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.
6 sections 5 terms 6 sources ~5 min
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.
Godet speeds: draw ratio and heat-setting
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.