Metal cans have been known for having a structural top flange wall secured to the upper edge of the can tubular body and including a depending inner skirt defining the opening for accessing the inside of the can. The skirt also serves as a seat, to which is pressure fittable the peripheral wall of a sealing lid. The lid is manually removable and also recloseable during the period the can is used.
In these prior art cans, the sealing and the axial retention of the lid in the mounted position are achieved by pressure seating a peripheral wall of the lid against the inner surface of the depending skirt of the can flange wall. This type of construction has some deficiencies resulting from the small amount of axial locking force of the lid to the can and also from the fact that the free lower edge of the skirt is in contact with the product in the can.
In order to eliminate the aforementioned deficiencies, there has been developed a can and lid as described in the copending patent application P19600454 currently to U.S. Pat. No. 5,899,352 of the same applicant, according to which the axial locking of the lid is achieved by providing, in one of the parts defined by the can skirt wall forming the opening and the lid peripheral wall of an annular rib that is fittable in a corresponding circumferential recess on the lid peripheral wall or can skirt and having a section similar to that of the annular rib. In this construction, the rib and the recess have substantially coincident profiles of a semicircular shape, defined so that said parts fit each other with a substantially complete contact between the confronting surfaces.
While this arrangement provides a substantial axial force for locking of the lid, avoiding its undue opening due to shocks, internal pressure increase, etc., and allowing an adequate degree of sealing of the can contents to be achieved, keeping the canned product out of contact with air, this construction has the disadvantage of requiring great precision in the formation of the annular rib and circumferential recess.
Due to dimensional imperfections that can exist in these cans, the fitting between the rib and the corresponding circumferential recess sometimes can have a radial gap, thereby reducing the contact between the respective confronting surfaces to only one point of tangency along a line that develops around the circumference of the rib and recess. This contact does not guarantee an adequate sealing for the can, allowing the canned product to deteriorate.
Another deficiency of the known constructions for the can and lid refers to the achievement of automatic closing of cans at the filling units. Cans in which the conventionally secured peripheral edge of the inner ring of the can is on a plane which is at the same level or slightly above the plane of the upper edge of the lid seated at the central opening in some cases have an inadequate closing of the lid. There will be insufficient introduction and pressure of the lid against the can when the latter is moved under the closure roll or piston of the filling machine.
Still another deficiency of the known constructions refers to the accumulation of the product which is spilled over the structural ring of the can during the progressive removal of the can contents, making subsequent closings of the lid more difficult and consequently making possible the contact of air with the product inside the can. This allows, for example, the oxidation of the product and also, in the case of products having volatile elements in their composition, such as paints, the evaporation of said elements, causing the hardening of the remaining product in the can.