The L/D ratio is one of the first numbers on any extruder's spec sheet — and one of the most misunderstood. It quietly governs how well the machine melts, mixes and delivers a stable output, so it is worth knowing exactly what it tells you.
What L/D actually means
L/D is the ratio of the screw's flighted length to its diameter. A 45 mm screw with roughly 1,170 mm of working length has an L/D of about 26:1. A higher number simply means a longer screw relative to its diameter — and therefore more processing length and residence time for the polymer as it travels down the barrel.
Why it matters
- Melting — a longer screw gives the material more length over which to fully melt and homogenise before it reaches the die.
- Mixing — more length means more opportunity for distributive mixing, so the melt arrives uniform in temperature and composition.
- Output and pressure — the extra length helps build and stabilise pressure, supporting consistent throughput.
- Residence time — the flip side: a longer screw keeps the polymer hot for longer, which matters a great deal for heat-sensitive materials.
Typical L/D ranges
| Process | Typical L/D |
|---|---|
| PVC insulation (single-screw) | 24:1 – 26:1 |
| PE / PP extrusion (single-screw) | 25:1 – 30:1 |
| Twin-screw compounding | 32:1 – 52:1 |
Key point: compounding needs length for mixing, so twin-screw compounders run long. PVC insulation is heat-sensitive, so single-screw PVC lines deliberately stay in the 24:1–26:1 band.
Longer is not always better
It is tempting to read a higher L/D as "more capable," but for a heat-sensitive polymer like PVC, too much length and residence time invites thermal degradation — discolouration, gas, and porosity in the insulation. The right L/D is the one matched to the material and the job, not the biggest number available.
L/D is only half the story
Two extruders with the same L/D can perform very differently, because what happens inside that length is set by the screw design — the feed, compression and metering zones, and any mixing elements. A screw profiled specifically for PVC will outperform a generic screw of the same or greater L/D. This is why the screw and barrel are engineered as a pair, to the material being run.