Production Line Leak Testing
This invention relates to the production line testing of sealed packages particularly sealed containers with lids on flanged edged cups.
Background to the invention
For food products poor seals mean the atmosphere in the container can change and affect the product as well as allowing the ingress of micro organisms or the leakage of contents. Thus the need to check for leaks in container seals is an important part of quality control for food packers.
There is a conflict between the requirements of fast testing and the ability to detect small leaks, since small leaks generally require more time to detect. A number of methods have been proposed to detect holes, which generally utilise the application of external pressure and measurement of the time rate of change of some parameter which indicates the flow of air into or out of the headspace above the container contents.
One established method of leak testing is to enclose the container within a space and either evacuate the space or increase the pressure within the space and then track changes in the pressure within the space over time. Variations in the pressure usually result from leaks out of or into the sealed container. USA patents 4593554, 4952496 and 5333492 are examples of this approach.
The introduction of a vacuum into such a test enclosure is a relatively slow operation, since a significant volume of air is typically trapped between the pack and the membrane, and must be removed after sealing of the enclosure through a pneumatic connection which is subsequently closed in order to perform the test. This is a major limitation to the speed of testing, and hence to the application of the method to high speed testing of packages in an automated operation. Typically in a test sequence it is found that a pumping time of 2-10 seconds is required to remove all the air from the test enclosure, whereas the test itself may be performed in as little as a fraction of a second.
In USA patents 5907093 and 6082184 disclose a method of testing for leaks in a packages having a head space which reduces the pressure in the enclosed space about the container seal to the vapour pressure in the container headspace. It has been difficult to adapt these techniques into viable production line testing systems as opposed to testing of randomly selected containers.
Another technique for vacuum packed goods is to measure wall or lid deformation due to an external pressure change applied to the package. USA patent 4747299 discloses a method wherein the body of the container as distinct from the sealed lid portion, is subjected to a reduction in pressure and the distention of the body is measured and measured again after a subsequent further pressure reduction. This senses if there is a leak by the further distention of the base or body of the container. USA patent 5249454 discloses a method in which a vacuum is used to cause lid deflection and the vacuum is held and any lid movement is monitored. European patent 473173 uses a similar method where the package is placed in a chamber and pressure is increased or decreased to deform the wall or lid and then the change in the wall deformation is indicative of a leak. USA patent 5111684 discloses a method where aseptic packages are stressed while the seal is subjected to atmospheric or greater pressure and the reaction to the stressing is detected.
Production line testing of all containers in a production line is not attractive if the time taken for the test inhibits the rate of production of containers or results in too large an increase in the floor space required for the production equipment when the on line tester is added. The additional capital cost and running cost of the equipment is also an important consideration.
Production line testing have tended to use cruder detection methods which are most effective with gross leaks. USA patent 5259237 measures response to the application of pressure to sort leaky packs from sound ones. USA patent 5786530 and 6105419 use a similar principal whereby leaky packages are more easily deformed and recover less quickly so that leaky packages can be removed for failure to activate an appropriate switch. USA patent 5226316 uses a force transducer to monitor positive wall pressure of the container over time.
It is object of this invention to provide a production line leak testing system for lidded containers that takes up minimal additional processing time. Brief Description of the invention
To this end the present invention provides a method for production line leak testing of sealed packages having at least one flexible wall or lid, in which one or more sealed packages are tested, which method includes the steps of a) applying a reference pressure to at least one wall or lid and measuring one or more of the wall or lid position, package volume or internal pressure b) applying a second pressure to the lid and optionally measuring one or more of the wall or lid position, package volume or internal pressure c) allowing the pressure to return to the reference pressure and again measuring one or more of the lid position, package volume or internal pressure d) analysing the data from a) and c) and optionally b) to identify whether the package has a leak.
This invention is predicated on the realisation that it is quicker and more accurate to measure lid deflection, volume change or internal pressure change at two different pressures and observe the difference, than to measure a decay in the applied pressure or vacuum over time. The pressure change produced in moving from the reference pressure to a second pressure induces a change in lid position or the internal volume or pressure. The pressure differential on either side of the flexible wall or lid means that if there is a leak this will tend to equalise. If the leak is large this occurs quickly and little change in the measurement is observed. If the leak is small a larger change occurs but when the pressure returns to the reference value the difference between this measurement and the first measurement at the reference pressure is greater than for non leaking containers. If a negative pressure or vacuum is applied as the pressure change the fine leak will register as a lower reading at the reference pressure. Where an increase pressure is used the fine leak will cause the second measurement to be greater than the first measurement at the reference pressure. This method works best when only a flexible wall or lid of the container is subjected to the change in pressure while maintaining the remainder of the container, which is preferably less flexible than the wall or lid, at atmospheric or some constant pressure. The portion subjected to the change in pressure preferably includes the seal between the lid and the container. This invention is particularly suited to testing for leaks in the seal between a flexible lid and a relatively rigid container such as those used for yoghurt or fruits. To determine both fine leaks and large leaks two different criteria are used. Large leaks are indicated by a measurement change less than a predetermined value and fine leaks by a positive change in the measurements at the reference pressure. Preferably the pressure change is positive as this will tend to unblock any holes in the seal by blowing product back into the container and increasing the internal pressure. The increase in internal pressure means that the return to the reference pressure will result in the second measurement [pressure, volume or lid height] at the reference position being higher than the first measurement when there is a fine leak. The reference pressure is preferably ambient [atmospheric]. The volume of gas in the head space may be measured by using contact or non contact position sensors to measure the position of the flexible region which is indicative of the volume of the container. Preferably a contacting sensor is used allowing an application of an initial force to locate the package against an external seal and enable the closure of the testing chamber. Alternatively the mass of the air in the package may be quantified by measuring the pressure exerted by the flexible region on a stationary force measuring system which has been maintained in contact with said flexible region at a fixed position.
It is preferred that the position of the lid be measured and this can be done by any suitable means such as by optical measurement or physical measurement of deflection of a sensor. It is preferred to use a magnetic deflection sensor which generates a voltage responsive to the deflection and can be used to give a measurement derived from lid height.
The method of this invention has the following advantages:
1. the position of the flexible region is measured twice at the same conditions of minimum strain so that its sensitivity to leakage and to geometric effects is maximised and it is able to detect small holes in shorter times than prior art methods
2. The test pressure if positive may be a significant value eg between 20 and 300kPa which helps to clear any blockages to the leakage path and assists the location of the pack against an external seal. The use of a positive pressure change also avoids the problem of product leaking into the test cavity which would create down time.
3. The magnitude of the measured leak is independent of minor variations in pack geometry and volume of the headspace above the product inside the package 4. The pressure application step is technically simple to perform and can be replicated within a single test system to allow longer dwell times without adding major cost to the system
5. Measurement is only essential before and after exposure to the pressure change and may be carried out in a separate location.
In another aspect the present invention provides a leak testing apparatus for testing packages having a flexible lid sealed to the rim of a body which includes a) means to convey the packages and providing a sealing area adjacent said rim b) a moveable cavity which seals on said sealing area of said support to enclose said lid and said seal c) means to apply a pressure to said lid and to release the pressure d) means to measure the change in the lid deflection induced by the pressure change. In this embodiment of the method one cavity encloses a plurality of containers so that the pressure change is applied to the whole cavity. Each container lid has associated with it a deflection sensor. Preferably the container is supported beneath the flange on the rim so that the seal between the lid and the flange is contained within the test cavity. The remainder of the container is subject to ambient pressure.
The apparatus of this invention provides the additional advantages that
1. the pressure differential across the package seal is kept at a maximum by the geometry of the testing enclosure
2. multiple packages may be tested in a single enclosure and simultaneously subjected to the test pressure without requiring pneumatic isolation which would be the case with methods that measure the sensed pressure within the chamber
3. test times are sufficiently short to enable the method to be used in production lines to detect fine leaks. Detailed Description of the Invention
A preferred embodiment will now be described with reference to the drawings in which :
Figure 1 is a schematic illustration of the principle of this invention; Figure 2 is a schematic illustration of the method of this invention;
Figures 3A, 3B and 3C are perspective views of the inlet side of an online testing machine in accordance with a preferred embodiment of this invention ;
Figures 4A and 4B are perspective views of the outlet side of the online testing machine shown in figure 3. Figure 5 is a side view illustrating the inlet side
Figure 6 is a schematic view of the tester cells
Figure 7 shows the test cell in a pressure testing position
Figure 8 is a graphical illustration of the transducer out puts from a non leaking container a container with a fine leak and one with a large leak
In one preferred embodiment the present invention tests for leakage in the seal between a flexible lid and a flanged container of a fluid product. The container is a semi rigid cup shaped container which is supported in the test equipment beneath the flange with a pneumatic seal being made against the container so that a sealed test chamber is formed above the lid. As illustrated in figure 1 the application of an external pressure change will result in a deflection of the lid. As shown in figure 2 pressure P1 is applied at least about the seal between the flange and the lid and force or pressure applied to the lid by the internal pressure P2 of the container is measured at F . A different pressure is applied to the lid to that applied to the body of the container. If the lid pressure P1 is greater than the body pressure P3 [preferably ambient] the body is able to expand to maximise the differential pressure between the headspace and the applied pressure. Example 1 A container of about 350 ml volume was placed in a test system . An initial pressure pulse of duration of 0.3 seconds was followed by a low pressure step of 0J seconds. The position of the sensor was measured at this reference pressure. The pressure was increased for 2.5 seconds and then reduced to the reference pressure . At 1.5 sends after the pressure reduction a second sensor measurement was made . Containers without a leak had a negative difference between the second and first readings. Containers with a hole of 4mm by 0.16mm were found to have a positive difference. Machine description
This describes one embodiment of the online testing machine of this invention. The machine has been customized to test the integrity of the seal of a metal cover sealed to the flange of a cup of the type used for yoghurt or fruit. Those skilled in the art will realize that the material handling aspects of the machine such as the conveyors and gripping mechanisms need to be customised for each installation. In this installation the filled and sealed cups exit the pasteurization unit in an inverted position and are tested and then packed. The cups exit the pasteurisation unit in lines of 9 abreast. The cups are separate and individual. In operation a line of containers [ nine are used] to be tested is advanced toward the test machine as shown in figures 3A 3B and 3C. The containers 18 are usually inverted following sterilisation and the row of cups are lifted from the inverted position, rotated through 90degrees and inserted in a horizontal position into the testing block which is then rotated a further 90 degrees into the vertical position for testing.
The test sequence is to clamp and seal around the top of the container by sandwiching an O ring between two plates so that the base is left exposed to atmospheric pressure and the top is in the test chamber. A seal is made around all 9 containers with an overhead plate into which is mounted nine transducers each positioned to seat on the lid of a corresponding container. With reference to figure 5, cups may be fed to the test machine from a number of alternative delivery systems. In the case of this embodiment they are taken from a conveyor 19 after treatment in an autoclave. The cups 18 are delivered in an inverted orientation, in rows. The first row of cups 18 is lifted using a set of vacuum cups on a rotating arm 20, and swung into a horizontal position. The top of each cup 18 is gripped by a profiled gripper 21 and the arm 20 is released and swings clear. The gripper 21 is then advanced to insert the cup 18 into a combined clamping and sealing mechanism in the testing block 30.
The block 30 consists of 4 sets of clamping plates each capable of holding 9 cups. The clamping plates hold cups at the insert position 31 the test position 32 and the withdraw position 33. With the conclusion of each test the block 30 rotates to move the cups to the next operative position. No cups are held in the inverted position at the bottom of the block 30
With reference to figures 6 and 7 the cup at the initial position 31 is clamped by compressing an "O" ring 34 between plates 35 and 36 under the action of cylinder 37, reducing the diameter of the "O" ring and forcing it against the wall of the cup. The gripper 21 then releases the cup and is withdrawn.
The testing mechanism rotates around the centre line 39 as shown in figure 6 to a second position 32. The testing assembly 38 is then clamped (Fig 7) against the plate 35, using the "O" ring 45 to create a sealed chamber 47 around the lids of all nine cups. The chamber is then subjected to a pressure sequence to enable the integrity of the seal to be evaluated. There are nine transducers 40, one for each cup which seat on the lid of each cup within the chamber 47. The plate 42 of each transducer 40 is urged against the lid 46 of each cup under the weight of the plate 42 and associated transducer structure 40, and complemented by the pressure of the optional spring 43 . The transducer magnet 41 then moves in direct relation to the movement of the lid, and causes the output of the transducer 40 to change. Said output is transmitted by the cable 44 to the system controller, and is analysed to detect leaking cups, which are identified by their position within the row. The testing pressure is then released, and the clamping plate raised to allow the mechanism to rotate until the cup is in position 33, at which point a second gripper 24 which may be identical in function to the gripper 21 holds the cup and extracts it after release of the aforesaid clamping mechanism consisting of plates 35 and 36 and O ring 34. The cup is then rotated to an upright position, by the gripper 24 and lowered into a screw feed 25 [see figure 4] on the block side with flights contoured to accept one cup per thread. The gripper is released and the screw rotates by a number of turns to move the row of cups in turn onto a conveyor 26, with each cup being identified by the number of turns required to reach the conveyor. When a faulty cup has been identified by the test procedure, it is identified by its position within the row and ejected by an ejection device 23 located close to the point of reaching the conveyor 26 and actuated by the system controller. Conveyor 26 conducts the tested containers in single file in the direction back toward the conveyor 19 to a packing station.
The transducers 40 are magnetic transducers which generate a voltage that corresponds to the movement caused to the transducer by deflection of the lid. An initial pressure pulse is provided to ensure that the containers are seated firmly. The deflection is then measured at the reference [atmospheric] pressure and the pressure increased by up to 300kPa. If the deflection caused by this pressure increase to any lid does not exceed a predetermined thresh hold, the control program registers the corresponding container as having a major leak and the container is registered for rejection. The pressure is returned to the reference [atmospheric ] pressure and the deflection measured once more. If the difference between the first and second reference pressure readings is positive the control program registers the corresponding container for rejection. An example of a test sequence for this machine is described in example 2. Example 2
A cup of approximately 400 mL capacity was tested by the application of a test pressure P2 of 100 kPa. The lid position was measured using a linear transducer with an output of 0 to 10V, corresponding to the deflection of the lid. After an initial delay of 0.1 seconds, pressure was applied to the testing chamber for 0.5 seconds and then reduced to P1 for 0.8 seconds, at which point the reference lid position R was measured. The pressure was then increased to P3 for a duration of 2 seconds and the peak deflection D1 was noted at the end of this period. The pressure was then returned to the original initial pressure P1 , and the lid deflection D2 was measured after a further 1.1 seconds.
As shown in figure 8, it was found that for cups with no leak the deflection D1 was always greater than 1.5V, and the deflection D2 was less than zero, and generally less than -0.15V. Cups with a large leak gave values of D1 less than 1.5V, whereas cups with a small leak gave values of D2 greater than -0.15V, and generally greater than 0V.
The entire test was completed in a duration of 4.5 seconds. The initial pressure step may be eliminated depending on the achievement of a very consistent clamping process, and the consistency of the cup when subjected to pressure.
From the above it can be seen that this invention provides a unique method and apparatus for detecting fine leaks at production line speeds. Those skilled in the art will realise that the method can be adapted to use with different conveyor systems and different sensor types depending on the needs of the packing plant. The machine described with reference to the drawings was customized for inverted cups entering the test machine as 9 individual cups in a row. Where the cups are not inverted those skilled in the art will realize that the cups can still be inserted into the testing block but without the need for inversion. The system can also be adapted for use with cups that are joined at their flanges usually in groups of 4 or 12. The testing block will not need a great deal of redesign because with individual cups one compartment is used for a group and each cup is tested individually. The controller will need to be programmed to identify which cup in a pack is faulty by identifying the correlation of the testing transducer and the cup. It will also be obvious that instead of using a rotary block the cups could be inserted into a holding plate which is then raised to locate against the testing plate. Similarly those skilled in the art will realize that the faulty cup ejection mechanism and the conveyor arrangement for the cups exiting the machine can be one of a range of possible designs to suit the circumstances of the packing plant and machinery already installed.