Direct Answer
There is no single figure, and any supplier who gives you one without asking what you process is guessing. Screw barrel service life is set mainly by your compound, then by throughput, process discipline and construction depth. The practical answer is not a number from a brochure — it is a wear rate you measure on your own line, and this guide shows how to calculate it.
This is the most common question we get from buyers, and the hardest one to answer honestly.
We rebuild worn screws and barrels. That gives us a view most people never see: the same alloy, on the same diameter, coming back from different plants in wildly different condition. One set arrives with square, bright flight lands after years of service. Another of identical specification arrives rounded off, and the customer is surprised.
The difference is almost never the metal. It is what ran through it. This guide replaces the brochure number with something you can actually use: a method for predicting service life on your own line, and the wear thresholds that tell you when to act.
Why the Published Numbers Contradict Each Other
Search this question and you will find confident figures. Read a few suppliers side by side and the confidence starts to look strange.
| Source type | Claimed advantage over nitrided |
|---|---|
| Supplier A | Around 1.5× the life |
| Supplier B | 3 to 5× |
| Supplier C | 2 to 5× |
| Supplier D | "Significantly longer" — no figure given |
They cannot all be right. The spread itself is the useful information: the answer depends entirely on what the machine runs.
On clean unfilled polyethylene, the difference between nitrided and bimetallic is small, because neither construction is under real attack. On agricultural film full of sand, or on 30 percent glass-filled nylon, the difference is large. A single multiplier averages two situations that have nothing in common, and the average describes neither.
We will not add another invented number to that pile. What follows is what can actually be established, and how you turn it into a date you can plan around.
What Actually Sets Service Life
Four variables do most of the work. The first one outweighs the other three combined.
| Factor | Effect | Under your control? |
|---|---|---|
| Compound | Dominant. Clean resin barely wears metal; glass fibre, mineral filler and contaminated recyclate attack it constantly | Partly — through sourcing and regrind policy |
| Throughput and running hours | Wear accumulates with material passed, not with calendar time. A line running three shifts wears roughly three times faster than one shift | Set by production demand |
| Construction depth and alloy match | More material in a correctly matched alloy takes longer to wear away | Yes — at specification stage |
| Process discipline | Running above design temperature, tolerating surging, or stretching screen change intervals all accelerate wear | Yes — day to day |
Notice what is missing from that table: brand, price, and country of manufacture. Those matter for whether you receive the alloy you ordered. They do not change the physics once the part is installed.
The One Figure That Is Checkable
Life multipliers are unverifiable. Layer depth is not. It appears on the drawing, it can be measured on a sectioned sample, and it is the physical basis for any life difference between constructions.
| Construction | Screw | Barrel |
|---|---|---|
| Nitrided | Case 0.4–0.7 mm | Case 0.4–0.7 mm |
| Bimetallic | PTA hardfacing 1.0–1.5 mm | Cast lining 2.0–3.0 mm |
Two things follow from this table, and both are worth understanding before you compare quotes.
A nitride case does not fail gracefully. It is hard and it holds up — until abrasive filler cuts through it. Underneath sits softer core steel that erodes much faster. Parts that seem fine for a long period can then deteriorate quickly.
A bimetallic layer wears down rather than through. There are millimetres of material to give, and the wear rate stays comparatively steady. That predictability is worth as much to a maintenance planner as the extra depth itself.
The alloy grade matters alongside the depth. A thick layer of the wrong alloy still fails early — a wear grade in a corrosive process gets pitted, and a corrosion grade under mineral filler gets ground away. Our materials guide sets out the full grade tables.
How to Measure Your Own Wear Rate
This is the part no brochure gives you, and it is the only way to turn "how long does it last" into a date you can plan around.
The method is simple arithmetic. It needs two measurement sessions and a running-hours figure.
Four steps to your own wear rate
- Take a baseline at installation. Before commissioning a new set, measure flight outer diameter with a micrometer and bore inner diameter with a bore gauge at several fixed positions along the length. Mark those positions so they can be found again. Record the machine hour meter.
- Log the reference run. At stable production, record output at a reference screw speed, drive current, melt temperature and head pressure. These four figures tell you later how much performance has drifted.
- Repeat at a later shutdown. Measure the same positions, the same way, with the same instruments. Record hours again.
- Calculate. Radial clearance is half the difference between bore inner diameter and flight outer diameter at each position. Subtract the baseline clearance from the current clearance, then divide by the running hours between the two sessions.
The result is a wear rate in millimetres per thousand running hours, for your compound on your machine. Extrapolate it to your replacement threshold and you have a planning date instead of a guess.
| Position along screw | Baseline clearance | Current clearance | Change | Hours elapsed | Rate (mm / 1000 h) |
|---|---|---|---|---|---|
| Feed zone | |||||
| Compression transition | |||||
| Mid metering | |||||
| Discharge end |
Measure at several positions, not one. Wear is not uniform. On the parts that come back to us for reconditioning, it concentrates in the metering section and around the compression transition, where contact pressure runs highest. A single reading taken at a convenient spot can understate the real condition by a wide margin.
Without a baseline, you are comparing against an assumption. The single most useful thing a maintenance team can do costs fifteen minutes: measure the new set before it goes in. Plants that skip this end up comparing worn measurements against a drawing value that may never have described their actual part, and every later decision inherits that uncertainty.
Replacement Thresholds and Planning
A wear rate only helps if you know what you are counting down to.
As-new radial clearance comes from the drawing where one exists. Where it does not, a widely used industry rule of thumb puts it near one thousandth of the screw diameter — roughly 0.09 mm on a 90 mm screw. Treat that as an approximation for orientation, not as a specification.
| Clearance vs as-new | What it usually means | Action |
|---|---|---|
| Near as-new | Healthy | Continue; re-measure at the next planned shutdown |
| Around 2× | Output loss and melt temperature drift become measurable | Put the set on the replacement plan and order ahead of need |
| Around 3× | Output, quality and energy cost all affected | Most processors replace or recondition at this point |
In practice, performance often makes the decision before the clearance figure does. Falling output at unchanged screw speed, rising melt temperature, black specks that return after purging — any of these can justify replacement while the measurement still looks tolerable.
One planning point matters commercially. Custom screw barrel production runs 20 to 70 days after drawing confirmation, plus transport. A set that hits your 2× threshold is telling you to start the procurement conversation now, not when the line stops.
What Cuts Life Short — Faults, Not Wear
When a set fails far earlier than a comparable one, the cause is usually a fault rather than the material. We see the evidence on returned parts: damage no resin could have caused.
| Cause | What it looks like |
|---|---|
| Misalignment between screw and drive | Wear concentrated on one side along the length |
| Bent shaft | Localised heavy contact; often follows a hard start or foreign object |
| Running above design temperature | Accelerated corrosion, degraded deposits, hardness loss in extreme cases |
| Undersized or bypassed screen changer | Abrasive particles reaching the screw that filtration should have removed |
| Geometry that never suited the compound | Chronic surging and unmelt from the first day, not a wear pattern |
| Cleaning with steel tools or an open flame | Gouges and blue heat marks; scratches later anchor degrading melt |
Diagnose before you upgrade. A bimetallic set on a misaligned machine still wears out early — more slowly, while the underlying fault continues. If a set failed well before a comparable one, look for the fault first. Buying better metal to cover a mechanical problem is the most expensive way to postpone it.
How to Get More Life From a Set
Four actions genuinely extend service life. None of them are exotic.
- Specify the right alloy at the start. The cheapest moment to fix metallurgy is before manufacture. A nitrided part on glass-filled nylon is the cheaper purchase and usually the more expensive decision. Tell your manufacturer the filler and regrind percentage, not just the polymer name.
- Protect the hardware with filtration. The screen changer is your first line of defence, not the screw. If it is undersized, blinding early, or being bypassed to hold output, abrasive particles reach metal that should never have seen them.
- Hold process discipline. Running above design temperature and tolerating chronic surging both accelerate wear. Surging in particular is a load cycle: it varies the side force pressing the screw against the bore.
- Replace as a matched pair. Clearance belongs to the pair, not to either part. A new screw in a worn bore runs at enlarged clearance from day one, and you lose most of what you paid for.
Reconditioning Resets the Wear Surface
Where the base steel and geometry remain sound, a worn set does not always need replacing. A screw can be re-welded with fresh PTA hardfacing and machined back to original geometry; a bore can be re-bored and re-lined by centrifugal casting. The original shaft, keyway and journal are retained.
This makes most sense when the OEM part is discontinued or new-build lead time is unacceptable. It makes least sense when the shaft is bent, flight roots are cracked, or so much material has gone that re-machining would take the part below its strength envelope.
Send photographs and clearance measurements for an assessment. Numbers get you an answer; adjectives get you a question in reply.
Frequently Asked Questions
How long does an extruder screw barrel last?
No honest manufacturer can give a single figure, because service life depends almost entirely on what you process. The same screw geometry in the same alloy can hold tolerance for years on clean unfilled polyethylene and need replacing far sooner on glass-filled nylon or contaminated recyclate. Throughput, process discipline and filtration change the answer again. The useful approach is to measure the wear rate on your own line rather than to accept a published number.
Why do suppliers publish different service life figures?
Because they are averaging situations that have nothing in common. Published claims for bimetallic versus nitrided life range from around one and a half times to five times across different suppliers, and they cannot all be right. On clean resin the difference between the two constructions is small, because neither is under attack. On abrasive filled compounds the difference is large. A single multiplier hides that spread, which is why a supplier who asks what you process before answering is more useful than one who quotes a number immediately.
How do I calculate the wear rate on my own extruder?
Record a baseline at installation: measure flight outer diameter with a micrometer and bore inner diameter with a bore gauge at several fixed positions, and log the running hours. Repeat the same measurements at the same positions during a later shutdown. Subtract to get the change in radial clearance, divide by the running hours between measurements, and you have a wear rate in millimetres per thousand hours. Extrapolating that rate to your replacement threshold gives a planning date rather than a guess.
At what clearance should a screw and barrel be replaced?
A widely used industry rule of thumb puts as-new radial clearance near one thousandth of the screw diameter, though the drawing value always takes precedence where one exists. Many processors put a set on the replacement plan when clearance reaches roughly twice the as-new value, and replace at around three times. Those are planning thresholds rather than hard limits, because output loss and melt quality often make the decision before the clearance figure does.
Does a bimetallic screw barrel last longer than a nitrided one?
On abrasive or corrosive compounds, yes, and the reason is checkable rather than promotional. A nitride case runs 0.4 to 0.7 mm deep, while bimetallic construction gives 1.0 to 1.5 mm of hardfacing on the screw and 2.0 to 3.0 mm of cast lining in the barrel, in a material chosen for that specific attack. More material in a better matched alloy takes longer to wear away. On clean unfilled resin the advantage is much smaller, because neither construction is being attacked.
What makes a screw barrel wear out earlier than expected?
Premature failure usually comes from a fault rather than from normal wear. Common causes are misalignment between the screw and the drive, a bent shaft, running well above design temperature, a screen changer that is undersized or being bypassed, a screw geometry that never suited the compound, and damage from cleaning with steel tools or an open flame. Each of these destroys hardware regardless of alloy, so diagnose the cause before assuming the material was wrong.
Does reconditioning restore the original service life?
Reconditioning restores the wear surfaces rather than making the part new. A worn screw can be re-welded with fresh PTA hardfacing and machined back to original geometry, and a worn bore can be re-bored and re-lined by centrifugal casting. Where the base steel is sound and the geometry suited the job, a reconditioned set can return to service on a similar wear cycle. Where the shaft is bent, flight roots are cracked, or too much material has been lost, new manufacture is the correct route.
