Successful application of threaded closures (unlike the "snap-on" variety) is highly dependent upon proper application torque. Many field application problems reported can be traced back to either excessive or insufficient closure torque. Some of the most common characteristics associated with application torque are listed below for your reference.
- Lid-fault: raising of the lid causing a visible gap between the lid and neck.
- Fracturing of closure at the neck.
- Scoring of closure body (fluted or smooth wall).
- Damage or breakage of hinge.
- Shearing of lid from body.
- Ineffective seal between bottle and closure.
- Stressing closure neck (bulging) that may result in future failure during distribution.
- Thread "jumping", or stripping of bottle.
- Closures "rattling" and falling off during transit.
- Product leakage.
- Failure of liner (induction) seal to bottle.
The application torque of closures must be controlled in order for the closures to perform properly in the field. Closures must be applied tight enough to maintain a seal and to resist closure back off, but must not be applied so tightly that the end user cannot remove the closure.
A general guide is that the application torque should equal the inch pounds of approximately ½ the closure millimeter size.
As application torque cannot be measured directly on an automatic capper, removal torque measurements are used as an indication of application torque. Since each package will have its own relationship of application to removal torque, the packager should determine this relationship for each of his packages. By applying caps by hand with a torque meter and immediately measuring the removal torque, a correlation between application and removal torque can be determined. This correlation can then be used as an indicator of the actual application torque generated by the capper. By checking removal torque regularly after capping, one can determine if there is a need for adjustment.