From Melt to Yarn: Melt Spinning and POY/FDY/HOY
A polyester filament's character is decided not at the knitting machine but in the few-second spinning line where melt turns to solid — and those seconds are governed by take-up speed.
Polyester (PET) yarn is made by a process in which a molten polymer is pushed through fine holes under pressure and solidifies as it cools: melt spinning. No solvent is involved; the filament is shaped by heat and tension alone. The yarn's tenacity, elongation, dye uptake and dimensional stability are all set within a few metres of this line. The crucial point: from the very same polymer, simply by changing the speed and geometry of the spinning line, you obtain completely different yarns — POY, FDY, HOY.
What happens along the line: from extrusion to godet
Dried PET chip is melted and homogenised at typically ~280–295 °C in a screw extruder with an L/D ratio typically of 30:1–36:1. A gear pump meters the melt at constant throughput to the spinneret. The spinneret is a precision-machined stainless steel plate carrying anywhere from a few dozen to a few hundred holes (more for industrial yarns), with hole diameters typically in the 50–300 µm range. Hole count sets filament count; cross-sectional geometry (round, trilobal, hollow) governs the yarn's luster, cover and hand.
The liquid filaments leaving the holes are cooled in a quench duct by conditioned air, typically at ~18–22 °C, in a crossflow or radial arrangement. The uniformity of this air's velocity and temperature directly governs denier/dtex CV% — i.e. fineness evenness — across the bundle. Once the surface has solidified, a spin finish (fiber lubricant) is applied to provide lubricity, antistatic protection and bundle cohesion; pickup is typically ~0.3–0.8% by weight. An unfinished bundle can shred from friction along the godet and winding path.
At the end of the line the filaments are wound onto one or more godet roller pairs. Godet speeds set the draw ratio and therefore the final orientation; heated godets additionally heat-set the structure to lock the crystalline order. This is where the variable that defines yarn type lives: the take-up/winding speed.
The speed regime sets the yarn's identity
The faster the filament is drawn off the line, the more the still-mobile polymer chains are stretched along the axis (molecular orientation) and the earlier stress-induced crystallization is triggered. So the industry names yarn by the orientation/crystallinity level at which it was 'frozen' on the way out of the melt. At low speed chains stay slack (high elongation, low tenacity); at high speed they align and partly crystallise (low elongation, high tenacity).
| Yarn type | Typical take-up speed (m/min) | Molecular orientation | Typical elongation at break | Typical use |
|---|---|---|---|---|
| UDY (undrawn) | < ~1,800 | Very low | Very high (>200%) | Intermediate for separate draw |
| LOY / MOY (low/medium) | ~1,800–2,800 | Low–medium | High | Intermediate / niche uses |
| POY (partially oriented) | ~2,800–3,500 | Medium (birefringence typically ~0.03–0.06) | ~100–160% | Feed for texturing (DTY) and draw |
| HOY/FOY (high/fully oriented) | ~4,500–6,000+ | High | ~50–90% | Direct use, no separate draw |
| FDY (fully drawn) | spin-draw line; typical take-up ~3,500–5,000 | High | ~25–40% | Direct to weave/knit, strong |
Why is POY a 'semi-finished' yarn?
POY is partially oriented yarn spun at ~2,800–3,500 m/min: chains are partly aligned but crystallization is not yet complete. Its high residual elongation (typically ~100–160%) and low crystallinity make it mechanically unstable but highly processable. This 'ready-to-draw' character makes POY an ideal feed for false-twist texturing machines, where it is simultaneously drawn (spending the residual elongation) and textured into bulky, elastic DTY. POY is therefore not a finished yarn on its own; it acquires its true end-use character in the next step.
FDY and HOY: shortening the line
FDY combines spinning and drawing in one continuous step (spin-draw): the filament is drawn directly in-line between heated godets, yielding a yarn with low residual elongation (~25–40%) and high tenacity that is dimensionally stable and ready to weave/knit without a separate draw step. HOY/FOY achieves a similar orientation in a single step purely through the tension created by very high winding speed — a way to produce high orientation with less equipment. The designer's choice is a trade-off between process economics and yarn character.
The physics, in essence: orientation, crystallinity and neck draw
PET's glass transition temperature (Tg) is roughly 72–80 °C; above this threshold chains gain mobility and orientation/crystallization become possible. In separate (off-line) drawing, when the filament is heated just above Tg and stretched, a localised neck draw appears: the structure is amorphous before the neck and aligns and stress-crystallises after it. In high-speed spinning the same stress-induced crystallization occurs on the line itself. Orientation is measured in practice via birefringence; for POY it is typically ~0.03–0.06 and rises with speed. Higher orientation also means a tighter structure — which hinders disperse-dye diffusion, so the spinning line even pre-sets dyeing behaviour.
How it is measured: the standards frame
Yarn linear density (fineness) is determined per ISO 2060 by weighing a skein, expressed in tex/dtex (the method applies to yarns up to ~2,000 tex); denier is another unit of the same quantity (denier = dtex × 0.9). Tenacity and elongation are determined per ISO 2062 by single-end testing of yarn taken from a package on a constant-rate-of-extension (CRE) machine: the standard defines a 250 mm or (by agreement) 500 mm gauge length and a time to break of ~20 ± 3 s (the extension rate follows from gauge length and this time), with breaking force preferably in centinewtons (cN) and elongation as a percentage of the original length. Tenacity is normalised by dividing force by fineness, in cN/dtex — typically ~2.0–3.0 cN/dtex for POY rising to ~3.5–5.0 cN/dtex for FDY. These two standards move the 'is it POY or FDY' question from a subjective label to a measurable number.
In short: as the melt-spinning line turns a polymer chip into yarn, the choice of speed and draw ratio pre-writes the yarn's entire textile life. When a buyer reads a spec like 'POY 150/48 SD', they are really reading a production regime — the orientation level, the residual elongation and therefore where the yarn will be fed next.