There are two types of pressed die shape of the ends for 1/2 lb. crab cans: one the ordinary type of flat centre and the other the special type of slightly convex centre such as shown in Fig. 1 on the right of it.
The filled cans with ends of the latter type are considered handicapped in that they are liable to be mistaken for swelled cans and that the lacquered film on the inner side of the ends may be easily stripped off due to repeated diaphragmatic action of the central convex part and this will be the source of black soiling of the contents.
However the author opines that in most normal cans of them in which an adequate vacuity is maintained the diaphragmatic action is not repeated so occasionally as generally considered, but on the contrary since the under vacuum cans are easily detected by the swelling out of the central part of the ends the cans with those ends of such type are of great advantage in being assorted according to the grade of vacuum in them.
The author has studied experimentally the d aphragmatic action of the central part of the can ends which are of the convex type and has f, and that for the 1/2 lb. crab cans the boun-dary zone of swelling of the central part of the ends lies between 201 mm and 148 mm in vacuum on the average.