A preform neck finish is the only part of a PET bottle that cannot be corrected after the mould is cut. Changing the container body is a new mould and a new blow recipe; changing the finish is a new mould, a new cap, a new capping chuck, a new torque recipe and often a new filling valve. That is why finish selection belongs at the beginning of a project, next to the product and the target output, and not at the end when the closure supplier finally sends a drawing.
The expensive mistakes repeat in the same order. A brand adopts PCO 38 because a co-packer already runs it, then discovers its own capping head needs a different chuck and torque setting. A water producer keeps a 28 mm cap on a 1.5 L bottle and finds the neck has to carry top-load through palletising, so the preform gains weight it never needed. Each of those is a decision taken with the wrong information.
Sailwin has delivered PET stretch blow molding machines into 500+ installations across 60+ countries over 15+ years, manufacturing under ISO 9001:2015 with CE marking. Engineering review starts from the neck finish because it constrains the mould, the cap, the filling head and the capping station at the same time. This guide compares PCO 28, PCO 30 and PCO 38 on the criteria that actually decide a line, and shows where each one stops being the right answer.
Key Takeaways
- Neck finish is a line decision, not a bottle decision. It fixes cap fit, the filling-head interface and the capping torque recipe together, so changing it later touches three departments and usually one purchase order too many.
- Product pressure and bottle size choose the finish, not supplier convenience. PCO 28 dominates still water and small still containers, PCO 30 sits between the two, and PCO 38 earns its place where top-load, flow area or existing capping equipment demand it.
- Sailwin PET machines cover PCO 28 / 30 / 38 plus wide-mouth necks to 130 mm, for containers from 0.1 L to 20 L — which means the finish can be settled before the machine model is fixed, rather than forcing the bottle to fit the equipment.
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1. What a Neck Finish Specification Actually Contains
A neck finish drawing is a set of dimensions described by a standard such as the PCO series or the ISBT finish list, plus the thread form that sits inside those dimensions. Four letters carry most of the meaning. T is the major thread diameter, E is the root diameter between the threads, H is the neck height from the support ring to the sealing surface, and S describes the support ring and tamper-evident band geometry. Two finishes can look identical in a photograph and differ by fractions of a millimetre in E and H.
Those fractions decide whether the closure seals. A cap that threads on but does not compress the sealing liner correctly will pass a visual inspection and fail a leak test after transit. The finish also fixes how the bottle is handled: the support ring is the handling surface for the whole life of the container, from the blow moulder through the filler to the consumer.
| Drawing element | What it controls | What happens when it is wrong |
|---|---|---|
| T — major thread diameter | Cap engagement and the repeatability of the capping torque setting | Caps cross-thread, or the capping head cannot hold a consistent torque band |
| E — thread root diameter | Interference with the cap thread and the compression force applied to the liner | Leaking closures in one direction, hard-to-open closures in the other |
| H — neck height | Seal compression on the liner and how the bottle registers in the filler and capper | Under-compressed seals that hold at the plant and weep in transit |
| S — support ring and tamper band | Handling through blow moulding, filling, capping and palletising | Bottles drop in transfer, or the tamper band bridges and fails to separate cleanly |
| Neck wall thickness | Top-load strength and resistance to ovalisation under stack load | Neck deformation on the pallet and seal failure in the warehouse, not on the line |
2. PCO 28 vs PCO 30 vs PCO 38 in a Direct Comparison
The three finishes differ mainly in opening diameter, and almost every practical consequence follows from that one number. A wider opening spreads vertical load over more material, gives the filler a larger flow area and takes a larger, more expensive closure. A narrower opening keeps closure cost and material weight down but asks more of the bottle body and base. Sailwin builds PET blow molding machines for the full set — PCO 28, 30 and 38 mm — plus wide-mouth necks up to 130 mm, so the comparison below is about product fit, not about machine availability.
| Criterion | PCO 28 | PCO 30 | PCO 38 |
|---|---|---|---|
| Nominal opening | 28 mm | 30 mm | 38 mm |
| Typical container fit | Still water and small still bottles where closure cost and weight matter most | Mid-size still formats where a slightly larger flow area is worth a slightly larger closure | Larger soft drink and juice formats, and bottles that need higher top-load |
| Top-load behaviour | Least neck area, so the body and base design carry more of the stack load | Between the two | Highest of the three because the wider neck spreads vertical load over more material |
| Flow area at the filler | Smallest, so fill time is the longest for a given head count | Intermediate | Largest, shortening fill time and helping viscous or pulpy products |
| Closure cost and supply | Generally the lowest, with the widest supplier base | Between the two | Higher material content, and fewer standard options to choose from |
| Capping equipment | The most commonly available chuck and torque recipe | Needs the matching chuck; 28 mm change parts will not transfer | Needs its own chuck and usually a heavier capping head |
| Where it goes wrong | Forcing a 28 mm finish onto a heavy large bottle to save closure cost | Assuming existing 28 mm tooling will adapt without new change parts | Adopting 38 mm for equipment reasons, then paying for it in every closure |
A neck finish is not a detail you can hand to a supplier and forget. It is the interface between four systems — preform, cap, filling line and capping station — and the only one of those four that is expensive to change after the mould exists.
3. How the Neck Finish Interacts With Preform Weight and Blowing
The finish is also why the neck of a PET bottle is treated differently from the body. The neck must stay dimensionally stable while the body is stretched, so it is shielded from the heating cycle that softens the rest of the preform. Sailwin machines heat preforms between 90 °C and 115 °C with PID control holding approximately ±1 °C, and that protection is what lets the body reach its stretch window without the finish deforming. Neck weight then follows from the finish rather than from bottle volume: a larger finish means more material above the support ring, carrying stack load instead of adding body wall.
- Blowing pressure is a separate decision from the finish. Sailwin machines operate at 25–40 bar, and the pressure required depends on container geometry and stretch ratio, not on closure size.
- Cycle time is set by heating and cooling, not by the neck. When a line runs slow, check the heating profile and the mould cooling circuit before blaming the finish.
- Energy performance scales with cavity count. Servo drives reduce power consumption by up to 30% against fixed-speed hydraulic machines, high-pressure exhaust recovery cuts compressor load by roughly 20%, and the PLC monitors 40+ parameters in real time so a neck-dimension problem can be traced to a station rather than guessed at.
Not Sure Which Finish Your Bottle Actually Needs?
Send the container drawing, target output and closure standard — our engineers confirm the finish, preform weight band and cavity count before you commit to a mould.
4. Case Study: A Two-Format Bottled Water Project That Started With the Wrong Finish
A beverage bottler planned two still water formats on one line — a small single-serve bottle and a larger family format — and had already ordered a closure tool for the smaller finish before the container drawings were finalised.
- One closure standard was already tooled before the large-format container drawing was settled
- Top-load on the larger format had to be carried by a neck specified for a much lighter bottle
- The proposal assumed one preform weight would serve both formats, which conflicted with the stack height on the pallet
- Neck finish reviewed against the closure standard first, then against stack load, before the mould specification was frozen
- Separate preform weights confirmed for the two formats instead of forcing one weight across both
- Machine selected from the Sailwin PET range with the cavity count fixed by the target output: SW-F4-650 at 5,500 bottles per hour up to 600 ml, or SW-F4-2000 at 4,500 bottles per hour up to 1,800 ml
- Neck protection in the heating profile verified during full-load FAT testing before shipment
- Closure tooling stayed valid because the finish was verified against the existing standard rather than changed late
- Pallet stability was addressed at the preform stage, where it costs resin, instead of at the warehouse, where it costs a recall
- Changeover between the two formats stayed inside the machine design, because one machine was specified for both bottle sizes rather than one format retrofitted later
Scenario based on a Sailwin customer project; site-specific figures available on request during engineering review.
Frequently Asked Questions
Get Your Neck Finish Confirmed Before the Mould Is Cut
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Related Reading:
• PET Blow Molding Machines — full model and capacity range
• PET Blow Molding Machine Buying Guide
• 4-Cavity vs 6-Cavity PET Blow Molding Machine
• Carbonated Bottle Blow Molding Machines
• Wide-Neck PET Jar Blow Molding Machines up to 130 mm
• Injection Molding Machines for preforms and closures
