Process and device for the ultrasonic testing for welds between plastics packaging and cover foils

A process and an apparatus for the ultrasonic testing of welds between plastic packaging like food trays and cover foils. The welded joint between the tray and cover foil is periodically exposed to pulses of ultrasonic radiation. The pulses pass through the weld and are picked up or received by a receiver. The amplitude of the received pulses is evaluated. The cross-section of the beam of ultrasonic radiation is so small that the radiation only passes through the weld itself. The packages and the ultrasonic tester are moved relative to each other in order to examine the whole circuit of the weld. The pulse rate is such that the weld areas covered by successive pulses overlap.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention pertains to a process for the ultrasonic testing of 
heat-sealed joints between plastic packages (especially trays for 
ready-to-eat meals) and cover films. The invention also pertains to 
equipment for performing this process. 
2. Description of the Related Art 
Dishlike plastic packages in various shapes for packaging foods are well 
known. After they have been filled, they are sealed with a cover film, 
which is not removed until immediately before the food is to be eaten. The 
stability of foods packaged in this way depends, among other factors, on 
the tightness of the heat-sealed joint between the cover film and the 
dishlike package. When the film is applied to the edge of the package, it 
frequently happens that wrinkles develop, plastic particles become 
heat-sealed in the joint bulges form, or food particles are trapped 
between the cover film and the package. If no bond forms between the 
plastic package and the cover film in such places, the package is not 
sealed, and then oxygen and contaminants can reach the food through these 
defects. Furthermore, the food may be able to run out etc. Therefore, to 
improve the probability that the packaged food will remain safe in 
accordance with applicable food laws until the expiration date that is 
printed on the package, it is necessary to make sure that the heat-sealed 
joint is completely closed all around the package, i.e., that there are no 
defects in the heat-sealed joint. 
Accordingly, the goal of the invention was to develop a process and 
equipment for testing heat-sealed joints of the type described above. A 
further goal of the invention was to be able to perform this process with 
both a high degree of accuracy and reproducibility and with as much speed 
as possible. 
SUMMARY OF THE INVENTION 
Surprisingly, it was found that by focusing a beam of ultrasonic radiation 
of relatively small cross section exclusively on the heat-sealed joint, 
very reliable information can be obtained about the quality of the joint. 
Since the ultrasonic radiation passes exclusively through the heat-sealed 
joint, boundary effects of the regions on either side of the heat-sealed 
joint, where air bubbles, pieces of food or other undesirable elements may 
be present between the cover film and package, do not enter into the 
measuring result. Due to the overlapping of the areas along the 
heat-sealed joint through which the ultrasonic radiation passes, 
continuous testing is achieved. The overlapping can be relatively broad, 
e.g., more than 50%, so that one and the same point of a heat-sealed joint 
is tested several times. Perpendicular transmission of the ultrasonic 
radiation, which is ensured by suitable measures in the course of the 
process and in the operation of the equipment, makes it possible to 
clearly distinguish signals caused by defects in the joint from signals 
caused by normal fluctuations in the area of the test system, e.g., in the 
thickness of the edge of the tray etc. In this connection, it should be 
noted that defects produce abrupt changes in the ultrasonic signal, even 
though the changes may be quite small, whereas geometric deviations cause 
no spontaneous change in the signal amplitude. Fluctuations in the 
amplitude of the transmitted ultrasonic pulse can be averaged out by 
sufficiently broad overlapping of the areas covered by the pulse. Finally, 
the plastic package and ultrasonic test device must move relative to each 
other in such a way that the entire heat-sealed joint, which forms a 
closed curve, is scanned once with complete certainty. 
The process of the invention and the equipment used to perform it allow 
relatively fast testing of a heat-sealed joint. For example, the testing 
of a heat-sealed joint between the tray and cover film of a ready-to-eat 
meal can be performed in less than a second. This has the advantage that 
the equipment of the invention can be integrated in an existing packaging 
plant without having to reduce the plant's cycle time. In a further 
modification with respect to the process, it is proposed that the 
ultrasonic testing be performed with its own cycle control running 
independently of the cycle time of the packaging plant itself. This 
eliminates the need for connections or coordination with the packaging 
plant itself, and the testing can be started and stopped relatively 
quickly. In accordance with the process, the heat-sealed packages are 
conveyed by a conveyor belt to a baffle plate, which diverts the packages 
one by one into the ultrasonic testing unit, in which the heat-sealed 
joints are tested. The entire passage takes only a few seconds, e.g., 
three seconds. The packages are then lifted back onto the conveyor belt. 
Untight packages are sorted out after they have been placed back on the 
convevor belt, while packages with intact heat-sealed joints remain on the 
conveyor belt. The acoustic coupling of the transmitting and receiving 
ultrasonic probes is effected through a water advance interval. Either 
both probes are located in a water bath, into which the heat-sealed joint 
or at least the portion of the heat-sealed joint currently being tested is 
also immersed, or the coupling is effected by water jets, i.e., for 
example, probes of the type described in U.S. Pat. Nos. 3,255,626, 
3,485,088, 3,908,455 and 4,403,510 or in European Patent 119 096. In both 
cases (immersion method or coupling by water jets), it has been found to 
be advantageous to work in a tank to catch or hold the water. 
In regard to the equipment, a so-called "bypass system" has proven to be 
very effective. It can be connected to existing packaging plants. The only 
thing needed is a short stretch of conveyor belt, up to which the system 
can be pushed. 
With respect to the equipment, the testing of circular plastic packages is 
especially simple and advantageous because in this case only the traylike 
package must be turned about its own axis, while the ultrasonic test line 
can remain stationary. However, the process of the invention also allows 
the testing of noncircular packages. In this case, the package to be 
tested is placed in a matching mount; this mount ensures geometric 
coordination. The mount with the package is turned about a quasi-center. 
In addition, the ultrasonic test line is moved transversely to the 
transport direction 44, so that deviations from the circular shape can be 
eliminated. Finally, rectangular packages can also be tested by covering 
heat-sealed joint areas running at right angles to the linear direction of 
transport by probes which are carried along in the direction of transport 
but which are simultaneously moved in the transverse direction. 
Transmission of the ultrasonic radiation through the heat-sealed joint was 
wound to be crucially important; testing on the basis of reflected sound 
signals does not yield satisfactory results. 
The ultrasonic testing of packaged foods is basically already known from 
EP-A-269 185. In that process, ultrasonic radiation is transmitted through 
the food itself for the purpose of detecting gas bubbles caused by 
fermentation before the fermentation has progressed so far that the 
package shows noticeable bulging. There is no preventive test that can be 
performed immediately after the food has been packaged, but rather 
deterioration of the packaged food due to fermentation must already have 
occurred for the previous ultrasonic test to be able to sort it out. 
Testing of the heat-sealed joint is not performed. 
Overlapping of the joint areas covered by the individual ultrasonic pulses 
is understood to mean the overlapping of a first ultrasonic pulse by the 
following ultrasonic pulse by at least 2% and preferably at least 20% of 
the ultrasonically covered area of the heat-sealed joint. The overlapping 
is usually less than 100%. 100% overlapping occurs when exactly the same 
area covered by a first pulse is covered again by a second pulse. 
Noncircular, e.g., oval, ultrasonic coverage areas, in which the 
cross-sectional length is greater than the cross-sectional width, have 
been found to be very advantageous. In this case, the greater length 
dimension of the ultrasonic coverage area should lie essentially parallel 
to the course of the heat-sealed joint line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Successively filled plastic packages 22 that have been sealed with cover 
films are located on a conveyor belt 20 moving perpendicularly to the 
plane of the drawing, i.e., into the plane of the drawing. These untested 
packages are conveyed towards an obliquely positioned baffle plate 24, 
which diverts them from their path and, as is shown for package 23, causes 
them to slide into the ultrasonic testing device, which will be described 
below. If the ultrasonic testing is not to be performed, the baffle plate 
24 is swung completely out of the path of the packages 22. 
As FIG. 3 shows, the ultrasonic testing device forms a feed hopper, which 
is bounded by a plate 26 and a U-shaped sheet-metal guide 28. As will be 
described below, the plate 26 holds all parts of the device. It is 
inclined at an angle of 30.degree. from the vertical. The packages 22 
slide on their bottoms on the plate 26, so that the cover film remains a 
certain distance from the surface of the plate 26 and is thus protected. 
The heat-sealed joint, whose tightness is to be tested, is located at 32. 
The package 22 to be tested is guided between guide jaws 34 by parts of the 
sheet-metal guide 28 that are positioned transverse to the direction of 
conveyance of the conveyor belt 20 and falls into the area between a 
conveyor belt 38 and the plate 26. The conveyor belt 38 has catches 40 
arranged at distances that are adjusted to the package 22. The system is 
designed in such a way that the package can only slide between two catches 
40 in the space between the conveyor belt 38 and the plate 26. The catches 
40 extend so far towards the plate 26 that a package that is sliding down 
first strikes them and then slides further only when a sufficiently large 
gap becomes available for it between two catches 40. This situation is 
shown in FIGS. 1 and 2. The package 22 is supported towards the bottom by 
a support ledge 36. It is arranged in such a way that the center of the 
packages 22, which are circular in the specific example shown here, lies 
on the center line of the conveyor belt 38. 
In a modification of the process, the conveyor belt 38 can also be stopped 
in the position shown in FIGS. 1 and 2 until a package 22 is located in 
the position shown at the far left. 
The conveyor belt 38 runs around in a closed loop. It is guided over two 
rollers 46, 47, at least one of which is driven. The rollers rotate about 
axes 42 that run parallel to the surface of the plate 26. As FIGS. 1 and 2 
show, the distance between the respective axes 42 of the two rollers 46 
and 47 is slightly greater than three times the diameter of the packages 
22. 
The conveyor belt 38 conveys the packages to the right in FIGS. 1 and 2 in 
the direction of arrow 44. The packages arrive at an intermediate 
position, which is shown in FIG. 1 as the middle position of the packages 
22, and then finally reach the actual test position, which is shown on the 
right in FIGS. 1 and 2. Thus the conveyor belt 20, the baffle plate 24, 
the feed hopper and the conveyor belt 38 cooperatively supply unrested 
packages successively to the test position. They are held at the top by 
suitably extended, now horizontally running guide jaws 34 and are 
supported at the bottom on the support ledge 36. 
The ultrasonic test position has respective axes 42 of the two rollers 46 
and 47 three driven, grooved rollers 46, 47, one of which is shown in 
cross section in FIG. 5. As FIG. 1 shows, two lower rollers 47 are located 
in a continuation of the support ledge 36 at a distance from each other 
that is selected so narrow that the ultrasonic test device, which will be 
discussed later, has just enough room between them. They are mounted at 
the same height. 
A single roller 46 is arranged centrally above the two lower rollers 47. 
The centers of the rollers 46, 47 form the vertices of an isosceles 
triangle, whose apex angle (at roller 46) is selected as acute as 
possible. 
The purpose of the rollers 46, 47 is to tune the packages 22 about their 
centers and at the same time to guide them exactly. During the turning 
movement of the package 22, the conveyance of the packages 22 by the 
conveyor belt 38 is momentarily stopped. 
As FIG. 1 shows, in the test position, the packages 22 are positioned 
somewhat lower than before, i.e., as long as they are supported by the 
support ledge 36. To allow them to enter the space between the three 
rollers 46, 47, the upper roller 46 is briefly raised, for which purpose a 
cylinder (shown schematically) is provided above this roller. As a package 
22 arrives at the test position, it drops down slightly onto rollers 47 
and triggers a control pulse, which in response thereto the roller 46 
drops back down and the ultrasonic test is performed with timing that is 
determined by the ultrasonic test device itself. After the roller 46 drops 
back down, the turning movement is performed. 
The three structurally identical rollers 46, 47 consist essentially of 
three disks, namely, two outer, beveled guide disks 48 and a middle, 
driver disk 50, which is knurled on the outside. The outer edge of the 
latter disk grips the edge of the tray by friction. The driver disk 50 is 
selected with such a thickness that the edge of the tray with the cover 
film on it is maintained between the two guide disks 48 with as little 
play as possible. 
The ultrasonic test will now be described especially with reference to FIG. 
4. An ultrasonic transmitter 52 is mounted in a drill hole in plate 26. It 
transmits short-duration ultrasonic pulses with a frequency of, for 
example, 10 MHz at a high pulse rate of, for example 1 to 5 kHz. A sound 
guide tube 54 with an inside diameter of 3 mm is mounted on the 
transmitter and extends to the immediate vicinity of the edge of the tray. 
It has an oval shape at its free outlet end, such that the major axis of 
the oval coincides with the direction of the heat-sealed joint to be 
tested. In this way, the ultrasonic transmission area on the heat-sealed 
joint is shaped such that the pulses have a cross-section which includes a 
dimension of about 2 to 2.5 mm in the direction transverse to the 
longitudinal course of the heat-sealed joint. The circular package 
disclosed have heat-sealed joints extended along and defining courses. The 
shape of the outlet end of the sound guide tube 54 and the exact geometric 
coordination between the sound guide tube 54 and the edge of the package 
22 with the heat-sealed joint ensure that only the actual area of the 
heat-sealed joint is exposed to the ultrasonic radiation. In this 
connection, the geometric coordination is guaranteed essentially by the 
two rollers 47, which are positioned in the immediate vicinity of the 
ultrasonic test. 
The ultrasonic pulses which have passed through the edge of the package 22 
and the cover film in the area of the heat-sealed joint are picked up or 
received by an ultrasonic receiver 56. The circumferential speed with 
which the package 22 is turned and the repeating frequency with which the 
ultrasonic pulses are transmitted by the transmitter 52 are mutually 
adjusted in such a way that the oval ultrasonic exposure areas on the 
heat-sealed joint overlap each other. 
Apparatus for the generation and evaluation of the ultrasonic pulses are 
already well known, i.e. state-of-the-art methods can be used. In the 
practical performance of tests with one type of packages 22, it was found 
that the ultrasonic signals fluctuate by 2 dB due to deviations from 
perpendicular transmission through the edge of the package 22 and other 
factors. Changes greater than 4 to 6 dB can be given as defects in the 
heat-sealed joint. Another decision criterion that can be used is based on 
the fact that defects lead to abrupt changes in the ultrasonic signal, 
while other geometric changes produce slower changes in the ultrasonic 
signal. 
In the specific embodiment of the invention shown here, acoustic contact or 
coupling between the probe and the test specimen is made through water. 
For this reason, as FIG. 3 shows, the test device is located in a water 
tank 58, which is filled to a sufficiently high level that at least the 
lower, currently tested regions of the edges of the trays are submerged in 
the water. 
During the test, the package 22 is turned at least once about its own axis. 
If enough time is available, the package can be rotated a second time. 
After the ultrasonic test has been performed, the conveyor belt 38 moves 
forward, and the package 22 advances to the area of a lifting device 60, 
which lifts the package in the direction opposite the direction in which 
it previously slid downward. The package 22 is thus lifted back to the 
plane of the conveyor belt 20 until it tips back onto this conveyor belt. 
The package then reassumes the position shown in FIG. 3 as the initial 
position. A baffle plate is then used to sort out those tested packages 
that were evaluated as defective during the ultrasonic test. In another 
modification, the lifting device can also be used to sort good and bad 
packages. 
The equipment of the invention can be coordinated with an existing 
packaging line in a bypass operation. Attachment and detachment can be 
accomplished by a few manual operations. Once the test equipment is 
connected to the packaging line, a decision can be made for each 
individual package on the conveyor belt 20 whether that package should be 
tested or not. This is accomplished by control of the baffle plate 24. 
To simplify maintenance of the system, the conveyor belt 38 is arranged so 
that it can be swung out around one of its axes 42. If one wishes not to 
use water (or another suitable liquid) as the acoustic coupling medium and 
would rather perform dry measurements, it is also possible to maintain the 
test probes 52, 56 in rubbing contact with the edge of the package 22 or 
cover film 30. Mixed forms, i.e., acoustic coupling of the transmitter 52 
through water but acoustic coupling of the receiver by rubbing contact, or 
the like, are also possible. 
Higher test frequencies, e.g., 15 MHz and higher, have been found to be 
more suitable for this test than lower frequencies, e.g., 3 MHz. The 
invention also allows the testing of yogurt containers, drug packages etc. 
In a modified embodiment of the invention, in which the equipment is 
otherwise identical, the probes 52, 56 are replaced by probes of types 
that are already known, for example, from the four US patents and the 
European patent cited above. The water jets are directed at the 
heat-sealed joint 32 from both sides in such a way that the degree of 
overlapping is as complete as possible, i.e., the ultrasonic pulse can 
pass through linearly. The probes 52, 56 are connected by well-known means 
to a water reservoir, from which they are continuously supplied with water 
by a pump. A sound guide tube is unnecessary under these circumstances. 
Aside from these modifications, the same equipment can be used as 
described above. 
However, as far as the process is concerned, when acoustic coupling by 
water jets is used, in contrast to the immersion method described above, 
the water in the tank 58 is maintained at such a depth that the water jets 
issuing from the probes 52, 56 are above the level of the water. 
The ultrasonic transmission areas are generally circular, but the use of 
oblong transmission areas is advantageous; in this case, the larger 
dimension of the transmission area is aligned with the longitudinal 
direction of the heat-sealed joint.