Nitrided vs bimetallic screw and barrel comes down to one question: what are you running? Nitrided sets are the cost-effective choice for standard unfilled PVC and PE. Bimetallic sets are for abrasive, filled or flame-retardant compounds, where nitrided steel wears out too quickly. This guide explains how each is made, why the hardness numbers can mislead you, and how to decide. We manufacture both, so the advice below is the same we give our own customers.
Quick Comparison
| Nitrided | Bimetallic | |
|---|---|---|
| How it is made | Steel surface hardened by nitriding | Hard alloy lining bonded inside the barrel; hard-facing on screw flights |
| Surface hardness | About 900–1000 HV | About 60–65 HRC |
| Hard layer depth | Thin case, a few tenths of a millimetre | Thicker liner, commonly quoted at 1.5–3 mm |
| Best for | Unfilled PVC and PE | Filled XLPE, HFFR/LSZH, glass and mineral-filled compounds |
| Upfront cost | Lower | Higher |
| Main risk | Rapid wear once the case is gone | Paying for life you do not need on easy compounds |
What Is a Nitrided Screw and Barrel?
Nitriding is a heat treatment that diffuses nitrogen into the surface of alloy steel, typically a grade such as 31CrMoV9, forming a very hard case. The part is not coated, so there is no layer to peel off. The hard zone is thin, though, and hardness falls away with depth. Suppliers commonly quote case depths of around 0.5–0.8 mm.
For unfilled compounds that do little abrasive work, this is usually all you need. It is why nitrided sets remain the standard on most building-wire lines.
What Is a Bimetallic Screw and Barrel?
A bimetallic barrel has a hard alloy lining metallurgically bonded to a steel body. The steel carries the pressure and the lining handles the wear. Bimetallic screws are built differently: the flights carry a hard-facing, because that is where the screw rubs against the barrel. Liner alloys vary, and the choice matters, since some resist abrasion best and others are chosen for corrosion resistance.
Read more on our bimetallic screw and barrel page.
Why Hardness Numbers Alone Can Mislead You
Compare the figures directly and nitrided looks harder. 900–1000 HV is above what 60–65 HRC converts to, which is roughly 700–830 HV. Yet bimetallic lasts longer on abrasive compounds. Two things explain this:
- Depth. Once a thin nitrided case wears through, softer steel is exposed and wear speeds up. A bimetallic liner has far more material to lose before it reaches that point.
- Structure. Many liner alloys contain hard carbide particles in a tougher matrix. Those particles resist the scratching action of fillers in a way a single surface-hardness number does not capture.
So compare wear depth and compound suitability, not just the hardness figure on the quotation.
Which Compounds Need Which?
| Compound | Our usual starting point | Why |
|---|---|---|
| Unfilled PVC insulation | Nitrided | Low abrasion; nitrided is cost-effective |
| LDPE, HDPE | Nitrided | Low abrasion |
| Heavily filled PVC | Review with supplier | Filler level decides it; high calcium loading moves you toward bimetallic |
| Filled XLPE | Bimetallic | Abrasive fillers wear a thin case quickly |
| HFFR / LSZH | Bimetallic | Very high mineral filler content |
| Glass or mineral-filled polymers | Bimetallic | Highly abrasive |
| Some masterbatches | Review with supplier | Abrasive pigments can justify an upgrade |
Treat this table as a starting point. Filler type and percentage, line speed and running hours can all change the answer. If you run PVC, also consider corrosion: overheated PVC releases acid that attacks steel surfaces, so temperature control and good purging practice protect either type of set.
How Much Longer Does Bimetallic Last?
Published figures vary widely. Manufacturers claim anywhere from about two times to several times the life of a nitrided barrel. Those numbers come mostly from suppliers, including us, and they depend heavily on the compound. On unfilled PVC the difference may never pay back. On HFFR it can be decisive.
The more useful approach is to work it out from your own records:
- How many months does your current set last on this compound?
- What does a replacement set cost, including fitting?
- What does a day of downtime cost?
- How much output and quality do you lose in the months before you replace it?
If a nitrided set lasts years on your compound, stay with it. If you are replacing every year or two, or your output drifts through a campaign, the upgrade usually justifies itself. Our guide on screw and barrel wear covers the warning signs.
Do the Screw and Barrel Need to Match?
Not always, but they need to be planned together. A common approach is to make the screw the sacrificial part, because it is usually cheaper and easier to rebuild than a barrel. A fresh hard-faced screw running in a badly worn barrel will not restore output, since clearance is set by both parts. Measure both before ordering a replacement.
Other Specifications to Get Right
Material is only one part of the order. A long-lasting set with the wrong geometry will still underperform. Include these in your enquiry:
- Screw diameter and L/D ratio
- Compression ratio and flight geometry for your compound
- Extruder make and model, or a drawing, for replacements
- Compound datasheet, including filler type and percentage
- Target output in kg/h
We make nitrided and bimetallic screws and barrels from 25 mm to 150 mm diameter, including replacements for extruders from other makers, and we export to customers in the UAE, UK, South Africa, Bangladesh, Nepal and Sri Lanka. Send us your compound and current screw details and we will recommend a suitable option.
Frequently Asked Questions
What is the difference between nitrided and bimetallic screw barrels?
A nitrided set has a thin hardened case on the steel itself. A bimetallic barrel has a separate hard alloy lining bonded inside it, and a bimetallic screw has hard-facing on its flights. Bimetallic has more wear material and suits abrasive compounds.
Is bimetallic always better than nitrided?
No. For unfilled PVC and PE, nitrided is usually sufficient and costs less. Bimetallic is worth paying for when you run filled, flame-retardant or glass-reinforced compounds that wear a nitrided case quickly.
How long does a nitrided screw and barrel last?
It depends on the compound, output and running hours, so there is no single figure. Unfilled compounds are far gentler than filled ones. Your own replacement records are the best guide, along with measured clearances and output trends.
Can I fit a bimetallic barrel to my existing extruder?
Usually yes. Replacement bimetallic screws and barrels can be made to suit extruders from other makers using a drawing or the existing part dimensions.
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