Today’s manufacturing operations are very concerned with loose or stripped caps which will clearly affect product freshness, product stability, shelf-life and possibly leakage.
Stability is of particular concern, since moisture sensitive or pressurised products require that the integrity of the container closure and seal be maintained.
Loose/stripped caps can lead to both package and product issues as the loss of nitrogen in non-carbonated drinks or carbon dioxide in sodas or addition of moisture to a dry product can cause contamination, discoloration, or significant changes in potency/taste. This can affect the quality of the product.
To maintain/improve process control, it is very important to monitor the closure integrity continuously. For this purpose, there are various torque testing devices available on the market. However it is rarely mentioned that closure integrity test results are influenced by a number of variables.
In the past, cap torque testing was as simple as grabbing a container from the production line and manually checking how tight the cap felt. Due to the subjective feeling of cap tightness, over time, the industry shifted toward using more objective measurement devices: spring-based benchtops and handheld torque testers.
It did not take long before quality assurance personnel realised the limitations of the spring-based devices. Additionally, entering the 1990s, there were so much more to consider: changes in materials, marketing themes, package aesthetics, transportation methods, packaging processes such as induction sealing and hot filling.
Product liability, versatility, compatibility and ergonomic issues made capping and torque testing much more than just placing a cap on a bottle and turning it by hand. As a result, automated torque testers emerged.
While most of these automated testers provide a great way to reduce operator variation, it is still crucial to understand all of the process variables that may influence the torque test results.
Closure Gripping Pressure
Bunches and Bits, Karina
Different cap finishes can be secured in a torque tester using different gripping methods. Especially with newer, lightweight closure designs, manual gripping can distort the cap and introduce an operator-dependent torque error between the container and closure threads. In order to overcome the variation introduced by manual gripping, custom moulded, 3D printed or machined ‘serrated’ chucks can be used for securing the caps during the torque test.
While these types of chucks are great for eliminating the torque error introduced by gripping pressure variation, there are also some disadvantages of using them. Due to the large variety of serration types and cap diameters, a quality department is challenged with the high cost of change parts and the management of the various serrated chucks. Other disadvantages include the difficulty of aligning the chuck with the clamped bottle/cap and the inability to work with smooth closures, pump dispenser caps and other unique closure designs.
Alternatively, mechanically actuated chucks are available on the market and with careful package analysis and pressure configuration they can address all of short– comings of the serrated chuck design.
For example, one air actuated chuck size works well for multiple serration types for the same cap diameter and by optimising the gripping pressure, the removal torque results are comparable to those measured with serrated chucks. While many packages are not sensitive to chuck pressure variation because of the harder material (phenolic) or more robust structural design of the cap (child resistant caps), others can be extremely sensitive to the gripping pressure (‘light weight’ caps and closures made of flexible materials).
In most cases the visible deformation can reduce the measurement sensitivity or results in higher release torque values. Decreased measurement sensitivity can cause torque testers to fail in detecting the drop in the torque after the thread break.
Bottle Gripping Pressure
It is not as common to see the torque readout influenced by the bottle clamping pressure. Nevertheless it can also ‘de–sensitise’ the automated release torque measurement and contribute to variations in the result.
The torque error introduced by the excessive clamping can be easily evaluated by rotating a loose cap on the bottle threads. A clamping pressure sensitive bottle will produce noticeably higher torque compared to the torque on a bottle that is clamped with optimised pressure.
When there is noticeable ‘drag’ between the bottle and cap threads, the operator should reduce the clamping pressure to the optimal level; the point at which the bottle is not slipping, but the cap rotates with minimal torque. The clamping pressure must be verified at the low and the high process limits to ensure the bottle clamping configuration will work for the normal measurement range.
Similar to the cap and bottle gripping sensitivity, the top load sensitivity of the release torque measurement largely depends on the specific package design (cap/ bottle/liner materials, dimensions).
The optimal top load should be evaluated and optimised for each product individually. Certain applications require minimised top load during the rotation of the cap, for example, when testing the thread break, seal break and bridge break torque of a closure with a tamper evident band. Minimised top load is also beneficial when measuring the snap torque of the tamper evident band during a cap tightening cycle or when measuring torque on a loose cap.
While some child–resistant (CR) closures only require an initial top load to engage the outer shell with the inner cap, it is best to optimise and maintain the vertical force during the measurement, and this is especially true for CR closures featuring a tamper evident band. One of the bigger challenges in automated torque testing is also related to top load sensitivity.
In many applications, there is a requirement to detect a loose child–resistant cap, however, when appropriate top load is applied on the CR closure, due to the top load introduced torque between the cap and bottle threads or between the opening of the bottle and the liner, the loose caps will produce torque values that meet or exceed the acceptable low process limit. While it is preferable to carefully specify the cap, liner and bottle materials and designs to avoid these kind of quality assurance problems during production, alternative process limits may be re-established and/or customised release torque validation methods can be developed to differentiate between top load sensitive loose and tight CR closures.
Torque variations are often introduced by variations in the packaging process.
As a result of the induction heating of the metal foil inside the cap, the bottle and cap may deform, expand then shrink. Because the cap and the bottle are typically made of different plastics, the timing and the rates of expansion/shrinking are different, as a result the torque between the threads can change considerably before and after the induction sealing process.
In order to ensure proper sealing and product integrity, it is important to apply enough torque on the cap so the foil is compressed firmly on to the opening of the bottle. One mistake often made is the overtorquing of the cap.
By overtorquing, it is possible to achieve good removal torque values after the induction sealing process, but high application torque increases the chance of the threads stripping during the tightening cycle and the excessive torque may break the cap or create wrinkles on the metal foil.
As a result, the wrinkled foil is not sealed properly and the product’s shelf-life is reduced significantly. A good approach to packaging induction-sealed products is to tighten the closures to a torque range that is optimal for seating the seal on the bottle so after the induction sealing, the cap can be secured on the bottle threads by utilising a shrink wrapper and/or re–torquer.
Another good example of torque variation introduced by a packaging process variable is presented by hot fill bottling and steam cap applications. During the hot–fill process, the 80–90 deg C product transfers heat directly to the bottle/cap. Fill temperature variation, the different thermal expansion contraction rates of different cap, liner and bottle materials, varying dwell times between cap tightening and release torque measurements can all result in different release torque values.
When the torque is applied right after hot–filling, the cap is soft and as the cap is cooling and shrinking the removal torque increases. Typically, products are sampled for fill volume/weight and cap removal torque values shortly after the hot–fill process. In order to ensure correct opening performance after the cool–down period, it is important to understand the effect of dwell time and thermal contraction on the removal torque value of the closure.
Certain products and materials are more sensitive to spills than others. During filling, it is possible that the product spills on the threads or the liner/seal. While the spilled product can act as a lubricant during the capping process, after storage/drying it can leave a residue with thread–locking properties.
The dwell is defined as the time interval between the cap tightening and the release torque measurements. 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 (hours to days). The rate of the release torque decay is greatest in the first couple of hours then it gradually decreases before reaching its stable level.
Transportation (vibration), thermal stress (varying storage temperatures) and production processes such as induction sealing, hot filling or the use of heat activated/ sterilised package designs can amplify or offset the torque decaying effect of the dwell time.
Variations are often introduced by inline and chuck capping equipment as well. Changes in magnetic/ electrical or pneumatic settings on chuck capper heads and the speed/pressure applied by the inline spindle/ belt type capping stations can influence the application and removal torque.
In a production environment the application torque is typically unknown but the capper chucks or spindle system is adjusted according to the release torque results. Generally, the higher the application torque, the higher the thread break torque. This is true up to the strip torque when the threads break/ deform irreversibly during the cap tightening cycle.
If the manufacturer of the cap/ bottle does not provide recommended torque specifications, it is best to start with an application torque setting that equals the cap diameter in mm divided by two (in lbfin). The release torque/applied torque quotient depends on the specific cap/bottle/liner design and the control of the previously listed variables. It is usually in the range of 0.6–0.9, higher for glass and lower for plastic bottles.
It is not uncommon to see values out of this range, but extremely high quotients typically indicate the presence of torque errors introduced in the torque measurement. As demand is continuously placed on all products to have extended shelf life, improved ergonomics, lower costs and improved appearance, application issues continue to emerge. In order to achieve maximum shelf life, the closure must create the best possible seal and the only way to ensure this is through continuous release torque monitoring at the production line.
A number of different variables can create a parasitic torque between the threads, changing the torque results or desensitising the automated release torque measurement of threaded closures. To overcome these issues it is essential to understand all the variables affecting the torque between threaded closures, optimise change parts and the configuration of the torque testing equipment accordingly.