ROPP Cap Torque Testing for Aluminum Roll-On Closures
By Lim Xu, Export Manager
ROPP cap torque is not one number. When a roll-on pilfer-proof closure is opened, the reading shows a slip torque as the cap first moves and a bridge torque as the pilfer bridges break. A good test records each peak on its own, plus the over torque needed to turn the cap the wrong way.
How is a ROPP cap applied to the bottle?
A ROPP closure starts as a plain aluminum shell with a liner inside. On the capping line, a capping head comes down over the bottle and closure and presses the liner onto the glass to form the seal.
As the head rotates, rollers move inward and form the thread in the aluminum, using the glass thread as a mandrel. Pilfer rollers tuck the lower band under the glass finish at the same time. The study described below gives 120 N as the load the capping head is calibrated to apply to the closure.
This is why ROPP testing differs from testing a screw cap. The cap is not tightened to an application torque. Its grip on the bottle comes from top load, liner compression and how well the rollers formed the thread.
What do the torque peaks on a ROPP cap mean?
A research group at the University of Sheffield, working on spirit bottles with aluminum ROPP closures, defined three torques. The first two come up in normal opening, and the third needs a separate turn in the wrong direction.
- Slip torque is the torque needed for the first movement of the cap when it is opened.
- Bridge torque is the torque needed to break the pilfer bridges between the cap and the pilfer band.
- Over torque is the torque needed to turn the cap the wrong way, clockwise, and strip the thread.
In that work, bridge torque stayed the higher of the two opening torques. The goal was to keep slip and bridge torque low for easy opening, while keeping over torque high and the seal intact.
The same group describes a defect called a spinner. It is a closure that turns without breaking the pilfer band, or one turned the wrong way that strips the thread while the band stays whole. A reading that never shows a bridge peak is a reason to check for spinners.
Have ROPP bottles to check? Tell us your test requirements. We source similar samples, run the test and send you the video. No need to ship samples.
What changes ROPP opening torque?
The Sheffield study tested at least 50 bottles for each change to the capping settings. Forty were opened to record the torques, and ten were cut and sectioned to examine the closure. Its main findings give a useful list of things to check.
More top load or roller pressure compressed the liner further and raised slip torque, though not above bridge torque. Roller pressure improved the seal but showed no clear link to opening torque. Closures from different sources gave significantly different opening torques, and the authors suspected the lacquer.
Rolling the thread first and tucking the pilfer band afterward appeared to lower opening torque and stopped thread pull-out. Product left on the sealing surface may have raised slip and over torque, but that sample was too small to be sure.
The authors also warned that one small change can have knock-on effects elsewhere in the closure system. When opening torque moves, review every capping setting and the closure lot, not just one variable.
How to set up a ROPP cap torque test
CETIE publishes voluntary data sheets and bottling guides for closures. Its data sheet DT38.00 is a removal torque test method for ROPP closures applied to glass finishes. Its Guide No. 11 lists ROPP capping defects, including removal torque that is too high or too low.
A practical setup records more than the highest reading. These points follow from the torques defined above.
- Open each cap in the normal direction and record slip torque and bridge torque as two values.
- Use separate bottles for over torque, because stripping the thread destroys the closure.
- Log the capping head load, the roller settings and the closure lot for every sample set.
- Test enough bottles at each setting to see the spread, not just one or two.
Once the readings are in, look at their spread as well as their average. Our guide on how to interpret torque test data explains mean, standard deviation and coefficient of variation.
Our NLY-20A digital cap torque tester measures both removal (opening) torque and application (locking) torque. Tell us your closure size and the torques you need to record before you choose a range.
Frequently asked questions
Is ROPP cap torque tested with ASTM D2063?
Not directly. ASTM D2063 covers torque retention of continuous thread closures where the application torque is known. A ROPP cap is not screwed on to a set torque, because its thread is rolled onto the glass. CETIE DT38.00 is written for ROPP removal torque.
Can ROPP closures be used on hot-filled products?
Usually not. CETIE Guide No. 2, which covers metallic tamper-evident closures for non-carbonated drinks and products, says this capping is not recommended for hot-filled products or for products heat treated after sealing.
Why does a ROPP cap feel tight even when the seal is fine?
The second peak is often the pilfer band, not the seal. In the Sheffield work, bridge torque stayed above slip torque, so the bridges can set how hard the cap feels to open.
Get a test video for your closures
Tell us your test requirements, such as the material, the size and the test you need. We source similar samples here, run the test on the instrument you are looking at and send you the video. No need to ship samples.
Questions about a model or a test? Email Amy Zhao, Export Manager, at [email protected].
