Screw Barrel Material Selection: Nitrided vs Bimetallic vs Full-Hard

Material Selection

Why the Alloy Decides How Long Your Barrel Lasts

A screw barrel fails in only three ways: abrasion (hard fillers cut the bore), corrosion (decomposition gases such as HCl from PVC attack the surface) and adhesive wear (metal-to-metal contact when the screw deflects). Which one kills your barrel depends on the resin you run — and each one is stopped by a different material.

That is why the cheapest barrel is not the cheapest choice. A nitrided set running 40 % glass-filled PA may lose 0.5 mm of bore in six months, while a bimetallic set in the same machine runs for years. The price difference is recovered the first time you avoid an unplanned line stoppage.

Nitrided — best price, for unfilled commodity resins
Bimetallic — for glass fibre, mineral fillers, flame retardant and regrind
Full-hard / stainless — for corrosive resins, food contact and medical grades
All options available as OEM replacement to your existing machine dimensions

Comparison

Nitrided vs Bimetallic vs Full-Hard: Side by Side

Item	Nitrided 38CrMoAlA	Bimetallic alloy	Full-hard / stainless
Base material	38CrMoAlA, 42CrMo	Carbon steel + cast alloy liner	H13, SKD61, 9Cr18MoV, stainless
Surface hardness	HV 900–1100	HV 1000–1300 with tungsten carbide	HRC 58–62
Hardened layer	0.30–0.60 mm diffusion layer	2–5 mm metallurgically bonded liner	Through hardened
Corrosion resistance	Moderate	Good (Ni/Cr based alloys)	Best
Abrasion resistance	Baseline 1×	3–8×	2–4×
Typical resins	PE, PP, ABS, PS, HIPS	GF-PA, GF-PP, FR compounds, PVC, regrind	PVC, PVDF, food and medical grades
Relative cost	Lowest	Medium	Higher
When to choose it	Standard production, tight budget, low filler	High output, abrasive or corrosive feed, long campaigns	Strong corrosion, product purity requirements

Resin Matrix

Recommended Barrel by Resin You Process

This is the matrix our engineers work from. If your resin is not listed, send us the grade — we will specify against the actual filler content and output.

Material you process	Main wear mechanism	Recommended barrel	Screw type
LDPE / HDPE / PP film and sheet	Mild abrasion	Nitrided	Single screw, barrier flight
PVC pipe, profile, window frame	Corrosion (HCl)	Bimetallic or full-hard	Conical twin screw
PET sheet, strapping, bottle flake	Abrasion + hydrolysis	Bimetallic	Single screw with barrier, vented
ABS, PS, HIPS	Mild abrasion	Nitrided	General purpose single screw
PC, PMMA, optical grade	Corrosion, purity	Full-hard stainless	Low-shear single screw
PA / PP with 10–50 % glass fibre	Severe abrasion	Bimetallic, WC alloy	Bimetallic single or parallel twin
Flame retardant compounds	Corrosion + abrasion	Bimetallic high-nickel	Corrosion resistant design
Recycled and regrind material	Abrasion + contamination	Bimetallic	High compression, reinforced flight
Masterbatch and compounding	Severe abrasion	Bimetallic	Parallel twin screw, segment elements
Wood-plastic composite	Severe abrasion + moisture	Bimetallic	Conical or parallel twin
XPS / EPS foam board	Corrosion from blowing agent	Bimetallic	Tandem or twin screw foaming
Rubber and elastomer	Adhesive wear, heat	Nitrided, hard chrome option	Rubber screw with high shear control

Timing

Three Signals That You Have Outgrown Nitrided

Output dropping

You raise the screw speed and the melt temperature creeps up, but throughput does not follow. The clearance between screw flight and bore has opened and melt is leaking backwards.

Melt quality drifting

Black specks, unmelted particles, colour streaks or gel in the product. Degraded material is hanging in the worn bore of the barrel.

Replacing every 6–12 months

If a set does not survive a year on an abrasive or corrosive resin, the next nitrided set will fail the same way. That is the point to move to bimetallic.

Common Mistakes

Four Specification Mistakes That Cost Money

1. Specifying hardness only

HV 1000 means nothing without the depth. A thin hard layer over a soft core collapses under load. Always specify hardness and case depth and core strength together.

2. Ignoring the screw

A bimetallic barrel with a nitrided screw just moves the wear to the cheaper part and contaminates your product with iron. Screw and barrel must be specified as a matched pair.

3. Copying the original drawing blindly

If the original set failed early, its geometry is part of the problem. A rebuild is the moment to correct compression ratio and clearance, not just to duplicate a worn design.

4. Quoting on price alone

Two quotations for the same drawing can differ by 30 % because of alloy system, layer thickness and whether nitriding is done in-house. Compare the specification, not the total.

FAQ

Material Selection Questions

Can you supply a barrel to my existing drawing?

Yes. Send the drawing or the worn sample and we build to your dimensions. If the original failed early we will also tell you what we would change and why.

How much longer does a bimetallic barrel actually last?

On glass-filled and mineral-filled compounds we typically see three to eight times the service life of nitrided, depending on filler content and output. On unfilled commodity resin the difference is smaller and nitrided is often the better value.

Do you do the nitriding and alloy casting in your own plant?

Yes. We run 4 nitriding furnaces and 4 bimetallic centrifugal casting furnaces in our own facility, which is why we can hold lead time and issue material certificates against each batch.

What is the largest diameter you can build?

Ø300 mm is our machining limit. Above that we will tell you honestly that we cannot take the job rather than quote something we cannot deliver.

Start Your Project

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Tell us your machine model, drawing or the worn part you need to replace. We will come back with a matching proposal and a quotation.

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