SankalpThe aluminium in a 28/15 mm ROPP cap weighs about 1.0 to 1.2 grams, estimated from the area of the shell at the 0.20 to 0.23 mm gauge commonly quoted for closure stock. That is roughly 830 to 960 shells to a kilogram of metal. The finished closure weighs more, because the liner, the coatings and any plastic ring sit on top of the metal. For a costing, weigh 100 caps from the actual lot and work from that figure.
What gauge means on an aluminium closure
Gauge on a ROPP cap is the thickness of the aluminium sheet the shell is drawn from, quoted in millimetres rather than as a gauge number. Closure stock is most often quoted at around 0.20 to 0.23 mm; one European data sheet for a 30 x 60 mm wine closure gives 0.23 mm plus or minus 0.01 mm.
The same property is sometimes asked for as gsm, grams per square metre. Aluminium weighs 2.70 grams per cubic centimetre, so every 0.01 mm of thickness adds 27 grams to a square metre of bare metal.
| Sheet thickness | Bare aluminium, grams per square metre | Where it sits |
|---|---|---|
| 0.20 mm | 540 | Lighter end of common closure stock |
| 0.21 mm | 567 | Common closure stock |
| 0.22 mm | 594 | Common closure stock |
| 0.23 mm | 621 | Heavier end, published for long wine skirts |
| 0.25 mm | 675 | Heavier than most ROPP shells |
Lacquer and print add to that by an amount that depends on the coating system, so ask whether a quoted gsm is bare or coated.
ROPP cap weight by size and gauge
Shell weight follows the amount of metal: the area of the top panel plus the area of the skirt, times the thickness, times the density of aluminium. With diameter D, skirt height H and thickness t all in millimetres:
Metal weight in grams = (3.1416 x D x D / 4 + 3.1416 x D x H) x t x 0.0027
The table runs it across the sizes in circulation, as an estimate of the metal only. R Vision manufactures one size, 28/15 mm; every other row is industry reference and not an R Vision product.
| Size (diameter x skirt) | Typical use | Metal at 0.20 mm | Metal at 0.23 mm | Shells per kg of aluminium |
|---|---|---|---|---|
| 22 x 18 mm | Miniatures, syrups | 0.9 g | 1.0 g | 990 to 1,140 |
| 28 x 15 mm (R Vision’s size) | Packaged drinking water, beverages | 1.0 g | 1.2 g | 830 to 960 |
| 28 x 18 mm | Water, soft drinks, juices | 1.2 g | 1.4 g | 730 to 840 |
| 28 x 44 mm | Deep finishes, liqueurs | 2.4 g | 2.8 g | 360 to 410 |
| 30 x 60 mm | Still wine, spirits | 3.4 g | 4.0 g | 250 to 290 |
| 31.5 x 24 mm | Spirits, standard finish | 1.7 g | 2.0 g | 510 to 590 |
| 31.5 x 44 mm | Spirits | 2.8 g | 3.2 g | 310 to 360 |
| 31.5 x 60 mm | Wine and spirits, long skirt | 3.6 g | 4.2 g | 240 to 280 |
| 38 x 18 mm | Wide-neck food bottles | 1.8 g | 2.0 g | 490 to 560 |
The skirt dominates. A 28 mm cap with a 44 mm skirt carries more than twice the metal of the same diameter at 15 mm, which is the cost side of the choice explained in long skirt vs short skirt ROPP caps. The diameters and heights themselves are listed in the ROPP cap size chart.
What the estimate leaves out
A finished closure adds the liner, the coatings, any print and on some designs a plastic ring; knurling changes the metal weight very little. The published 30 x 60 mm wine closure makes a useful check: the formula puts its shell at about 3.9 g, and its data sheet gives 4.6 kg per thousand caps with a 2.0 mm liner. The gap of roughly 0.7 g is the liner and coatings on that design, and it is why a lined cap always weighs more than the metal estimate for its size.
Pieces per kilogram, and turning a kilo price into a price per thousand
Pieces per kilogram is 1,000 divided by the weight of one finished cap in grams. The price per thousand caps is the price per kilogram multiplied by the weight of a thousand caps in kilograms, and a cap weighing X grams makes exactly X kilograms per thousand.
| If one finished cap weighs | Caps per kilogram | Kilograms per 1,000 caps | Price per 1,000 caps |
|---|---|---|---|
| 1.2 g | 833 | 1.2 | 1.2 times the per-kg rate |
| 1.5 g | 667 | 1.5 | 1.5 times the per-kg rate |
| 2.0 g | 500 | 2.0 | 2.0 times the per-kg rate |
| 3.0 g | 333 | 3.0 | 3.0 times the per-kg rate |
| 4.6 g | 217 | 4.6 | 4.6 times the per-kg rate |
To get the weight, take 100 caps from a sealed bag, weigh them on a scale that reads to 0.1 g, and divide by 100. Repeat on two more bags from different cartons, because liner weight varies a little from run to run. A per-kilogram quote and a per-thousand quote only compare once the piece weight is written into the specification, which is also where what drives ROPP cap cost starts: metal is the largest line in the quote.
The same sum runs in reverse for volume: R Vision’s lowest minimum, 50,000 plain silver caps (200,000 for plain colours, 300,000 for litho printing), is 50 kg of closures for every gram a cap weighs. The tiers are in MOQ for bottle caps, and carton and pallet weights in ROPP cap packing and storage.
What gauge changes on the capping line
Thicker stock resists denting but needs more roller force to form the thread and more force to break the pilfer band. Thinner stock forms and parts readily and is more exposed to handling damage. The gauge has to match the capping head settings and the band design.
| Property | Thinner shell | Thicker shell |
|---|---|---|
| Metal per cap | Less | More, in direct proportion |
| Thread forming | Forms with less roller pressure | Needs more pressure to take the thread |
| Pilfer band break | Parts easily on opening | Needs more force to part |
| Dents in transit | More exposed | More resistant |
A gauge that shifts between lots can look like a machine fault, because the head was set for the previous lot, which is why why tamper bands fail treats lot-to-lot consistency as a cause of its own. Gauge is measured with a micrometer on the panel and the skirt as part of incoming inspection, covered in cap dimension tolerances and QC sampling.
Where R Vision’s closures sit
R Vision manufactures 28/15 mm aluminium ROPP closures in two designs. Aqua Cap is the packaged drinking water closure with a three-part break-system ring. Bev Cap is the beverage closure whose tamper-evident ring stays attached after opening. Both have an aluminium shell with a PE ring and a TPE liner and are rated to 8 bar. The weights and gauges on this page are industry reference, not R Vision’s specification; ask for a piece weight with the quotation through bulk ROPP cap orders or contact us.
Frequently asked questions
How much does a 28 mm ROPP cap weigh?
The aluminium in a 28/15 mm shell comes to about 1.0 to 1.2 grams at the 0.20 to 0.23 mm gauge commonly used, and a 28/18 mm shell to about 1.2 to 1.4 grams. The finished cap weighs more once the liner, coatings and any ring are added. Weigh 100 caps from the lot and divide by 100 for the working figure.
What gauge of aluminium is used for ROPP caps?
Closure stock is most often quoted at around 0.20 to 0.23 mm thick. One European data sheet for a 30 x 60 mm wine closure gives 0.23 mm with a tolerance of plus or minus 0.01 mm. The right gauge depends on the size, the skirt length and the capping head settings, so it belongs in the specification.
What is the gsm of the aluminium in a ROPP cap?
Bare aluminium weighs 2.70 grams per cubic centimetre, so sheet 0.20 mm thick is 540 grams per square metre, 0.21 mm is 567, 0.22 mm is 594 and 0.23 mm is 621. Lacquer and print add to that, by an amount that depends on the coating system, so ask whether a quoted gsm is bare or coated.
How many ROPP caps are in a kilogram?
Divide 1,000 by the weight of one finished cap in grams: caps of 1.5 grams give about 667 to the kilogram. Metal alone for a 28/15 mm shell comes to roughly 830 to 960 shells per kilogram; the finished count is lower because the liner and coatings add weight. The price per thousand is the per-kilogram rate multiplied by the cap weight in grams.
Sources
- BT-Watzke, vinotwist 30 x 60 technical details, for the published shell thickness and tolerance, liner thickness and weight per thousand of a 30 x 60 mm aluminium wine closure.
R Vision Pvt. Ltd manufactures 28/15 mm aluminium ROPP closures at MIDC Malegaon, Sinnar, Nashik. Call +91 72620 02754 to discuss a specification.