Method and apparatus for testing vacuum insulation panel quality

A method of verifying the integrity of the walls and seals of a sealed container is provided in which a low molecular weight gas is introduced into the interior of the container before the container is sealed. After sealing, the container either alone, or in batches with others, is placed into a chamber which is evacuated of gases to a pressure level below that interior of the container and is evacuated of the low molecular weight gas to a partial pressure below that which the gas has within the container. Subsequently the gas within the chamber is sensed to determine whether the rate of increase of the low molecular weight gas in the chamber exceeds a predetermined rate.

BACKGROUND OF THE INVENTION 
The present invention relates to a sealed container and a method and 
apparatus for testing the integrity of the sealed container and more 
particularly relates to an evacuated container and a method and apparatus 
for testing an evacuated container. 
In many applications it is desireable to evacuate a container and to 
hermetically seal the container and maintain the evacuated and sealed 
nature of the container over a long period of time. An example of such a 
container is a vacuum insulation panel such as those disclosed in U.S. 
Pat. Nos. 4,668,551 and 4,681,788. Such vacuum insulation panels are 
suggested for use in refrigerator/freezer cabinet walls to reduce heat 
load on the refrigeration system; thereby achieving substantial product 
energy usage reductions. Successful application of vacuum insulation 
panels in the refrigerator/freezer walls require that an acceptable vacuum 
level inside the panel be maintained for twenty years or longer. Any loss 
of vacuum beyond acceptable limits would seriously degrade the product 
performance. Once the vacuum insulation panels are placed into the 
refrigerator/freezer walls, it would be extremely difficult to determine 
the integrity of the insulation system (other than catastrophic failures), 
short of testing every product for energy consumption. Since there are 
more than ten million refrigerators produced in the United States on an 
annual basis, such testing would be impractical. Further, even if a 
defective panel were located by such testing, replacement of the panel 
would cause severe damages, if not destruction, of many components of the 
refrigerator/freezer. 
Therefore there is a need to provide a technique to test vacuum insulation 
panels before they are applied to the refrigerator/freezer cabinets. There 
is a similar need to provide a technique to test other types of evacuated 
containers. Since such vacuum insulation panels need to be manufactured in 
very large numbers at very high speeds, any test technique has to be 
accurate enough to test the vacuum insulation panels in a very rapid 
manner. 
SUMMARY OF THE INVENTION 
The present invention provides an improved vacuum insulation panel and a 
method for testing vacuum insulation panels for leaks by inserting into 
the panel, at low pressure, a test gas and then after the panel has been 
sealed, measuring any escaping gas. The test gas preferred would be a low 
molecular weight, easily detectable gas such as helium. Helium is chosen 
as a test gas due to the fact that it is completely inert and is present 
at very low concentrations in ambient air. Also, because of helium's low 
molecular weight, it flows more rapidly through very small leaks and 
permeates more rapidly through solid barrier plastics than other gases. 
This invention is not limited to using helium as a test gas for leak 
detection, yet, helium is a preferred gas and will be used in the 
discussion of the present invention. 
Similarly, the method described herein as to how to fabricate and test a 
thermal vacuum panel is an example only and the invention is not limited 
to such specific methods described herein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 there is illustrated a refrigeration appliance generally at 10 in 
which the present invention finds utility, and which comprises a 
refrigeration compartment 12 and a freezer compartment 14 located above 
the refrigerator compartment 12. 
Each of the refrigerator compartments 12, 14, are accessed by means of a 
separate door 16, 18. The compartments are defined by an interior wall or 
liner 20 which is spaced within an exterior outer shell 22, comprising the 
refrigeration appliance cabinet. A space 24 between the interior wall 20 
and the shell 22 is generally filled with an insulation material such as 
polyurethane foam which is injected into the space 24 in a liquid state 
where it expands and hardens into a porous solid state to form a 
structural part of the cabinet as well as providing a thermal barrier 
necessary to prevent rapid warming of the interior compartments. 
The present invention, in a preferred embodiment, provides an improved 
vacuum thermal insulation panel 28 (FIG. 2) which is to be inserted in the 
space 24 (FIG. 1) between the liner 20 and the shell 22 to enhance the 
insulation property of the insulation system. Vacuum insulation panels may 
be placed in other thermally sensitive areas throughout the appliance 10 
as required. Polyurethane foam is used to provide additional insulation, 
the structural support it normally provides and to assist in holding the 
panels 28 in place between the walls 20, 22. 
The particular vacuum thermal insulation panel illustrated is only a single 
example of a type of device that could utilize the present invention. 
Other environments where the sealed integrity of an enclosure or container 
must be assured could also benefit from the use of the present invention 
irrespective of whether the container is evacuated. 
The particular placing and usage of such vacuum panels is disclosed in 
pending patent application Ser. No. 452,063 entitled "VACUUM INSULATION 
PANEL SYSTEM FOR INSULATING REFRIGERATION CABINETS", now U.S. Pat. No. 
5,082,335, assigned to the assignee of the present invention. That 
disclosure is incorporated herein by reference and further discussion of 
the use of such panels will not be contained herein. 
Each vacuum insulation panel 28 is constructed with a pair of highly 
permeation resistant outer film walls 30, 32 (FIG. 3) and may have one or 
more gas permeation resistant inner wall(s) 34. Although the barrier film 
walls are defined as being gas impermeable or permeation resistant, in 
reality they do have a very limited permeability. In a typical vacuum 
insulation panel, permeability of oxygen should be such that less than 10 
mm Hg pressure is built up over the life of the product (for most 
insulation fillers used in vacuum panels). 
If a single gas impermeable inner wall 34 is used, then two adjacent 
compartments 36, 38 (FIG. 3) are created enclosing one or more insulation 
filler materials 40 that support the film walls 30, 32, 34 of the panel 28 
when atmospheric gases are evacuated from the interior compartments 36, 38 
of the panel 28. The pressure differential across the inner wall 34 is 
very small, while the pressure differential across either of the outer 
walls 30, 32 will be approximately equal to atmospheric pressure. The 
insulating filler material(s) 40 may be in the form of individual sheets 
of fibrous material such as glass fiber insulation as illustrated in FIG. 
3 or may be in the form of a microporous powder material (FIG. 4), or a 
mixture of various insulation materials, as described in pending 
application Ser. No. 451,830, now U.S. Pat. No. 5,018,328 entitled 
"MULTI-COMTMENT VACUUM INSULATION PANELS", assigned to the assignee of 
the present invention. The disclosure of that application is incorporated 
herein by reference. Of course, any other form and combination of 
microporous, other porous, or any other filler insulation material may be 
employed in a compartment or various compartments of the vacuum insulation 
panel 28. 
The two outer walls 30, 32 and inner wall 34 (FIG. 3), are hermetically 
sealed to each other about their edges or periphery 42 to define the two 
vacuum sealed compartments 36, 38 (FIG. 3). The primary function of the 
compartment walls 30, 32, 34, made of flexible barrier films, is to 
inhibit gas and water vapor permeation into the vacuum panel 28, thereby 
maintaining the proper vacuum level achieved during its manufacture 
through evacuation prior to the sealing of the peripheral edges 42 of the 
panel to form the vacuum insulation compartments. Particular barrier films 
best suited for forming a vacuum insulation panel are described in pending 
application Ser. No. 451,830, entitled "MULTI-COMTMENT VACUUM 
INSULATION PANELS". Such laminate barrier films have the best permeability 
characteristics, that is, will maintain the vacuum conditions within the 
panel for the longest period of time. 
An alternative construction for the vacuum insulation panel is shown at 52 
in FIG. 5 wherein four separate layers 54, 56, 58 and 60 are formed and 
comprise two completely independent sealed bags. An outer bag 62 defining 
an outer compartment 64 is formed by layers 54 and 56 which completely 
encapsulates and surrounds an internal bag 66 defining an inner or main 
compartment 68 formed by layers 58 and 60. Within each bag 62, 66 there is 
a porous filler material 70 as described above. 
It may be desirable to provide within the sealed panels 28, 52 materials to 
absorb or otherwise interact with gases and vapors that are able to 
permeate the film walls 30, 32, 54, 56 of the panel. Such materials are 
known as getters 72 (FIG. 5) and are described in greater detail in 
pending patent application Ser. No. 452,068, now U. S. Pat. No. 5,091,233, 
entitled "GETTER STRUCTURE FOR VACUUM INSULATION PANELS", assigned to 
assignee of the present invention. The disclosure of that patent 
application is incorporated herein by reference. 
To test the hermetic seal and integrity of the panel film to assure that 
the manufactured panel 28 will remain hermetically sealed and thus 
impervious from the external environment for the life of a refrigerator 
(approximately 20 years or more), a low molecular weight, easily 
detectable gas 73 (FIGS. 3, 4 and 5), such as helium is introduced into 
the interior 36, 38, 64, 68 of the vacuum panel before it is sealed 
(although the depiction of the gas 73 in the figures is schematically 
shown as a bubble, in fact the gas will be evenly dispersed throughout the 
entire interior area). Of course, this application is not limited to the 
use of the helium, any low molecular weight gas may be used, but 
preferably one that is inert and easily detectable. 
The helium is introduced into the interior 36, 38, 64, 68 of the vacuum 
panel at a pressure substantially less than atmospheric pressure and at a 
pressure level not high enough to damage the thermal performance of the 
panel. The helium pressure, however, must be high enough such that a test 
pump can rapidly evacuate a test chamber below the helium pressure. 
Generally less than 2 millimeters (mm) of mercury (Hg), and preferably 1 
mm Hg will satisfy these requirements for precipitated silica fillers. For 
fillers such as fumed silica, the optimum pressure of the helium may go as 
high as 5 mm Hg. The vacuum insulation panel is then hermetically sealed 
about its periphery and tested for helium leaks. Because helium has a high 
mobility nature, the gas 73 will escape the interior 36, 38, 64, 68 of the 
vacuum panel 28 either by permeation or hydrodynamics. 
In FIG. 8 there are illustrated, schematically, three different levels of 
the rate of helium gas escape from the interior of the vacuum panel. A 
first amount indicated by arrow 80 represents a normal permeation rate of 
gas escape for a panel, thus representing a leak rate indicated by line 82 
which Applicants have determined to be approximately in the range of 
1.times.10.sup.-7 atm cc/sec. (actually this rate can vary based on the 
size and the number of vacuum panels tested and barrier material 
characteristics). A second, higher leak rate indicated by arrow 84 
represents an excessive amount of permeation generally caused by some 
fault in the film material forming the walls of the panels. This film 
fault does not rise to the level of an actual pin hole leak, but rather is 
a defect in the chemical characteristics of the film allowing a larger 
than acceptable permeation rate. The third arrow indicator 86 represents a 
rate of gas escape significantly greater than that permissible and would 
represent a hydrodynamic leak generally caused by an actual pin hole in 
the wall of the film or in the seal area. 
In order to check the panels for leakage, Applicants have developed a 
method for individually testing each of the panels in a manner which can 
be accomplished in a very quick and yet reliable fashion. In this method 
the vacuum panels are filled with the microporous insulation and evacuated 
in a vacuum chamber, then a small amount of low molecular weight gas, such 
as helium, is introduced into the panel at a pressure level substantially 
less than atmospheric pressure and below that which would cause damage to 
the thermal performance of the panel. Since helium and other low molecular 
weight gases have high thermal conductivity compared to large molecular 
gases, it is necessary to keep the pressure of the introduced gas very 
low. The gas pressure, however, must be high enough that a test pump can 
rapidly evacuate a test chamber below the introduced gas pressure and also 
the gas pressure should result in a partial pressure greater than that 
which the gas has in the ambient atmosphere of the test chamber. 
A number of different approaches to introducing such a gas, preferably 
helium, to the interior of the panel are available. In a first embodiment, 
illustrated in FIG. 6, an evacuated assembly chamber 100 is provided in 
which the vacuum insulation panel is first filled with the evacuated 
insulating material, such as microporous powder, and then a probe 102 of a 
gas cylinder 104 is introduced into the interior of the panel to introduce 
the gas into the partially assembled, but not yet sealed panel. This first 
step is illustrated at step 106 in FIG. 10. 
The next step illustrated in FIG. 10 at 108 is to heat seal the remaining 
open edge of the panel to seal the helium within the panel. 
A second embodiment of the invention regarding the filling of the panel 
with helium is illustrated in FIG. 7 wherein an assembly chamber 110 can 
be evacuated while the panel is filled with the insulation material. 
Subsequently the chamber is flooded with the helium gas through a conduit 
112 operated by a valve 114. A pump 116 is used to evacuate the chamber 
and to allow a full flushing of helium gas into the chamber. Again, the 
pressure of helium gas in the chamber will be well below one atmosphere 
and preferably will be in the range of approximately 1 mm Hg. 
Further alternate methods for introducing helium to the interior of the 
vacuum panel are illustrated in FIGS. 11-21. In FIGS. 11-16 there is shown 
an arrangement whereby a helium source in the form of a thin strip of 
helium blown foam 120 (FIG. 11) is produced in an extruder 122 which 
receives a material, such as a thermoplastic, for example, polystyrene, 
which is fed in from a hopper 124 to the extruder. A source 126 of helium 
at high pressure communicates with the extruder barrel 122 and is mixed 
with the plastic. The mix is then extruded from a die 128 as in 130 into a 
sheet, strand or some other convenient shape. Upon exiting the die 128, 
the molten plastic foams due to the high pressure helium contained it. The 
foam is directed through a series of chill rolls 132 and is directed past 
a cutter mechanism 134 and then assembled in stacks or other transportable 
form. During the assembly of the vacuum panel, the foam strip 120 will be 
placed in the interior of the vacuum panel (FIG. 13) prior to introduction 
of the filler material 40 as shown in FIG. 14. The vacuum panel is 
subsequently heat sealed as schematically illustrated in FIG. 15. 
A still further embodiment of the invention is illustrated in FIGS. 17-21 
in which a helium source in the form of a helium pellet 140 is formed by 
exposing a piece of plastic to helium which will rapidly dissolve into the 
plastic so as to form a reservoir of helium that can be transferred into 
the vacuum panel pouch. This occurs since helium is soluble and it easily 
permeates plastics. A strip of a plastic laminate material having an upper 
and lower diffusion control layer 142, formed for example of PET, 
surrounds an interior helium solution layer 144 which for example may be 
EP rubber. The pellet is first placed into a helium saturation chamber so 
that the interior layer 144 will absorb a desired amount of helium. Then 
the pellet 140 will be introduced into the vacuum panel as shown in FIG. 
18 prior to insertion of the filler material 40 as illustrated in FIG. 19. 
Subsequently, the vacuum panel will be heat sealed closed as schematically 
illustrated in FIG. 20. 
In a preferred embodiment of the invention, regardless of the manner in 
which the helium is introduced into the interior of the panel, the 
remaining open edge of the panel is heat sealed to seal the helium gas, at 
pressures below 10 mm Hg, within the evacuated panels. Then the vacuum 
panel in the form of a pouch is transferred, as illustrated in FIG. 9 to a 
flat conveyor 150. The panel is conveyed on the flat conveyor 150 through 
a pinch type flat upper conveyor 152 to uniformly distribute the powder 
within the panel. If filler materials other than powder are utilized, this 
step may not be necessary. 
Following the panel flattening conveyor 152, an elevator 154 with a pallet 
156 is staged in an upper position to accept a first panel. When a panel 
is sensed in the transfer position, a gripper (not shown) closes on a 
leading edge 158 of the panel and positions the panel on the pallet 156 
and the transfer device moves to the incoming pallet and plate position. A 
plate 160 is then picked up and placed on top of the panel and the 
elevator is lowered to accept the next panel of the stack. This process 
continues until a stack of panels and plates are complete. When the pallet 
stack is complete, the elevator lowers to the pallet transfer position and 
an air lock door 162 opens. Once the pallet transfer is complete a new 
pallet is transferred onto the elevator and the process is repeated. 
The opening of the air lock door 162 is indicated at step 164 in FIG. 10. 
The panels are then transferred to a hydraulic press mechanism 166 in an 
air lock chamber 167 as indicated at step 168 in FIG. 10. The air lock 
door 162 is then sealed as indicated at step 170 in FIG. 10. Once the 
completed pallet is conveyed into the air lock chamber 167, the press 
mechanism 166 extends to exert pressure on the panel and plate stack. This 
force is required to keep the panel stack flat during the time that the 
air lock chamber 167 is returning to atmospheric pressure, the hydraulic 
press 166 retracts and the pallet is conveyed to a position for further 
processing. 
Three approaches are available for leak testing. A first is a sniffer 
testing approach which utilizes a probe to take a continuous sample of 
gases surrounding the tip of the probe. Such an approach is suited for 
testing parts which have a common potential leak site and is not 
particularly useful in testing the overall integrity of the panel. For a 
vacuum enclosure, this sniffer method can detect only the leaks due to 
permeation, not the hydrodynamic leaks. A second method of testing is 
accumulation testing which is a version of sniffer testing. This approach 
relies on enclosure of the panels and all of the emitted helium from all 
of the leaks. The actual test consists of closing the enclosure once a 
specified number of panels are inside, waiting for the concentration of 
helium to increase to a detectable level, then sampling the contents of 
the enclosure through a sniffer probe that is located within the 
enclosure. Mixing of the gas inside the enclosure is required to aid in 
the diffusion of the helium so that the concentration at the sniffer probe 
is an accurate representation of the total level. The required dwell time 
for such a testing procedure is sufficiently high so as to cause a serious 
slow down in the manufacturing process. 
A third approach, which is preferred for high speed, high volume testing, 
is vacuum assisted testing. Two major advantages in evacuating a chamber, 
such as chamber 182 of FIGS. 16 and 21, in which to test the panels are: 
first, the dispersion rate of the helium within the enclosure is increased 
by eliminating the dense air; second, the background concentration of 
helium can be dramatically reduced within the vacuum chamber. Thus, the 
interference caused by the naturally occurring helium in air can be 
controlled. This method is suited for a production application because the 
vacuum pumps can be sized given the chamber tare volume and the cycle time 
required to maintain throughput. 
The relative layout of an example of this type of equipment is shown in 
FIG. 22. It consists of a vacuum chamber 210, a leak detecting device 212, 
one or more auxiliary vacuum pumps 214 and valves 216, 218, 220 and 
controls (not shown in FIG. 22). 
In order to reduce the background helium concentration to make leak 
detection possible, the stack of panels are placed in the test chamber 210 
and pressed together by a press mechanism, similar to that shown in FIG. 
9, to constrain the panels as the pressure within the test chamber 
fluctuates. This step is illustrated at 190 in FIG. 10. Subsequently the 
test chamber is flushed and pressurized to 760 mm Hg with a non-helium 
gas, preferably dry nitrogen as indicated at step 192 and then the test 
chamber is again evacuated as shown at step 194. As indicated at step 196, 
this process may be repeated one or two times to assure a complete 
flushing of background helium from the test chamber. 
A final flush of the test chamber with dry nitrogen is indicated at step 
198, this time the flushing with dry nitrogen will occur at a pressure 
level below the internal pressure of the panels. The hydraulic press is 
then released as indicated at step 200 and the panels are then tested for 
leakage as indicated at step 202. 
As indicated above, the leakage may be one of two types. A hydrodynamic 
leak, generally caused by an actual hole in the panel or the seal is 
caused by a flow in the direction of the total pressure gradient and is 
proportional to the difference in pressures. Thus, if the test chamber 
pressure is below that of the internal pressure of the panel, gas will be 
caused to flow out of the panel signifying a hydrodynamic leak. 
A permeation-related gas flow is characterized by flow in the direction of 
a concentration gradient. Such a gas flow would indicate a higher rate of 
permeation through the panel than is acceptable and would indicate 
defective laminate film. In order to drive permeation gas flow from the 
interior panel to exterior of the panel, the concentration of helium 
within the test chamber should be significantly less than that inside the 
panel. 
A leak detector 184, such as Leybold UL 500 Ultra Test Leak Detector would 
then be used to determine the level of helium within the chamber. If the 
level of helium increases at a rate which exceeds a range around 
1.times.10.sup.-7 atm cc/sec, then as indicated at step 204, this would 
indicate a failed test and the stack would be transferred to a dry 
atmosphere area for individual testing of the panels within the stack as 
indicated at step 206. A flow diagram of the vacuum panel test operation 
is shown in FIG. 23. The acceptable helium leak rate will very much depend 
on the panel design and the barrier films used in the panel construction. 
A failure is indicated by a flow greater than the film's specific 
permeability. If the stack of panels passes the test, then as indicated at 
step 208, the panels would again be repressed by the hydraulic press to a 
one atmospheric level, the chamber would be filled with normal atmosphere, 
and the panels would be released as indicated at step 210 and de-stacked 
and moved to an area for inclusion within the refrigerator cabinets. 
It should be noted that the helium gas will also continuously diffuse out 
of the vacuum panels during the life of the package. This will eventually 
eliminate the slight degradation of the panel thermal conductivity caused 
by the presence of helium gas. 
Thus it is seen that the present invention provides a container, which in a 
preferred arrangement is an evacuated container, having hermetically 
sealed walls forming at least one inner compartment in the compartment 
containing an amount of low molecular weight gas sufficient to raise the 
partial pressure of the gas to a predetermined level above its atmospheric 
partial pressure level but at a substantially lower pressure than 
atmospheric pressure. The invention also provides a method of verifying 
the integrity of a sealed container in which a low molecular weight gas is 
introduced to the interior of the container before sealing, the container 
is placed in a chamber which is then evacuated of the low molecular weight 
gas to a partial pressure level below that of the gas in the container and 
the chamber is also evacuated of all other gases to a total pressure level 
below that of the container. Then the gas within the chamber is sensed 
after the evacuations to determine whether the rate of increase of the low 
molecular weight gas in the chamber exceeds a predetermined rate. If the 
container is an evacuated container, then it of course is necessary to 
evacuate the chamber to a pressure level below that of the evacuated 
container. This method is enhanced by introducing the low molecular weight 
gas to the interior of the container at a concentration level such that 
the partial pressure of the gas will be in excess of the normal 
atmospheric partial pressure of that gas. Further, the present invention 
provides an apparatus for verifying the integrity of the sealed container 
and for carrying out the steps of the above described method. 
As is apparent from the foregoing specification, the invention is 
susceptible of being embodied with various alterations and modifications 
which may differ particularly from those that have been described in the 
preceding specification and description. It should be understood that we 
wish to embody within the scope of the patent warranted hereon all such 
modifications as reasonably and properly come within the scope of our 
contribution to the art.