Index marks for PET bottles

Par N. Harris
Le 17/01/2018

Plastic screw caps and finishes at Cetie


Cetie has just published a new recommendation sheet, FS 18, for index marks on PET bottle screw caps and finishes.
Developed by the "PET Finishes" working group, these recommendations aim to encourage the adoption of these index marks by the industry in a consistent manner, taking into account all the collectively identified constraints.
These index marks facilitate measurement of the application angle, defined as the angular position of the cap relative to the finish after capping, and enable this measurement to be made in a non-destructive manner.
As index marks visually materialise the take-up of the cap thread on the finish thread, the application angle gives an indication of the correct positioning of the sealing feature of the capped bottle in accordance with the specifications of the cap supplier.

 To perform on samples

The group considers in particular that the application angle provides a rapid and direct means of assessing the correct application of the caps and is more appropriate than measuring the removal torque, the method often used until now.
The reason for this is that even if the opening force is in principle linked to the compression force of the sealing feature exerted by the thread take-up, the relationship between these two forces more specifically involves the coefficients of friction between the components, which can vary according to the composition of the resins used or the presence of product residues, for example. Furthermore, the removal torque can be affected by the relaxation of the plastic material, depending on the conditions. As the closure system specifications can of course include the upper and lower limits of the removal torque, this would need to be measured when closed bottles are quality controlled. These are, however, in principle, destructive tests to be performed on samples, which is also the case with the application angle if there are no index marks. Moreover, the index marks enable automatic reading of the application angle by a vision system and thus online 100% inspection after capping. In this case, the measurement provides in particular continuous feedback on the consistency of the capping operation for each capping head and enables early detection and correction of drifts in settings. Such inspection systems are still under development and Cetie document F18 has been published as an "experimental" data sheet, which implies that the working group will be monitoring its implementation in the field and review it within a year in order to take any developments in industrial practice into account. The possible use of regulation loops acting directly on the settings of the capping heads may also be considered in the future. As a general rule, aesthetic considerations require these index marks to be relatively discrete. They are usually materialised on the cap by a modification in the knurls - a shorter knurl (Figure 1) or a missing knurl, or a solid area forming a bridge between two or three knurls (Figure 2).
The document gives dimensions and other recommendations to ensure these marks may be recognised by optical detection systems. The marks are preferably positioned to correspond to each thread start, therefore with as many marks as there are threads. In the case of PET bottle finishes, the marks are situated on the only part of the bottle finish that is visible once the cap is applied, that is on the outer edge of the support ledge. They can be discrete marks that are either protruding (Figure 3) or recessed (Figure 4), moulded with the preform.


Geometrical constraints

This positioning introduces both geometrical constraints associated with the functions of this support ledge when blowing the preforms (sealing with respect to the blowing head) and possible interaction with handling devices during transfers. In practice, protruding marks are recommended in order to minimise the moulding costs and allow positioning on the mould parting line. They are positioned on the upper edge of the support ledge, outside the zone in contact with the blowing head.

The mould-release constraint however implies that for a three-thread finish, there can only be one mark, in which case the three marks on the cap are all the more useful. Certain precautions are necessary to enable the vision systems to distinguish the index marks and avoid spurious measurements. Product or water droplets on the edge of the support ledge can cause such false readings, which, according to the experts, are observed in particular with recessed marks. This is a further reason for favouring protruding marks and adopting as square a shape as possible. On the caps, ink-jet printing in the proximity of the index marks is to be avoided. For further details, we suggest that you consult sheet FS 18 which, like all the Cetie documents, you can download from after opening a user account simply by completing the proposed form.
Please do not hesitate to send us your comments by e-mail at

Nicholas Harris - for french Magazine Liquides & Conditionnement N°392
PDF (FR) :
- Liquides & Conditionnement N°392 (FR)
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