Across commercial PET strapping extrusion lines equipped with 120 mm barrier screws, consistent melt pressure is a coupled outcome of resin moisture control, screw geometry, gear pump compensation, and screen changer pressure management. Pre-dried bottle-grade PET is introduced through a gravimetric hopper feeder at a residual moisture content below 30 ppm; moisture above this threshold accelerates hydrolytic chain scission at melt temperatures above 270 °C. A desiccant-wheel dryer with air dew point of -40 °C to -50 °C and outlet air temperature of 160 °C to 170 °C is routinely specified for virgin resin, with residence time of 4 h to 6 h. The 120 mm barrier screw, usually configured at 30:1 L/D, separates the solid bed from the melt pool by means of a secondary barrier flight; this separation reduces solid-bed breakup and lowers discharge pressure pulsation compared with a conventional metering screw. Melt leaving the screw enters a gear pump that isolates die pressure from screw-speed instabilities. Pressure transducers installed directly before the screen changer and after the gear pump provide closed-loop feedback to the gear pump drive. Industrial practice on PET strapping lines of this scale commonly sets gear pump inlet pressure stability at ±0.5 bar and die pressure stability at ±0.2 bar for consistent strapping width and thickness. Barrel temperature profiles from feed to discharge are typically 250 °C, 260 °C, 270 °C, 275 °C, and 275 °C, while the adapter and gear pump are held at 275 °C to 280 °C. Head pressure before the screen changer is generally maintained between 80 bar and 140 bar, and screw speed is limited to 40 rpm to 80 rpm at 120 mm diameter to prevent excessive viscous heating. Post-extrusion intrinsic viscosity retained above 0.78 dL/g to 0.85 dL/g is normally required for high-tenacity strapping, and incoming PET with 0.84 dL/g to 0.95 dL/g is therefore selected. The complete operating band shifts with screw wear, regrind fraction, screen pack condition, and ambient humidity.

What Limits Head Pressure Stability When Barrier Screws Exceed 120 mm Diameter?

When barrier flight clearance opens beyond 0.9 mm due to abrasive wear, the barrier screw loses its ability to maintain a sealed melt chamber and suction pressure at the gear pump inlet becomes erratic. For 120 mm barrier screws processing PET, radial barrier flight clearance is typically specified at 0.6 mm to 0.8 mm; clearance above 1.0 mm permits melt to recirculate back into the solids channel, producing pressure spikes of ±1.0 bar to ±1.5 bar at the pump inlet. The barrier section in a 30:1 L/D screw places the secondary flight undercut below the main flight by a defined depth, forcing melt across the barrier into a separate channel while the remaining solid bed advances. If the feed zone is too cold, the solid bed is overly stiff and the barrier flight can generate local melt temperatures above 300 °C, causing rapid thermal degradation. If the feed zone is too hot, premature melting before the barrier entry destabilizes the solid bed and reduces the screw's pressure-generation capability. Axial pressure at the barrier entry is influenced by channel depth: engineering literature for 120 mm barrier screws reports feed channel depths of 12 mm to 15 mm and metering depths of 5 mm to 7 mm, with compression ratios of 2.5:1 to 3.5:1. Gear pump inlet pressure should remain between 20 bar and 50 bar to avoid pump cavitation and to ensure complete filling; inlet pressure below 10 bar is generally associated with throughput instability and die pressure oscillation. Strain-gauge melt pressure transducers with full-scale accuracy of ±0.25% are preferred for closed-loop control because their response time is compatible with the 0.1 s to 0.5 s pressure corrections executed by the gear pump controller. On production equipment, wear-related suction pressure swings are often mistaken for feed problems; the diagnostic distinction is that feed problems appear as periodic low-frequency variation tied to screw rotation while worn barrier clearances produce non-periodic spikes lasting 2 s to 10 s.

Moisture-induced hydrolysis is initiated when residual moisture above 30 ppm passes through the barrier section and gear pump into the die. Each water molecule cleaves one ester linkage along the PET chain, generating one carboxyl end group and one hydroxyl end group; the number-average molecular weight decreases in direct proportion to the number of scission events. Intrinsic viscosity is related to molecular weight by the Mark-Houwink-Sakurada equation, using constants K = 2.1 × 10^-4 dL/g and a = 0.82 for PET dissolved in 60/40 phenol/1,1,2,2-tetrachloroethane at 30 °C as specified in ASTM D4603-18. A decline in IV from 0.84 dL/g to 0.74 dL/g reduces hot-drawing uniformity in the orientation stage, causing width variation and tensile strength loss in the finished strapping. Thermo-oxidative degradation proceeds simultaneously in the presence of dissolved oxygen, but its rate is slower than hydrolysis when moisture is properly controlled. Resin stored at ambient relative humidity above 60% requires pre-drying before hopper drying to avoid exceeding the moisture-removal capacity of the desiccant bed. Melt temperature should not exceed 290 °C; residence time in the barrel, adapter, and gear pump at temperatures above 285 °C should be kept below 3 min to limit thermal degradation. The combination of PET with amine-based additives is incompatible because amines catalyze chain scission and produce rapid molecular weight loss during extrusion. A compliance matrix for the extrusion and finished strapping is shown below.

Standard designation	Measured parameter	Relevance to 120 mm barrier screw extrusion
ASTM D4603-18	Intrinsic viscosity	Verifies molecular weight retention after melt processing
ISO 1133-1:2022	Melt mass-flow rate	Monitors viscosity consistency under capillary shear
ASTM D3950-17	Nonmetallic strapping classification	Defines break strength, elongation, and width requirements
ASTM D638-14	Tensile properties of plastics	Provides tensile yield and elongation for base polymer comparison
ISO 527-2:2012	Tensile properties of rigid plastics	Supplies comparative tensile modulus data for quality control

Melt Filtration and Screen Changer Pressure Drop as Coupled Variables

Continuous screen changer performance on 120 mm PET strapping lines couples melt pressure stability with contamination level and filter pack condition. A typical PET filter pack uses layered mesh screens of 20/40/60/100 or 40/60/100/150 mesh depending on regrind content and end-use surface quality. As the screen pack blinds, pre-screen pressure increases while post-screen pressure remains controlled by the gear pump; the control loop compensates by raising screw speed or reducing pump throughput, which increases melt residence time and can raise melt temperature by 2 °C to 5 °C. Pressure drop across the screen changer should be recorded continuously; a differential pressure above 60 bar on a 120 mm line is a typical maintenance trigger for screen replacement. The gear pump absorbs moderate screen-changer pressure variation, but when differential pressure changes at a rate greater than 5 bar/min, the pump suction pressure may fall below the cavitation threshold and produce streaking in the extruded sheet. Melt temperature at the die inlet should be maintained within ±1 °C of setpoint by adjusting barrel cooling blowers and screw temperature-control circuits. Pressure oscillation from a worn screen changer seal can appear as cyclic variation at 0.2 Hz to 1.0 Hz, which is distinguishable from screw-induced pulsation by its independence from screw speed. On production equipment, dual-bolt hydraulic continuous screen changers are preferred over manual slide-plate units because the flow interruption during screen change is reduced to less than 1 s, preserving die pressure within ±0.3 bar of setpoint.

When Regrind Fraction Exceeds 30% at 120 mm Diameter

At regrind levels above 30%, bulk density, granule shape, crystallinity distribution, and intrinsic viscosity variability in the feed stream to a 120 mm barrier screw are altered. Post-industrial edge trim and rejected strapping are mechanically ground to a nominal particle size of 8 mm to 12 mm; fines below 2 mm should be limited to less than 5% by mass because fines can pack the feed throat and create periodic feed interruptions. The barrier screw responds to feed-density changes by shifting the location of the melting front along the barrel; when regrind fraction changes by more than 5% per 15 min, the gear pump inlet pressure can drift by 2 bar to 4 bar before the control loop restores equilibrium. Using a gravimetric blender with component accuracy of ±0.5% is standard practice. The IV of regrind is typically 0.02 dL/g to 0.04 dL/g lower than virgin resin after one heat history; blending beyond 40% regrind can reduce the average melt IV below the required strapping threshold unless the virgin resin IV is raised accordingly. The melt pressure trace at the die adapter often displays a bimodal disturbance pattern when high regrind fractions are processed: a low-frequency component tied to feeder refill and a high-frequency component tied to barrier flight passage. Reducing screw speed by 5 rpm to 10 rpm while allowing the gear pump to maintain throughput can stabilize the suction pressure at ±0.5 bar if melting capacity is sufficient. Published data for this specific regrind configuration in 120 mm PET strapping barrier screw extrusion is limited, so start-up trials with a given regrind source are required to set alarm thresholds for pressure deviation.

Downstream of the gear pump, pressure consistency in the die adapter and orientation stage is governed by the melt pump, melt filter, and die resistance more than by the barrier screw alone. Once the melt enters the die adapter, pressure must be held steady enough to produce a flat ribbon with width variation below 0.5 mm and thickness variation below 0.03 mm before hot drawing. The die gap is typically set at 1.5 mm to 3.0 mm with the final strapping thickness achieved by orientation at draw ratios from 4:1 to 6:1. The melt pressure transducer at the die inlet is calibrated against a dead-weight tester at operating temperature; an accuracy of ±0.2 bar is required for reliable width control. Finished strapping is tested according to ASTM D3950-17 for break strength and elongation, with additional tensile property data generated under ASTM D638-14 or ISO 527-2:2012 for resin and extrudate comparisons. Orientation-induced crystallinity in the hot-drawn strap increases stiffness and tensile strength, but non-uniform melt pressure during extrusion produces thickness-driven draw ratio variation that appears as gauge bands and inconsistent joint weldability in strapping tools. The operating boundary for high-tenacity PET strapping on 120 mm barrier screw lines is therefore set by the combined requirements of post-extrusion IV above 0.78 dL/g, gear pump inlet pressure between 20 bar and 50 bar, die pressure stability within ±0.2 bar, and melt temperature no greater than 290 °C.