Before specifying and purchasing a torque tester it is best to fully understand the process and variables involved in torque testing. For example all of the following variables or inputs can impact your results;
1. Initial applied torque
2. Sensitivity of automatic thread break torque measurement
3. Cap squeezing pressure
4. Container squeezing pressure
6. Speed of torque ramp, update rate of torque digitization
7. Dwell time
8. Product variations (dimension, liner variations, etc)
1. Applied Torque
Generally speaking, the higher the applied torque is the higher the thread break torque will be. This is true up to the strip torque where the threads break/deform irreversibly in a cap tightening cycle. Without the cap/container manufacturer’s guidance, it is recommended to start with an application torque that is equal to the cap’s outer diameter in mm divided by two (in inch-pounds (in-lbs, lbf-in)). The release torque / applied torque quotient depends on the specific cap design. It is usually in the 0.6-0.9 range and higher for glass and lower for plastic bottles. It is not uncommon to see values out of this range and dwell time usually decreases the quotient over time.
2. Sensitivity of automatic thread break measurement
When measuring thread break torque two methods may be used to validate the result.
a) Fallback based peak torque validation: this is the fastest and most cost effective way to measure thread break torque on a cap. In CR cap applications, special attention must made when fine tuning the fallback value in order to avoid validating the shell engagement as the thread break torque.
b) Rotation based peak torque validation: to overcome the problem introduced by the torque drop during the CR engagement an additional encoder can be used to validate the thread break torque. The rotation limit must be set according to the worst case scenario for shell engagement.
In the following example or fallback based validation, the fallback is set at <1.5lbfin, the tester stops in <25 degrees and displays the resulting engagement torque. To avoid this false readout, either the fallback must be increased above 1.5lbfin (the recommended fallback for CR caps is 2.5lbfin), or rotation based validation must be used and the rotation limit set at ~40 degrees.
See the top-load vs. rotation and torque vs. rotation trends below to understand the variations during a CR cap removal cycle. The vertical axis represents both toque (lbfin) and top-load (lbf), while the horizontal axis is the rotation in degrees.
3-5. Cap and container squeezing pressure, top-load
The variation in cap/container squeezing pressure and the top-load on the cap may also affect the torque reading. The pressure variation is caused by either force or contact area variation. The larger the contact area and/or the higher the force compressing the cap and the container threads, the higher the torque readout will be. Thus in some applications it is important to monitor the container and/or cap squeezing pressures and the top-load force. Both the ST-120 and ST-S3 utilize a load cell to control or evaluate the effect of top-load on your packaging.
6. Speed of torque ramp (the acceleration of the cap) vs. the update rate of the torque digitizer
There are two phenomenons a packaging engineer must be aware of when configuring the torque ramp:
a. When the torque ramp is fast compared to the A/D conversion time of the digitizer, the removal torque readout on the digital machine can be considerably lower than the real peak. This error is caused by the slow analog to digital conversion speed and it is not to be confused with the quantization error. To understand the error originating from inappropriate digitization, look at the graphs below and/or find more information on the internet about the Nyquist-Shannon sampling theorem and resolution /quantization noise.
b. When the torque ramp up time is slower (resulting in lower cap acceleration), the removal torque tends to be lower because torque is proportional to the acceleration of the cap. It is also possible that during a slower torque ramp, the gradually increasing torque slowly deforms the threads and lowers the peak torque required to finally break the threads. If the torque ramp is faster (the cap’s acceleration is higher) the thread break torque is usually higher.
Note: Even if a manual torque tester had a fast digitizer circuit, the lack of torque ramp control may still cause considerable variation from one operator to another depending on how fast he/she manually applied torque on a specific package.
Analog torque signal (red) sampled with a 12 bit, 100ms AD converter. The peak readout with this digitizer is ~4.9 lbfin at 200ms.
The same analog signal (red) sampled (green) with a 12 bit, 10ms AD converter. The peak readout with this system is ~5.5 lbfin at 160ms.
7. Dwell time
In various experiments it has been established that release torque levels are highest immediately after application and then gradually decrease to a stable level over a period of time (days/weeks). The rate of the release torque decay is greatest in the first couple of hours/days and then reduces at a decreasing rate before reaching its stable level. Production processes such as hot filling or using heat activated glue cap systems can produce a big difference in the release torque readout when compared with results measured in a laboratory environment.
8. Product variations
Minor changes in mold, material and liner can also be major contributing factors in torque varation. See an example of different liner alignments and how it relates to the contact surface area and the release torque.