A bottle is only as good as the seal it makes with its closure. In IVD packaging, the neck finish, meaning the threaded or shaped top of the container, determines which caps, liners, dropper inserts and stoppers will fit, how reliably the package seals during transport, and whether a capping machine or an analyzer probe can work with it. Yet finish codes such as 28/410, GL 45 or 20 mm crimp are often copied from an old drawing without anyone checking what they actually specify.
This guide explains the main naming systems, the dimensions behind them and the questions to ask before you freeze a finish in a specification. Exact tolerances come from the relevant standard or the supplier drawing; the values below are typical and should always be confirmed against the actual bottle and closure drawings.
How GPI/SPI finish codes work
Most plastic bottles and many glass bottles use the finish system originally defined by the Glass Packaging Institute (GPI) and the Society of the Plastics Industry (SPI). A designation such as 28/410 (also written 28-410) has two parts:
- First number (28): the nominal diameter in millimetres, which corresponds approximately to the T dimension, the outside diameter across the threads.
- Second number (410): the finish style, which defines the thread profile, the number of thread turns and the height of the finish.
The three styles you will meet most often in reagent and laboratory packaging are:
- 400: a short finish with roughly one full thread turn. Common on wide-mouth bottles and jars, e.g. 38/400, 45/400 and 53/400.
- 410: a taller finish with roughly one and a half turns. Typical for narrow-neck bottles, dropper bottles and dispensing closures, e.g. 20/410, 24/410 and 28/410.
- 415: the tallest of the three, with roughly two turns. Used where a longer cap skirt or more thread engagement is wanted, e.g. 20/415 and 24/415 on tall narrow-neck bottles.
Within a given diameter, the thread pitch is usually the same across 400, 410 and 415; a 28 mm finish, for instance, typically has 6 threads per inch. That is why a 28/400 cap can sometimes be screwed onto a 28/410 neck. It will not necessarily seal properly: a shorter 400 cap on a 410 neck leaves exposed thread and a visible gap above the shoulder, while a taller cap on a shorter neck may bottom out on the shoulder before the liner is compressed.
The key dimensions: T, E, H, S and I
| Dimension | Definition | What it affects |
|---|---|---|
| T | Outside diameter across the threads | Thread engagement with the cap, cap fit and resistance to stripping |
| E | Diameter at the root of the thread | Thread depth (T minus E) and resistance to jumping threads under torque |
| H | Height from the top of the finish to the shoulder or bead | Cap skirt length, gap between cap and shoulder, TE band position |
| S | Distance from the top of the finish to the start of the thread | Point of first engagement and liner compression at a given torque |
| I | Minimum inside diameter of the neck bore | Fit of plug seals, dropper inserts and orifice reducers; filling nozzle and probe access |
Two further features are often specified on the drawing: the sealing land, the flat top surface the liner presses against, whose width and flatness matter for liners and induction seals, and, on tamper-evident finishes, the bead or ratchet that retains the TE band.
DIN and GL threads for laboratory and pharmacy containers
European laboratory glassware and many pharmacy containers follow DIN conventions rather than GPI. The most important one for diagnostic users is the GL thread (from the German Gewinde Laborglas), a rounded thread profile defined in DIN 168. The number after GL is approximately the outside thread diameter in millimetres.
- GL 45: the standard neck of laboratory media bottles, typically from about 100 mL up to several litres. A large range of screw caps, pouring rings, multi-port caps and dispensers is built around it.
- GL 32 and GL 25: used on smaller laboratory bottles and some reagent containers.
- GL 18 and GL 14: screw-cap tubes, small bottles and tubing connectors, often with a septum or an open-top cap.
GL threads are not interchangeable with GPI finishes of similar diameter. A GL 45 cap will not reliably seal a 45/400 neck, or vice versa, because the thread profile and pitch differ.
Another frequently quoted European finish is DIN 18, the standard neck of amber glass dropper bottles (typically 5 to 100 mL) used in pharmacy and for reagents. It accepts a wide range of dropper inserts, pipette caps and tamper-evident screw caps. DIN 18 and 18/410 are both 18 mm finishes, but they are different thread forms, so check the drawing before assuming that a GPI closure will fit a DIN bottle.
Crimp, snap-fit and special-purpose finishes
Crimp necks for serum vials
Serum and injection-type vials have no thread at all. A rubber stopper, typically chlorobutyl or bromobutyl, is pressed into the bore and held by an aluminium seal crimped under the neck bead. The two common sizes are 13 mm, used on smaller vials, and 20 mm, the most widespread size across a broad range of vial volumes. A flip-off seal adds a plastic button that protects the stopper and shows whether the vial has been opened. Stopper fit depends on the bore and the blowback geometry of the neck, so stopper, vial and seal should be qualified together.
Snap-fit and press-fit closures
Snap caps engage a bead on the neck instead of a thread, while press-fit plugs and dropper tips rely on an interference fit in the bore. They are quick to apply and open, but their sealing depends heavily on the I dimension and on the stiffness of both parts, so the tolerance stack-up must be checked across the storage temperature range.
Child-resistant and tamper-evident closures
Child-resistant (CR) closures, such as push-and-turn or squeeze-and-turn designs, are tested as a complete package; reclosable CR packaging is assessed under ISO 8317. Tamper-evident (TE) closures carry a band that breaks on first opening and needs a matching retaining bead or ratchet below the thread. Both usually require a finish variant designed for them, not just a standard thread.
Analyzer-specific necks and pierceable closures
Reagent bottles for automated clinical chemistry and immunoassay analyzers often use necks defined by the analyzer's reagent compartment rather than by GPI or DIN. Neck position, opening diameter and cap style must suit the probe path, the bottle holder and any barcode reader. Closures may be screw caps removed before loading, pierceable caps or slit septa that the probe passes through, or caps that limit evaporation. For compatible replacement bottles these geometries have to match the analyzer's requirements, which is why they are usually produced in dedicated moulds.
Common finishes at a glance
| Finish | Typical diameter | Typical use | Typical closure types |
|---|---|---|---|
| 18/410, 20/410 | 18–20 mm | Small dropper and dispensing bottles, about 5–60 mL | Screw caps with dropper tip insert, nozzle caps |
| 24/410 | 24 mm | Narrow-neck bottles, about 30–250 mL | Lined screw caps, disc-top and dispensing caps |
| 28/400, 28/410 | 28 mm | Reagent and general-purpose bottles, about 60–500 mL | Lined screw caps, induction seals, TE caps |
| 38/400 | 38 mm | Wide-mouth bottles and jars, about 100 mL–1 L | Lined screw caps, induction seals |
| GL 45 | about 45 mm | Laboratory media bottles, about 100 mL to several litres | GL 45 screw caps, pouring rings, multi-port caps |
| GL 18, GL 32 | about 18 / 32 mm | Screw-cap tubes, small lab bottles, connectors | Screw caps with septum or open top, tubing connectors |
| DIN 18 | 18 mm | Amber glass dropper bottles, about 5–100 mL | Dropper inserts, pipette caps, TE screw caps |
| 13 mm / 20 mm crimp | 13 / 20 mm | Serum and injection-type vials | Rubber stopper with aluminium or flip-off seal |
| Analyzer-specific | Proprietary | R1/R2 reagent bottles for automated analyzers | Screw, pierceable, septum or evaporation caps |
Checklist: how to choose a neck finish
Before you fix a finish in a specification, work through the following points with your bottle and closure suppliers:
- Seal type: will the seal be made by a liner (foamed PE, PE- or PTFE-faced), an induction heat seal, a plug or valve inside the bore, or a stopper? Induction seals need a flat, smooth sealing land and a seal laminate matched to the bottle material.
- Application and removal torque: define the application torque for the diameter. A common rule of thumb is about half the closure diameter in mm, expressed in in-lb; for 28 mm that is roughly 12–17 in-lb (about 1.4–1.9 N·m). Check removal torque after storage, because plastics relax over time.
- Dropper and insert compatibility: the I dimension and its tolerance decide whether a dropper tip or orifice reducer stays in place and seals. Test with the actual insert, not a generic one.
- Automated capping: capping chucks, torque heads and cap feeders are set up for a specific closure. Finish height, the S dimension and the thread start affect cross-threading and cycle time.
- Analyzer probe access: for on-board reagent bottles, check the opening diameter, neck position and dead volume, and whether the probe pierces a septum or needs an open bottle.
- Materials and chemistry: confirm that bottle, closure and liner materials are compatible with the reagent, and run leak, stability and transport tests on the final combination.
Neck finishes in the Linden range
Linden Grup produces plastic bottles, dropper bottles and glass vials with standard finishes, as well as compatible analyzer reagent bottles with the neck geometries their reagent compartments require. Examples include the 12 mL clear glass vial with aluminium crimp cap and the 10 mL PET dropper bottle. Where a standard finish does not suit your closure or filling line, a custom mould can be considered. To discuss a finish or to request drawings and samples, contact us.