Unscrewing or stripping? How bottle cap moulds release threaded caps and tamper-evident bands
A screw cap has an internal thread, and in the mould that thread is formed around a steel core. When the mould opens, the cap has to come off that core without tearing the thread, stretching the tamper-evident band or marking the sealing surfaces. Stripping the cap off, or unscrewing it, is one of the first choices on a cap mould. It affects the cost of the mould, the cycle, the maintenance and which cap designs are practical.
Stripping, or jump-off, ejection
In stripping ejection, also called jump-off, the mould usually opens first. The cap comes out of the cavity still sitting on the core, and a stripper ring around that core then pushes it straight off. While the cap is warm, a flexible polyolefin can expand over a shallow thread and spring back. Polypropylene and polyethylene are the resins usually chosen for bottle caps released this way.
Stripping is a practical choice when:
- the thread is relatively shallow and rounded, so it can flex over the core without taking a permanent set;
- the material is a flexible polyolefin, such as the PP and PE grades usually used for bottle caps;
- the design has no rigid undercut that cannot expand.
This is the simpler mould of the two. It has no rotating drive to maintain, and the cycle does not have to wait while a core rotates. High-volume water and beverage closures are often released this way.
The limit is the part. A deep or sharp thread, a thick wall or a stiffer material can be marked or permanently stretched if it is forced off the core. The thread profile and the ejection have to be designed together.
Unscrewing ejection
When the thread cannot be stripped without that damage, the core, or a threaded sleeve, is rotated so the cap unscrews from the steel. The rotation is usually driven by a hydraulic or electric motor through gears, or by a rack and pinion.
Unscrewing is the usual choice when:
- the thread is deep or sharp, and forcing it over the core would mark or stretch it;
- the material is stiffer, or the wall is thicker, than a jump-off thread can accept;
- the thread should stay as moulded, and should not be stretched over the steel thread.
The cost is complexity. An unscrewing mould has gears, bearings and seals to maintain. It needs a drive on the moulding machine or built into the mould, and the time spent unscrewing has to fit in the cycle. The mould is also more expensive to build.
Where the tamper-evident band comes in
Many beverage and water caps have a tamper-evident band. It locks under the tamper bead on the neck and breaks away on first opening. Two routes are common.
Moulded skirt, then slitting and folding
The cap is moulded with a plain skirt where the band will be. After moulding, a slitting and folding machine cuts the slits that leave small bridges, and folds the band inward so the flaps can catch under the tamper bead. The mould stays simpler. The number of slits and the size of the bridges can be changed on that machine, rather than by cutting the mould again.
Band formed in the mould
The bridges, flaps or other retaining features are moulded in the tool. That can remove a later slitting or folding step. It also makes ejection more demanding, because those bridges have to leave the mould intact. Designs of this kind typically use a controlled, two-stage ejection. An inner sleeve or the core moves first, so the inner features have room to flex. The stripper then pushes the cap off at the band, so the bridges are not the part that has to stretch. Changing the band afterwards means changing the mould.
Which route fits depends on the cap, the volume and how often the band may change. A stable, high-volume cap can justify forming the band in the mould. If the band is still being changed, or several caps share one slitting line, slitting and folding keeps that change off the mould steel.
Questions to settle with your mould maker
Before a cap mould is quoted, these points keep two quotations comparable.
- Cap drawing or sample. Name the neck finish, for example PCO 1881, PCO 1810, 29/25, 30/25 or 38 mm, plus the thread profile, wall thickness and the liner or sealing design.
- Material. PP, HDPE or another resin, and the grade if it is already chosen.
- Tamper-evident band. Slit and folded after moulding, or formed in the mould.
- Ejection. Whether stripping is acceptable for this thread, or unscrewing is required, and why the mould maker says so.
- Machine. The moulding machine model, tonnage and platen size, and whether it already has the core-pull, unscrewing or ejector functions this mould needs.
- Downstream equipment. If the band is slit and folded, which slitting and folding machine will run it.
- Output. How many good caps per hour are needed. Together with the machine, that frames the cavity count and the hot runner layout. It does not by itself fix either one.
If a PET preform mould is part of the same project, the cap and the preform have to use the same neck finish. The closures are not interchangeable. A PCO 1881 bottle needs a PCO 1881 closure, and a PCO 1810 bottle needs a PCO 1810 closure.
PCO 1881 and PCO 1810, and why the closures are not interchangeable
How many cavities to specify on a bottle cap mould
What to send OSLON
OSLON Precision Mold designs and makes bottle cap moulds, PET preform moulds and hot runner systems at its factory in Taizhou, China. Send the cap drawing, the neck finish and the output you need with the inquiry, or bring them to Plastics & Rubber Indonesia 2026.
The show is 17–20 November 2026 at JIExpo Kemayoran, Jakarta, booth A3-3026. You can also email the drawing to frank@oslonmold.com.
Send the cap drawing with your inquiry
Plastics & Rubber Indonesia 2026, booth A3-3026, 17–20 November, JIExpo Kemayoran, Jakarta
Questions buyers ask
When is stripping, or jump-off, ejection used?
The mould usually opens first, and a stripper ring then pushes the warm cap straight off the core. A flexible polyolefin can expand over a shallow, rounded thread and spring back. A deep or sharp thread, a thick wall or a stiffer material can be marked or permanently stretched if it is forced off.
When does the mould unscrew the cap instead?
When the thread cannot be stripped without that damage, the core or a threaded sleeve is rotated so the cap unscrews from the steel. The rotation is usually driven by a hydraulic or electric motor through gears, or by a rack and pinion. The time spent unscrewing has to fit in the cycle.
How is a tamper-evident band made?
One route moulds a plain skirt, then slits and folds the band so the flaps can catch under the tamper bead. The number of slits and the size of the bridges can be changed on that machine. The other route moulds the bridges or flaps in the tool. That can remove a later slitting or folding step. Designs of this kind typically use a controlled, two-stage ejection, so the bridges are not the part that has to stretch.
What should you settle before a cap mould is quoted?
Send the cap drawing or sample, the resin, whether the band is slit after moulding or formed in the mould, and whether stripping is acceptable for this thread. Name the moulding machine, including any unscrewing or ejector function it has, the slitting machine if you use one, and how many good caps per hour you need. A cap and a preform mould in the same project have to use the same neck finish.