Acetaldehyde in PET Preforms: Causes, Measurement, and Control
Acetaldehyde (AA) is tasteable in still water at concentrations below 40 ppb. Understanding what generates it, how to measure it, and — critically — how mould design controls it is essential knowledge for any quality-focused preform operation.
What Is Acetaldehyde and Why Does It Matter?
Acetaldehyde (CH₃CHO) is a naturally occurring aldehyde that forms as a thermal degradation by-product when PET is heated above its melting point. It has a characteristic fruity or solvent-like odour and taste. In still water — where there is no flavour, carbonation, or colour to mask it — AA is detectable at concentrations as low as 15–40 ppb in the water itself.
Even with a preform AA content of only 2–3 ppm, migration into still water during storage can exceed taste thresholds depending on fill temperature, storage conditions, and bottle wall thickness. This is why still water brands specify preform AA levels below 1 ppm — and why mould design is a direct quality lever.
The Four Primary Drivers of AA Generation
AA Measurement Methods
| Method | What Is Measured | Standard | Use Case |
|---|---|---|---|
| Headspace GC (gas chromatography) | AA vapour from preform headspace | ISO 14895 / ASTM F2013 | Production quality control — most common method |
| Migration testing | AA in water after contact with bottle | EU Regulation 10/2011 | Regulatory compliance testing |
| Simulation (Moldflow) | Predicted thermal history → AA risk | CAE-based | Design stage — Hexamech uses this before mould is built |
How Mould Design Controls AA
| Mould Design Variable | AA Impact | Hexamech Approach |
|---|---|---|
| Hot runner manifold volume | More volume = longer residence time = more AA | Minimum necessary volume; no dead zones in channels |
| Gate geometry (diameter, land) | Incorrect sizing causes localised shear heating spike | Gate sized for IV and flow rate; validated via Moldflow |
| Gate type | Thermal gate tip stays hot between shots; valve gate closes positively | Valve gate standard for AA < 1 ppm applications |
| Temperature zoning | Uniform, controlled temperature prevents hot spots | Individual zone control per nozzle in all Hexamech moulds |
Many preform producers try to control AA purely through processing adjustments (lower barrel temperature, higher screw speed). While these help, the mould itself is a major variable. A hot runner with dead zones, oversized nozzle tips, or poor temperature uniformity will generate AA regardless of how carefully the machine is set. At Hexamech, AA control is a design objective from the first drawing — not an afterthought.
Engineering for Low AA — From Mould Design Onwards
Hexamech designs hot runner systems with AA control as a primary engineering objective — validated through Moldflow thermal analysis before manufacturing.
Discuss Your AA Requirements