In stretch blow moulding PET, the final container properties are controlled less by resin choice alone than by three process variables: preform reheat, stretch rod stroke, and hoop ratio. Amorphous injection-moulded preforms are produced from bottle-grade polyethylene terephthalate with an intrinsic viscosity typically between 0.80 dL/g and 0.84 dL/g when measured by ASTM D4603-18 in 60/40 phenol/1,1,2,2-tetrachloroethane at 30 °C. The preforms are fast-cooled below the glass transition temperature of approximately 78 °C to 80 °C, then reheated offline in a two-stage process. Preform reheat establishes local material temperature; the stretch rod imposes axial strain; and hoop ratio, which is the ratio of blown container internal diameter to preform internal diameter at the same axial coordinate, imposes circumferential strain. The process window is bounded by strain-hardening behaviour, thermal crystallization kinetics, and stress relaxation. A small shift in one variable propagates into wall thickness, base clearance, residual stress, and gas barrier. Industrial qualification therefore addresses reheat infrared profiles, servo-rod kinematics, and axial mapping of hoop ratio simultaneously, not as isolated settings.

What Reheat Temperature Window Prevents Premature Spherulitic Crystallization?

In two-stage reheat-stretch-blow moulding, preform reheat is performed with quartz halogen or near-infrared emitters whose peak irradiance lies between 0.9 µm and 1.6 µm. Heat transfer is deliberately non-isothermal because PET has a thermal conductivity of roughly 0.15 W/(m·K) to 0.24 W/(m·K) and a specific heat capacity near 1.3 J/(g·K) at ambient temperature. Surface temperature is acquired with short-wavelength pyrometers calibrated against a transfer standard per ASTM E1256; emissivity is set between 0.90 and 0.95 depending on preform colour and surface finish. If the outer skin exceeds 115 °C for more than a few seconds, cold crystallization can nucleate spherulites and produce permanent sidewall haze. If the body temperature remains below 85 °C, the material is too close to the glass transition for stable high-pressure inflation and final blow pressures of 3.5 MPa to 4.0 MPa generate stress whitening and uneven thickness. The working body temperature window for bottle-grade PET is narrow, typically 95 °C to 110 °C for single-serve water and carbonated soft drink preforms.

The infrared absorption across the preform wall is sensitive to pigment loading and wall thickness. Transparent water-grade preforms heat more evenly than coloured carbonated soft drink preforms, which may require adjusted lamp intensity, longer oven residence, or additional air cooling to prevent surface overheat. Reheat ovens are segmented into 8 to 16 axial zones; each lamp zone is controlled by thyristor power controllers with pyrometer feedback. For a 28 g 500 mL carbonated soft drink preform, overall reheat time is typically 20 s to 40 s, with an equilibration period of 10 s to 30 s before the blow mould closes. The axial thermal profile is intentionally non-uniform: the neck support ring and thread region are maintained below 70 °C to avoid deformation under clamp force, while the body is heated to the stretching window and the gate area is often set 5 K to 10 K cooler to prevent base crystallinity and gate push-out. Circumferential temperature spread should be held within ±2.5 K; wider spread is associated with sidewall thickness variation above ±0.1 mm in production.

Preform injection conditions also establish the reheat response before the preform reaches the blow moulder. A preform with uneven wall thickness, unbalanced runner filling, or gate crystallinity will not reheat uniformly even with correct lamp zoning. Injection moulding of preforms therefore uses hot runner systems with individually controlled valve gates, cooling times short enough to avoid significant crystallinity but long enough to set geometry. The resulting preform wall thickness distribution is measured before reheat; wall thickness variation across the body should remain below 0.05 mm for stable thermal profiling. Residual stress from injection can be partially relaxed during reheat, but excessive gate crystallinity cannot be reversed and leads to base haze after blowing.

During axial extension, the stretch rod enters through the neck after preform seating and advances toward the blow cavity bottom to impose the first stage of biaxial strain. In a rotary SBM wheel operating at 1,200 to 2,200 containers per cavity per hour, the rod contact-to-full-stroke interval can be completed in 0.15 s to 0.40 s depending on preform length and machine pitch. The rod is usually water-cooled or thermally isolated from the preform gate to prevent localized sticking. If the rod speed is too high, the engineering axial extension rate exceeds the relaxation response of oriented PET chains and strain localization may occur in the transition region between the cylindrical body and the base; the defect appears as thin bands, base eccentricity, or inconsistent drop-impact resistance. If the rod speed is too low, the preform cools before final inflation, reducing axial orientation and increasing oxygen transmission. Preblow pressure is introduced after the rod has travelled 60% to 80% of its total stroke; preblow air pressure is typically 0.5 MPa to 1.0 MPa, followed by final blow pressure of 3.5 MPa to 4.0 MPa. The axial stretch ratio—the final sidewall length divided by the original preform body length—is generally 2.0:1 to 3.0:1 for pressurized single-serve containers. The stretch rod therefore functions as both a kinematic and thermal boundary, not merely an initial inflation guide. Load-cell monitoring of rod force is used on modern machines as a comparative control because the force signature reflects the onset of strain hardening and can separate preforms with incorrect reheat from normal preforms.

The selection of axial stretch ratio and hoop ratio is constrained by the preform geometry and the target bottle geometry. If the hoop ratio is fixed by the cavity diameter, the axial stretch ratio must be adjusted through preform length and rod stroke. For a given preform, a longer rod stroke increases axial orientation but reduces the remaining heat and may thin the shoulder; a shorter stroke leaves more material in the sidewall and base but can reduce top-load capacity. Process engineers use measured bottle thickness profiles to infer whether the rod or inflation dominates each region. In a typical 28 mm PCO 0.5 L carbonated soft drink bottle, the shoulder is rod-dominated, the sidewall is inflation-dominated, and the base is influenced by both.

Hoop Ratio, Planar Extension and Local Thinning in Non-Axisymmetric Cavities

Hoop ratio is calculated from the internal diameter of the blown container divided by the internal diameter of the corresponding preform at the same axial coordinate. Most single-serve water and carbonated soft drink bottles are blown to hoop ratios between 3.0:1 and 4.5:1. Wide-mouth food, pharmaceutical, and hot-fill containers may operate below 2.5:1 because the neck finish diameter approaches the body diameter; in those geometries, axial stretch from the stretch rod controls orientation more than radial inflation does. The planar stretch ratio—the product of hoop ratio and axial stretch ratio—commonly falls between 8.0 and 12.0 for beverage bottles. Increasing hoop ratio at constant preform weight reduces sidewall thickness and increases circumferential chain orientation; this lowers oxygen permeability measured by ASTM D3985-24 at 23 °C and 0% relative humidity because oriented tie-chain-rich regions obstruct gas diffusion more effectively than amorphous regions. However, hoop ratio cannot be increased without limit: wall thickness eventually falls below the threshold for top-load and drop-impact resistance, and internal pressure retention becomes non-linear at 38 °C storage.

In paneled or contoured containers, the local hoop ratio is not constant along the circumference or axial direction. A blow mould cavity with a local expansion ratio above the material's natural draw ratio can produce stress concentration at panel edges and sharp corners. The natural draw ratio of PET in the rubbery state is approximately 3.0 to 3.5 at 90 °C to 100 °C; below this value, orientation remains low and properties are only weakly improved. Above this value, strain hardening increases apparent stiffness and stabilizes the bubble against rupture. Measurement of hoop ratio from sectioned containers uses optical profilometry or computed tomography, with data acquired at axial increments of 1 mm to 5 mm from the neck to the base. In a round 500 mL container, hoop ratio is maximum at the sidewall mid-height and decreases near the shoulder and base. For lightweight containers below 10 g, hoop ratio is increased to retain orientation while reducing wall thickness, but the processing window narrows because preform wall thickness is simultaneously reduced and reheat uniformity becomes more critical.

For hot-fill containers, the hoop ratio and reheat settings are shifted by the need for heat-set crystallinity. Blow moulds are held at 60 °C to 80 °C and the blown container is held against the mould wall to increase crystallinity and reduce volume collapse. This heat-set route changes the thermal window because the preform may be reheated slightly hotter than conventional cold-mould SBM, but the same hoop ratio principles apply. Published data for local hoop ratio limits in heat-set PET is more limited than for cold-mould carbonated soft drink bottles, so development relies on mould-specific qualification.

When Stretch Rod Speed Is Mismatched to Preblow Timing

When the stretch rod reaches full stroke before the preblow valve opens, residual heat in the preform base is conducted into the rod and the colder mould components, leaving the gate area with reduced mobility. High-pressure inflation cannot fully redistribute the cooled material, so the base becomes thick while the sidewall falls below target thickness. By contrast, when preblow pressure is introduced before the rod has travelled 60% of its stroke, the preform expands into a free bubble that experiences hoop strain without sufficient axial orientation; the resulting container may show low axial strength and elevated oxygen transmission measured by ASTM D3985-24. The rod servo profile and preblow delay are therefore programmed as paired parameters, with rod velocity adjusted in 5 mm/s to 10 mm/s increments on modern rotary machines. Process timing is narrow: a preblow delay shift of 20 ms can change sidewall thickness distribution by approximately 0.05 mm to 0.10 mm in a 0.5 L container. When local hoop ratio is recalculated from measured thickness distribution, the shoulder region can exceed nominal hoop ratio by 0.3 to 0.5, increasing susceptibility to environmental stress cracking during alkaline conveyor-lubricant exposure.

Closed-loop process control on high-output lines combines preform pyrometer arrays, rod position feedback, and blow pressure transducers to maintain a reproducible reheat-stretch-blow sequence. A typical control loop: pyrometer data are acquired at the oven exit, compared to a thermal profile, and lamp outputs are adjusted by zone; rod servo position is compared to the ideal motion profile; blow pressure is recorded as a function of time. When one variable is adjusted, the other two are monitored for interaction. For example, increasing lamp output to compensate for lower ambient temperature can reduce the blow pressure required to reach full sidewall contact, but if the rod timing is unchanged, the hoop ratio profile can shift. This confirms that preform reheat, stretch rod, and hoop ratio are intrinsically coupled in two-stage SBM.

Qualification of preforms and blown containers requires separation of resin degradation from orientation effects. The following test matrix is used as a control framework on production lines.

Operational boundaries are set by combined effects of intrinsic viscosity loss and molecular orientation. Injection moulding of preforms requires resin moisture below 50 mg/kg; at higher moisture, hydrolysis at 280 °C reduces intrinsic viscosity and shifts the stretch rod force signature during blowing. Preforms stored at relative humidity above 60% for extended periods may absorb surface moisture; rapid infrared reheating can then generate microvoiding or surface blisters, particularly on the sidewall. The oriented sidewall should not be exposed to conveyor lubricants with pH above 9.0 for prolonged intervals because high hoop strain increases environmental stress cracking. Acetaldehyde generation is controlled at the injection stage by holding melt temperature between 270 °C and 285 °C and limiting melt residence time below 5 min; residual acetaldehyde in water-contact applications is measured by ASTM F2013-10 and commonly specified below 4 µg/L. Within these operational boundaries, the stretch blow moulding process can be held stable by closed-loop preform pyrometry, servo stretch rod position feedback, and periodic sidewall thickness mapping.