What Is a Torque Tester? A Guide to Cap Torque Testing for Packaging
A torque tester is a measuring instrument that determines the rotational force needed to tighten or loosen a screw cap on a container. It measures the torque applied by the capping machine and the torque the consumer will need to open the package. Torque testers are standard equipment in quality control laboratories in the beverage, food, cosmetic and pharmaceutical industries.
How does a torque tester work?
The container is held firmly on a measuring table while the cap is turned, by hand or by a motor. On electronic testers, a torque sensor converts the twisting force into an electrical signal, and the instrument displays the value, usually the peak reached during the test. Mechanical testers, such as the TMV-ECO, use a spring instead.
Opening torque: the torque needed to unscrew the cap, also called removal torque.
Closing torque: the torque used to tighten the cap, as a capping head does on the line, also called application torque.
Release and break torque: on caps with a tamper-evident band, the opening curve shows two peaks. The first is the release torque, when the cap starts to turn. The second is the break torque, when the bridges between the cap and its band break.
A torque tester should measure only the torque around the axis of the cap. AT2E torque testers use a special mechanical system that cancels most non-coaxial and top load influences on the measurement, so that the result depends as little as possible on the way the operator holds the container.
Main applications
Beverages: checking the capping of PET and glass bottles with screw caps, sport caps and tamper-evident closures.
Cosmetics and pharmaceuticals: jars, bottles and vials with small screw caps, for which a high-accuracy measuring mode at low torque values is useful.
Capping line set-up: checking that every capping head applies the right torque, directly on the line.
Calibration: checking the accuracy of torque testers against a reference torque.
Key concepts: application torque, removal torque and accuracy
Application torque and removal torque are not the same. After capping, the cap and its liner relax, so the removal torque is usually lower than the application torque, and it continues to change during the first hours and days. Laboratories therefore record when each measurement was made, and many follow a test method such as ASTM D2063 for torque retention.
A torque that is too low can cause leaks and, for carbonated drinks, a loss of CO2. A torque that is too high makes the package hard to open and can damage the cap or its threads.
Torque is expressed in kgf·cm, N·m, N·cm or in·lbs. 1 N·m equals 10.2 kgf·cm, or 8.85 in·lbs.
Accuracy is usually given as a percentage of full scale. On the AT2E TMV7, which measures 0 to 99.00 kgf·cm (0 to 85.9 in·lbs) with an accuracy of 0.2 % of full scale, the possible error is about 0.2 kgf·cm (0.17 in·lbs) across the whole range. For very small caps, the TMV7 also has a high-accuracy mode from 0.030 to 4.000 kgf·cm.
Manual or automated testing?
Anyone who has opened a jar at home knows that the result depends on the hand: turn fast or slowly, squeeze the jar or not, and the effort feels different. The same is true in the laboratory. With a manual tester, speed and grip vary from one operator to another.
Automated torque testers remove this human influence. The cap is clamped pneumatically, the measuring head adjusts to the height of the sample, the rotation speed is set (0.3 to 10.0 rpm on the ADATMV-ECO) and the zero is reset automatically before each cycle. The operator places the sample and presses the start button.
A third approach measures the torque where it is applied. A dynamometric torque bottle, such as the BT ETA TORQUE, is installed on the line after the filler and passes under the capping heads like a real bottle. It measures the torque in real conditions of speed and top load, without dismantling the machine.
Calibration of a torque tester
A torque tester is calibrated against a torque reference that is more accurate than the tester itself. Between two calibrations, a quick check confirms that it still reads correctly:
Install a torque reference, such as the BT ETA TORQUE ECO (accuracy ±0.5 in·lbs), on the measuring table of the tester.
Turn slowly and smoothly in the opening or closing direction.
Compare the value displayed by the tester with the torque setting of the reference, and record the deviation.
AT2E TMV and ADATMV torque testers are delivered with a COFRAC calibration certificate. If a tester shows a deviation above the acceptable limit, it must be adjusted and recalibrated before it is used again.
AT2E torque testers
AT2E has built its own torque testers since 1991, at first at the request of a cosmetics manufacturer. The range covers manual, automated and on-line measurement:
TMV7: touch screen electronic torque tester, 0 to 99.00 kgf·cm (0 to 85.9 in·lbs), accuracy 0.2 % of full scale, release and break torque mode.
TMV8: electronic torque tester, 0 to 99.00 kgf·cm, accuracy 0.5 % of full scale.
TMV-ECO: fully mechanical spring torque tester, 0 to 25 in·lbs, no electricity needed.
ADATMV-ECO: semi-automated torque tester with one-button start; its software is designed to meet the requirements of FDA 21 CFR Part 11.
ADATMV5: automated monopost torque tester with 7 measuring cycles.
BT ETA TORQUE: dynamometric torque bottle for capping heads, 0 to 60.00 kgf·cm (0 to 52.1 in·lbs).
TORQUE CHECK: hand-held torque controller for torque testers, caps and capping heads.
BT ETA TORQUE ECO: torque reference bottle from 1 to 15 in·lbs, to check any torque tester.
Conclusion
Torque is one of the simplest measurements on a closed package and one of the most telling: it shows whether the capping machine works correctly and whether the consumer will be able to open the product. Measured with a stable, calibrated instrument, at a known time after capping, it gives results that can be compared from one shift, line or facility to another.
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AT2E is an ISO 9001 certified company (AFNOR Certification).
Request a quote: info@at2e.com
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