PET Bottle and Preform FAQ

1. Preform to bottle

PET bottles are made in two steps: injection moulding produces a preform — a small, thick-walled tube with the neck finish already formed at the top — and a blow moulder reheats the preform and inflates it in a mould to the final bottle shape. The preform is the bottle's starting point, and its weight and wall distribution decide the bottle's strength, weight and price.

Everything about the bottle — capacity, weight, neck, panel design — is fixed by the preform and the blow mould. Change the bottle and you change the preform and the blow set, not just the label.

2. Neck finishes

The neck finish is the threaded mouth of the bottle, formed in the preform. Its diameter, thread and tamper-evident ring set the cap type — and the capper on the line. Standard finishes (such as the common 28 mm and 30/25 caps) are cheap and universal; custom finishes are possible but cost more and lock you to one supplier.

Standardisation is a quiet cost saver: a plant with one neck finish across formats buys caps in volume and changes capper change-parts less.

3. Bottle weight

Bottle weight is the grams of PET per bottle, and it is the biggest recurring packaging cost. Lighter bottles save material on every bottle, but they are weaker and less forgiving — and they need the right preform design and filling conditions to survive. A combiblock enables lighter bottles because the bottle is filled hot-off-the-blow, never handled in between.

Weight is optimised with the preform supplier, not on the line. The machine enables the saving; the preform design realises it.

4. Blow moulding

The blow moulder reheats the preform and blows it to shape in a mould. Its output is set by cavity count and cycle time: more cavities mean more bottles per hour, and the cycle time is set by the preform weight and the bottle design. The blow moulder needs a steady supply of compressed air — a utility that is part of the plant design, not an afterthought.

Confirm the blow moulder's rated output on your preform and bottle, not on a generic figure. The difference between a 1 L and a 5 L bottle at the same cavity count is significant.

5. Hot-fill bottles

Hot-fill product (juice, tea) fills the bottle hot, and the bottle must survive the temperature and the vacuum that forms as the product cools. A hot-fill PET bottle is heavier, with a panel design that absorbs the vacuum without distorting — visibly different from a cold-fill bottle of the same capacity. Glass is the alternative where brand weight justifies it.

The bottle and the filling line are specified together for hot-fill: the filler, the capper and the cooling tunnel all follow the bottle's heat behaviour.

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 preform supply: local stock, lead time for moulds, and the quality consistency. A line is only as reliable as its preform supply.

7. What the line needs from the bottle

The line supplier needs five bottle facts: capacity, neck finish, weight, the drawing (with wall distribution), and the product — still, carbonated or hot-fill — because that decides the valve, the infeed and the capper. Send these with the enquiry and the quote is real; without them it is a guess with a price.

The bottle ties the line together: the combiblock blows it and fills it, the filling machine fills bought-in bottles, and the cap and label finish it.

Frequently asked questions