From haze to clarity: how to optimise rPET stretch blow moulding for consistent bottle quality

The transition from virgin PET to rPET in stretch blow moulding is far from plug and play. Mechanically recycled PET undergoes repeated exposure to heat during processing, which causes polymer chain scission, a reduction in molecular weight, and ultimately diminished mechanical strength. The result is variation in melt viscosity, crystallisation rate and thermal stability that prevents a straightforward substitution of virgin PET with rPET on the production line.

Using BMT's material characterisation capabilities, our Head of Material Characterisation, Josh Turner, set out to quantify exactly how those differences show up in practice, and what it takes to design them out.

What the data showed

Using BLOWSCAN, BMT's instrumented lab scale stretch blow moulding machine, virgin PET and rPET preforms were free blown under matched conditions while high speed imaging and three dimensional Digital Image Correlation tracked deformation across the entire preform.

The results were clear. A larger blowing diameter was associated with the rPET preform throughout the test window, with greater out of plane displacement observed at equivalent time steps compared to virgin material. Extracting the true stress versus stretch response in both the hoop and axial directions confirmed why: the onset of strain hardening, known as the Natural Draw Ratio, was delayed by approximately 6% in the hoop direction for the rPET preform compared to virgin PET under identical processing conditions. That shift in deformation behaviour points to an inherent variation in wall thickness distribution in the finished bottle if left unaddressed.

Pinpointing pearlescence

Pearlescence, the milky surface haze that diminishes the visual quality of a PET bottle, occurs when material is stretched beyond its capacity, forming microscopic cracks within the structure. It does not compromise the structural integrity of the bottle, but it strongly affects how consumers perceive product quality.

rPET is more prone to this defect. Having already undergone thermal degradation during recycling, it carries increased crystallinity and a correspondingly higher propensity for pearlescence to form. To preserve clarity, manufacturers need tighter control over processing parameters than virgin PET typically demands. The most effective lever is increasing the temperature of the region undergoing maximum stretch, although this has to be balanced carefully to avoid triggering thermal crystallisation instead.

Using the same DIC techniques, BMT mapped the exact stretch ratio at which pearlescence begins to form, across a range of preform temperatures and mass flow rates, for both virgin and rPET resin. Across every combination tested, the surface plot for rPET consistently sat below that of virgin PET, confirming a heightened sensitivity to pearlescence throughout the stretch blow moulding process. Low temperature, high rate deformation was the combination most strongly associated with early pearlescence onset at lower stretch ratios, while higher preform temperatures combined with lower mass flow rates reduced its formation.

The clear difference

With the specific stretch ratio responsible for triggering pearlescence identified for each resin, BMT moved to single cavity mould blow trials. Internal and external preform temperature profiles were measured and matched using THERMOSCAN following IR heating, and prototype bottles were produced on BLOWSCAN.

The first comparison was stark. A bottle produced entirely from virgin PET resin showed clean optical clarity with no visible defects. A bottle produced from rPET, using exactly the same processing parameters, displayed a pronounced area of pearlescence concentrated around the base, precisely where the highest stretch ratios occur. Switching materials without adjusting for their distinct processing windows had visibly compromised the bottle.

Using the data gathered from the earlier free blow trials, BMT then adjusted the processing parameters specifically for the rPET resin and produced a third prototype. Comparing this optimised rPET bottle against the original, defective rPET bottle showed complete removal of pearlescence induced optical haze, with bottle performance maintained throughout.

Why this matters for manufacturers

This work demonstrates why understanding the behaviour and processing characteristics of mechanically recycled PET cannot be skipped before substituting it into an existing production line. By systematically quantifying mechanical properties, the onset of strain hardening and susceptibility to pearlescence, manufacturers can tailor their stretch blow moulding process specifically for rPET rather than discovering the gap through defective production runs.

The approach reduces waste and machine downtime while ensuring a more efficient transition to sustainable packaging. As demand for recycled content grows, this kind of material characterisation and validated simulation work provides a robust framework for achieving high quality rPET based products without sacrificing performance or appearance.

Want to find out how characterisation could help eliminate defects in your own rPET transition? Get in touch with our team