Tethered closures testing attachment force

Case study

Custom fixture for bottle preform with the tethered closure being pulled and a selection of test specimens

Typical tethered closure design components

Solution

Motorised tensile tester with a digital force gauge
Grips adapted to hold plastic closure and bottle preform

Benefits

Ease-of-use to repeatably batch test plastic closures used for single-use beverage containers
Comply with the testing methods described in EN 17665:2022

Requirement

As consumers, manufacturers and legislators demand greener packaging and the growing use of recyclable materials is commonplace, attention has turned to the humble plastic closure. Regulatory agencies are setting important sustainability targets to reduce the environmental impact of certain plastic products particularly marine litter. The European Union directive 2019/904 has introduced the requirement that plastic caps and lids, used for single-use plastic beverage containers under 3 litres, shall remain attached. Legislation to ensure compliance is being enforced from 2024.

Closure manufacturers and their customers have worked closely together to create new innovative designs which meet this new requirement. Consumers looking for innovative products and options with improved sustainability are likely to consider these new designs. A typical example is the ‘integrated hinge' or 'moulded-in bridge' closure. As the cap is unscrewed the flap makes contact with the hook and the arms are stretched with enough tension to lock the shell in place. The maximum opening angle is 180 degrees ensuring the shell is out of the way and not likely to hit the cheek of the consumer as they drink from the bottle.

To characterise exactly what constitutes ‘remaining attached’ and how closures and lids should be tested, the EN standard 17665:2022 has been introduced.

At the heart of EN 17665:2022 is the need to test for a) the strength and b) the reliability of the attachment with minimum acceptance criteria being defined for both.

To test the attachment strength of the caps and lids, a pre-defined tensile force is applied. This varies from either 25N (bottles with integral neck finishes) or 12N (cartons and containers where the neck finish is not integral) .

As state legislators in the USA, plus those at the national level in Canada and Australia, are poised to do the same as the EU, it is widely expected that similar test standards will also be implanted shortly in those regions.

Solution

Mecmesin developed a solution to test the attachment strength of closures, based around the MultiTest 2.5-dV motorised test stand and VFG Touchscreen Digital Force Gauge.

EN 17665 calls for 2 subtests within method 5.1 Resistance to a tensile force. Each subtest requires a batch of 10 samples to be tested. Subtest 1 measures the tensile strength in the direction of the neck finish (180°) and subtest 2 measures perpendicularly to the neck finish (90°). Each test is conducted at 200mm/min until the pre-defined tensile force is reached.

By using the VFG force gauge mounted to the MultiTest-dV motorised stand it is possible to apply load at the designated speed and stop at the exact tensile limit defined in the standard. Tests are normally conducted using closures fitted to plastic preforms, which are gripped in a universal V-jaw holder for the 90° test and an adjustable sleeve for the 180° test. The closure itself is held securely in an upper vice grip when conducting tests in both orientations.

The minimum separation force in newtons (N) is recorded on the display of the force gauge. The VFG Touchscreen Force Gauge has the added benefit of grouping measurements by batch and performing a simple statistical calculation of results.

This testing solution is flexible enough to be readily applied or adapted to other tethering solutions, such as slit bands and other locking and snapping designs.