1. Field of the Invention
This invention relates to a method for inspecting leakage of a sealed container, and more particularly to an inspection method effectively used for inspecting a leaking place such as a pin hole, a crack, etc. of a sealed container containing foods, pharmaceuticals, chemical industrial products or the like.
2. Description of the Prior Art
In the food industry, there are generally used containers of a type which are sealed after foods are filled therein and then sterilized by heating, or containers of a type which are sealed after foods are filled therein under a bacteria-free condition. In these containers, if there occurs a poor sealing, a pin hole or the like, air tends to flow into the containers through the pin hole, etc. from outside and a bacterial contamination takes place. As a result, the food contents of the containers are oxidized, changed in quality, or corrupted. In some cases, it turns out to be a significant problem endangering human life.
Therefore, in the food industry or container manufacturing industry, many attempts have been made in order to develop an improved art for sealing with a hope to provide a perfect sealing of a container, an improved art for inspecting the sealing of a container, and a related instrument, device, apparatus, etc. Representative examples thereof are as follows.
(1) One of the representative methods for inspecting leakage of a food can using a metallic can is known as a beat inspecting method. According to this method, an objective container is beaten with a metallic bar and the sealing is inspected by checking with the beating sound. However, this beat inspecting method is inapplicable to a compound container of plastic material with aluminum foils laminated thereon.
(2) As another example, there is a method for inspecting the sealing of a container, in which an objective container is formed with a hole, the peripheral area of the opening portion of the hole is sealed with a rubber material or the like and then, the container is sunk into water, and thereafter, air is sent into the hole in order to raise the internal pressure of the container and finally, generation of an air bubble from the container is checked. This method, however, has the shortcoming in that breakage of the objective container is unavoidable.
(3) In a retort food, there is employed a method in that a bag-shaped container formed of a laminated film of aluminum foil and plastic, or a plastic film vapor deposited with aluminum is used, and air is removed from the container after the container is filled with foods, or otherwise an inert gas such as a nitrogen gas or a carbon dioxide gas or a mixture of a nitrogen gas and a carbon dioxide gas is charged into the container after air is removed from the container, and then the container is sealed.
As a seal inspecting method of a retort food, there is employed a method in that an objective container containing food therein is kept for two to three weeks at a normal temperature of 25.degree. C. to 40.degree. C. and any expansion of the objective container caused by bacterial contamination or any oozing-out of a liquid content filled in the container, which tends to occur when imperfect sealing, pin-hole, etc. are taken place, is inspected by eye. However, any of the above-mentioned methods has the problems in that many days are required for carrying out this inspection and a large stock of the objective containers is required.
(4) As still another method for inspecting any leakage, there is known a method in that a container having a confirmed perfect sealing and an objective container are accommodated in two vacuum chambers which are perfectly sealed and separated, and thereafter, any change of an internal pressure of the vacuum chambers, which tends to occur when an imperfect sealing of the objective container, a pin-hole, etc. are detected, by using a so-called pressure sensor under a predetermined reduced pressure. However, this method again has the shortcoming in that when moisture is attached to the external part of the container, a correct leakage inspection is difficult to obtain because the internal pressure of the chambers is also readily changed owing to evaporation of the moisture when the pressure is reduced.
(5) There is also a method for determining any leakage in that a vacuum chamber is filled with water, and after the objective container is dipped in the water, the internal pressure of the chamber is reduced, and finally, any bubble or any oozing-out of the content of the objective container is inspected. However, this method has the problem that an additional procedure for drying the container is required after inspection because the objective container is wet with the water.
As described above, the conventional inspecting methods have the various problems in that the objective container is limited, a correct inspection is difficult to obtain, the manner for carrying out the method is complicated and a long period of time is required for carrying out the method, and so on.