The L/D ratio of an injection molding screw is its flighted length (L) divided by its nominal diameter (D), and the compression ratio is the depth of the screw channel in the feed zone divided by its depth in the metering zone. Together they describe how much work the screw does on the plastic as it melts it. General-purpose injection screws typically have an L/D of about 20:1 to 25:1 and a compression ratio between about 2:1 and 3:1, but the right values depend on the material and the machine.
Both ratios describe the screw, item 4 in the drawing. For the screw's parts and its three zones, see the screw guide.
How to calculate the L/D ratio
Divide the flighted length by the nominal diameter:
L/D = flighted length ÷ screw diameter
For example, a 40 mm screw with 880 mm of flights has an L/D of 880 ÷ 40 = 22, written 22:1.
Not every data sheet measures L the same way. Most use the flighted length; some use the effective length, from the front edge of the feed opening to the front of the flights. Check which one a machine or screw specification uses before comparing two screws.
What the L/D ratio changes
A longer screw (higher L/D) gives the plastic more time and more flights to heat, melt and mix in. It produces a more uniform melt, handles higher outputs and mixes colors better. The drawbacks are a longer residence time, which can degrade heat-sensitive materials, more torque to turn the screw, and a longer screw that is harder to keep straight.
A shorter screw keeps the plastic in the barrel for less time, which suits heat-sensitive materials. Screws for thermosets are short, with little or no compression, because the material must not cure in the barrel.
Effective L/D during screw recovery
As the screw recovers, it moves backward, and the rear flights pass behind the feed opening. The length of flights doing work in front of the feed opening shrinks by the length of the stroke. With a long shot stroke, the last part of the shot is melted by a shorter screw, so melt quality can fall at the end of recovery. This is one reason why very large shots relative to the barrel are avoided; see the shot size calculation.
The compression ratio
In its simplest form, the compression ratio is:
Compression ratio = feed zone channel depth ÷ metering zone channel depth
For example, a screw with a 6.0 mm deep channel in the feed zone and a 2.4 mm deep channel in the metering zone has a compression ratio of 6.0 ÷ 2.4 = 2.5:1. Some screw makers calculate it from the channel volumes per turn instead of the depths, which gives a slightly different figure.
A higher compression ratio squeezes and shears the plastic more, which helps melting and mixing but adds heat. A lower ratio is gentler, for materials that degrade with heat or shear. The length of the compression zone matters as well: a long, gradual compression zone suits materials that soften over a wide temperature range, and a short one suits materials that melt over a narrow range.
Screw geometry by material
| Material type | Typical screw choice |
|---|---|
| General-purpose plastics such as PP, PE, PS and ABS | General-purpose three-zone screw |
| Heat-sensitive plastics such as rigid PVC | Lower compression with a gradual compression zone, often with a free-flow tip |
| Semi-crystalline plastics that melt over a narrow range, such as nylon | Short compression zone |
| Thermosets | Short screw with little or no compression |
These are general tendencies. The machine maker or the screw supplier sets the actual geometry, and the material supplier can tell you whether a standard screw is suitable.
How to measure a screw's L/D and compression ratio
- Pull the screw (see how to remove the screw) and clean it.
- Measure the flight diameter with a micrometer in a part of the screw with little wear, or use the nominal size from the machine data.
- Measure the flighted length with a tape along the screw.
- Measure the root diameter in the middle of the feed zone and of the metering zone with a caliper or micrometer.
- Work out each channel depth as (flight diameter − root diameter) ÷ 2, then the two ratios.
- Count the flights in each zone: the compression zone is where the root diameter changes.
Choosing L/D and compression for a replacement screw
For a replacement, keep the original geometry unless you have a processing problem that a different screw would solve. The L/D is fixed by the barrel: a longer screw needs a longer barrel. Within the same barrel, the screw maker can change the zone lengths, the compression ratio and the mixing sections for a particular material.
The bimetallic barrel and screw assemblies we supply are listed with L/D ratios of 20:1 to 22:1 for screw diameters up to Φ50 mm, and 20:1 to 24:1 for Φ60 mm and Φ70 mm. When you request a quote, send the materials you run along with the screw dimensions listed on the screw page.
Updated by the IMSPARES technical team.
