Key takeaways
- The neck finish standard (e.g. 24/410) must match the closure exactly.
- A liner or plug seal decides whether a liquid leaks in transit.
- Screw caps loosen under vibration; a break tab or a pump lock helps.
A closure costs cents and causes more complaints than any other component. It is a sealing system made of a neck finish, a thread, an orifice and a liner, and every one of those has a tolerance. Most leaks come from a mismatch between two parts that were each individually within specification.
1. The neck finish
The neck finish is specified as a diameter, a thread style and a height, for example 24/410. A 24/410 bottle needs a 24/410 closure. A 24/415 closure will thread on and leak. The number is the whole specification, and a “close enough” match is a leak.
2. Liners
A liner is the sealing element inside the cap: a foam liner for a cream, a pressure-sensitive liner for a liquid, a plug seal for a bottle that must not leak at all. A cap without a liner on a liquid bottle leaks in transit, however tight it is screwed.
3. Plug seals
A plug seal presses into the bottle neck and seals on the inside. It is the most reliable option for a low-viscosity liquid and the standard for a toner or a micellar water. A plug seal requires the neck bore to be within tolerance, which is a bottle specification issue.
4. Orifice and flow
The orifice controls the dose. A 3 mm orifice suits a toner; a 6 mm orifice suits a lotion; a 10 mm orifice suits a shampoo. An orifice that is too small clogs with a viscous product, and one that is too large dumps the product. The orifice is chosen with the viscosity, not after it.
5. Vibration
A screw cap loosens under transport vibration over hours. A cap with a break tab holds better, and a pump with a lock collar cannot be actuated accidentally. A product that leaks in a parcel is usually a screw cap that worked free rather than a bad seal.
6. Material
PP is the default closure material because it tolerates most formulas and is cheap. A metal cap on an acidic formula corrodes; a metal cap on a PET bottle with a high-alcohol formula can stress the bottle. The closure material must be compatible with the formula as well as with the bottle.
7. Child resistance
Some actives at high concentration require child-resistant closures in some markets. A child-resistant cap is a different mechanism and costs more. The requirement depends on the active and the market, and it should be checked before the pack is chosen.
8. Recycling
A closure of the same polymer as the bottle is recyclable with it; a different polymer is a contaminant. A PP cap on a PP bottle is the best combination. A closure with a metal spring or an elastomer seal is not recyclable as a unit.
9. Leak testing
Fill, cap, invert and shake. Then place in a vacuum chamber at 300 mbar for 30 seconds. A leak shows as bubbles. Do this on ten units per lot, and again after a vibration test. A leak test at ambient pressure misses most failures.
10. Cost
A stock closure costs a few cents and a custom moulded one costs thousands in tooling. For a first run, a stock closure matched to a stock neck finish is the cheapest reliable option. Custom closures arrive with the second run and a higher MOQ.
11. Where it fits
Closure decisions run across the toner range; the tamper variant continues in the tamper evidence entry.
Reference: Closure supplier data and leak test records, 2025
