This invention relates generally to the sealing of containers and more particularly to the application of a recessed membrane seal to a container of the type used to hold food products.
Food containers are typically sealed by a flexible membrane type seal which keeps the product clean, protects it from bacterial contamination, prevents it from being spoiled or otherwise harmed by air and foreign matter, keeps moisture either in or out, and also provides evidence of any tampering with the product that has taken place. The seals that are most often used are flat membranes which extend across the top of the container and are sealed to its rim. A replaceable lid normally covers the seal. This type of membrane is level with the top of the container and is not recessed below the container rim.
Membranes that are recessed into the container are used less frequently than flat membranes even though they are more advantageous for many types of products. The container is usually underfilled somewhat because of the difficulty of achieving a complete fill and also because liquid and semi-liquid products and even dry products can spill out of a completely filled container during subsequent handling before the seal can be applied. If the container is less than full and a nonrecessed membrane is applied, a considerable amount of air is entrapped in the top of the container and can degrade and otherwise damage the product. For example, any foods containing oils such as butter can become rancid if exposed to entrapped air. A recessed membrane also allows the lid to have a recessed top surface. This facilitates stacking and convenient display of the containers because the recessed surface of the lid provides space for stacking lugs.
Despite these advantages, recessed seals are not as popular as flat seals, primarily because of the expense and difficulty involved in forming, applying and sealing a recessed membrane. The recessed membranes that are used at present are preformed as individual pieces which are subsequently applied to the container and sealed in a separate operation. The membrane is usually formed off line on a separate machine, and this requires additional machinery, added factory space and extra manufacturing operations, all of which contribute to the overall cost. There are some recessed seal machines which preform the membrane seal from a continuous strip on the sealing machine itself. However, whether the membrane is formed on a separate machine or on the sealing machine itself, it must be made from a material which is stiff enough to hold its shape while the membrane is being applied to the container and sealed. Thin and/or non-ductile film materials cannot be used because of their inability to retain their preformed shape.
Another type of recessed seal currently in use is a flat disc membrane which is applied and sealed to an upwardly facing shoulder formed on the interior wall of the container at a location recessed below the rim. Shoulders of this type are practical only on plastic containers and are normally not used on paper or metal containers. Again, it is necessary for the membrane to be relatively stiff so that it can hold its flat shape until it is sealed to the shoulder.
Thus, the recessed membranes that have been used in the past are all relatively stiff because they must hold their shape until they have been sealed to the container. Consequently, thin films have not been used even though they are more desirable in a number of respects such as cost and performance. The stiffer materials are generally more costly and more difficult to remove and sometimes even require the use of a knife or other tool. Tear marks and grooves can be formed on the membrane to permit removal without tools, but this increases the manufacturing cost and machine complexity. Some seals require tearing in several places for removal, and this increases the inconvenience to the consumer. Pull tabs are for the most part impractical because they present additional difficulties in handling of the preformed membrane.
The stiff construction of the recessed membranes currently in use also generally eliminates transparent materials. For a number of reasons, it is desirable to provide the lid with a window having a transparent patch to keep out dirt which permits the contents to be viewed while the lid remains in place on the container. This increases the sales appeal of the product and eliminates the temptation for consumers to remove the lid so they can visually inspect the contents. Increased sanitary protection and enhanced tamper evident safeguards are also provided with this type of arrangement. However, it requires that the membrane be transparent and is thus not feasible with recessed seals formed by stiff opaque membranes.