By Chris Grant, Sales & Marketing Co-ordinator, Mecmesin Limited, Force & Torque Test Solutions, Mecmesin Limited
SubscribeWhenever an end-user encounters the packaging around a product, the objective is usually in little doubt - opening the package! With the possible exception of the traditional brown paper bag, this will almost certainly involve pushing, pulling or twisting part of the package.
Force and torque measurement therefore has a critical role to play both in optimising the design of a package and in evaluating the quality of a particular packaging sample.
The article 'Quality Control Testing of Packaging Force' deals with the application of tensile or compressive load to a package. This article covers the contribution torque measurement makes to quality control testing in the packaging industry.
Containers such as bottles or jars are generally sealed by screw or lug-style caps or lids. At various stages during the life cycle of such closures their performance can be characterised by a number of critical torques.
Manually-operated instruments, such as the Mecmesin "Orbis" a digital closure torque tester with a capacity of 6N.m (53lbf.in), have proved extremely popular for capturing the torques associated with the first opening, resealing, and re-opening of a closure.
Orbis has been designed with busy production environments in mind. The universal gripping fixture is able to accommodate containers with a wide range of sizes, shapes and symmetries. A custom-designed precision-moulded polyurethane housing dramatically increases the ability of Orbis to resist those occasional spillages which almost inevitably happen.
Exterior surfaces are sloped and channelled to assist and control run-off, while recesses are radiused for easy cleaning. Furthermore the internal loadcell is located in an elevated position to minimise damage should fluid ingress occur. Other exposed components are also suitable for withstanding demanding environments, grip runners and pillars are stainless steel, the recessed baseplate is anodised aluminium, and grip faces are silicone rubber.
Orbis with its integral handle and NiMH power pack is a portable instrument, ideal for transportability between production line and quality laboratory or visiting suppliers. Product specifiers could, for example, test samples on-site then archive data to a laptop via the Orbis RS232 port.
Tamper-evident Closures
While the ability of the basic Orbis to measure clockwise torque (the 'application torque') and the anti-clockwise torque (the 'release torque') and capture each peak value is sufficient to meet the needs of many packaging professionals, the torque characteristics of tamper-evident closures are more complex.
Therefore Mecmesin also supplies an additional manually-operated instrument especially designed for testing tamper-evident closures the "Orbis-TE". Consider, for example, the roll-on pilfer-proof (ROPP) aluminium cap used for sealing bottles of alcoholic drink including spirits, fortified wines and, increasingly, table wine. Alternatively consider the aluminium lid and tamper-evident drop-ring sealing a jar of baby food, or the plastic closure sealing a PET soft drink bottle.
When increasing release torque is applied to a closure, a peak torque value is associated with the point at which the closure begins to move (the 'slip torque'). However, in the case of TE-closures, torque must continue to be applied both to overcome friction and to apply tensile load to the 'bridges' connecting the closure to the tamper-evident ring. Eventually one bridge will fail, and a second (lower) torque peak will occur (the 'bridge torque'). The Orbis-TE will capture both peaks. A single button push will then allow both values to be recorded on a PC or printer.
Child-resistant Closures
As the packaging industry develops more sophisticated closures, Mecmesin has responded by introducing new testing systems. A child-resistant package can be described as one sealed in such a manner that it can be opened only by a person with a combination of dexterity, co-ordination, strength and experience well within the capabilities of a 'legitimate user', but beyond the capacity of a small child. One example of such a package is a container with a 'Type IA or IIA' closure (see ASTM D3472-97), i.e. a screw cap which requires a simultaneous pushing and twisting action before opening is possible.
Closures of this type are in widespread use for protecting children from: paints, solvents, thinners, household chemicals, adhesives, cleaning fluids, garden chemicals and particularly pharmaceutical products.
The Mecmesin child-resistant closure tester is able simultaneously to measure the release torque and the downward force (the 'top-load') applied to a closure. This makes it a particularly suitable instrument for identifying designs (or batches) of closures which either present an insufficient challenge to a child, or too great a physical challenge to an elderly or infirm adult.
An operator may apply to a closure both a top-load of up to 500N (110lbf) and a torque of up to 10N.m (90lbf.in). The CRC tester can also measure 'reverse-ratchet torque', i.e. the torque peak developed when rotating a closure in a anti-clockwise direction without any top-load being applied.
Motorised Torque Testing Systems
Although manually-operated closure torque testers have proved to be extremely popular because of their portability and low cost, all such systems do have an inherent drawback. The rate of application of torque during each test will depend upon the technique of individual operators, and a systematic error between different shifts may be detectable.
This is kept to a minimum with Mecmesin digital instruments because of the high speed internal sampling rates used (1000Hz for Orbis, and 5000Hz for CRC tester). However for the most demanding torque testing applications motorised systems deliver better reproducibility.
The Mecmesin 'Vortex', for example, is a motorised instrument with a capacity of 10N.m and a speed range of 0.5 to 15revs/min. Upper and lower universal mounting tables capable of holding closures and containers with diameters of 8 - 55mm and 10 - 160mm respectively are available, making the system suitable for a wide range of food, household chemical and beverage containers; custom-designed fixtures can also be supplied.
The upper display unit which gives a digital reading of application/release torque, also features a peak hold facility and programmable pass/fail alarms. The base unit displays digitally either speed or total rotation during the test.
To enable child-resistant containers of the 'push-and-twist' type to be evaluated at constant speed, the Vortex is able to apply release or application torque and fixed static top-load simultaneously.
Test curves of torque as a function of angular displacement, in either degrees or revolutions, can be generated by means of Mecmesin DataPlot software operating in conjunction with a Vortex.
The flagship of the Mecmesin range of torque testing systems is the 'Vortex-i'. This fully computer-controlled system is driven by 'Emperor', Mecmesin's new generation product testing software.
This powerful, flexible, user-friendly software enables sophisticated test procedures and analyses to be developed. An extensive range of features of particular interest to quality control laboratories includes: automatic pass/fail analysis, batch reports/statistics, graphical display of data with tolerance band, global 'out of tolerance' detection, and data export.
With a Vortex-i a quality manager could, for example, develop a program to meet the testing requirements of a particular standard, customise this 'library program' to match the tolerances of each product to be tested to the standard, protect the program from being modified by other personnel, save each new library program (under its part number), then make this suite of programs available on the shopfloor.
However whenever closure torque testing is mentioned, it is usually a manually-operated, portable digital system generically similar to the Orbis that will spring to the mind of most packaging quality engineers. Such instruments continue to be the mainstay of the industry, indeed the Orbis range has been expanded. In addition to the 6N.m capacity system, new instruments with capacities of 10N.m and 1500mN.m have now been introduced.
The latter instrument is particularly suitable for measuring the relatively delicate torques which characterise packages in the pharmaceutical and cosmetics industries.