Pairing the Rubber Stopper and Aluminium Seal on a Vial

An injectable vial is closed by two components working as a pair: a rubber stopper that seats in the vial mouth and forms the resealable barrier, and an aluminium (or aluminium-plastic) seal crimped over the stopper to hold it compressed against the glass. The pairing is one to one by nominal size: a 13 mm stopper on a 13 mm ISO 8362-1 neck takes a 13 mm seal, and the same relationship runs through 20, 28, 32 and 34 mm. Neither part closes the vial alone. The stopper provides the seal a needle can pierce and that reseals afterwards; the metal seal is the clamp that keeps that stopper under load for the product’s shelf life.

This is the reference page for that pairing: why both parts are needed, how they are matched by size, the controlled dimensions and opening forces at each size, what the crimp does, how to specify the seal, and how it is supplied.

Key takeaways

A vial closure is a pair: the rubber stopper (the barrier) plus the aluminium or aluminium-plastic seal (the clamp). Autofits manufactures the seal, not the stopper.
The size match is one to one: stopper, vial neck and seal share one nominal finish across 13, 20, 28, 32 and 34 mm.
The vial neck is the reference dimension, standardised under ISO 8362-1; the closure that seats in it under ISO 8362-2; the aluminium-plastic cap over both under ISO 8362-6.
Maximum opening force runs 30 N at 13 mm, 35 N at 20 mm and 28 mm, and 65 N at 32 mm, rising with the number of bridges.
Total seal height runs 7.62–8.38 mm at 13 mm up to 15.00–16.00 mm at 34 mm, and must suit the seated height of the stopper the customer has qualified.
The crimp rolls the aluminium skirt under the vial neck flange, which is what keeps the stopper under load and the package sealed.
A correctly paired and crimped closure is what passes container closure integrity testing to USP <1207>.

Why a vial needs both a stopper and a seal

A vial needs both because each part does a job the other cannot: the rubber stopper makes a piercable, resealing barrier, and the aluminium seal holds that stopper in place so the barrier stays under compression. A rubber stopper on its own would not stay reliably compressed against the glass over time, especially through handling, transport, and temperature changes; it could back out or loosen, breaking the seal. A rigid metal cap on its own could not be pierced by a needle and would not reseal. The terminology around the rubber half is untangled in rubber stopper versus elastomer.

The combination solves both needs at once. The soft elastomer seals and reseals; the rigid crimped metal clamps it down. This is why injectable vials are universally closed with a stopper-and-seal pair rather than a single component. The two together, with the glass vial, make up the container closure system, and it is that assembled system, not either part, that is qualified against the drug.

How the stopper and seal are matched by size

The stopper and seal are matched to each other and to the vial neck by their nominal size, so all three components share the same finish and assemble cleanly. Vials, stoppers, and caps are made in standard neck sizes designated 13, 20, 28, 32 and 34 mm, and a closure is built by selecting all three from the same size family.

The vial neck is the reference finish that both the stopper and the cap are built around, standardised for tubular glass vials under ISO 8362-1.
The stopper must seat fully in the vial mouth, with its flange resting on the vial rim. Its dimensions are standardised under ISO 8362-2, and slotted freeze-drying closures under ISO 8362-5.
The seal must fit over the seated stopper and crimp under the vial neck flange. Plain aluminium caps are standardised under ISO 8362-3 and aluminium-plastic combination caps under ISO 8362-6 and ISO 8362-7.

The nominal figure is a finish designation, not a measured diameter of any single part. That is why the inner aluminium diameter of a 20 mm seal reads 20.22 mm minimum rather than exactly 20.00 mm: the skirt has to pass over the seated stopper flange before it is rolled in. Two dimensions carry the fit. The inner diameter governs whether the skirt clears the stopper and reaches the flange; the inner aluminium height governs whether the crimp lands on the neck rather than on the stopper shoulder or below the flange. Matching also extends to the filling line: the chosen stopper and seal have to run together on the customer’s capping equipment at speed.

Cross-size reference: seal dimensions by stopper size

Controlled dimensions for the Autofits FlipTop Optima aluminium-plastic seal at each nominal stopper and neck size, taken from the engineering drawings. Select the row that matches the nominal finish of the vial and the stopper already qualified.

Stopper and neck size	Inner Ø aluminium seal (D1)	Outer Ø plastic disc (D2)	Disc height	Inner aluminium height	Total seal height	Aluminium thickness	Bridges	Max opening force
13 mm	13.36 mm (min)	14.99 mm (max)	2.90–3.30 mm	6.15–6.40 mm	7.62–8.38 mm	0.16–0.20 mm	6 taper	30 N
20 mm	20.22 mm (min)	23.14 mm (max)	3.20–3.60 mm	7.37–7.62 mm	9.02–9.91 mm	0.16–0.20 mm	6 taper	35 N
28 mm	27.60–28.20 mm	30.10–31.10 mm	Available on request	8.70–9.30 mm	11.06–12.06 mm	0.17–0.23 mm	8	35 N
32 mm	32.10–32.90 mm	35.50–36.50 mm	Available on request	Available on request	14.30–15.50 mm	0.21–0.25 mm	12 taper	65 N
34 mm	33.60–34.00 mm	35.50–36.50 mm	Available on request	Available on request	15.00–16.00 mm	0.20–0.25 mm	8 taper	Available on request

Opening force is the force the end user applies to break the bridges and lift the plastic disc at point of use. It rises with diameter and with the number of bridges: six tapered bridges at 13 mm give 30 N, the same six at 20 mm give 35 N, and twelve tapered bridges at 32 mm give 65 N. It is a usability figure, not the compression the stopper is held under. Crimp force is a machine setting on the customer’s capping head and is established during capping qualification rather than supplied as a product attribute.

Each size has a landing page listing every format available at that finish: 13 mm, 20 mm, 28 mm, 32 mm and 34 mm.

Which Autofits seal family pairs with which stopper

All five families crimp over a standard rubber stopper on a standard neck; they differ in how the user opens them, not in how they seat. The opening action is a market and product decision, and it does not change the stopper.

Seal family	Construction	Sizes	How it opens
FlipTop Optima	Aluminium-plastic flip-off	13, 20, 28, 32, 34 mm	Flip the polypropylene button up to expose the stopper
FlipTop Pull Ring	Aluminium-plastic	20 mm (bridge), 32 mm (flower)	Pull the aluminium ring, open anticlockwise, whole seal detaches
FlipTop Tear-Down	Aluminium-plastic, PP disc riveted to aluminium	13, 20, 32 mm	Open the disc, pull and rotate anticlockwise, whole seal comes off
Tear-off	All-aluminium, no plastic	13, 20, 32 mm	Tear the perforated panel away
Tear-down	All-aluminium, no plastic	20 mm, further sizes on request	Twist off anticlockwise

The Optima Button design carries intrinsic tamper evidence from a 12-stem flower moulded under the button disc. Pull Ring seals are frequently marked for oral use. Tear-Down seals are mainly used for oral vaccination presentations. Aluminium pilfer-proof (ROPP) caps sit outside this table because they thread onto a screw finish rather than crimping over a stopper; common ROPP sizes are supported, with the full range available on request from the pilfer-proof caps page.

How the crimp holds the pair together

The crimp is what locks the stopper and seal together: the capping station rolls the lower edge of the aluminium skirt under the vial neck flange, compressing the stopper against the glass and holding it there permanently. Until the seal is crimped, the stopper merely rests in the vial mouth. The crimp turns the loose assembly into a sealed package.

A good crimp compresses the stopper just enough to maintain the barrier without damaging the rubber or the glass. Too loose, and the stopper is under-compressed and the package can leak; too tight or uneven, and it can damage the stopper or crack the glass. Residual seal force, the load the stopper keeps holding after the head retracts, depends on the stopper’s hardness and compression set as much as on the machine setting, so two compounds of the same nominal dimensions can leave different residual force at identical settings. That is why capping is treated as a critical step in sterile manufacturing, and why guidance such as EU GMP Annex 1 expects capping to be controlled and sealed vials to be inspected. Verification of the result belongs to container closure integrity testing, for which USP <1207> sets out the deterministic and probabilistic method families. The mechanics of the crimp are covered in how flip-off caps work, and the parameters themselves in vial crimping explained.

What to consider when pairing a stopper and seal

Pairing is about more than matching the nominal size: the stopper material, the seal type, the line equipment, and the regulatory filings all have to align. Points that matter in practice:

Size match. Stopper, vial, and seal share the same nominal neck finish, one of 13, 20, 28, 32 or 34 mm.
Seated stopper height. The inner aluminium height of the seal has to suit how tall the stopper sits once seated, which moves with compound hardness.
Stopper material and function. The elastomeric closure is moulded from a rubber compound chosen from the elastomer families used for closures for compatibility with the drug, a laminated halobutyl for a biologic for instance, and qualified under USP <381> for material and USP <382> for function, which became official on 1 December 2025 and whose text now in force carries a Revision Bulletin official 1 February 2026.
Seal type. Flip-off, tear-off, tear-down, or pull-ring, chosen for the opening action the market and product require.
Line compatibility. The stopper and seal must feed, place, and crimp reliably on the customer’s filling and capping equipment.
Qualification. Once a stopper-and-seal pairing is qualified with a product, changing either component generally triggers re-validation, so qualified pairings tend to persist.

Forms of supply, shelf life and packing

Autofits supplies the seal half of the pairing in three forms. Regular is the standard non-sterile presentation for customers who wash and sterilize in house. Ready-to-Use is sterilized by gamma irradiation or by ETO, washed to control particles, assembled in an ISO Class 8 cleanroom from double-lacquered aluminium, and supplied double-bagged so the outer bag is stripped at the airlock into the aseptic area. Ready-to-Sterilize is supplied clean and bagged for the customer’s own cycle. Tyvek bagging is available in place of the standard bag where the customer’s transfer procedure calls for it.

Shelf life follows the form of supply. Non-irradiated seals carry 3.5 years before capping, plus a further 5 years after capping. Ready-to-Use seals carry 2 years before capping, the shorter window reflecting the sterilization dose the aluminium and polypropylene have already absorbed.

Standard packing is set by size, in a 420 × 270 × 320 mm shipper box.

Size	Standard packing per box	Bag split
13 mm	15,000	5,000 × 3 bags
20 mm	6,000	2,000 × 2 bags
28 mm	3,000	1,500 × 2 bags
32 mm	1,400	700 × 2 bags
34 mm	Available on request	Available on request

To specify a seal against a stopper you have already qualified, send the nominal neck finish, the stopper reference and its seated height if you have it, the seal family and top design, the finish and colour, and the form of supply. Samples, drawings and a quotation are available through the contact page. Minimum order quantities and lead times are available on request.

How this works in practice at Autofits

Autofits manufactures the aluminium and aluminium-plastic seal half of the pairing, designed to crimp over a standard rubber stopper on a standard vial neck. It does not manufacture rubber stoppers. The range includes FlipTop Optima flip-off seals in 13, 20, 28, 32 and 34 mm, FlipTop Pull Ring seals, FlipTop Tear-Down seals, tear-off and tear-down all-aluminium seals, and aluminium pilfer-proof caps; the full products page lists the range and the by-size index sorts it by neck finish.

Because the seals are produced to standard neck finishes with tight dimensional control, they pair with the matching stoppers and crimp cleanly on customers’ capping lines. The aluminium is AA8011 alloy, epoxy lacquered to 21 CFR 175.300, with polypropylene discs to 21 CFR 177.1520 moulded in-house on Japanese Toyo injection machines. Heavy metals are held to a maximum 100 ppm total and components are BSE/TSE-free to EMEA/410/01 rev. 3. Every seal passes 100% high-speed camera inspection at an AQL of critical nil, major not more than 2.5% and minor not more than 4%. Production runs under an ISO 15378:2017 quality system, alongside ISO 9001:2015 and ISO 14001:2015 certification and a Drug Master File, in a 75,000 sq ft Nashik facility with an ISO Class 8 cleanroom. The full certification set is on the quality page.

Frequently asked questions

Why does a vial need both a rubber stopper and an aluminium seal?

A vial needs both because each does a job the other cannot. The rubber stopper forms a barrier that a needle can pierce and that reseals after the needle is withdrawn. The aluminium seal crimps over the stopper to hold it compressed against the glass so the barrier stays under load through the product’s shelf life. A stopper alone would not stay reliably compressed; a rigid cap alone could not be pierced or reseal.

What size aluminium seal goes with a 20 mm rubber stopper?

A 20 mm seal. The match is one to one by nominal size: a 20 mm stopper seats on a 20 mm vial neck and takes a 20 mm seal, just as 13, 28, 32 and 34 mm stoppers take seals of the same nominal size. The nominal figure is the neck finish designation shared by all three parts, so the measured inner aluminium diameter of a 20 mm seal is 20.22 mm minimum rather than exactly 20 mm.

How are the stopper and seal matched to the vial?

They are matched by nominal neck size, with the vial neck as the reference dimension. The stopper must seat in the vial mouth with its flange on the rim, and the seal must pass over the seated stopper and crimp under the neck flange, so all three are dimensioned to the same standards. Beyond diameter, the seal’s inner aluminium height has to suit the seated height of the stopper, which changes with the hardness of the rubber compound.

What holds the stopper and seal together?

The crimp holds them together. At the capping station, the lower edge of the aluminium skirt is rolled under the vial neck flange, compressing the stopper against the glass and locking the assembly permanently. Until it is crimped, the stopper just rests in the vial mouth. Residual seal force after capping depends on the stopper’s hardness and compression set as well as the machine setting, which is why capping is qualified rather than simply set.

Can any seal be used with any stopper?

No. The stopper and seal must share the same nominal neck size and must run together reliably on the filling and capping line. The stopper material is also chosen for compatibility with the drug. Once a particular stopper-and-seal pairing is qualified with a product, changing either component usually requires re-validation, so pairings tend to stay fixed.

What standards apply to the stopper and seal pairing?

The rubber stopper’s material is covered by USP <381> and its function by USP <382>, with dimensions under ISO 8362-2 and freeze-drying closures under ISO 8362-5. The aluminium or aluminium-plastic seal is covered by ISO 8362-3 for plain aluminium and ISO 8362-6 and ISO 8362-7 for aluminium-plastic combinations. The vial dimensions are under ISO 8362-1, and the assembled package is verified by container closure integrity testing, whose method families are described in USP <1207>.

Sources

USP-NF: General Chapters <381> and <382>, Elastomeric Components in Injectable Pharmaceutical Product Packaging/Delivery Systems (https://www.usp.org/)
USP-NF: General Chapter <1207>, Package Integrity Evaluation, Sterile Products (https://www.usp.org/)
ISO: ISO 8362-1, Injection containers and accessories, Part 1: Injection vials made of glass tubing (https://www.iso.org/)
ISO: ISO 8362-2:2024, Injection containers and accessories, Part 2: Closures for injection vials (https://www.iso.org/standard/87251.html)
ISO: ISO 8362-6:2010, Caps made of aluminium-plastics combinations for injection vials (https://www.iso.org/standard/52806.html)
European Commission: EudraLex Volume 4, EU GMP Annex 1, Manufacture of Sterile Medicinal Products (https://health.ec.europa.eu/medicinal-products/eudralex/eudralex-volume-4_en)
Autofits FlipTop Seals dossier and engineering drawings (working/spec-sheets/)

*Last updated: 2026-08-18. This article is general technical information, not regulatory or compliance advice; confirm current standard editions and requirements with the issuing bodies.*