Understanding PET: Material Properties, Grades & IV Specification for Preform Manufacturing
PET resin is not a commodity. The grade you select, the IV you specify, and the catalyst system you choose directly determine how your mould must be designed and what quality your preforms will achieve. Here is what every buyer and engineer needs to know.
1. What Is PET?
Polyethylene terephthalate (PET) is a linear, semi-crystalline thermoplastic polyester produced through the polycondensation of purified terephthalic acid (PTA) and monoethylene glycol (MEG). The result is a long-chain polymer whose average molecular weight — and therefore every important processing and performance property — is controlled precisely during manufacture.
Crystal Clear
Glass-like transparency for water, juice & CSD bottles
High Strength
Excellent tensile & impact resistance after biaxial orientation
Barrier
Good CO₂ and O₂ barrier; excellent moisture barrier
Chemical Resistance
Safe for food, beverage, pharma & household chemicals
Recyclable
Fully recyclable; supports rPET circular economy
Lightweight
Significant weight advantage over glass; logistics savings
2. Intrinsic Viscosity (IV): The Most Important Specification
IV measures the average molecular chain length of PET in decilitres per gram (dL/g). Higher IV = longer chains = higher melt viscosity = stronger material. It is the single most important number on any PET resin data sheet.
3. Amorphous vs. Semi-Crystalline PET
PET exists in two structural states. Managing which state appears where in the preform is a core mould engineering discipline.
| State | Appearance | Where Required in Preform | How Achieved |
|---|---|---|---|
| Amorphous | Clear, transparent | Preform body — essential for blow moulding | Rapid quench cooling in the mould |
| Semi-crystalline | Opaque, white | Neck / finish — required for stability and hot-fill | Post-mould neck crystalliser |
A hazy or cloudy preform body means crystallisation has occurred in the mould — a direct sign of insufficient cooling, excessive wall thickness for the given cycle time, or hot spots in the cooling circuit. This requires a mould engineering solution, not a process adjustment.
4. PET Catalyst Systems
| Catalyst | Colour | AA Generation | Market Position | Best For |
|---|---|---|---|---|
| Antimony (Sb) | Slight blue-grey | Moderate | Low cost; globally dominant | CSD, household, general packaging |
| Titanium (Ti) | Near water-white | Low | Premium; growing share | Still water, pharma, high-clarity |
| Germanium (Ge) | Water-white | Very low | Premium; Japan + specialists | Highest-end water and cosmetics |
Engineering-Driven Mould Design — Matched to Your Resin
Hexamech specifies hot runner geometry, gate design, and cooling circuits around your PET grade and IV specification from day one.
Contact Our Engineering Team