Closures and caps must be applied tight enough to maintain a seal and to resist working loose, but they must not be applied so tightly that the end user cannot reasonably remove them.
The application torque of closures is best controlled and monitored at the point of production, in order for the closures to perform properly during their life-cycle in the field.
Closure Torque Testing is a measure of the rotational force required to either apply or remove a threaded screw-cap, stopper or lid on a container.
This is frequently a two-stage measurement. If the closure incorporates a tamper-evident (TE) band or a security ring, then removal torque will involve measuring the initial torque to start the closure moving plus the 'bridge torque' required to break the TE band.
In the case of a child-resistant closure (CRC), the axial push force to overcome the safety mechanism must first be applied before the removal torque is subsequently measured.
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application torque
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removal torque
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slip torque
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bridge torque
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strip torque
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reverse-ratchet torque
To obtain the best measurement repeatability it is recommended that closures always be tested using a motorised device which operates at a constant speed. Manual torque testers, although practical, may exhibit variation as operators test at different speeds.
Where possible care should be taken to avoid distorting the closure when gripping during the testing process, as such distortion can dramatically influence the accuracy of the torque measurement particularly for thin-walled plastic closures.
The use of a mandrel, specifically tailored to the dimension and profile of the closure, is highly recommended as a means of gripping the closure without undue distortion when it comes to applying torque.
