What Is a Flip-Off Seal? A Plain-English Guide
A flip-off seal is the cap on an injectable vial that combines an aluminium shell crimped over the rubber stopper with a coloured plastic button on top that you flip off to expose the injection point. When the button is flipped, it snaps away at a thin scoreline and cannot be put back, while the aluminium underneath stays crimped and keeps the vial sealed. It is the most common closure on vials of vaccines and injectable medicines, recognisable by the bright plastic top you press with a thumb.
This guide explains what a flip-off seal is, how it works, what the parts are, where it is used, and how it differs from other vial seals.
Key takeaways
- A flip-off seal is an aluminium-plastic vial cap: an aluminium shell crimped over the stopper, topped by a plastic button that flips off.
- Flipping the button exposes the centre of the stopper for the needle; the aluminium stays crimped and the vial stays sealed.
- The button snaps at a scoreline and cannot be reseated, so a missing or broken button is tamper evidence.
- The coloured plastic top is often used for colour-coding different products or strengths.
- It is the standard closure on injectable vials, including vaccines.
- For the formal definition see the flip-off seal glossary entry; for the mechanism see how flip-off caps work.
What is a flip-off seal?
A flip-off seal is a combination (combi) seal made of an aluminium shell and a moulded plastic button, applied over the rubber stopper of an injectable vial. It is the top component of the vial’s container closure system, sitting over the stopper that seals the glass vial.
The name describes how it opens: you flip off the plastic button. That button covers the centre of the seal, and lifting it uncovers the small area of the rubber stopper that a needle will pierce. The aluminium shell beneath the button is crimped tightly around the vial neck and does the actual sealing work, holding the stopper compressed against the glass. Because the plastic top and the metal crimp have separate jobs, you can open the access point without ever loosening the seal.
How a flip-off seal works
A flip-off seal works by separating two functions: the aluminium crimp permanently seals the vial, and the plastic button gives single-use access to the stopper. The aluminium shell’s lower edge is rolled under the vial neck flange during capping, compressing the stopper to form the sterile barrier. The plastic button is moulded with a thin scoreline so a thumb can snap it up and off, exposing the stopper centre, while the crimped aluminium stays in place.
This means flipping the button does not break the seal. The sterile barrier is the stopper compressed against the glass, held by the crimp, and that is untouched until a needle is introduced. The full mechanism, including the crimp and the role of the stopper, is explained in how flip-off caps work.
The parts of a flip-off seal
A flip-off seal has three elements working together, two of them part of the cap itself and one the stopper underneath.
- Aluminium shell. The thin formed cap that covers the vial top and crimps around the neck. It does the mechanical sealing.
- Plastic button. The moulded, often coloured top with a scoreline, which flips off to expose the stopper and provides tamper evidence.
- Rubber stopper. Not part of the cap, but the surface the needle pierces; the cap crimps over it. It is an elastomeric closure, usually a butyl rubber stopper.
The aluminium-plus-plastic construction is why the product is described as an aluminium-plastic seal. Flip-off seals come in sub-types (button, bridge, and flower or scoreline) that differ only in the shape of the plastic flip feature, compared in bridge-type vs button-type seals.
Where flip-off seals are used
Flip-off seals are used on injectable vials across pharmaceuticals, most visibly on vaccines and other parenteral medicines supplied in glass vials. They suit any vial that has to stay sterile, allow needle access, and show tamper evidence. The coloured plastic top is often used to colour-code different products, strengths, or formulations, which helps clinical staff pick the right vial quickly.
The seals are made in standard vial neck sizes (commonly 13 mm and 20 mm, with larger sizes for bigger vials), so they match the vials and stoppers of the same finish. Aluminium-plastic combination caps like these are standardised under ISO 8362-6 and ISO 8362-7.
How a flip-off seal differs from other vial seals
A flip-off seal is opened by flipping a plastic button, while other vial seals are opened by tearing the aluminium. All of them crimp over the stopper to seal the vial; the difference is the opening action and the presence of the plastic top.
| Seal type | How it opens | Plastic top |
|---|---|---|
| Flip-off | Flip the plastic button | Yes |
| Tear-off | Tear the whole aluminium top off | No (all aluminium) |
| Tear-down | Tear down a section | Usually no |
| Pull-ring | Pull a ring to remove the top | Often combination |
The flip-off design is preferred where a quick, tool-free, colour-codeable opening is wanted, which is why it dominates vaccine and injectable vials. A side-by-side with the tear-off design is in flip-off vs tear-off seals.
How this works in practice at Autofits
Autofits manufactures flip-off seals as its core product: the FlipTop Optima aluminium-plastic flip-off seals, in 13, 20, and 32 mm sizes, in several flip-feature sub-types (bridge with groove, bridge without groove, button, and flower or scoreline). The plastic disc comes in matte or glossy finish and can be printed and embossed, and the metal can be coloured and printed, supporting product-specific colour-coding. The broader range also includes tear-off and tear-down seals and pilfer-proof caps; see the full products page. 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 and high-speed visual inspection. The full certification set is on the quality page.
Frequently asked questions
What is a flip-off seal?
A flip-off seal is the cap on an injectable vial that combines an aluminium shell crimped over the rubber stopper with a coloured plastic button on top. You flip off the plastic button to expose the centre of the stopper for a needle, while the aluminium stays crimped and keeps the vial sealed. It is the most common closure on vaccine and injectable vials.
Does flipping the button break the seal?
No. The sterile barrier is the rubber stopper compressed against the glass and held by the crimped aluminium, not the plastic button. Flipping the button only uncovers the central injection area of the stopper. The vial stays sealed until a needle pierces the stopper, which is why the stopper is wiped with alcohol before use.
Why are flip-off seals different colours?
The coloured plastic top is used for colour-coding. Manufacturers use different colours to distinguish products, strengths, or formulations so that clinical staff can identify the right vial at a glance. The colour is on the plastic button (and sometimes the metal); it does not change how the seal works.
What is the scoreline on a flip-off seal?
The scoreline is a thin, deliberately weakened line moulded into the plastic button that lets it snap off cleanly by hand without a tool. It controls where the button separates and ensures the button cannot be reattached, which provides the visual tamper evidence.
What is the difference between a flip-off seal and a tear-off seal?
A flip-off seal has a plastic button you flip up to expose the stopper, while the aluminium stays crimped. A tear-off seal is all aluminium with no plastic top; you tear the whole metal top away to expose the stopper. Both crimp over the stopper to seal the vial, but they open differently and a tear-off seal removes more of the cap.
Related reading
- Flip-off seal: definition
- How flip-off caps work
- Bridge-type vs button-type flip-off seals
- Flip-off vs tear-off seals
- FlipTop Optima flip-off seals
Sources
- ISO: ISO 8362-6:2010, Injection containers and accessories, Part 6: Caps made of aluminium-plastics combinations for injection vials (https://www.iso.org/standard/52806.html)
- USP-NF: General Chapter <1207>, Package Integrity Evaluation, Sterile Products (https://www.usp.org/)
*Last updated: 2026-06-18. This article is general technical information, not regulatory or compliance advice; confirm current standard editions and requirements with the issuing bodies.*
