SankalpThe skirt is the cylindrical wall of a ROPP cap that runs down the bottle neck below the top panel, and its length is the second number in a size like 28/15 mm: 28 is the shell diameter, 15 the skirt length in millimetres. Skirt length is not a preference, it follows the depth of the bottle’s neck finish. IS 7511 (Part 4), the Indian Standard for roll-on pilferproof neck finishes, names four depth classes: standard, shallow, deep and extra deep.
Key takeaways
- The finish depth class sets the skirt length. A cap cannot be shorter than the thread it covers, nor so long the band misses the locking ring.
- A longer skirt gives more thread engagement and more tamper band material, at the cost of more aluminium and a different roller position.
- Indian spirits sit at the long end, under the IS 1662 clause allowing deep and extra deep finishes on IMFL bottles.
- R Vision’s 28/15 mm closures sit at the short end, where water and beverages live.
Where the skirt is on the cap
The flat top of a ROPP cap is the panel, with the liner inside it. The skirt is the wall dropping from the panel’s edge, smooth and thread-free on an uncapped shell, its lower portion becoming the pilfer band once the rollers tuck it under the locking ring. Parts of a ROPP cap identifies each feature. Two things follow: the skirt is the only part that becomes a thread, so its length limits the turns the closure holds, and it is the only source of tamper band material, so a short skirt splits a fixed budget between the two.
The finish decides, not the cap
A ROPP closure has no thread until the rollers make one against the bottle, so the depth of the glass or PET finish sets the working length of the skirt.
| IS 7511 (Part 4) finish type | Nominal sizes it covers | Typical position |
|---|---|---|
| Shallow finish | 38, 46 | Wide-neck bottles, broad but not tall |
| Standard finish | 22, 25, 28, 31.5, 53, 70 | The workhorse depth across most segments |
| Deep finish | 31.5 | Taller neck, more thread, more band |
| Extra deep finish | 30 | The tallest class, markedly longer skirt |
Read that with the diameters in mind. Extra deep exists only at 30 mm and deep only at 31.5 mm, while 28 mm is standard only. That is why a 28 mm water cap and a 30 mm spirits cap differ so much in height: two finish classes, not two choices for one bottle.
IS 1662, the Indian Standard for glass liquor bottles, applies the same logic commercially. It fixes 28,0 mm as the nominal finish diameter for 750 ml, 500 ml and 375 ml bottles and 25,0 mm for 250 ml and 180 ml, then allows a 750 ml Indian made foreign liquor bottle to take 31,5 mm standard or deep, or 30,0 mm extra deep. The tall closures on Indian whisky are that clause on shelf; liquor bottle caps in India covers the segment.
Long skirt against short skirt
| Aspect | Shorter skirt | Longer skirt |
|---|---|---|
| Thread engagement | Fewer turns, adequate on a standard finish | More turns, better resistance to backing off |
| Tamper band material | Enough on a matched finish, less margin if rollers drift | More for a full, clean band |
| Aluminium per cap | Less | More, scaling with skirt length |
| Shelf presence | Discreet, reads as functional | A visible band of metal, the spirits look |
| Decoration area | Limited side area for litho printing | Larger side panel for print or embossing |
| Capping setup | Standard height and roller position | Set for the taller shell |
| Where you see it | Water, soft drinks, juices | Spirits, premium and export bottles |
The cost is not only metal. A longer skirt spends more time under the rollers, and research at the University of Sheffield showed that rolling thread and pilfer together can cause thread pull-out, where cap material is drawn out of the first thread form to make the second. The taller the skirt, the more thread there is for that to happen in, which makes a long skirt more sensitive to roller sequence. The ROPP capping machine guide covers the settings.
Which formats use which
| Segment in India | Typical skirt | Why |
|---|---|---|
| Packaged drinking water | Short | Standard 28 mm finish, high line speed |
| Carbonated soft drinks and juices | Short | Same 28 mm family, liner does the pressure work |
| Pharma syrups and small bottles | Short | Small diameters at standard depth |
| Edible oils and wide-neck food | Short for the diameter | Shallow finish at 38 mm and 46 mm |
| Wine and still beverages in glass | Medium | Taller decorative necks |
| Spirits and Indian made foreign liquor | Long to extra long | Deep finishes, full band, printed panel |
Spirits is the clearest case. The Sheffield study describes its subject finish, an industry standard spirit bottle thread profile, as one using an extra deep aluminium ROPP closure. The long skirt is a specification.
The 15 in 28/15 mm
R Vision manufactures one size: 28/15 mm. The 28 is the shell diameter, matched to a 28 mm neck finish. The 15 is the skirt length, which puts the closure in the short-skirt class where Indian packaged drinking water and beverages sit. A 28 mm finish is published only at standard depth, so a 15 mm skirt is what the geometry asks for.
Aqua Cap and Bev Cap are both 28/15 mm, with a TPE liner and an 8 bar rating, adhering to USFDA and EU standards. Bev Cap’s tamper-evident ring stays attached after opening so the bottle reseals, which shows how much function a short skirt carries when the band geometry is right. Other depths named here are industry reference: see the ROPP cap size chart for diameters in circulation and 28mm ROPP caps for the size R Vision makes.
Frequently asked questions
What does the 15 in 28/15 mm mean?
It is the skirt length in millimetres. In a ROPP size the first number is the shell diameter, matched to the bottle’s neck finish, and the second is the cap height, which is the skirt running down the neck. So 28/15 mm is a 28 mm shell with a 15 mm skirt, the short-skirt format used for packaged drinking water and beverages.
Is a longer skirt better?
Only if the bottle calls for one. A longer skirt gives more thread engagement, more tamper band material and a larger printed side panel, but it uses more aluminium and needs a different roller position and working height. A long skirt on a shallow finish will not seat correctly, so the finish depth decides.
Why do liquor bottle caps look so much taller?
Because the finish under them is deeper. The Indian Standard for roll-on pilferproof neck finishes publishes deep and extra deep classes alongside the standard one, and the Indian Standard for glass liquor bottles allows a 750 ml Indian made foreign liquor bottle to take a 31,5 mm deep finish or a 30,0 mm extra deep finish.
Does R Vision make long skirt ROPP caps?
No. R Vision manufactures 28/15 mm aluminium ROPP closures, a short-skirt format matched to the standard 28 mm neck finish used across packaged drinking water and beverages. Longer formats appear here as industry reference.
Sources
- BIS, IS 7511 (Part 4): 1986, Roll-on Threads Pilferproof, for the four finish types and their sizes.
- BIS, IS 1662: 1974, Glass Liquor Bottles, for finish diameters by capacity and the IMFL clause.
- J. Langley, A. Yoxall and J. Yates, Understanding the roll-on-pilfer-proof process, 21st IAPRI Symposium, 2003, for the extra deep spirits closure and thread pull-out.
- CETIE, Finish data sheets (GME), for the European finish geometry.
R Vision Pvt. Ltd manufactures 28/15 mm aluminium ROPP closures from Sinnar, Nashik. Send the drawing or a sample bottle through contact us.