How to Determine Torque Process Limits to Achieve a Perfect Seal
If you have ever tried to open a bottle and struggled like the man in this video, you have witnessed firsthand the undeniable truth that, sometimes, brand owners and bottlers overtorque their caps. When caps are difficult to open like the one we see in the video, the capper chuck applied excessive torque on them. This could be for various reasons, such as the fear of not sealing the bottle properly and losing pressurizing/preservative gases. Bottlers often choose to overtorque caps, but out of spec/undersized caps, material contraction after a hot-fill process and several other factors can contribute to tight closures as well.
How can we better control the capping process?
Many facilities are turning to torque testing, with devices such as the Mesa ST-120S and ST-S3, to check the release torque of the caps from time to time. If the results indicate a creeping torque value, the technicians can then make an adjustment to the capping head. There are two basic methods they can use to monitor the process variable (torque):
- Measure the release torque of the final package. (Please note: the dwell time between the cap tightening and removal torque testing cycles must be controlled.)
- Measure the application torque directly at the capper head with a tester such as Mesa’s ST-FT1.
Ideally, torque process limits are determined after conducting a number of tests:
- Strip torque (Tstrip) – It is very important for production line technicians to be aware of the torque value at which the cap/bottle threads fail during the capping process. Recognizing stripped caps and understanding the consequences (wrinkles on induction seal, damaged threads, loose cap, etc.) will result in improved package integrity.
- Maximum Seal Torque (Tsealmax) – This is the maximum torque that may be applied without wrinkling induction seals or damaging other liners.
- Torque Setpoint – This is the torque a cap is tightened to. A common rule of thumb is that the setpoint equals the cap diameter in mm divided by 2 (lbf-in).
- Test Batches – To conduct the tests below, tighten batches of closures to 25%, 50%, 75%, 100%, 125%, 150%, 175% of the recommended setpoint.
- Minimum Seal Torque (Tsealmin) – This is the minimum torque that must be applied in order to provide sufficient seal. Seal integrity should be tested using bubble emission or dye leak detection methods. Seal integrity should be evaluated considering transportation (vibration, internal pressure changes) and dwell time (shelf life, accelerated aging).
- Convenient Opening Torque Range – To evaluate the consumer group’s opening experience, caps must be tightened to different test batches, and the target consumer group must rate the samples in each batch for ease of opening. This evaluation will result in Tusermin and Tusermax values.
Torque Process Limits:
Tusl = Tsealmax
Tlsl = Tsealmin
For a well-controlled process, the following equations should also apply:
T setpoint = Tsealmin+(Tsealmax-Tsealmin)/2
Mesa offers a unique line of torque testers and accessories that can help you achieve the perfect seal. Please contact us today for more information.
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