Quick Question Monday : How To Make Sure Your Cap Is Tight Enough?

When it comes to screwing on a continuous thread (CT) cap of any kind, application and removal torque should be properly measured to ensure a proper engagement of a threaded closure to a bottle or jar.  It is a common misconception that the tighter you apply a closure to the bottle or jar, the less likely it will leak, but in reality it is quite the opposite. Over-tightening of the cap, commonly referred to by the packaging industry as excessive torque, can cause the cap to shear from the body, thread "jumping", or stripping, or even cracking the closure. On the other hand, closures that are screwed on too loose, with insufficient torque, can lead to closure "rattling" and product leakage.

Proper application of screw-on caps to bottles or jar, aka application and removal torque, should always be measured to ensure proper function of these screw cap, it's liners, and the bottle or jar. So let's first look at what application and removal torque is, how to measure it, and common recommendations from closure manufacturers.

Definition - Application Torque

Application torque is the rotational force with which a closure is applied to a bottle or jar. Measured in lb / in, rule of thumb for application torque measurement half (50%) of the diameter of the closure. For example, a 43mm closure should have an application torque of between 21 to 22 lb /in.

Great! application torque : 50% of the closure diameter, that's it? Shortest quick question Monday article ever right?

Removal Torque, the other half of the equation

Since application torque cannot be measured directly on any automatic capping machines; the only way to really tell the screw cap was applied properly is to measure the removal torque. Removal torque is the maximum force required to remove or unscrew a closure.  Generally speaking, removal torque should be between 40 to 60% of the application torque 24 hours after the closure has been applied.

Why 24 hours, you may ask. Closures can back off it's application torque as time goes by, which is largely depends on the type of materials the closure and bottle. This is commonly referred to as the closure "Back off".

Ok, removal torque is : 40 - 60% of the application torque, so 43mm = 21 - 22 lb / in application torque,= 8 - 13 lb / in removal torque... So we are done now right?

Common Measuring and Testing Technique

While the industry rule of thumb seems fairly straight forward, it is difficult to actually implement as application torque, as mentioned above, cannot be directly measured and require removal torque measurement to validate. To further complicate the issue, closure back-off will reduce the removal torque of a closure as time progress. While removal torque is commonly measured 24 hours after the cap is applied, most fillers will first select a batch of 30 packages to measure, apply the closures with varying application torque, select a small number of samples from that batch of 30 and measure for removal torque after 5 minutes. Then measure the rest after 24 hours. After correlating the application torque the best option is then selected. Finally, the optimal 5 minute removal torque correlation is identified and utilized.

On the production line, you can now randomly remove capped packages 5 minutes after application and measure removal torque to determine whether they are in range specified in the previous technique.

Machine that Measures

Since measuring application and removal torque has to be precise, torque measuring machines, manual or automated, have been developed to help with this process. They are commonly referred to as torque testers. For small scale, hand tighten applications, a manual torque tester can be used. For high speed, automated fill line application, it is best to use automated torque tester to eliminate manual variances.

Common Closure Application Torque Chart

Keep this table as a reference point as you determine the best application and removal torque for your package, keep in mind that different materials, content, and filler environment all plays a role in proper application of closures.

Neck Finish (mm) Application Torque (inch-pounds) Removal Torque (inch-pounds)
8 3 - 7 2 - 4
10 4 - 8 2 - 4
13 5 - 9 3 - 5
15 5 - 9 3 - 5
18 7 - 10 3 - 5
20 8 - 12 4 - 6
22 9 - 14 4 - 6
24 10 - 18 4 - 11
28 12 - 21 5 - 12
30 13 - 23 6 - 14
33 15 - 25 6 - 15
38 17 - 26 7 - 16
43 17 - 27 7 - 20
48 19 - 30 8 - 18
53 21 - 36 9 - 22
58 23 - 40 10 - 24
63 25 - 43 10 - 26
66 26 - 45 11 - 27
70 28 - 50 12 - 30
83 32 - 60 16 - 36
86 40 - 65 16 - 39
89 40 - 70 18 - 42

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