ROPP stands for roll-on pilfer-proof. It is an aluminium closure with an unusual property: when it arrives at your filling line it has no thread at all. It is a plain, smooth aluminium shell with a liner inside and a knurled skirt outside. The thread is cut into it at the last possible moment, by your own capping head, against your own bottle.
That single fact explains most of what follows — why these caps seal well, why they cannot be reused, why the size on the box refers to the bottle and not the cap, and why two suppliers' caps can behave differently on the same machine.
How a ROPP cap is applied
The capping head does three jobs in one stroke. A pressure block pushes down on the cap top, compressing the liner against the rim of the bottle — this is the part that actually creates the seal. At the same time, thread rollers press into the side of the aluminium skirt and follow the contour of the glass thread underneath, forming a matching thread in the metal. A second pair of rollers tucks the bottom edge of the skirt under the locking ring on the bottle neck, which anchors the pilfer band.
When the head lifts, you have a threaded, sealed, tamper-evident closure that was shaped around that specific bottle. A pre-threaded cap can only ever be an approximation of the neck it lands on. A rolled-on cap is a copy of it.
What the pilfer band does
The lower section of the skirt is joined to the rest of the cap by a ring of thin bridges. When the consumer twists the cap for the first time, the cap body rises on its new thread while the band stays locked under the bottle's locking ring. The bridges break. The band drops loose and stays on the neck, and there is no way to put it back.
That is the entire tamper-evidence mechanism, and its value is that it needs no interpretation. A pharmacist, an excise officer or a customer does not have to check a seal number or hold anything up to the light. Either the band is intact or it is not.
Why aluminium
The alloy used for these closures is normally 8011, at a sheet thickness of medical-grade. It is soft enough to be formed cleanly by the thread rollers without tearing, and stiff enough to hold the formed thread afterwards. It does not rust, it takes an anodised or coated finish well, and it carries print.
Thickness is a real variable rather than a detail. A slightly heavier gauge helps where the glass neck finish is uneven or the capping head runs hard; a lighter one brings the cost per thousand down when the neck finish is consistent. It is worth telling your supplier which problem you have.
The liner is what seals
It is easy to focus on the shell and forget the wad — the thin disc pressed inside the cap top. The aluminium holds the wad in position; the wad does the sealing. Most closures use expanded polyethylene (EPE), which compresses evenly and suits aqueous products, syrups, oils and most chemicals. Latex is a flowed-in alternative that conforms more closely to an uneven glass sealing surface.
If a bottle leaks, the liner is the first thing to examine, not the cap. We compare EPE and latex in more detail here.
Where ROPP caps are used
- Pharmaceuticals — syrups, tonics, homeopathic and ayurvedic preparations, reagent bottles and vials, where tamper evidence is a regulatory expectation.
- Distillery and spirits — the standard closure for liquor bottles, where the first twist is both the excise seal and the consumer's opening.
- Food and edible oil — oils, sauces and additives that have to survive a long, warm distribution chain without seeping.
- Agro-chemicals and speciality chemicals — pesticides, laboratory reagents and cleaning products, where the liner has to resist the contents.
- Flavours and fragrances — concentrates where a small loss of volatiles or a trace of contamination ruins the batch.
Sizing, briefly
ROPP caps are specified by the bottle's neck diameter in millimetres — not the bottle's volume and not the cap's own outer diameter. The common Indian sizes are 20, 22, 25 and 28 mm, and a 100 ml and a 200 ml syrup bottle will often take the same 22 mm cap. Our guide to the four sizes covers what fits where.
A note on names
You will see the same closure sold as a ROPP cap, a PP cap, a pilfer-proof cap, an aluminium screw cap, a roll-on closure or simply a metal cap. In India, PP cap almost always means pilfer-proof cap rather than polypropylene — a genuine source of confusion when comparing quotations. We untangle the terminology here.