about the bend lines  22  to form the box portion  12 . The free edges  20  of the flaps  18 , which are parallel to the bend lines  22 , are bent to form an edge joint  24 . The edge joint  24  of the panel  10  is a formed lip or flange which protrudes outwardly from the side wall  18 , preferably in a direction parallel to the bottom  16  of the panel  10  and provides a flange like surface for clamping the panel  10  to a frame  26  of the freezer or container as best shown in FIG.  5 . 
     Referring now specifically to FIGS. 1,  3  and  4  the edge joint  24  which is formed from the free edges  20  of the side wall flaps  18  is shown in greater detail. The free edges  20  are bent first outwardly from the side walls in a direction parallel to the bottom  16  and then bent back again in the opposite parallel direction toward the side walls  18 , so as to form a “U” shaped slot  30  which is open at the portion  32  of the slot  30  immediately adjacent to the walls  18 . When the box  12  is formed from the sheet  14  and the edge joints  24  are formed in the free edge portions of the side walls  18 , the corner portions  34  of the edge joint  24  are missing. Therefore it is preferable to secure corner edge portions  34  of the edge joints  24  to the edge joints of the side walls  18  along the joint lines  36  or  37 . These corner edge portions  34  of the edge joint  24  are preferably welded along the joint lines  36  or  37  to the abutting edges of the flange  24  in the side walls  18 ; however, any desired conventional joining technique could be used. This provides a continuous flange  24  in order to have better sealing and support for the panel  10 . In this embodiment, the corner portions  34  are preferably welded only along one of the joint lines  36  or  37  so that after forming the box  12  from the sheet  14  it is possible to spread the walls  18  apart in order to insert the cover sheet  38  into its flange  24  support. 
     The edge joint  24  is adapted to receive a preferably rectangular, floating cover sheet  38  which is slideably secured to the panel  12  by being captured in the peripheral slot  32 . In order to avoid warping of panel  10 , the cover member  38  floats within the peripheral edge joint  24  of the panel  10  at the face of the panel opposed to the panel bottom  16 . The cover member  38  may be formed of any desired material, but it is preferably formed from a metal such as  18  gauge stainless steel. 
     Referring to FIGS. 3 and 4 the peripheral edge portion  40  of the cover member or sheet  38  has attached to it a peripheral metal seal  42 . The seal  42  may be attached by any desired means, but preferably is spot welded by conventional means to the outer surface of the sheet  38 . The seal has a first section  44  which overlies the sheet  38  in a parallel abutting manner. The welding of the seal  42  to the sheet occurs in this section  44 . Extending inwardly from the first section  44  of the seal  42  is a spring section  46 , which in this example has an inverted “V” shape with its free end  48  slideably engaging the sheet  38 . While the metal seal arrangement  42  is the preferred embodiment of this invention any desired conventional seal  42 , metal or polymeric, could be employed. 
     The outer edge  49  of the cover member  38  has a “U” shaped polymeric sealing gasket  50  comprised of a sealing tape secured to it by any desired means such as by an adhesive. The purpose of this gasket is to seal off the inside of the panel  12  to inhibit moisture and the atmosphere from entering. The gasket may be formed of any conventional gasket material but is preferably formed of a polymer such as ultra high molecular weight polyethylene. 
     To insert the floating cover sheet  38  into the edge joint  24  of the panel, the side walls  18  of the box  12  are pulled or sprung apart and the floating cover sheet  38  with the attached gasket  50  and peripheral metal seal  42  is then inserted into the edge joints  24 . The edges  55  between the respective side walls  18  of the box  12  are then closed and welded to seal them, although any desired conventional sealing technique could be used, including without limitation the use of structural adhesives. The edge joint  24  is then crimped to sealingly engage the sliding gasket  50  and cover sheet  38 . 
     Having completed the envelope of the panel  10  the insulation is then inserted to fill up the inside of the panel  10 . The insulation may be any desired conventional insulation but is preferably a polymeric foam such as a closed cell polyurethane foam. The polyurethane foam preferably is foamed in place through holes  54  which may then be sealed with a suitable metal or polymer plug (not shown) such as a screw on plug. Alternatively the insulation may be placed in the box  12  prior to welding the edges of the box. 
     FIG. 3 shows a section of the panel  10  with the cover sheet  38  at a low temperature which would be experienced on the cold side of a freezer or cryogenic container, while the box  12  is at an ambient or room temperature. In this condition the cover member  38  shrinks more than the box  12  due to the temperature difference, so that its peripheral edge  49  moves inwardly of the panel  10  toward the side walls  18 . Since the cover sheet  38  is floating there is no warpage of the panel as the aforenoted shrinkage is taken up by sliding movement of the cover sheet  38  relative to the panel box  12 , within the edge joint  24 . By contrast, FIG. 4 shows a section of the panel  10  with both the cover sheet  38  and the box  12  at an ambient room temperature before cooling down the freezer (not shown). In this condition the cover member  38  expands within the edge joint  24  without warpage of the panel since it is not rigidly attached to the side walls  18 . 
     Referring to FIG. 5 a typical mounting arrangement in a freezer environment for the panel  10  of this invention is shown. The cold side of the freezer is on the side of the mounting arrangement shown by arrow  56 . In this example the temperature could be at a temperature of about −100° F. The side of the freezer which is exposed to ambient room temperature is shown by arrow  58  and would be at a temperature of about 70° F. An insulated frame member  26  of the freezer is used to support one side of a panel  10 . The panel is clamped to the frame member  26  by a series of clamps  59 . Each clamp includes a clamp member  64  which is bolted by bolt  60  and washer  62  to the frame  26  using nut  66 . The clamp member  64  is shaped like an inverted “L” so that a leg  68  has a face  70  which engages the edge joint  24  of the panel  10  to secure it to the frame member  26 . The face  72  of the clamp member  68  serves as a stop to prevent over tightening of the clamp member  26  against the edge joint  24 . If the edge joint  24  is squeezed too much it might prevent the cover sheet  38  from sliding thereby causing panel  10  to buckle. 
     The frame member  26  is composed of two pieces, a channel member  74  and a cover member  76  secured thereto by welding, bolting, adhesively or by any conventional securing means. Before the cover member  76  is secured access is provided to the bolts  60  and nuts  66  so they can be tightened. After the cover member is in place insulation such as a closed cell polyurethane foam is inserted into the member  26  to insulate it. The insulation  52  in the panel  10  in this support arrangement is sealed from the cold side of the freezer by the insulated frame member  26 , the cover sheet  38 , the peripheral metal seal  42  and the sealing tape gasket  50 . The side wall of the freezer (not shown) comprises a plurality of such frame members  26  supporting a plurality of insulated panels  10 . 
     The design of a second preferred embodiment of the insulation panel  100  of this invention will be described by reference to FIGS. 6 through 9. Elements corresponding to the previous embodiment have been given the same reference numbers. The panel  100 , of this embodiment, which is shown by way of a further example, also comprises a rectangular box or pan portion  112 , which is preferably filled with closed cell polyurethane foam. 
     The bottom portion  16  of the box  112 , as best shown in FIG. 16, is made by bending a flat sheet  14  of (e.g.  18  gauge) stainless steel. The bottom  16  of the box portion  12  has a rectangular shape in this exemplary embodiment. Four side wall flaps  18  extend out from the bottom  16  and are adapted to be bent upwardly from the bottom  16  about the bend lines  22  to form the box portion  112 . The free edges  20  of the flaps  18 , which are parallel to the bend lines  22 , are bent to form an edge flange  150 . The edge flange  150  of the panel  100  is a formed lip or flange which protrudes outwardly from the side wall  18 , preferably in a direction parallel to the bottom  16  of the panel  10  and provides a flange like surface for sliding within the edge joint  124  formed in the cover member  138 . 
     Referring now specifically to FIGS. 6,  8  and  9  the edge joint  124  which is formed from the peripheral edges  120  of the cover member  138  is shown in greater detail. The peripheral edges  120  are bent 180° to form a “U” shaped slot  130  and any excess material is cut away. The slot  130  is open at the portion  132  of the slot  130  immediately adjacent to the walls  18 . When the box  12  is formed from the sheet  14  and the flanges  150  are formed in the free edge portions of the side walls  18 , the corner portions (not shown) of the flanges  150  are missing. Therefore it is preferable to secure corner edge portions (not shown) to the flanges  150  by welding however, any desired conventional joining technique could be used. This provides a continuous flange  150  in order to have better sealing for the panel  100 . In this embodiment, the corner portions (not shown) are preferably welded to the flanges  150  so that after forming the box  12  from the sheet  14  it is possible to push the walls  18  inwardly in order to insert the flanges  150  into the edge joint  124  of the cover sheet  138 . 
     The edge joint  124  is adapted to receive the peripheral flange  150 , which is slideably secured within the peripheral slot  132 . In order to avoid warping by panel  100  the cover member  138  floats about the flanges  150  of the panel  100  at the face of the panel opposed to the panel bottom  16 . The cover member  138  may be formed of any desired material, but it is preferably formed from a metal such as  18  gauge stainless steel. 
     Referring to FIG. 9 the peripheral edge portion  140  of the cover member or sheet  138  in this embodiment is preferably secured to a frame member  156  by discrete “L” shaped clamps  164  which are bolted to the frame member  156  by bolt  160  and nut  166 . If desired the clamps  164  could be turned over and the side of the frame member  156  to which they are clamped made shorter to minimize the projection of the clamps  164  above the surface of the sheet  138 . This would provide a less obstructed surface for cleaning. The peripheral edge portion  140  in this embodiment is sealed to the frame member  156  by a polymer seal  165  preferably formed of ultra high molecular weight polyethylene. These panels  100  are particularly suitable for the floors of freezer tunnels wherein it is desired that they be easily cleanable. While the polymer seal arrangement  165  is the preferred approach for this embodiment of the invention any desired conventional seal  165 , metal or polymeric, could be employed. 
     The outer edge  49  of the flange  150  has a “U” shaped polymeric sealing gasket  50  comprised of a sealing tape secured to it by any desired means such as by an adhesive. The purpose of this gasket is to seal off the inside of the panel  100  to inhibit moisture and the atmosphere from entering. The gasket may be formed of any conventional gasket material but is preferably formed of a polymer such as ultra high molecular weight polyethylene. 
     To insert the flange  150  of the box  112  into the edge joint  124  of the cover member  138 , the side walls  18  of the box  12  are pushed inwardly and the flange  150  with the attached gasket  50  is then inserted into the edge joints  124  of the cover member. The edges  55  between the respective side walls  18  of the box  112  are then closed and welded to seal them, although any desired conventional sealing technique could be used, including without limitation the use of structural adhesives. The edge joint  124  in the cover member  138  is then crimped to sealingly engage the sliding gasket  50  and flange  150 . 
     Having completed the envelope of the panel  100  the insulation is then inserted to fill up the inside of the panel  100  as described in the previous embodiment. 
     FIG. 9 shows a section of the panel  100  with the cover sheet  138  at a low temperature which would be experienced on the cold side of a freezer or cryogenic container, while the box  112  is at an ambient or room temperature. In this condition the cover member  138  shrinks more than the box  112  due to the temperature difference, so that its peripheral edge  149  moves inwardly of the panel  100  toward the side walls  18 . Since the cover sheet  138  is floating there is no warpage of the panel as the aforenoted shrinkage is taken up by sliding movement of the flange  150  relative to the cover member  138 , within the edge joint  124 . By contrast, FIG. 8 shows a section of the panel  100  with both the cover sheet  138  and the box  112  at an ambient room temperature before cooling down the freezer (not shown). In this condition the cover member  138  expands without warpage of the panel since it is not rigidly attached to the side walls  18  flange  150 . 
     The second embodiment of this invention set forth in FIGS. 6 through 9 with the edge joint  124  forming part of the floating cover member provides an inside surface which is easy to wash for applications where this is required, while still maintaining the floating low warpage type design. The first and second embodiments are particularly useful for use in CO 2  type freezers where hermetic sealing of the insulation panels  10  or  100  is not absolutely required. 
     Third and fourth designs will now be described by reference to FIGS. 10 through 15, which comprise fully enclosed all welded panels  200  and  300 . These panels would be more suitable for use with liquid nitrogen freezers or containers where the tendency to encounter thermal warpage problems described above is the most pronounced, due to the much lower temperatures involved in the freezer. The all welded designs prevent oxygen enrichment of condensation on the inside of the panel  200  or  300 . The designs utilize a low coefficient of thermal expansion (CTE) material such as Invar  36  for the portion of the panel exposed to the low temperatures and a stainless steel such as type  304  for the portion of the panel exposed to room temperature to achieve a very low warpage under extreme conditions. Since Invar is expensive and is available in limited shapes, using Invar for preferably only the portion of the panel exposed to significant temperature variation from ambient reduces the cost of the panel  200  or  300 . 
     The low coefficient of thermal expansion (or contraction) material used in accordance with this invention preferably has a linear thermal expansion (or contraction) comprising as determined by the formula (L t −L a )/L a  of from about −60×10 −5  inch/inch to about 0 inch/inch, at a temperature of about −300° F., where L a  is the ambient temperature to which a portion of the panel  200  or  300  is exposed, (e.g. a 68° F. room temperature) and L t  is the temperature to which the remaining portion of the panel  200  or  300  will be exposed. Invar  36  has a linear thermal expansion of about −39×10 −5  inch/inch and therefore is quite suitable for this application. 
     The design of a third preferred embodiment of the insulation panel  200  of this invention will now be described by reference to FIGS. 10 through 12. Elements corresponding to the previous embodiments have been given the same reference numbers. The panel  200 , of this embodiment, which is shown by way of a further example, also comprises a rectangular box or pan portion  212 , which is preferably filled with closed cell polyurethane foam. 
     The bottom portion  16  of the box  212 , as best shown in FIG. 16, is made by bending a flat sheet  14  of (e.g.  18  gauge) stainless steel. The bottom  16  of the box portion  12  has a rectangular shape as in the previous embodiment. Four side wall flaps  18  extend out from the bottom  16  and are adapted to be bent upwardly from the bottom  16  about the bend lines  22  to form the box portion  212 . The free edges  20  of the flaps  18 , which are parallel to the bend lines  22 , are bent to form an edge flange  150 . The edge flange  150  of the panel  200  is a formed lip or flange which protrudes outwardly from the side wall  18 , preferably in a direction parallel to the bottom  16  of the panel  10  and provides a flange like surface for welding to the special metal cover sheet  238 . 
     When the box  212  is formed from the sheet  14  and the flanges  150  are formed in the free edge portions of the side walls  18 , the corner portions (not shown) of the flanges  150  are missing. Therefore it is preferable to secure corner edge portions (not shown) to the flanges  150  by welding. This provides a continuous flange  150  in order to have better sealing of the panel  200 . 
     In order to avoid warping of panel  200 , the cover member  238 , which is the portion of the panel  200  exposed to low cryogenic temperatures, is preferably formed out of Invar  36  metal sheet. The box  212  is preferably made of SS  304  stainless steel, and is adapted to be placed on the warm side of the Invar cover member  238 . The metal seal  42  in this embodiment is preferably made of either half hard SS  301  spring steel or Invar. As in the previous embodiments it is placed on the cold side of the Invar cover sheet  238 . A seam weld  250  is then applied to the joint between the flange  150  and the edge  252  of the cover sheet  238  along the entire perimeter of the cover sheet  238  and flange  150 . The edges  55  between the respective side walls  18  of the box  12  as in FIG. 16 are closed and welded to seal them and provide a hermetically sealed panel  200 . 
     Invar  36  is composed of iron and 36% nickel. Due to this composition it is very weldable with most stainless steels including SS  304 . During cool down in a freezer the Invar has very small contraction due to its low CTE (about a factor of  8  less than stainless steel) and thus the panel has very low thermal generated distortion. 
     Referring to FIG. 12 the peripheral edge portion  252  of the cover member or sheet  238  has attached to it a peripheral metal seal  42  which will be described in greater detail. The seal  42  may be attached by any desired means, but preferably is spot welded by conventional means to the outer surface of the sheet  238 . The seal  42  has a first section  44  which overlies the sheet  238  in a parallel abutting manner. The welding of the seal  42  to the sheet occurs in this section  44 . Extending inwardly from the first section  44  of the seal  42  is a spring section  46 , which in this example has an inverted “V” shape with its free end  48  slideably engaging the sheet  238 . While the metal seal arrangement  42  is the preferred approach for this embodiment of the invention any suitable seal  42  could be employed. 
     Having completed the envelope of the panel  200  the insulation is then inserted to fill up the inside of the panel  200  as described in the previous embodiments. However, due to the weld position, if desired, the insulation could be inserted (e.g. in sheet form) prior to welding on the cover sheet  238 , preferably with, if required, appropriate cooling of the portions of the panel away from the weld zone. 
     The design of a fourth preferred embodiment of the insulation panel  300  of this invention will now be described by reference to FIGS. 13 through 15. Corresponding elements to the previous embodiments have been given the same reference numbers. The panel  300 , of this embodiment, which is shown by way of a further example, also comprises a rectangular box or pan portion  312 , which is preferably filled with closed cell polyurethane foam. 
     This embodiment is similar to the third embodiment except the inside cover body  338  is box shaped similar to the box in FIG.  16  and is formed out of bent Invar sheet with the corners welded closed. The box is then insulated and a SS  304  stainless steel outside base member  312  is welded to the cover body  338  to completely and hermetically seal the insulation panel. This design has the advantage of locating the Invar to SS  304  weld joint  350  on the warm side of the panel  300 . In the previous design if the weld joint  250  gets sufficiently cold it can be a source of warpage of the panel  200  due to the different CTE&#39;s of Invar and SS  304 . Again, during cool down the Invar has very small contraction due to its low CTE and the panel  300  of this embodiment has very low thermal generated distortion. 
     The box shaped cover member  338  of this embodiment is made in a manner similar to that shown for the previous embodiments in FIG. 16 for the box shaped member  12  except that the material of the cover member  338  is Invar instead of stainless steel. The base member  312  also has a box type shape except that the side walls  318  are shorter and the base member  312  is adapted to nest within the cover member  338  providing a peripheral seam  352  where the cover member  338  is welded at  350  to the base member  312 . 
     As in the previous embodiment in order to avoid panel  300  warping, the cover member  338 , which is the portion of the panel  200  exposed to low cryogenic temperatures, is preferably formed out of Invar  36  metal sheet. The base member  312  is preferably made of SS  304  stainless steel, and is adapted to be placed on the warm side of the Invar cover member  338 . The metal seal  42  in this embodiment is preferably made of either half hard SS  301  spring steel or Invar. As in the previous embodiments it is welded to the cold side of the Invar cover sheet  338 . A seam weld  350  is then applied to the joint  352  between the cover member  338  and the base member  312  along the entire perimeter of the cover member  338 . The edges between the respective side walls of the box  338  are closed and welded to seal them and provide a hermetically sealed panel  300 . 
     Referring to FIG. 15 the peripheral edge portion  356  of the cover member  338  has attached to it a peripheral metal seal  42  which will be described in greater detail. The seal  42  may be attached by any desired means, but preferably is spot welded by conventional means to the outer surface of the sheet  238 . The seal  42  has a first section  44  which overlies the member  338  in a parallel abutting manner. The welding of the seal  42  to the sheet occurs in this section  44 . Extending inwardly from the first section  44  of the seal  42  is a spring section  46 , which in this example has an inverted “V” shape with its free end  48  slideably engaging the sheet  238 . While the metal seal arrangement  42  is the preferred approach for this embodiment of the invention any suitable seal  42  could be employed. 
     The designs of this invention differ from prior art approaches in that they use either a sliding seal design shown in FIGS. 1 through 9 or they use an all welded design based on the low CTE material Invar  36  as shown in FIGS. 10 through 15. The sliding seal design shown in FIGS. 1-5 will limit infiltration of water, air and other unwanted material into the panel  10  due to the taped joint  24  and its location on the inside of the freezer. The sliding seal design shown in FIGS. 6-9 will limit infiltration of water, air and other unwanted material into the panel due only to the taped joint. The all welded design of FIGS. 10 through 15 are completely welded and thus eliminate any infiltration. They utilize the unique material Invar  36  which has very low CTE and is easily weldable to stainless steel  304 . Care must be taken to preferably prevent the Invar to stainless weld joint  250  or  350  from getting cold enough to cause warpage. This is accomplished by either adding frame  26  insulation as in FIG. 5 when using the welded design shown in FIGS. 10-12 or by simply moving the stainless to Invar weld joint to the warm side of the panel as shown in FIGS. 13-15. 
     While a rectangular box  11  is preferred, the box  11  may have any desired shape or form that could be used for insulation panels. While closed cell polyurethane foam is the preferred insulation material, any desired insulation material conventionally used for insulation panels can be employed, including without limitation, fiber glass, ceramics, other low thermal conductivity materials or vacuum. While stainless steel is the preferred material for use in the panel embodiments of FIGS. 1-4,  6 - 9  and in portions of the panels of FIGS. 10-12 and  13 - 15 , any desired metal or alloy conventionally used for insulation panels could be employed. While Invar  36  nickel iron alloy is the preferred material for use in the cold portions of the panels of FIGS. 10-12 and  13 - 15  any desired metal or alloy or other material having a low coefficient of thermal expansion (e.g. less than about −60×10 −5  inch/inch as described previously) could be employed. 
     The panel designs of this invention can be extended to shapes other than flat rectangular panels. The panels could be any plane shape or curved in shape and still benefit from this invention. The insulation panels of this invention can be used to provide insulation for applications in which one side is exposed to temperatures below −320° F. These panels could also be used for hot applications where the inside surface is hotter than ambient, such that the opposed sides of the panel are exposed to a sufficient difference of temperatures that warping could otherwise result. Other low CTE alloys (for example Invar  45 ) could be used in the fully welded designs. 
     It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the spirit of the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the spirit and scope of the appended claims.