If you are specifying or purchasing an extruder for cable or wire production, one of the first numbers you will encounter is the L/D ratio. It is printed in every extruder datasheet and quoted in every technical discussion — but what does it actually mean, and why does it matter?
What L/D Ratio Means
L/D stands for Length to Diameter ratio. It describes the length of the extruder screw relative to its diameter.
For example:
- A screw that is 25D long and 60mm in diameter = L/D of 25:1
- A screw that is 30D long and 90mm in diameter = L/D of 30:1
The formula is simple: L/D = Screw Length ÷ Screw Diameter
A higher L/D means a longer screw relative to its width. A lower L/D means a shorter, more compact screw.
Why L/D Ratio Matters
The extruder screw has three jobs: feed solid pellets, melt them, and pressurise the melt into the die. These three functions happen in three zones along the screw — the feed zone, the compression zone, and the metering zone.
A longer screw (higher L/D) gives each zone more length to do its job:
- Better melting: More residence time means more heat input and better homogeneity
- Better mixing: Longer metering zone mixes colour, additives, and compounds more thoroughly
- Better pressure consistency: Longer metering zone reduces surging and pressure fluctuations
- Lower melt temperature: More gradual melting generates less frictional heat, which matters for heat-sensitive materials like PVC
A shorter screw (lower L/D) processes material faster but with less mixing and less consistent output.
Typical L/D Ratios for Cable Extrusion
Different cable materials and applications require different L/D ratios:
| Material | Recommended L/D | Reason |
|---|---|---|
| PVC (standard) | 20:1 – 25:1 | PVC is heat sensitive; shorter residence time preferred |
| PVC (rigid/compound) | 25:1 – 30:1 | More mixing needed for filled compounds |
| XLPE (silane crosslinkable) | 20:1 – 24:1 | Prevent premature crosslinking |
| HFFR / LSZH | 28:1 – 32:1 | Highly filled material needs longer mixing zone |
| Polyethylene (HDPE/LDPE) | 25:1 – 30:1 | Good melt behaviour; longer screw improves output |
| Nylon (PA) | 20:1 – 25:1 | Hygroscopic; short screw to minimise degradation |
| TPR / TPE | 25:1 – 28:1 | Needs good mixing for consistent properties |
For most cable insulation and sheathing applications, an L/D of 24:1 to 28:1 is the industry standard starting point.
L/D and Screw Design Work Together
L/D ratio alone does not tell the full story. The compression ratio, feed zone depth, metering zone depth, and helix angle of the screw all interact with the L/D to determine actual extruder performance.
A long screw with the wrong compression ratio can over-shear a material and cause degradation. A short screw with the right design can outperform a longer one for a specific material.
This is why screw selection should always be done together — specifying the L/D, the compression ratio, and the material to be processed as a package, not each in isolation.
Common L/D Mistakes
Mistake 1: Using a PVC screw for HFFR/LSZH compound
PVC screws are typically 20–24:1. HFFR compounds are highly filled and need a 28–32:1 screw for proper dispersion. Using the wrong screw causes poor mixing, surface defects, and inconsistent mechanical properties.
Mistake 2: Using a long screw for silane XLPE
Silane crosslinkable XLPE begins crosslinking when exposed to heat and moisture. A very long screw increases residence time and raises the risk of scorch (premature crosslinking inside the barrel). Keep L/D at 20–24:1 for silane XLPE.
Mistake 3: Assuming longer is always better
Some operators assume a 32:1 screw will always outperform a 24:1 screw. This is not true. For heat-sensitive materials, a longer screw can cause more degradation, not less. Match the L/D to the material.
Screw and Barrel from Sai Extrumech
Sai Extrumech manufactures screws and barrels for cable, wire, and pipe extrusion across a full range of L/D ratios and diameters. We manufacture screws in nitrided steel and bimetallic (for abrasive compounds), with custom flight geometry matched to your material and line speed.
If you are replacing a worn screw or upgrading to a new compound, send us your current screw specification and material datasheet — we will recommend the correct replacement.
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Frequently Asked Questions
Q: Can I increase the L/D ratio of my existing extruder?
A: In most cases, no. The barrel length is fixed by the extruder frame. To change L/D, you would need a new barrel and screw — effectively a new extruder. Some extruder designs allow extension barrels to be added, but this is not common.
Q: Does a higher L/D ratio mean a higher output rate?
A: Not necessarily. Output rate depends more on screw diameter, screw speed, and metering zone depth. A higher L/D can improve melt quality and reduce surging, which can indirectly allow higher stable output — but it is not a direct relationship.
Q: What L/D ratio does Sai Extrumech recommend for PVC cable extrusion?
A: For standard PVC cable insulation and sheathing, we recommend 24:1 to 25:1 as the standard, with 28:1 available for heavily filled or rigid PVC compounds that need more intensive mixing.
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