Injection-moulded poly(ethylene terephthalate) preforms intended for stretch-blow-moulded beverage containers are exposed to pneumatic conveyors, hopper transitions and transfer chutes where a free-fall height of 2.0 m onto a steel or aluminium guide rail has been adopted as a pass/fail boundary. The 2.0 m drop criterion is not a universal harmonised test height within the main packaging standards; it is a private specification value derived from observed preform handling damage on high-output blowing lines. Under free-fall conditions and negligible air drag, a preform released from 2.0 m reaches an impact velocity of 6.26 m/s according to v = (2gh)^0.5 with g = 9.81 m/s². The kinetic energy of a 25 g preform is 0.49 J, and for a 50 g preform it is 0.98 J. Total energy, however, is a weak predictor of damage because impact failure initiates at local stress concentrations: the gate vestige, the thread root, the neck support ring ledge, and the transition radius between dome and body. Drop impact resistance is therefore evaluated as a system response that includes PET molecular weight, residual moisture, injection thermal history, cooling rate, cavitation-specific hot-runner balance, and gate geometry. Acceptance criteria are commonly written as: after conditioning at 23°C ±2°C and 50% ±5% RH per ASTM D618-21, each preform is dropped from 2.0 m onto a flat steel plate and shall show no crack longer than 2 mm when inspected under polarised light. The supporting test methods ASTM D5276-19, ASTM D2463-15 and ISTA 1A are used for containers or packaged loads, but the preform-specific criterion is a derivative test because no international standard fixes the 2.0 m height for unblown preforms.

Before the mould cavity is filled, the molecular architecture of the PET must be preserved in a narrow IV window because the amorphous preform responds to impact through chain entanglement and the early stages of shear yielding. Bottle-grade PET pellet IV is generally 0.80–0.84 dL/g for carbonated soft drinks and 0.74–0.78 dL/g for still water. The 2.0 m drop criterion tends to impose a moulded preform IV floor of 0.78 dL/g, with higher values required for heavy preforms and aggressive distribution. Intrinsic viscosity is measured according to ASTM D4603-18 in 60/40 phenol/1,1,2,2-tetrachloroethane at 30°C. Hydrolytic, thermal and oxidative degradation in the barrel and hot runner can reduce preform IV by 0.02–0.04 dL/g when the melt exposure time exceeds 180 s at 285°C. The loss is not uniform across the cavitation; end cavities of the manifold may see higher residence time and lower molecular weight when the melt temperature difference exceeds ±2°C. A preform with IV below 0.72 dL/g may pass top-load screening but fail a 2.0 m edge strike after a thread-root crack propagates longitudinally. The impact-energy penalty of IV loss is larger in the 0.70–0.78 dL/g range than near 0.84 dL/g; this nonlinearity makes process control of residual moisture and melt temperature more effective for drop performance than a single IV measurement. Regrind addition above 25% is therefore controlled with a separate stream because the lower average IV and mixed thermal history introduce drop-failure variability that cannot be assigned to a single cavity.

Preform geometry interacts with the drop criterion through the wall-thickness distribution, the length-to-thickness ratio, and the radii of the transitions. For a 20 g preform with a 38 mm finish, sidewall thickness commonly lies between 2.2 mm and 3.0 mm, while the dome thickness is reduced to 1.8–2.2 mm to enable preferential stretching. The transition from the dome to the sidewall must be smooth; a radius below 3 mm acts as a notch under side-body impact at 6.26 m/s. High-speed video of drop failures shows that cracks frequently initiate where the wall thickness changes by more than 0.3 mm over an axial distance of 5 mm, especially when the melt filling pattern creates a weak knit line. The length-to-thickness ratio of the preform body, typically 10:1 to 15:1 for beverage preforms, controls the vibrational response after impact. A longer preform sustains a first bending mode that amplifies stress at the neck thread root; a shorter preform transfers more energy to the gate region. For this reason, the 2.0 m drop criterion cannot be applied as a single wall-thickness minimum without considering preform length and mass distribution.

Post-Mould Cooling Rate Controls Impact-Active Skin Morphology

Cooling in the injection mould determines whether the preform wall is a uniformly amorphous glass or contains microcrystalline domains that reduce crack initiation resistance. Preforms are typically injected in tools of 96 or 144 cavities on injection moulding machines with clamp force between 350 t and 500 t and a 25:1 L/D reciprocating screw. Cavity water temperature is set at 8–12°C in high-output multicavity systems, but the effective cooling rate varies with wall thickness, proximity to the gate, and water-circuit design. The wall solidifies as a skin first and the core later, creating a residual stress gradient: the surface is in compression while the interior remains in tension. The thickness of the tensile core is a direct function of cavity temperature, holding pressure and cooling time. If the mould temperature is raised above 20°C to control gate-vestige deformation, the cooling time required to maintain stable dimensions increases by 20–40%. A preform demoulded at a surface temperature above 75°C may possess a soft core that deforms upon ejection and becomes more notch-sensitive during a subsequent 2.0 m impact. Differential scanning calorimetry per ISO 11357-3:2018 on a correctly cooled preform shows a cold-crystallisation exotherm at approximately 135–150°C with an enthalpy proportional to the amorphous content; induced crystallinity above 2–3% is detectable as a reduced exotherm and increased haze. Crystallised PET microdomains act as stiff inclusions in the amorphous matrix and can accelerate catastrophic crack propagation at the strain rates associated with a 6.26 m/s impact. Cooling-time reductions below 18 s for a 45 g preform with a maximum wall thickness of 6 mm are therefore rejected if the preform immediately enters an impact evaluation and the failure is observed at the inner gate region.

When the Gate Vestige Is Subjected to a 2 m Free-Fall Edge Strike

If a preform falls from 2.0 m and first contact occurs at the injection gate vestige, the impact force is amplified by the small contact area and the sharp transition between the vestige and the preform body. Valve-gated hot-runner systems produce a residual vestige whose height and diameter are controlled by stem retraction timing, gate-insert temperature, and cooling time. For a 38 mm neck-finish preform, the gate vestige is usually specified below 0.50 mm in height and below 3 mm in diameter; when the vestige exceeds 0.75 mm, vertical and 45° impact orientations show an increase in failure rate. The observed failure mode is frequently a crescent-shaped crack that initiates at the boundary between the crystallised vestige surface and the amorphous body, then propagates through the gate well. Production measurement of vestige geometry uses laser profilometry or vision systems with 0.01 mm axial resolution, integrated into the robot extraction cell. The gate insert temperature is commonly maintained 5–10°C below the melt temperature to reduce stringing while preserving a short vestige. A cold-gated preform has a different risk: the cold slug can separate from the gate and create a particle contaminant during the drop test, which is counted as a failure under the 2.0 m criterion even if the preform body remains intact. Because gate-vestige impact is a local stress concentration, cavitation-to-cavitation differences in gate geometry are monitored continuously, and a gate insert with visible wear or a vestige-height trend above 0.40 mm is removed from service before the lot is submitted to drop evaluation.

What Limits Comparability Between Unblown Preforms and Stretch-Blow-Moulded Containers?

The central limitation in using a 2.0 m preform drop test as a proxy for finished-container toughness is that the preform is amorphous and unstretched, while the bottle wall is biaxially oriented and strain-crystallised. A preform that survives the 2.0 m fall may not guarantee acceptable container drop performance per ASTM D2463-15 or ASTM D5276-19, because the final container properties are determined by the stretch ratio, preblow pressure and blow-mould temperature. Conversely, a preform that fails the 2.0 m criterion may still be suitable for low-pressure still-water containers if downstream handling is mild. The preform drop test is therefore a screening method for preform handling damage, not a replacement for finished-container impact evaluation. Comparability is particularly weak for preforms below 22 g because the impact energy is less than 0.43 J and may not discriminate between molecular weight or cooling differences. For preforms above 40 g, the higher kinetic energy can produce neck-thread fractures that are not representative of bodywall toughness. Transferability improves only when drop orientation, impact surface and conditioning are fixed, and when high-speed video confirms that the fracture origin is in the preform body rather than at a mould seam or gate vestige. Published data for a harmonised correlation model between preform drop height and container burst pressure or container drop impact energy is limited; production sites therefore maintain separate in-house thresholds for preform drop testing and finished-bottle impact testing.

Residual moisture in dried PET is a direct predictor of hydrolytic degradation during melt processing and therefore of reduced drop impact resistance at 2.0 m. Amorphous PET pellets are dried in hot-air desiccant dryers at 160–180°C for 4–6 h to reach a residual moisture content at or below 30 ppm, measured by Karl Fischer titration according to ISO 15512:2019. The equilibrium moisture content of PET can exceed 2,500 ppm at 23°C and 50% RH; therefore improperly dried regrind or open silo storage can rapidly contaminate the melt stream. At melt temperatures above 270°C, water attacks the ester linkage and lowers molecular weight. A residual moisture increase from 50 ppm to 100 ppm can reduce moulded preform IV by 0.03–0.06 dL/g depending on residence time and venting efficiency. The resulting drop-failure pattern is often concentrated in the first shots after a feeder bridge or in material transferred from a wet silo. Dryer dew point is therefore maintained at or below −40°C; an increase to −20°C indicates that the desiccant bed is saturated or the air flow is bypassing the hopper. The operational boundary for the 2.0 m criterion is that dried material must pass a residual moisture check below 30 ppm before machine feed, and any granulate held in an open feed hopper for more than 20 min at 50% RH is either re-dried or segregated from the drop-certified material stream.

Drop orientation is a confounding variable because the same preform can survive a neck-first impact and fail a body-first impact at 2.0 m. Multi-impact specifications therefore define the number of impacts per orientation: one flat side, one gate end, and one neck end. A side impact places the maximum bending stress in the middle of the preform body where the wall is thinnest; a neck-end impact produces a compressive stress pulse through the finish and can damage the thread root; a gate-end impact is the most severe for the gate-vestige defect. Orientation-specific failure rates must be tracked separately, not averaged, because averaging can hide a cavitation-specific thread-root problem. The impact surface itself is also fixed: a 3 mm thick polished steel plate with a Rockwell hardness of 58–62 HRC is a typical production choice. If the impact surface is replaced by an aluminium plate or a vinyl-covered conveyor rail, the measured failure rate changes and the 2.0 m criterion is no longer comparable to the original validation. Temperature conditioning therefore matters less than mechanical alignment and surface finish when the product is already close to the acceptance boundary.

Measuring Peak Force–Time Signatures on Instrumented Drop Towers

Instrumented drop-tower testing is used to resolve why a specific preform survives or fails the 2.0 m criterion, because total energy alone obscures local strain-rate response. A production-scale drop tower is equipped with a piezoelectric load cell, a velocity sensor and a high-speed camera operating at 5,000–20,000 frames/s. The preform is held by a vacuum gripper and released from 2.0 m onto a polished steel anvil. The force signal is acquired at 100 kHz. A surviving preform typically shows a single peak force in the range of 200–600 N with a contact duration of 0.5–1.5 ms, depending on preform mass and wall thickness. A failing preform often produces a double peak: the first peak corresponds to initial contact, and the second peak to unloading after crack initiation. The area under the force–time curve is the impulse, which is used to compare cavities across a hot-runner tool. Post-impact rebound height is recorded with a linear encoder; a rebound greater than 30% of the initial drop height indicates that less energy was dissipated in the preform. Standard free-fall methods such as ASTM D5276-19 do not require force instrumentation, but the force-time data are necessary to distinguish geometry effects from material degradation when a batch approaches the 2.0 m acceptance limit.

Hot-Runner Thermal Imbalance and Cavitation-Specific Drop Criteria

On a multicavity platform, the 2.0 m drop criterion is applied to a destructive sampling plan rather than to 100% inspection because the test consumes the preform and requires conditioning. A common sampling approach is derived from ISO 2859-1:1999, with an AQL of 0.65 for critical cracks and 1.5 for minor gate-vestige deformation. Preforms are conditioned at 23°C ±2°C and 50% ±5% RH for 40 h per ASTM D618-21 before the drop. If low-temperature performance is required, a second set is conditioned at 5°C ±2°C for 24 h because impact resistance of PET decreases as the test temperature approaches the beta transition. The lot is accepted only if all selected cavities pass the no-crack criterion after one drop; some specifications require three impacts per preform from different orientations. Production data from 96-cavity and 144-cavity lines often show that cavity-to-cavity variation in impact performance exceeds batch-to-batch variation, which indicates manifold imbalance or blocked cooling circuits rather than resin variation. A retained-cavity SPC chart of peak force or crack length is used to set control limits. The operational boundary is that any cavity with a gate-vestige height trend above 0.40 mm or a drop failure in two consecutive lots is removed for gate insert and cooling inspection before the next production run.