Figure 2-6. Body dent involving the end seam.
2-10. EXTERNAL DEFECTS--INTERNAL PRESSURES
External examination of cans will also reveal defects caused by internal
pressures being either negative or positive. A negative internal pressure is the result of
excessive exhausting or excessive head space following the filling operation and it
causes inward movement of the can sides. Positive internal pressure causes an
outward distention of the can end. As the pressure increases, the distention becomes
more evident. Defect identification is dependent on the amount of the distention
a. Paneling. Paneling is caused by negative internal pressure, causing one or
more flat surfaces on the sides of the can. This is similar to the dent causing a sharp
ridge; however, instead of one ridge, there will be two or more, with evidence or
suspicion of fracture. It is a result of excessive exhausting or excessive headspace. It
is a major defect.
b. Flipper. This is a can with little or no vacuum. It appears normal, but when
one end is struck on a flat surface, the opposite end will distend and remain distended
until forced back into position. It will then stay in that position until the can is struck
again. This condition may be the result of overfill-ing, insufficient exhausting, or
chemical or bacterial action. It is a critical defect.
c. Springer. A springer has one end distended at all times. Both ends cannot
be pushed in at the same time. When the distended end is depressed, the other end