In order to get a good hermetic seal, it's necessary for the foil innerseal (inside of the cap) to be in contact with the lip of the container and have consistent pressure around the circumference. This is accomplished by applying correct application torque to the cap. When the application torque is inconsistent, the result is inconsistent seals. Causes of inconsistent seals are either cappers that are out of adjustment or have worn tightening wheels, or by poor bottle/closure tolerances resulting in caps and bottles not fitting properly together. Sometimes these irregular fits can be solved with proper types of foil innerseals.
Good Container Lip/Liner Contact
Good contact between the liner material and the container lip is very important. This goes hand in hand with the amount of torque applied, but can be a problem even when the torque levels are good. The land area of the container should be wide enough and rigid enough to support the pressures and heating required for a good seal.
Poor Container Lip/Liner Contact
A thin or weak land area can cause poor contact between the liner material and the container lip. When the land area is too thin there may not be enough rigidity in the land area to support the pressures and heat required for a good seal.
Container Lip Deformations
Gaps between the land area and liner material can be the result of any number of problems, from mold imperfections to container material issues. Regardless of the cause, any gap between the liner and container land area may cause overheating and failure of the seal. A gap caused by fallaway of the land area, as well as a gap caused by protusion. These defects can be detected on the container land area before the container is capped.
A cocked cap is usually due to a problem with the capping process or a cap or container problem. The gap created can cause overheating and the cap itself may actually jam the container under the sealing head causing the liner to overheat severely.