What Is a 13 mm Crimp Neck and Flip-Off Cap?
A 13 mm crimp neck is a standardised glass vial finish whose neck flange measures 13 mm in nominal outer diameter, closed by a three-part system: a rubber stopper seated in the neck bore, an aluminium ring crimped under the flange to compress it, and a plastic flip-off disc on top. Flipping the disc removes only that disc — roughly 7–8 mm of stopper surface is exposed, and the aluminium ring stays crimped in place holding the seal. The 13 mm finish is used on 2 mL and 3 mL formats; 5 mL and larger generally step up to a 20 mm finish.
The part that gets misread is the aluminium. Because the coloured plastic button lifts away with a thumbnail, the metal underneath reads like packaging that should also come off — a ring to be peeled, prised, or cut away to "open" the vial. It is not packaging. The aluminium ring is the mechanical component that holds the closure shut. Remove it and the vial is no longer sealed, regardless of how intact the stopper looks.
What is a crimp neck finish?
A crimp finish is a glass neck that ends in an outward flange, or bead, rather than a screw thread. It is not designed to be closed by twisting anything on. Instead a rubber stopper is pushed into the neck bore, an aluminium seal is placed over the top, and a crimping tool rolls the skirt of that seal inward and under the flange. The metal deforms permanently, pulling the stopper down against the glass sealing surface and holding it there under continuous compression.
That compression is what makes the seal, not adhesive and not friction alone. The stopper's plug is manufactured slightly oversized relative to the neck bore, so it is squeezed on insertion; the crimp then locks that squeeze in place permanently. Because the deformation is one-way, a crimped seal cannot be reapplied by hand once it has been removed.
| Component | Material | Typical dimension | Comes off by hand? |
|---|---|---|---|
| Glass neck flange | Type I borosilicate | 13 mm nominal OD (12.6–13.0 mm measured, varies by manufacturer) | No — it is part of the vial |
| Stopper | Bromobutyl or chlorobutyl rubber | 13 mm nominal, plug ~7 mm across, ~6–7 mm tall | No — held under crimp compression |
| Aluminium seal ring | Aluminium, ~0.2 mm gauge | 13 mm, crimped under the flange | No — needs a decapping tool |
| Flip-off disc | Polypropylene | ~8–11 mm across the button | Yes — the only part designed to be removed |
What does 13 mm actually measure?
The 13 mm figure is the nominal outer diameter of the neck flange — the widest point of the glass at the top of the vial. It is not the diameter of the vial body, and it is not the diameter of the opening. On a 13 mm finish the internal bore that the stopper plugs is closer to 7 mm, while the vial body itself is typically 14–17 mm across.
Because the figure is standardised, it functions as a compatibility code rather than a measurement anyone needs to take. A 13 mm stopper and a 13 mm aluminium seal will fit any 13 mm vial finish, from any manufacturer, on any format. Measured flange diameters cluster around 12.6–13.0 mm in practice; the nominal number is the one that matters for fit.
What is the difference between a 13 mm and a 20 mm neck?
Only two crimp finishes cover the overwhelming majority of small injection vials: 13 mm and 20 mm. They are not interchangeable in either direction — stoppers, seals, and crimping tool heads are all sized to one or the other. An 8 mm finish exists for very small formats and a 32 mm finish for large ones, but neither is common in this size range.
| Property | 13 mm finish | 20 mm finish |
|---|---|---|
| Nominal flange outer diameter | 13 mm | 20 mm |
| Measured flange OD (varies by manufacturer) | 12.6–13.0 mm | 19.8–20.2 mm |
| Neck bore the stopper plugs | ~7 mm | ~12.6 mm |
| Typical vial body outer diameter | 14–17 mm | 20–30 mm |
| Vial formats that use it | 2 mL and 3 mL (ISO 2R, 4R) | 5 mL and up (ISO 6R, 8R, 10R, 20R) |
| Stopper surface exposed after the disc flips off | ~7–8 mm circle | ~12–13 mm circle |
What does the flip-off cap actually remove?
The flip-off disc is a moulded polypropylene button bonded to the centre of the aluminium seal, sitting over a pre-cut circular opening in the metal. Levering it with a thumb snaps the bond and takes the plastic away, leaving that circular window in the aluminium and the top face of the rubber stopper visible through it — a circle roughly 7–8 mm across on a 13 mm closure.
- Removed: the plastic flip-off disc, roughly 8–11 mm across, and nothing else.
- Left behind: the aluminium ring, crimped under the 13 mm flange, still compressing the stopper.
- Exposed: a ~7–8 mm circle of stopper surface at the centre of the ring.
- Unchanged: the seal itself. Flipping the disc does not open the vial.
Why is the aluminium ring not meant to come off?
The stopper is held in place by the compressive load the crimp applies, and by nothing else. Prising the ring off releases that load. The stopper is then retained only by the interference fit of oversized rubber in a 7 mm bore, which is far weaker and gives up entirely if there is any pressure differential across it — and sealed vials frequently sit below ambient pressure, so the differential is real.
There is also no way back. The crimp is a permanent plastic deformation of the metal; re-forming it needs a crimping tool sized to the finish, and a ring that has been bent open will not re-seat cleanly even with one. Once the aluminium is off, the closure is spent.
How does a crimp closure show tamper evidence?
The tamper evidence is structural rather than printed. Three things are worth reading on an intact closure: the flip-off disc is present and unlifted, the aluminium skirt is smooth and evenly rolled all the way around the flange, and the ring does not rotate or lift when handled. A skirt that is scored, creased, partly unrolled, or visibly re-crimped indicates the closure has been worked on.
The disc cannot be reseated once flipped. Its bond to the aluminium breaks on removal and the plastic itself distorts, so a flipped disc that has been pressed back on sits proud, sits crooked, or falls off when the vial is inverted. That irreversibility is the point — the closure is designed so that the first opening is permanently visible.
Which vial formats use which neck size?
The step from 13 mm to 20 mm happens at the 5 mL mark. Everything below it — 2 mL and 3 mL formats, corresponding roughly to the ISO 2R and 4R tubing designations — uses a 13 mm finish. Everything from 10 mL upward uses 20 mm. The 5 mL format sits on the boundary and is the least consistent: most use a 20 mm finish, but 13 mm versions exist, so it is the one size where the finish should be confirmed rather than assumed.
Note that the neck finish tracks the diameter of the glass, not the amount of material inside. A 2 mL and a 3 mL vial share a 13 mm finish because they share a body diameter of roughly 14–17 mm, and they take identical stoppers and seals regardless of what they hold.
Common questions
What does 13 mm refer to on a vial?
The nominal outer diameter of the neck flange, 13 mm. Not the vial body, which is typically 14–17 mm, and not the opening, which is roughly 7 mm.
Should the aluminium ring be removed?
No. Only the plastic flip-off disc is designed to come off. The aluminium ring supplies the compressive load that holds the stopper against the glass, and the crimp cannot be re-formed by hand once opened.
Is a 13 mm stopper the same as a 20 mm stopper?
No, and they do not interchange. A 13 mm stopper has a plug of roughly 7 mm; a 20 mm stopper has a plug of roughly 12.6 mm. Seals and crimping tool heads are likewise finish-specific.
How much of the stopper is exposed after flipping the cap?
A circle of roughly 7–8 mm on a 13 mm closure, and roughly 12–13 mm on a 20 mm closure — the size of the pre-cut opening in the aluminium seal.
Can a flip-off cap be put back on?
Not securely. The bond between the disc and the aluminium breaks on removal and the plastic deforms, so a replaced disc sits proud or crooked. That is deliberate — it is what makes the closure tamper-evident.
Which vial sizes use a 13 mm neck?
2 mL and 3 mL formats. The 5 mL format is the transition point and is usually 20 mm, and 10 mL and larger are 20 mm throughout.
References
- 01International Organization for Standardization ISO 8362-1: Injection containers and accessories — Part 1: Injection vials made of glass tubing. ISO, 2018.
- 02International Organization for Standardization ISO 8362-2: Injection containers and accessories — Part 2: Closures for injection vials. ISO, 2015.
- 03International Organization for Standardization ISO 8362-3: Injection containers and accessories — Part 3: Aluminium caps for injection vials. ISO, 2018.
Citations are listed by title so they can be verified directly on PubMed. Identifiers are omitted deliberately rather than reproduced from memory.
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