A production-scale heat set stretch blow molding operation for hot fill juice and tea containers hinges on a controlled elevation of polyethylene terephthalate crystallinity beyond the strain-induced levels achieved in conventional cold-fill bottle manufacturing. The base polymer—typically a copolymerized PET resin with intrinsic viscosity measured at 0.78–0.84 dL/g via ASTM D4603—undergoes two distinct crystallization modes during biaxial orientation: strain-induced crystallization develops during stretching at draw ratios between 8x and 12x, while thermal crystallization proceeds during the extended mold residence phase when the blow mold is maintained at elevated temperatures. Amorphous PET exhibits a density of approximately 1.335 g/cm³, whereas the fully crystalline phase approaches 1.455 g/cm³; heat set hot fill containers typically register bulk wall densities between 1.370 and 1.385 g/cm³ when measured by the density gradient column method per ASTM D1505, corresponding to volume-fraction crystallinities of 28–38%. The practical consequence is that hot fill juice bottles must withstand product introduction at 85–88°C without volumetric shrinkage exceeding 1.5% of original dimensions, while ready-to-drink tea products, depending on preservative formulation and pH classification under 21 CFR 113, may be filled at temperatures up to 90°C. Published data for single-stage injection-stretch-blow heat set configurations operating at fill temperatures above 92°C is limited, and most dual-stage reheat blow molding equipment manufacturers specify an upper mold temperature boundary that still requires laboratory validation against specific container geometry. The preform conditioning phase establishes the thermal foundation for subsequent crystallization control. In dual-stage reheat stretch blow molding systems such as the Sidel Universal series or Krones Contiform platforms equipped with modular IR ovens, preform temperature profiling across the axial length determines whether spherulitic crystallization initiates before stretching begins. Preform surface temperatures entering the mold typically range from 95°C to 115°C for heat set operations, approximately 10–15°C higher than cold-fill settings, with the temperature differential between preform inner wall and outer wall constrained to less than 5°C to avoid asymmetric crystallinity distribution. The conditioning parameter window is narrow because insufficient preform temperature prevents adequate chain mobility for subsequent strain-induced orientation, while excessive input calories promote haze formation through premature thermal crystallization in the preform wall prior to stretch initiation—a defect visible as a white opalescent ring pattern concentrated near the gate region and transition zone. Industrial infrared pyrometer arrays positioned at the oven exit provide closed-loop feedback by comparing measured temperature profiles against pre-established setpoint curves; deviation beyond ±2°C on any individual zone of a 10-zone oven configuration typically triggers automatic preform rejection or line speed adjustment.

What Mold Temperature Range Delivers Adequate Thermal Crystallinity Without Haze Penalization in the Side Wall?

The blow mold temperature functions as the primary driver of thermal crystallinity development during the heat set phase. Mold surface temperatures for hot fill PET production are conventionally maintained between 120°C and 160°C via closed-loop thermal oil circulation systems, with mold cooling water channels replaced by high-temperature heat transfer fluid circuits rated for continuous service at 180°C. The residence time in the mold during which the stretched bottle wall remains in contact with the heated mold surface ranges from 1.5 to 4.0 seconds, depending on container volume, wall thickness distribution, and target crystallinity level. During this interval, the molecular chains in the oriented bottle wall undergo additional thermally activated rearrangement, converting a portion of the mesomorphic and amorphous phase into a more thermodynamically stable crystalline arrangement. The critical process conflict emerges when mold temperature is pushed toward the upper boundary: thermal crystallization accelerates approximately linearly with temperature within the 120–160°C range, but haze (measured per ASTM D1003) increases disproportionately once the mold temperature exceeds approximately 145–150°C because spherulitic growth begins competing with oriented crystallite refinement. The processing window for clear juice bottles therefore narrows to approximately ±5°C around the optimal setpoint, while tea bottles with lower clarity specifications may tolerate mold temperatures 5–10°C higher without exceeding acceptable haze thresholds of 8–10%. Production-scale failure modes at the low-mold-temperature boundary manifest as elevated volume shrinkage during hot fill simulation testing, with the bottle body developing visible panel deformation and shoulder collapse when fill temperature exceeds the thermal stability threshold imparted by insufficient crystallinity. Conversely, excessive mold temperature produces embrittlement in the base and neck regions where orientation is lowest, with drop-impact resistance per ASTM D2463-15 declining below acceptable minimums of 2.5 J at 4°C for 500 mL containers.

Preform Design Parameters and Axial Crystallinity Gradients in Hot Fill Container Engineering

Preform geometry exerts a first-order influence on the crystallinity distribution achievable in the final blown container. Hot fill preforms are typically designed with a wall thickness 20–30% greater than equivalent cold-fill preforms for the same container volume, reflecting the need to maintain final side wall thickness sufficient to resist the vacuum forces generated during product cooling. The stretch ratio profile—longitudinal stretch ratios between 2.5x and 3.2x combined with hoop stretch ratios between 3.5x and 4.5x—produces biaxial orientation factors measured by wide-angle X-ray diffraction that directly correlate with strain-induced crystallinity levels. In the transition zone between neck support ring and side wall, orientation is inherently lower due to geometric constraints, creating a crystallinity deficit that manifests as localized shrinkage bands when containers are subjected to hot fill simulation. Preform gate thickness and cooling time during injection molding control the residual crystallinity present before blow molding; for hot fill applications, preform intrinsic viscosity retention must remain above 0.72 dL/g after both injection and blow molding operations, corresponding to hydrolytic degradation levels not exceeding 0.02 dL/g across the entire thermal history. Published data from industrial mold trials indicates that the axial crystallinity gradient in a typical 500 mL hot fill bottle varies from approximately 15–20% in the neck base region to 35–40% in the mid-panel side wall, with the base gate area exhibiting intermediate crystallinity of 25–30% due to compression effects during mold contact. This gradient fundamentally limits the maximum fill temperature achievable for any given bottle design, since shrinkage initiates preferentially at the lowest-crystallinity zone and propagates along the panel. Processing conditions for tea containers involve additional thermal compensation because many tea beverages incorporate preservatives that permit lower fill temperatures but often require amber-tinted PET formulations containing UV-blocking additives such as carbon black or benzotriazole derivatives. These additives alter the reheat characteristics of the preform by increasing IR absorption, which shifts the effective oven temperature requirements downward by 5–10% relative to clear formulations. The interaction between pigment loading and thermal crystallization kinetics is not trivial: carbon black particles serve as heterogeneous nucleation sites during thermal crystallization, accelerating the rate of spherulitic growth and narrowing the effective mold temperature processing window to approximately ±4°C. Tea bottle producers commonly specify wider panel geometries with vacuum-compensating ribs or nitrogen-dosed headspace to accommodate the higher thermal expansion coefficient mismatch between the container and the filled product at 88–90°C.

Crystallinity Quantification Via Density Gradient and Differential Scanning Calorimetry in Production Quality Control

Routine crystallinity measurement in hot fill bottle production relies on two complementary analytical methods that provide different information about the structural state of the polymer. Density gradient column testing per ASTM D1505 uses a column prepared with aqueous calcium nitrate solutions or toluene-carbon tetrachloride mixtures spanning a density range from 1.32 to 1.42 g/cm³, with calibration floats traceable to NIST-certified density standards. Wall specimens cut from defined bottle locations—typically shoulder, mid-panel, and base—are conditioned at 23°C for 16 hours per ASTM D618 before immersion. The position at which the specimen floats corresponds to its bulk density, which is converted to weight-fraction crystallinity using the two-phase density model with amorphous and crystalline densities as endpoints. A density value of 1.375 g/cm³, for example, corresponds to approximately 29% crystallinity by weight. Differential scanning calorimetry per ASTM D3418-21 provides an independent measurement by integrating the melting endotherm over the temperature range from 220°C to 260°C and dividing by the theoretical heat of fusion of fully crystalline PET, conventionally taken as 140 J/g. DSC also detects the presence of secondary crystallization exotherms in the 100–140°C range, which indicate incomplete heat setting and predict poor hot fill performance despite acceptable density measurements. The dual-method approach is necessary because density gradient columns cannot distinguish between strain-induced and thermal crystallinity, whereas DSC thermograms reveal distinct melting peak shapes: strain-induced crystallites melt with a broad endotherm between 230°C and 250°C, while thermally grown crystallites produce a sharper peak near 255–260°C. Production quality control protocols typically specify density as the primary release criterion—with acceptance limits of 1.365–1.385 g/cm³ for mid-panel locations—while DSC serves as a diagnostic tool for process deviation investigations. The hot fill performance of heat set containers is validated through a battery of standardized test methods that simulate fill-line conditions. Volume shrinkage measurement per ASTM D2659 involves filling the container with water at a specified temperature, holding for a defined period, and measuring dimensional changes against reference marks scribed on the bottle wall. The standard specifies a fill temperature that mirrors the intended product processing condition; for juice and tea applications, test temperatures of 85–90°C with 5-minute hold times are typical. Volume shrinkage acceptance criteria vary by bottle design, but industrial specifications generally require less than 1.0–1.5% total volume change, with localized panel deformation limited to 2.5 mm maximum inward deflection measured by gauge at the panel center. Top load testing—performed on an Instron or Zwick universal testing machine equipped with environmental chamber capability—measures the axial compressive force required to produce 5 mm deflection at both ambient temperature and elevated temperature (70°C for simulated warehouse conditions). Heat set hot fill bottles must maintain a minimum ambient top load of 250 N for 500 mL containers, while elevated-temperature top load typically declines by 30–40% relative to ambient values due to modulus reduction. Side wall vacuum resistance tests simulate the internal negative pressure of 0.2–0.5 bar generated during product cooling, with pass/fail determined by visual inspection for panel collapse or shoulder deformation exceeding geometric tolerance limits of ±1.5 mm. When fill temperatures exceed 90°C, side wall thickness compensation becomes mandatory for the vacuum panels that accommodate product volume contraction during cooling. The thermal contraction mismatch between PET and aqueous products is quantified by the difference in volume-temperature coefficients: PET exhibits a volumetric thermal expansion coefficient of approximately 2.1 x 10⁻⁴ K⁻¹ below the glass transition temperature of 78°C, while water-based products contract with a coefficient near 2.1 x 10⁻⁴ K⁻¹ above 20°C but exhibit non-linear behavior as temperature approaches boiling range. The vacuum panel must be engineered to flex inward without permanent creasing at the geometric boundaries; finite element analysis typically models panel deflection using hyperelastic material models calibrated against bi-axial tensile test data from the anticipated wall thickness range of 0.25–0.45 mm. The crystallinity level in the panel ribs is critical—insufficient crystallinity leads to stress relaxation during hot filling, while excessive crystallinity impairs the ductility required for controlled panel flexure. Industrial experience indicates that panel region crystallinity should be maintained in the 30–35% range for optimal balance between dimensional stability and flexural compliance.

Process Windows Narrow to ±3°C at the High-Temperature Boundary of Heat Set Molding

The operational tolerance of heat set blow molding narrows disproportionately as mold temperature approaches the upper boundary of the processing envelope. At mold temperatures of 130–140°C, a ±6°C variation produces measurable but acceptable changes in bulk crystallinity of approximately ±1.5%, with corresponding hot fill shrinkage variations remaining within specification. When the mold setpoint is elevated to 155–160°C, the crystallinity response becomes significantly more sensitive: the same ±6°C variation produces crystallinity swings of ±3% or greater, pushing the lower boundary below minimum shrinkage resistance requirements and the upper boundary into haze territory exceeding 10%. The practical processing window therefore compresses to approximately ±3°C at the high-temperature boundary. Mold temperature control systems for heat set applications must consequently be specified with temperature uniformity across the mold face of ±1.5°C or better, achieved through multi-zone thermal oil manifolds with independent flow control to each mold half. Mold-to-mold variation across a 10-cavity blow molding station introduces additional complexity: thermal oil flow balancing typically achieves cavity-to-cavity temperature variation of ±2°C, which consumes a portion of the available processing window and necessitates conservative setpoint selection. Production line audits of heat set operations frequently identify mold temperature drift as the dominant source of batch-to-batch performance variation, ahead of preform temperature variation and ambient humidity effects. The relationship between mold residence time and thermal crystallinity development follows non-linear kinetics consistent with Avrami-type crystallization models. Published kinetic studies of PET thermal crystallization between 120°C and 160°C report half-times for crystallization ranging from approximately 20 seconds at 160°C to 120 seconds at 130°C for unoriented PET. The oriented polymer in the stretched bottle wall, however, crystallizes substantially faster due to chain alignment and reduced entropy: half-times in the heat set mold temperature range are reduced by approximately one to two orders of magnitude relative to unoriented material. This kinetic acceleration partially offsets the thermodynamic driving force limitation and permits adequate thermal crystallinity development during the brief 1.5–4.0 second mold residence time. The practical implication is that mold residence time and mold temperature are partially interchangeable within a bounded window: increasing residence time from 2.0 to 3.0 seconds at 140°C achieves a crystallinity increment equivalent to approximately 5°C of mold temperature increase at constant residence time. However, residence time extension beyond 4.0 seconds generally fails to produce further crystallinity gains because the thermal gradient across the wall thickness approaches equilibrium, and productivity penalties become unsustainable for high-volume beverage packaging operations.

Mold Temperature (°C)	Wall Density (g/cm³) per ASTM D1505	Crystallinity (wt%) by DSC per ASTM D3418	Volume Shrinkage at 85°C (%) per ASTM D2659	Haze (%) per ASTM D1003	Elevated Temp Top Load (N at 70°C)
115	1.358–1.365	20–24	3.5–5.0	1.5–2.0	120–160
125	1.365–1.372	25–29	1.8–2.8	2.0–2.5	160–200
135	1.372–1.380	29–34	0.8–1.5	2.5–3.5	200–240
145	1.380–1.388	34–38	0.3–0.8	3.5–5.0	240–270
155	1.388–1.395	38–42	0.1–0.4	5.0–8.0	260–290
165	1.395–1.402	42–46	0.05–0.2	8.0–12.0	270–300

The data presented in the preceding table represent aggregated ranges from industrial process characterization studies and should be understood as indicative envelope values rather than universally applicable specifications. Actual performance values depend on preform design, stretch ratio configuration, resin formulation (including comonomer content and catalyst systems), and container geometry. The haziness increase beyond 145°C mold temperature is particularly geometry-sensitive: containers with high stretch ratios in the side wall can maintain acceptable clarity at higher mold temperatures because the oriented structure suppresses large-scale spherulitic growth, while thick-walled base regions inevitably develop visible haze first due to lower orientation levels. Quality specifications for clear juice containers typically cap haze at 5% measured on flattened side wall specimens, which corresponds to a practical mold temperature ceiling of approximately 150°C for clear formulations. Amber tea bottles with carbon black or iron oxide pigments tolerate haze levels up to 12%, permitting mold temperatures of 160°C or slightly higher while maintaining acceptable sensory and visual properties for the filled product. Humidity and ambient moisture conditions introduce an often under-appreciated variable into heat set processing. PET is hydrolytically sensitive at elevated temperatures, with hydrolysis rates accelerating sharply above the glass transition temperature. During the IR conditioning phase, surface moisture on the preform vaporizes and may locally cool the preform or contribute to hydrolytic chain scission if the resin has been inadequately dried. Preform moisture levels must be maintained below 50 ppm (measured by Karl Fischer titration on ground preform material) to prevent intrinsic viscosity loss exceeding 0.01 dL/g during the blow molding operation. Facilities operating in tropical climates with ambient relative humidity exceeding 70% require additional controls: preform storage silos should be purged with dehumidified air at RH < 10%, open-floor time for preforms between silo and oven feed must be minimized to less than 30 minutes, and blow molding line enclosures may require conditioned air supply to prevent moisture condensation on preform surfaces. Published data from manufacturing plants in Southeast Asia indicates that crystallinity distribution variability increases by approximately 20–30% during monsoon season operations when humidity controls are not actively managed, manifesting as intermittent hot fill shrinkage failures concentrated in the afternoon shift when ambient dew point is highest. The neck and closure geometry of hot fill containers demands crystallinity control distinct from the side wall. The neck finish is typically crystallized during preform injection molding via a thermal crystallization station operating at 160–180°C for 90–180 seconds, producing a whitened neck ring with crystallinity exceeding 40% that resists thermal deformation during hot fill capping operations. The neck crystallization step is essential because the neck region experiences no biaxial orientation during blow molding and would otherwise deform at fill temperatures approaching 90°C under capping torque application ranging from 20–30 N·m for conventional 28 mm and 38 mm closures. Neck finish dimensions per ISBT standards specify tight tolerances of ±0.15 mm on inner diameter dimensions that must be maintained after neck crystallization, requiring preform neck designs that compensate for the 1.5–2.0% volumetric shrinkage accompanying the crystallization step. Closure system selection interacts with crystallinity control: lined closures with polyethylene or EVA-based gasket materials provide adequate sealing at fill temperatures up to 92°C, while linerless closures with molded plastisol foam gaskets may require fill temperature reduction to 85°C to prevent leakage under post-fill inversion testing.

Vacuum Panel Dynamics, Oxygen Barrier Requirements, and Shelf Life Implications for Crystallinity-Controlled Containers

The crystallinity level in hot fill containers directly affects gas barrier properties, with practical consequences for juice and tea product shelf life. Oxygen permeability of oriented PET decreases with increasing crystallinity because the crystalline domains are impermeable to gas transport and simultaneously increase the tortuosity of the diffusion path through the amorphous phase. Published oxygen transmission rate data for oriented PET films at 23°C and 50% RH ranges from 2.5–4.5 cm³·mm/(m²·day·atm) for low-crystallinity samples to 1.5–2.5 cm³·mm/(m²·day·atm) for highly crystallized heat set material, representing a barrier improvement of approximately 40–50%. For hot fill juice applications requiring 12-month ambient shelf life with oxygen-sensitive vitamin C retention above 80% of initial concentration, the intrinsic PET barrier of even highly crystallized containers is often insufficient, necessitating oxygen scavenger additives such as MXD6 nylon blends or cobalt-catalyzed polybutadiene-based scavengers compounded at 3–5 wt%. The interaction between crystallization processing and oxygen scavenger functionality is complex: thermal crystallization during heat setting can alter the morphology of the scavenger phase, potentially reducing scavenging capacity by 15–25% relative to cold-fill conditions due to domain coarsening and reduced interfacial area. Tea products, particularly green tea formulations containing catechins and polyphenols, are similarly sensitive to oxidative degradation but present additional challenges related to color stability: browning reactions catalyzed by residual oxygen in the headspace are accelerated at the elevated fill temperatures characteristic of hot fill processing, placing a premium on headspace oxygen control below 0.5% by volume. The vacuum compensation mechanisms in heat set bottles interact with crystallinity levels in predictable but non-intuitive ways. When the hot-filled product cools from 88°C to 25°C, the volume contraction of the liquid product and the headspace generates internal negative pressure that must be accommodated by controlled panel deflection. The magnitude of this vacuum force scales with the temperature differential, the container filling height, and the headspace volume established during capping. For a typical 500 mL bottle filled at 88°C with 8% headspace by volume, the post-cooling internal pressure stabilizes at approximately 0.35–0.45 bar below atmospheric. The vacuum panel must deflect inward by 3–5 mm at the panel center to compensate for the volume change without inducing permanent deformation at the panel boundary. Crystallinity levels in the panel region that are too low (below approximately 25%) lead to permanent creasing when the panel flexes, since the material lacks sufficient elastic recovery capability under sustained flexural stress at the elevated temperatures encountered during cooling. Conversely, crystallinity above 38% in the panel makes the material excessively stiff, increasing the force required for panel actuation and potentially causing stress concentration at the panel rib boundaries where wall thickness transitions create geometric discontinuities.

Compliance Standard	Measurement Parameter	Typical Acceptance Criterion for Hot Fill PET
ASTM D1505-18	Density gradient column method for wall crystallinity	1.365–1.385 g/cm³ (mid-panel)
ASTM D3418-21	DSC crystallinity and melting behavior	28–38% crystallinity; no secondary exotherm > 0.5 J/g
ASTM D2659-16	Volume shrinkage under simulated hot fill	≤ 1.5% at product fill temperature
ASTM D2463-15	Drop impact resistance	≥ 2.5 J at 4°C (500 mL container)
ASTM D1003-21	Haze and luminous transmittance	Haze ≤ 5% (clear juice); ≤ 12% (amber tea)
ASTM D4603-18	Intrinsic viscosity of PET resin	≥ 0.72 dL/g after processing; 0.78–0.84 dL/g as-received
ISO 1133-1:2022	Melt flow rate verification of incoming resin	Consistent with resin supplier specification ± 15%
21 CFR 177.1630	PET food contact compliance	Conformance to extractive limits per FDA protocol
21 CFR 113	Thermal processing requirements for low-acid foods	Fill temperature and hold time validated per process authority filing
EU Regulation 10/2011	Overall migration and specific migration limits	OML ≤ 10 mg/dm²; SML for antimony ≤ 0.04 mg/kg

The compliance matrix above consolidates the standards framework governing crystallinity-controlled hot fill PET container production. Note that several of these standards test finished article properties rather than crystallinity directly; the crystallinity measurement serves as an internal process control parameter that correlates with—but does not replace—finished article performance testing. Regulatory compliance under 21 CFR 177.1630 requires that PET containers intended for food contact be manufactured from resins meeting the specified compositional requirements, including total extractables limits determined by extraction testing with food-simulating solvents. The thermal history experienced during heat set processing may alter migration behavior: extended exposure to elevated mold temperatures can promote surface segregation of low-molecular-weight species such as cyclic oligomers (predominantly cyclic trimer) that are inherently present in PET resin. Surface concentrations of cyclic trimer, measured by HPLC extraction per published analytical methods, typically range from 0.2–0.8% by weight of the surface layer in conventional stretch blow molded containers, and heat set processing at mold temperatures above 150°C may increase surface segregation by 20–40% due to enhanced chain mobility during the thermal crystallization phase. For tea and juice applications with aqueous and slightly acidic product contact, cyclic trimer migration into the product is negligible, but the surface deposit can interfere with label adhesion and ink jet coding legibility. Product line changeover and equipment startup protocols represent critical operational boundaries for heat set crystallinity control. Transition from cold-fill to hot-fill production on shared blow molding assets requires mold temperature ramp-up from ambient to operating temperature over a period of 2–4 hours, with thermal oil circulation establishing equilibrium before production preforms are introduced. During this stabilization phase, the first 50–100 bottles produced typically exhibit sub-optimal crystallinity and are automatically rejected by in-line density or temperature monitoring systems. Similarly, transition between different hot fill SKUs with different container designs necessitates mold setpoint adjustment and validation of the new crystallinity profile through sampling and laboratory confirmation before full-speed production resumes. Published data for specific configuration changeover time losses in juice and tea packaging facilities is limited, but industrial experience suggests that crystallinity-related startup waste can be reduced by 30–40% through implementation of mold temperature logging with automated setpoint verification and statistical process control charts monitoring density of first-pass production samples every 15 minutes. The base and shoulder regions of heat set containers present crystallinity management challenges distinct from the mid-panel side wall due to their complex geometries and reduced thickness uniformity. The base, particularly the injection gate area and the standing ring, experiences compressive deformation during mold contact that enhances thermal crystallization in the contact zone while adjacent areas with thinner walls and lower contact pressure develop less thermal crystallinity. This differential crystallization generates residual stress patterns that remain frozen into the container and influence drop impact performance and environmental stress crack resistance. Stress relaxation during subsequent hot filling partially relieves these residual stresses, but the crystallinity gradient remains and can concentrate deformation at the boundary between high-crystallinity contact zones and lower-crystallinity adjacent regions. For petaloid bases with five or six standing legs, the crystallinity variation between leg walls and leg valleys frequently exceeds 8%, requiring base design features such as tapered leg transitions and rounded valley radii to distribute strain and prevent crack initiation during top load application. Published data from finite element simulations validated against experimental strain gauge measurements indicates that maximum principal strain concentrations at the base during hot fill cooling can reach 1.8–2.5%, approaching the yield strain of the semi-crystalline PET material in the base region. Recycling compatibility and environmental performance metrics impose additional constraints on crystallinity control strategies. The high crystallinity levels achieved in heat set containers do not fundamentally alter their compatibility with established PET recycling streams, since the mechanical recycling process involves re-melting that erases the thermal history of the original container. However, the compositional requirements of EU Regulation 10/2011 and 21 CFR 174.5 for post-consumer recycled content limit the inclusion of certain additives that may be used in heat set container formulations. Multilayer constructions with oxygen barrier layers, while offering superior gas barrier performance, introduce recyclability concerns unless designed as compatible bilayer structures where the barrier phase can be incorporated into the PET recycling stream at controlled levels below 5% of the total bottle weight. Monolayer heat set containers remain the preferred configuration for most juice and tea applications, with oxygen barrier enhancement achieved through passive crystallinity control and, where necessary, oxygen scavenger chemistry dispersed directly in the PET matrix. The crystallinity-related density increase in heat set containers does not affect conventional PET flake recovery processes operating with sink-float separation systems, since the density difference between heat set material (1.370–1.385 g/cm³) and cold-fill material (1.360–1.370 g/cm³) is insufficient to trigger separation in water-based density baths.