Two numbers decide whether a PET preform blows well, and neither is the price. The first is the intrinsic viscosity, or IV, of the resin; the second is the weight tolerance of the moulded preform. Programmes that specify neither get parts that blow acceptably in a trial lot and reject badly in production, because both numbers drift, and both drifts are invisible until the bottle line stops.
Intrinsic viscosity: the number behind the blow
IV measures the polymer chain length, and it decides how far the preform stretches before it breaks. Too low and the preform blows through, producing thin sidewalls and a bottle that fails its drop test; too high and the preform resists stretching, producing thick bases and a bottle that costs more resin than it should. The working range for most bottle programmes is 0.76 to 0.84 dl/g, quoted as a range with a stated test method rather than a single number, because the blow process tolerates a band, not a point.
| IV (dl/g) | Behaviour on the line | Typical use |
|---|---|---|
| 0.72 to 0.76 | Stretches easily, thin walls, low heat stability | Carbonated soft drink, thin-wall bottles |
| 0.76 to 0.84 | Balanced stretch and strength | Water, juice, food containers |
| 0.84 plus | Resists stretch, thick walls, higher resin cost | Hot fill, wide-mouth jars, returnable |
IV and the pigment
Coloured preforms complicate the IV picture, and this is where programmes are most often surprised. The masterbatch load changes the thermal history the preform sees in the tool, and a dark colour from the same base resin does not carry the same IV after moulding as a light one. A coloured preform programme therefore specifies IV on the moulded preform, not on the incoming resin, and the lot card carries the moulded result. If the order specifies IV on the resin alone, the blowing line inherits a drift it cannot see.
Weight tolerance is the blowing window
The preform weight is not a dimensional nicety; it is the blowing window. A preform that is 0.5 g heavy blows into a bottle with a thin sidewall, and one that is light blows short and fails the fill level. The standard family holds plus or minus 0.2 g, verified per lot on a sampled basis, and the lot card carries the weight distribution rather than only the average, because the line's reject rate follows the spread, not the mean. Programmes with tight blow processes quote tighter tolerances and pay for them, correctly.
Neck finish and the sealing surface
The neck finish is the interface to the closure, and it is the part of the preform that cannot be corrected by blowing. A scored finish is a blown bottle that leaks at the sealing surface, which is why preforms are packed with the finish protected and why the thread gauging is verified on the preform rather than assumed from the tool. Wide-mouth jar blanks add the thread runout as a critical feature, because sink at the runout is a leak path that only appears after the closure is applied with torque.
Writing the specification
A complete preform specification has six lines: resin grade and food-contact declaration, IV range with test method, weight range with tolerance, neck finish drawing, colour with delta-E target where coloured, and packing with finish protection. Everything else on a preform quotation is commercial. Programmes that take the six lines to two suppliers get comparable quotes, and programmes that take a drawing get parts that are dimensionally identical and blow differently.
Related reading
Our coloured preform and jar blank programme covers the production route, and the preform category lists the full range. The blow moulding process guide covers the downstream step.
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
