Polyethylene terephthalate moves from chip to yarn through a sequence of drying, plastication, filtration, metering, extrusion, quenching, drawing, and package build. The phrase “From Chip to Yarn: How PET Reaches a Spinning Line” describes a process in which polymer molecular weight, moisture content, carboxyl end group level, and thermal history determine whether the threadline survives high-speed winding. Fibre-grade PET is typically supplied with an intrinsic viscosity of 0.62–0.68 dL/g measured in 60/40 phenol/1,1,2,2-tetrachloroethane at 30 °C per ASTM D4603-18. Bottle-grade PET near 0.80–0.84 dL/g is sometimes processed into industrial yarn or monofilament but requires higher melt temperatures and imposes a narrower processing window. Chip bulk density is commonly 0.80–0.85 g/cm³; the melting range is 245–255 °C and the glass transition is near 70–80 °C. Diethylene glycol content in standard textile PET is commonly 1.2–1.5 wt%; carboxyl end groups are usually 15–30 mmol/kg. Fines and angular chip fragments are controlled because they create feed inconsistency, and hopper bridging on production-scale extruders has been observed when fines exceed 0.1 wt%. The chip is conveyed to the extruder hopper under dried air or nitrogen; if ambient relative humidity exceeds 60% and hopper blanketing is absent, moisture regain can exceed 30 ppm before plastication.

Why Is Chip Moisture Controlled Below 30 ppm Before the Extruder Hopper?

Each water molecule can hydrolyse one ester linkage in the PET backbone, producing a hydroxyl and a carboxylic acid chain end. The reaction is kinetically slow at room temperature but becomes significant above the melt temperature of 280–295 °C. Production-scale spinning experience shows that chip moisture above 50 ppm commonly produces measurable intrinsic viscosity loss and increased filament breaks in pre-oriented yarn spinning; above 100 ppm, bubble formation and die-face contamination are reported. Amorphous PET chip must be pre-crystallised before drying to prevent agglomeration in the hopper; a fluidised bed or stirred crystalliser at 120–140 °C for 15–30 min raises crystallinity enough to avoid sticking and protect downstream desiccant beds. Desiccant dryers using molecular sieve wheels are specified to deliver air at a dew point of −45 °C to −35 °C and a supply temperature of 170–180 °C. A drying residence time of 4–6 h reduces chip moisture to 20–30 ppm for fibre-grade resins; high-IV PET or granulate stored in cold warehouses may require 8 h or a two-stage hopper because crystallinity and pellet size slow moisture diffusion. The hopper and transfer line are blanketed with dried air at a dew point of −40 °C or lower. Moisture is measured by Karl Fischer titration according to ISO 15512:2019; loss-on-drying methods over-report volatile content. When plant relative humidity exceeds 60%, open conveying systems represent a process boundary because rehydration of dried chip can occur in minutes.

Extruder Barrel Zones and Residence-Time Limits During Melt Preparation

Single-screw extruders with 25:1 to 33:1 L/D ratio and barrier screws are common in PET filament operations. Barrel set-points typically progress from 260–285 °C in the feed zone to 275–290 °C in the compression zone and 280–295 °C in the metering zone, with melt temperature at the discharge held between 285–295 °C. The melt temperature is kept below 300 °C; above this threshold thermal degradation accelerates, producing acetaldehyde and increasing carboxyl end groups. Total residence time from hopper to spinneret is normally limited to 12–15 min. Longer residence time broadens molecular weight distribution and shifts colour b* yellowness upward, which is a production bottleneck for optically bright filament. Extruder discharge pressure is commonly 80–150 bar, depending on throughput and filter pack condition. Apparent melt viscosity for textile PET at 285 °C and 1,000 s⁻¹ shear rate is typically 120–180 Pa·s; inline melt pressure transducers are used to detect viscosity drift. A reverse temperature profile—feed zone hotter than metering zone—is sometimes used to minimise shear heating on high-speed lines. Melt temperature is measured at the discharge flange with a thermocouple inserted flush to the melt stream; surface-mounted barrel thermocouples alone are not adequate because they lag polymer temperature by several seconds.

Between extruder discharge and spinneret, the melt passes through a positive-displacement gear pump. Metering pump displacement is selected to deliver 0.5–2.0 cm³/rev per spinning position; pump speed is adjusted to maintain constant throughput because pressure fluctuations produce denier drift. Filtration in the spin pack uses sintered metal or multilayer screen media rated at 20–40 µm, with upstream coarse filtration at 100 µm to protect the gear pump. Differential pressure across the spin pack increases as the media plugs; replacement is commonly scheduled at 150–250 bar, depending on resin cleanliness and throughput. The spin pack distributes melt to spinneret holes of 0.20–0.40 mm diameter with land lengths of 0.6–1.0 mm. Hole geometry controls pressure drop and extrudate swelling; inadequate pressure drop produces unstable melt streams and filament denier variation. Spinneret face temperature is maintained within ±1 °C across the beam using liquid-phase heat transfer, because a temperature difference of 2 °C across a 48-end beam can change melt viscosity enough to create measurable denier drift. Spinneret plates are cleaned by fluidised bed oxidation or vacuum pyrolysis; residual carbon in the capillaries is a common cause of broken filaments on start-up.

When Quench Air Velocity Falls Below 0.30 m/s in Crossflow Chimneys

In a crossflow quench system, extruded filaments pass through a conditioned air stream supplied horizontally through a perforated plate. Uniform air velocity of 0.30–0.50 m/s, temperature of 18–22 °C, and relative humidity of 65–85% are typical for PET pre-oriented yarn; fully drawn yarn may use 0.40–0.60 m/s at winding speeds above 4,500 m/min. If air velocity falls below 0.30 m/s, cooling is asymmetric and filament fusion can occur; above 0.60 m/s, filament vibration and drag increase tension variability. The quench delay distance from spinneret face to first air contact is usually 30–80 mm. Longer delay reduces stress-induced orientation, while shorter delay increases skin-core orientation differences. Crossflow air is filtered to 10 µm absolute to prevent hard particles from being drawn into the fibre bundle. The quench air plenum pressure drop is commonly 100–250 Pa. Spinneret hole arrangement and monofilament spacing are designed so that air can penetrate to the centre of the bundle; quench air distribution is checked with a velocity grid across the full chimney face, and deviations greater than ±10% are corrected before production because local low velocity correlates with undrawn filament defects.

Spin finish is applied before the first godet at an add-on of 0.4–0.6% by weight of yarn. Aqueous emulsions containing lubricant and antistatic components are diluted to 5–15% concentration and applied by ceramic kiss roll or metered dosing pump. Oil pickup is verified by extraction according to ASTM D2257-20. Insufficient finish increases filament-to-metal friction and can generate broken filaments at high godet speeds; excessive finish deposits on draw panels and package rings. Interlace nozzles operating at air pressure of 1.5–3.5 bar introduce 10–20 nodes/m for package stability. Interlace pressure is adjusted at the winder because excessive nodes reduce dye uptake and can produce a harsh fabric hand.

What Draw Ratio Converts Low-Oriented Yarn into Fully Drawn Flat Yarn?

The as-spun threadline is collected on a first godet at 2,500–3,500 m/min for POY, or drawn in-line for FDY at 4,500–5,500 m/min. In-line drawing between heated godets is carried out at a draw ratio of 1.6–2.5 for FDY, depending on spinning speed and target elongation. POY is drawn during subsequent texturing at a draw ratio of 1.55–1.75, with a primary heater temperature of 160–210 °C and a secondary heater of 140–180 °C. The first draw godet is typically heated to 80–95 °C, and the second to 120–150 °C, to promote molecular orientation and partial crystallinity. The draw point must be fixed by the temperature gradient between godets; if the draw point moves upstream, necking defects appear. A draw ratio that is too high for the as-spun orientation causes filament breaks; too low a draw ratio leaves elongation above specification and may create dye-uptake unevenness. Tensile testing is performed after conditioning at 20 °C and 65% RH for 24 h according to ISO 139:2005, using ISO 2062:2009. Fully drawn PET yarn typically shows tenacity of 4.0–7.5 cN/dtex and elongation of 20–35%, depending on draw ratio and polymer IV.

Texturing converts POY into draw-textured yarn by passing it through a primary heater, a friction disc unit, a cooling plate, and a secondary heater. Disc arrangement and surface material control yarn tension; common production lines use polyurethane or ceramic discs with a D/Y ratio of 1.8–2.2. Package density is controlled to 0.90–0.95 g/cm³ for downstream dyeing uniformity. Wound packages of FDY or DTY are tested for oil pickup, linear density by ISO 1973:2021, tenacity and elongation by ISO 2062:2009, hot-air shrinkage by ASTM D2259-21, and Uster evenness. Hot-air shrinkage at 180 °C for 15 min is commonly 5–8% for FDY, depending on draw ratio and heat setting. Package build uses a traversing winder with a crossing angle of 5–10° to avoid ribboning. Too low a traverse ratio increases package hardness beyond the dye-bath flow limit; too high a ratio causes edge collapse. Published line data for very high-speed POY above 5,500 m/min with spinneret holes below 0.16 mm remains limited.