Anatomy of a preform
What a preform is made of and what each part does: neck, support ring, body, base and gate.
- Threaded neck
The finished part of the bottle that does not change during blowing. Caps and the capper are matched to it.
- Support ring
The machine holds and carries the preform by this ring. Everything below it gets stretched.
- Body wall
The material reserve of the future bottle. A thicker wall means a heavier preform and a stiffer bottle.
- Base and gate
This is where the melt entered the mould. A clean gate with no strings or bubbles shows careful moulding.
Neck: the finished part of the bottle
The neck is formed when the preform is moulded and does not change during blowing: heating leaves it alone and the machine holds the preform by the ring below it. The neck carries the cap thread and a bead that catches the cap tamper band on first opening.
Neck dimensions define the standard: PCO 1881, PCO 1810 or 38 mm. The cap and the capper settings depend on the standard, which is why the neck is chosen first.
Support ring
The machine carries and holds the preform by the ring. The distance from the neck top to the ring is one of the key dimensions of a standard: about 17 mm for PCO 1881 and about 21 mm for PCO 1810.
Transition zone and body
Below the ring the wall widens into the body. The transition zone becomes the bottle shoulder, the body becomes its walls. The body is the material reserve: the thicker the wall, the heavier the preform and the stiffer the bottle.
Base and gate
The centre of the base shows the point where the melt entered the mould. It should be clean, with no strings, bubbles or white spot. A white spot means crystallisation; a string can puncture the bottle during blowing.
Weight equals bottle weight
All the PET in a preform goes into the bottle: blowing only redistributes it. So the bottle weighs exactly as much as the preform, and choosing the weight means choosing the PET used per bottle.
What to check on delivery
- Neck: thread with no flash or chips, an even ring.
- Body: no bubbles, black specks or streaks.
- Gate: a clean point with no strings or white spot.
- Weight: weigh 10 preforms on precise scales and compare with the nominal.
