Child-resistant (CR) closures can pose several challenges. Below are the top 5:
1. Repeatable topload: Topload or, in other words, the push-down force on the outer shell, can create excessive and varying friction between the closure and bottle threads. Varying friction translates to varying torque readout, therefore it is critical to apply repeatable topload.
2. Measuring torque on loose caps: Depending on closure type and the level of topload, loose caps can produce acceptable release torque values due to topload-induced friction between the cap and bottle threads.
3. Seal concerns: To properly engage the outer shell of a CR closure, vertical force is applied on the outer shell. The higher the vertical force that is applied, the more friction the capper/torque tester has to overcome to apply the same amount of torque between the threads. When excessive torque/force is applied on liners, the release torque will be higher and seal deformation may occur.
Left: indicates correct seal. Right: indicates excessive torque/topload, resulting in wrinkled seal.
4. The higher the application torque is, the more topload will be required to open the closure without the outer shell ratcheting on the inner cap. Ratcheting may be incorrectly triggering the automatic detection of a torque peak (false torque result). High topload may not allow the “loose” cap torque to fall enough, not allowing the system to automatically detect the torque peak.
5. CR caps and test methods must be carefully evaluated due to additional variables:
- CR cap design (there are 20+ CR geometry designs)
- Liner type/material (induction seal vs non-induction seal, liner materials)
- Bottle and cap plastic materials (slip agent vs no-slip closures)
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