There are a variety of applications in which it is desirable to provide a hermetic seal across the open end of a container. As one example, food products to be sold in a plastic container are often provided with a metallic foil lid or cover which extends across the open end of the container and is heat sealed to the open-top end of the container; typically, the container will be provided with a flange about its open end to which the heat seal can be formed. Heat sealing has been accomplished by applying a generally cylindrical heating tool to the portion of the lid which overlies the flange, and urging the hot tool against the lid material whereby the heat from the tool is conducted through the metallic lid to melt the adjacent underlying plastic material and, upon subsequent cooling, forming the desired heat seal between flange and metallic lid. Typically, the pressure applied by the heat-sealing tool is counter-balanced by a support on the opposite side of the container flange.
It has also been known to utilize a so-called "dished" lid, which is formed with a depression or dished-out portion throughout most of its extent so that the edge portions may provide the desired seal but the remainder of the lid extends downwardly more nearly into contact with the contents of the container, thus reducing the amount of air or other gas within the container after sealing and thereby reducing the adverse effects of the gaseous material upon the product and/or any tendency for expansion of the gas upon subsequent heating to loosen the lid. Because of its dome-like shape it also resists bending forces due to increases of internal pressure, and thus preserves its pleasing smooth appearance.
While such procedures and containers have been found suitable for various purposes, they have been found to be subject to defects which sometimes result in loosened seals, at least at certain points along the circumference of the seal, and therefore in a breach of the desired hermetic condition. This is believed to be at least in part due to variations in thickness of the plastic material of the flange. In any event, it has commonly been found necessary to form a heavier seal, as by increasing one or more of the parameters of temperature, time and applied pressure, so as to assure that even the portions less likely to seal well are in fact well sealed. While this is capable of providing the desired strength and completeness of seal, the resultant seal is typically so strong that it is difficult for the user to peel off the lid, as is customarily done later to gain access to the product in the container.
Accordingly, it is an object of the present invention to provide new and useful method and apparatus for sealing a lid or closure to the open end of a container.
Another object is to provide such method and apparatus which provide complete, high quality seals and which, nevertheless, are relatively easy to peel off and remove when access to the interior of the container is desired.
Another object is to provide such method and apparatus in which the time for forming the seal is reduced, thus enabling a higher rate of sealing of a series of successive containers.
It is also an object to provide a container assembly comprising a container and a closure heat-sealed thereto, in which the seal is complete and strong yet the closure can readily be peeled off when desired.