Bottle Preform Selection and Neck Finish Standards

1. From preform to bottle

PET bottles are made in two steps: injection moulding produces the preform — a thick-walled tube with the neck finish already formed — and a blow moulder reheats and inflates it in a mould to the final bottle. The preform's weight and design set the bottle's strength, weight and price; the blow mould sets its shape. Change the bottle and you change the preform and the blow set, not just the label.

Everything downstream — the blow moulder's output, the filler's infeed, the capper — is specified around the preform and the bottle. They are the first documents a line supplier asks for.

2. Neck finish standards

The neck finish is the threaded mouth of the bottle, formed in the preform. Its diameter, thread profile and tamper-evident ring set the cap type — and therefore the capper, the cap sorter and the infeed. Standard finishes (the common 28 mm and 30/25 cap families are the volume standards) are cheap and universally supplied; custom finishes cost more and lock you to one cap supplier.

Standardisation is a quiet cost saver: one neck finish across formats means volume cap buying and fewer capper change-parts.

3. Bottle weight

Bottle weight is the grams of PET per bottle — the biggest recurring packaging cost, and the lever the market feels in the shelf price. Lighter bottles save material on every bottle but are weaker and less forgiving; the weight is optimised with the preform supplier against the filling conditions (still, carbonated, hot-fill) and the handling the bottle must survive. A combiblock enables lighter bottles because the bottle is filled hot-off-the-blow, never handled in between.

Weight optimisation is a preform-design exercise with the line as the constraint — the machine enables the saving, the preform realises it.

4. Wall distribution

The wall distribution — how the PET is spread between the base, the body and the shoulder — is set by the preform design and the blow process. A bottle that is thick where it is stressed (base, shoulder, handle) and thin where it is not uses less PET for the same performance. The distribution is invisible to the eye and central to the cost — and it is why the preform, not the bottle drawing alone, is the spec.

Confirm the wall distribution requirement with the preform supplier and the line supplier together — the preform that blows well on the line is the one both approved.

5. Hot-fill preforms

Hot-fill product (juice, tea) fills the bottle hot, and the bottle must survive the temperature and the vacuum that forms as it cools. The hot-fill preform produces a heavier bottle with a vacuum-panel design — visibly different from a cold-fill bottle of the same capacity. The preform and the filling line are specified together for hot-fill, because the filler, the capper and the cooling tunnel all follow the bottle's heat behaviour.

For hot-fill PET, the preform's heat resistance and the panel design are part of the product's success — not a packaging detail.

6. Sourcing preforms

Preforms come from dedicated injection moulders or from a preform supplier with standard stock. Standard preforms are cheap and quickly available; custom preforms (own weight, colour, finish) need a mould — an investment with a lead time. The preform price is part of the bottle cost and belongs in the business model, not just the line budget. Confirm the supply: local stock, mould lead time and quality consistency.

The line is only as reliable as its preform supply — a plant that cannot source preforms cannot run, whatever the machine's rating.

7. The preform spec

Five inputs define the preform and the line: capacity, neck finish, weight, wall distribution (from the preform design) and the product (still, carbonated or hot-fill, which decides the bottle's duty). Send these with the enquiry and the blow moulder, the filler and the capper are quoted around them — the blow moulding machine and filling machine ranges are the starting points.

Frequently asked questions