- 1. What a screw actually does
- 2. The three zones, and what happens in each
- 3. Mixing sections: when you need one and which
- 4. The design numbers that matter
- 5. Wear: where it happens and what it costs
- 6. Screw-related problems and their signatures
- 7. Specifying a screw for a new profile
- 8. Running the screw well
- 9. Frequently asked questions
What a screw actually does
An extruder screw does four jobs simultaneously, and it does them with one continuous piece of steel:
- Conveys solid pellets from the hopper to the melt zone.
- Melts them, using both conducted heat from the barrel and frictional heat from the screw's own work.
- Mixes the melt, so that temperature and composition are uniform across the channel and along the screw.
- Pumps the melt against the die's resistance at a stable rate and pressure.
Those four jobs compete for the same length. A screw optimised for conveying is bad at mixing; a screw optimised for mixing generates too much heat for a heat-sensitive material. Almost every screw design decision is a trade between them.
The number that describes a screw: L/D — the working length divided by the diameter. A 90 mm screw with a 2,700 mm working length is 30:1. Profile extrusion screws run 25:1 to 33:1; the longer the screw, the better the mixing and the more stable the output, at the cost of more torque, more residence time and a bigger machine.
The three zones, and what happens in each
| Bölge | Share of length | Flight depth | What happens |
|---|---|---|---|
| Feed | 20 – 30 % | Deep | Pellets conveyed and compacted; some preheating by conduction |
| Compression (transition) | 20 – 30 % | Tapers from deep to shallow | Pellets compact, air is pushed back, melting begins |
| Metering | 40 – 50 % | Shallow, constant | Remaining solids melt; melt is homogenised and pumped |
These proportions are for a general-purpose profile screw. Heat-sensitive materials shift length toward the feed zone; mixing-intensive compounds shift it toward metering.
Feed zone
Deep flights, often with a slight taper, and a smooth or grooved barrel wall. The feed zone's job is to deliver a consistent volume of compacted solids. The most common feed-zone problem is surge — the solids bed slips and then grips, producing a periodic variation in output that shows up downstream as a thickness wave in the profile.
Fixes for surge are almost always in the feed zone: a grooved feed throat, a slightly warmer feed zone, or a screw with a deeper feed section. Changing the die never fixes surge, even though the symptom appears at the die.
Compression zone
The flight depth reduces from deep to shallow. The ratio of feed depth to metering depth is the compression ratio, and it is one of the most consequential numbers on the screw.
| Malzeme | Typical compression ratio | Neden |
|---|---|---|
| Rigid PVC (dry blend) | 2.0 – 2.8 : 1 | Low — PVC is shear-sensitive and degrades easily |
| Flexible PVC | 2.5 – 3.2 : 1 | Slightly higher; plasticiser aids compaction |
| HDPE | 3.0 – 4.0 : 1 | Bulk density changes a lot on melting |
| PP | 2.8 – 3.8 : 1 | — |
| ABS | 2.5 – 3.5 : 1 | — |
| PC | 2.2 – 3.0 : 1 | High viscosity; too much compression overheats |
| PMMA | 2.5 – 3.2 : 1 | — |
| PA | 3.0 – 3.8 : 1 | Crystalline, sharp melting |
| POM | 2.8 – 3.5 : 1 | — |
| Filled compounds (talc, CaCO₃) | 2.0 – 2.8 : 1 | Higher bulk density; less compression needed |
| Glass-filled | 1.8 – 2.5 : 1 | Low, and wear protection is more important than ratio |
A compression ratio that is too high generates excess shear heat and can degrade PVC; too low and the melt is not fully compacted, producing unmelt and poor surface.
Metering zone
Shallow constant depth. This is the pump, and its depth determines the screw's output characteristic — how much output changes when die pressure changes.
A deep metering zone gives high output but a "soft" characteristic: output falls noticeably as die pressure rises, which makes the line sensitive to die condition. A shallow metering zone gives lower output but a "hard" characteristic: output stays nearly constant as die pressure changes, which is exactly what a profile line wants for dimensional stability.
Why profile lines prefer a shallow metering zone: a profile die's resistance drifts as it fouls, as its temperature moves and as the screen pack loads. A screw with a hard characteristic absorbs those drifts without changing output; a soft screw transmits every one of them into the profile's wall thickness.
Mixing sections: when you need one and which
A plain three-zone screw leaves a temperature gradient across the channel and often carries unmelted pellets into the metering zone. A mixing section fixes both, at the cost of pressure drop and some output.
| Mixing element | How it works | İçin en iyisi | Pressure penalty |
|---|---|---|---|
| Maddock / fluted mixer | Melt is forced through flutes and over a barrier, then recombined | General purpose; excellent for colour and unmelt | Orta düzeyde |
| Pineapple / pin mixer | Rows of pins interrupt the flow | Simple, cheap, good for heat-sensitive materials | Düşük |
| Barrier screw (maillefer) | A secondary flight separates solid from melt along the whole length | High output, crystalline materials (PE, PP) | Low — but a completely different screw |
| Static mixer (in the adapter) | No moving parts; the melt passes through twisted elements | Retrofit; excellent thermal homogenisation | Yüksek |
| Cavity transfer mixer | Melt is repeatedly split and recombined in cavities | Difficult dispersive mixing, colour concentrates | Yüksek |
For profile extrusion of PVC, a pineapple or a simple fluted mixer is usually enough. For filled or coloured engineering resins, a Maddock section earns its pressure penalty.
Do you always need one?
No. A well-designed three-zone screw running a single unfilled material at stable conditions can produce an acceptable profile without any mixer. The cases where a mixer pays for itself:
- Colour masterbatch at low let-down ratios (below 2 %), where dispersion is difficult.
- Filled compounds where agglomerates must be broken up.
- High output rates, where residence time in the metering zone is short.
- Any process where surface defects track back to unmelt or to temperature streaks.
The design numbers that matter
| Parametre | Typical range | Effect of increasing |
|---|---|---|
| L/D ratio | 25:1 – 33:1 | Better mixing and stability; more torque, longer residence time |
| Compression ratio | 2.0:1 – 4.0:1 | Better compaction; more shear heat |
| Feed depth / D | 0.12 – 0.20 | Higher output; weaker solids conveying |
| Metering depth / D | 0.04 – 0.08 | Higher output, softer characteristic |
| Flight pitch | 0.8 – 1.2 × D | Higher convey rate; less mixing per turn |
| Flight width | 0.08 – 0.12 × D | Stronger flight; less channel volume |
| Flight clearance | 0.001 – 0.002 × D | Less leakage, better output; more wear risk |
| Helix angle | 17 – 25° | Optimum around 17 – 20° for most materials |
These are starting points for a general-purpose profile screw. A screw for a specific compound is normally designed against that compound's viscosity curve, not from a table.
Two of these deserve a note:
- Flight clearance is the wear indicator. A new screw runs 0.10 – 0.15 mm clearance on a 75 mm screw. At 0.30 mm the output has typically fallen 10 – 20 % and the output stability has degraded. Measuring clearance at service is how you decide whether to re-cut or replace.
- Helix angle is a real design choice, not a default. A steeper flight (25°) conveys more per turn but mixes less; a shallower one (17°) mixes better and pumps more stably. Profile screws usually sit at the shallow end.
Wear: where it happens and what it costs
Screws wear, and they wear unevenly. Knowing where tells you what the compound is doing.
| Wear location | Neden | Consequence |
|---|---|---|
| Flight tips in the compression zone | Abrasive filler, high screw speed | Increased clearance, lost output |
| Flight leading faces in the feed zone | Hard or irregular pellets, glass fibre | Worn flight profile, poorer conveying |
| Screw root in the metering zone | Corrosive compounds, HCl from PVC | Pitting, poor heat transfer, hang-up points |
| Barrel wall opposite the feed throat | Solids bed pressure | Barrel wear — much more expensive than screw wear |
| Check valve / non-return | Backflow erosion | Unstable output, especially at high pressure |
Barrel wear is the expensive one: a barrel is 2 – 4× the cost of a screw, and replacing it means a full teardown. Protecting the barrel is a reason to keep screw speed and back pressure within design limits.
Wear protection options
- Nitrided screw — a hard surface case, good for PVC and moderate fillers, but the case is thin and cannot be repaired.
- Bimetallic (tungsten carbide) flights — welded hard-facing on the flight tips. Standard for filled and glass-containing compounds.
- Through-hardened tool steel — for high-torque, high-wear applications; can be re-cut.
- Barrel liner — a centrifugally cast bimetallic liner, which protects the more expensive component.
| Symptom | Screw-related cause | How to confirm |
|---|---|---|
| Output surges periodically | Solids conveying instability in the feed zone | Output varies at a fixed period independent of die |
| Output falls gradually over months | Flight clearance wear | Measure clearance; check motor load for same output |
| Unmelt / specks in the profile | Insufficient melting length or compression | Section the extrudate; look for unmelted cores |
| Temperature streaks across the section | Poor cross-channel mixing | Thermal image the extrudate; add a mixer |
| Colour variation | Insufficient dispersive mixing | Change let-down ratio; if it improves, mixing is the issue |
| Black specks | Hang-up on a worn screw root or dead spot | Pull the screw; inspect root and flights |
| Motor load high at low output | Excess back pressure or worn check valve | Compare specific energy (kWh/kg) against baseline |
| Degradation at the metering end | Too much shear heat, too shallow metering | Measure melt temperature at the die; compare to setpoint |
Specific energy — kWh per kilogram — is the single best indicator of screw health. It rises as clearance wears and as the screw works harder to deliver the same output.
Specifying a screw for a new profile
When you order a screw, the supplier needs six pieces of information. Without them, you get a general-purpose screw and then spend a year working around its compromises.
| Information | Why it matters |
|---|---|
| Material and grade, with filler type and loading | Sets compression ratio, wear protection, mixing needs |
| Bulk density of the pellet or dry blend | Sets feed depth and compression ratio |
| Target output in kg/h | Sets diameter and metering depth |
| Die head pressure at that output | Sets metering depth and motor sizing |
| Melt temperature limit | Caps shear heat; constrains compression ratio |
| Colour and additive requirements | Decides whether a mixing section is needed |
A screw specified without the die pressure figure is a guess. The same screw delivers very different output into a 10 MPa die and a 35 MPa die.
Running the screw well
Most screw problems in service are operating problems, not design problems. Five habits prevent most of them:
- Keep the screw full. Starve-feeding a profile extruder produces output variation and poor melt quality. The screw should be full and the metering zone should be generating pressure, not just conveying.
- Do not use screw speed as the primary control. Set the speed for the output you want and control dimensions with haul-off, temperature and die — not by hunting the speed up and down.
- Watch specific energy. Log kWh/kg at every setup. A 10 % rise means something has changed — wear, fouling, or a colder barrel.
- Purge properly on shutdown. PVC and PC left in a hot screw degrade and leave carbon that becomes black specks for weeks.
- Measure clearance at every major service. It is a 20-minute job with the screw out, and it is the only way to plan a replacement instead of reacting to a failure.
Frequently asked questions
What is the L/D ratio of an extruder screw?
The working length divided by the diameter. Profile extrusion screws typically run 25:1 to 33:1. A longer screw gives better mixing and more stable output, at the cost of more torque, longer residence time and a larger machine.
What is compression ratio and why does it matter?
The ratio of flight depth in the feed zone to flight depth in the metering zone. It controls how much the material is compacted as it melts. Too high and you generate excess shear heat and can degrade the polymer; too low and the melt is not fully compacted, giving unmelt and poor surface.
What compression ratio should I use for PVC?
2.0 – 2.8:1 for rigid PVC dry blend, and 2.5 – 3.2:1 for flexible PVC. PVC is shear-sensitive and degrades if it gets too much mechanical work, so the ratio is deliberately lower than for polyolefins.
Do I need a mixing section?
Not always. A well-designed three-zone screw handles a single unfilled material well. Add a mixer if you use colour masterbatch below 2 % let-down, run filled compounds, need high output, or see surface defects that track back to unmelt or temperature streaks.
Why does my extruder output surge?
Almost always solids conveying instability in the feed zone — the pellet bed slips and grips. Check the feed throat for grooving, the feed zone temperature, and the bulk density of the material. Changing the die will not fix surge.
How often should a screw be replaced?
Depends entirely on the compound. An unfilled PVC screw can run 10 – 15 years; a 30 % glass-filled PA screw may need replacement in 2 – 3 years. The decision point is flight clearance: when it reaches about twice the original value, output and stability have degraded enough to justify replacement or re-cutting.
Can a worn screw be repaired?
Sometimes. Flight tips can be hard-faced and re-ground, and a nitrided screw can be stripped and re-nitrided once. A screw worn more than about 0.4 mm on the flights is usually beyond economic repair, and a screw with root corrosion cannot be repaired at all.
What is a barrier screw and do I need one?
A screw with a second flight that separates solid from melt along most of its length, so the solids never enter the metering zone. They give high output with good melt quality on crystalline materials like PE and PP. They are a different screw design, not an add-on, and they are rarely used for PVC.
Does screw speed affect profile quality?
Yes. Higher screw speed raises output, melt temperature and shear, which changes both dimensions and surface. On a profile line the screw speed should be set once for the target output and left alone; dimensions are controlled downstream.
What is specific energy and why should I log it?
Kilowatt-hours consumed per kilogram of output. It is the single best indicator of screw health, because it rises as flight clearance wears, as the die fouls, or as barrel temperatures drift. A 10 % rise from the commissioning baseline means something has changed.
Buying a screw, or chasing an output problem? Tell us the material and the numbers
We will tell you what L/D, compression ratio and mixing section your compound needs — and whether the problem you have is the screw or something upstream of it.
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