A closure for a glass bottle is specified in three parts: the neck finish moulded into the glass, the way the closure makes its seal, and the liner or compound that touches the product. Each part has to be right on its own and matched to the other two, because a cap chosen on diameter alone can still leak, taint the contents or fail to show tampering. The tables below give the finish codes and liner options to put in a request for quotation; treat the figures as typical planning values, since certified gauge drawings are issued per mould and per closure lot.
If you would rather start from the product category than from the engineering, use our closure selection pages organised by sealing mechanism.
Reading a neck finish drawing
Buyers meet two naming systems. GPI/SPI codes are numeric, such as 28-400 or 38-430. Metric descriptive codes name the finish family and size, such as DIN18, ROPP 31.5 or twist-off 63. Because finishes are so standardised, people assume compatibility, and that assumption is where most mismatches begin.
Five dimensions define a finish on the drawing:
- T – outside diameter across the thread
- E – root diameter of the thread
- I – bore
- H – height of the finish
- S – distance from the top of the finish to where the thread starts
Matching a closure on T only is not enough. In a GPI/SPI code the first number is T in millimetres and the second identifies thread style and depth. A 28-400 carries one full thread turn, a 28-410 has a deeper thread for taller closures such as pumps, and a 28-415 is deeper again. A 400 cap therefore bottoms out loose on a 410 finish even though both are 28 mm, so always write the complete code.
Finish codes and what they are used for
Continuous-thread (CT) finishes
| Code | T (mm, typical) | Thread style | Closures that fit | Where it is used |
|---|---|---|---|---|
| 18-400 / 18-410 | 18.0 | Single turn / deep | Caps, droppers, sprayers | Essential oils, serums, miniatures |
| 20-410 / 24-410 | 20.0 / 24.0 | Deep | Pumps, sprayers, disc-tops | Lotions, room sprays, cleaners |
| 28-400 (GPI) | 28.0 | Single turn | Screw caps, bar-top adapters | Spirits, beverages |
| 28-410 | 28.0 | Deep | Trigger sprayers, pumps | Sprays, foodservice |
| 38-400 / 38-430 | 38.0 | Standard / buttress | Caps, juice finishes | Juices, syrups, chemicals |
| 70-400 / 89-400 | 70.0 / 89.0 | Single turn | Wide-mouth caps | Cream jars, foodservice |
Lug, ROPP and specialty finishes
| Finish | Sizes (mm) | How it seals | Where it is used |
|---|---|---|---|
| Twist-off lug | 38, 43, 48, 53, 58, 63, 70, 82 | Plastisol vacuum seal | Hot-fill foods, jams, sauces, juices |
| ROPP | 18, 22, 25, 28, 30, 31.5 | Rolled thread plus tamper-evident band | Value spirits, wine, edible oils, pharma syrups |
| Cork (wine) | 18.5 bore | Cork compressed radially | Still and fortified wines |
| Bar-top | 18.5, GPI 28-400, 21.5 | Shank compressed in the bore | Spirits, gift olive oil |
| Crown | 26, 29 | Crimped skirt compressing a liner | Beer, carbonated drinks |
| FEA crimp (perfume) | 15, 20 | Crimped pump ferrule | Fragrance pumps |
Five ways a closure seals against glass
The mechanism decides what your filling line has to control, so pick it before you pick a cap style.
- Compression – corks, bar-tops and crowns squeeze an elastic element against the bore or the rim. What matters is compression set, meaning how well the material springs back. Natural cork keeps recovering for years, while a low-grade synthetic shank can relax within months. Specify the shank material of a bar-top as well as its head.
- Torque – a CT cap clamps a liner onto the sealing surface. It works only inside a torque window: below it the cap backs off, above it the thread strips or the liner wrinkles.
- Vacuum – a plastisol twist-off is sealed by the pasteuriser, with the capper merely positioning the cap.
- Formed thread – ROPP rolls a plain aluminium shell onto each bottle on the line.
- Crimp – crowns and FEA ferrules bend metal around a locking ring. The result is fast, holds pressure and cannot be undone.
ROPP compared with a moulded screw thread
A CT bottle arrives with its thread already moulded, and the cap thread has to match it. A ROPP (roll-on pilfer-proof) bottle has a smooth shell zone above a locking ring instead. The capping head forms the thread to that individual bottle and locks the band under the ring, so the first opening is visible. This tolerance of glass variation at very high line speeds explains why value spirits, wine and edible oil settled on 18 mm, 28 mm, 30 mm and 31.5 mm ROPP.
How a plastisol lug cap pulls a vacuum
Inside the cap sits a ring of plastisol compound. The cap goes on at moderate torque, and during pasteurisation, typically at 85–95°C, the compound softens and moulds itself to the glass sealing surface. As the contents cool the headspace shrinks, the resulting vacuum seats the compound and the safety button is drawn down. The pack stays hermetic until the first twist, when the button pops up and tells the consumer it was unopened.
Hot-fill products are therefore specified with lug finishes in the 38–82 mm twist-off range, and the specification sheet has to record fill temperature and capper torque alongside the cap itself.
Choosing the liner by product chemistry
The liner is the only closure part in contact with the contents, so its facing material is chosen by how aggressive the formula is.
| Liner or compound | Seal type | Compatible products | Usual application |
|---|---|---|---|
| Plastisol | Vacuum, formed by heat | High-acid foods, hot-fill | Twist-off food caps |
| PE foam | Torque compression | Water-based products, food oils, low alcohol | General CT caps |
| PTFE-faced | Torque compression | Solvents, essential oils, aggressive formulas | Chemicals, aromatherapy |
| Induction foil | Hermetic membrane | Juices, dairy-style products, tamper evidence | Beverages, pharma |
| PVDC / tin-saran | Torque with controlled oxygen transmission | Wine | Wine screw caps |
| Pressure-sensitive | Adhesive tamper evidence | Dry products only | Spices, supplements |
Two points from the table deserve emphasis. PE foam is not suitable for high-alcohol products (20%+): it degrades and can taint, so those need an alcohol-resistant facing. And in a wine screw cap, the oxygen transmission rate of the PVDC or tin-saran liner effectively sets how the wine ages.
Tell us the alcohol percentage, oil content and pH of the formula. We arrange a 4-week compatibility soak and confirm the liner before a volume order is placed.
Torque settings and validation before production
Application torque varies with finish diameter and closure type. As a typical reference, a 28-400 CT cap goes on at 12–18 inch-pounds and a 63 mm twist-off at 8–14 inch-pounds. Removal torque measured after 24 hours should stay above a set floor, commonly 40–60% of the application figure. Too little torque lets the cap work loose under transit vibration; too much stresses the finish, strips threads or creases the liner until it leaks.
We provide a torque window with each matched bottle-and-closure set. Check it with a torque meter during line trials on the first production day and keep checking, because capper chucks wear and torque drifts over a season.
Before committing to volume, we coordinate a five-step validation on the matched set and keep the file together with signed master samples, so that reorders begin from proven data:
- Gauge bottle and closure against their drawings on T/E/I/H/S.
- Apply closures across the whole torque window, then measure removal torque at 24 hours and at 7 days.
- Leak-test by inverted stand, vacuum chamber or dye penetration, whichever your industry uses.
- Put filled packs through real distribution stresses: temperature swings, vibration and stacking load.
- For food and beverage, run the actual hot-fill or pasteurisation cycle on a pilot batch.
Stock, modified or fully custom closures
| Route | When it fits |
|---|---|
| Stock finish with a stock closure | Launch and validation |
| Modified closure (liner, colour, tamper-evident print) | Brand-matched versions of a proven seal |
| Full custom finish and closure system | Proprietary dispensing systems |
Whichever route you take, buying bottle and closure as one validated set removes the risk of tolerances stacking up between two separate suppliers.
What to send for a closure review
To have a closure system specified or checked, send us the project details with the following:
- the bottle drawing, or a physical sample, with its finish code
- product chemistry: alcohol %, oils, pH and any solvents
- fill and pasteurisation parameters
- capper type and the torque it is currently set to
- annual closure quantity and any decoration required
When you are matching a closure you already use, a physical sample is the most reliable input. We gauge its T, E, I, H and S dimensions against the drawing and confirm the exact variant before a quotation is prepared.
Frequently asked questions
Are two 28 mm finishes interchangeable?
No. The digits after the hyphen describe thread style and depth, and a cap made for one will not seat correctly on another. Quote the full GPI/SPI code rather than the diameter.
Which liner suits essential oils and solvent-based products?
These need a PTFE-faced or foil-faced liner. PE foam is limited to water-based products and food oils.
Why would a bar-top loosen a few months after bottling?
The usual cause is the shank. A cheap synthetic shank loses its spring-back within months, whereas natural cork goes on recovering for years, which is why the shank material belongs in the specification.