When purchasing a replacement screw and barrel, one common question is:

Should I choose a nitrided screw barrel or a bimetallic screw barrel?

There is no single answer for every application. The correct choice depends on the polymer, filler content, abrasion, corrosion, production conditions and expected service life.

For standard plastics, a nitrided screw and barrel may provide good performance at a reasonable cost. For recycled, highly filled or abrasive materials, a bimetallic screw barrel can often provide better wear resistance.

Here are the main differences buyers should understand before ordering.

What Is a Nitrided Screw Barrel?

A common material for extrusion and injection molding screws and barrels is 38CrMoAlA nitriding steel.

After machining, the components undergo nitriding treatment. Nitrogen diffuses into the steel surface and creates a hard wear-resistant layer.

Typical advantages include:

• Good surface hardness
• Good wear resistance
• Good dimensional stability
• Reasonable manufacturing cost
• Suitable for many general plastic applications

Nitrided screw barrels are widely used for PE, PP, PS, ABS and other conventional plastics.

However, the nitrided layer has a limited effective depth. When processing strongly abrasive materials continuously, this surface layer can gradually wear away.For more technical information about nitriding and other surface treatments, see this surface engineering guide from TWI.

What Is a Bimetallic Screw Barrel?

A bimetallic screw barrel uses a wear-resistant alloy in the critical working area.

For a barrel, an alloy layer is typically applied to the internal bore. For a screw, wear-resistant alloy can be applied to the flight surfaces or other high-wear areas depending on the design and manufacturing process.

The purpose is to improve resistance to abrasion and, depending on the alloy, corrosion.

Bimetallic construction is commonly considered for applications such as:

• Recycled plastics
• High CaCO3 compounds
• Glass-fiber-reinforced plastics
• Engineering plastics
• Masterbatch
• Long continuous production

The exact alloy specification should be selected according to the processed material rather than simply requesting “bimetallic.”

Which One Has Better Wear Resistance?

In demanding abrasive applications, a correctly selected bimetallic solution will generally provide greater wear resistance than a conventional nitrided surface.

But this does not mean every customer should automatically buy the more expensive option.

Consider an extruder processing relatively clean virgin PP with little abrasive filler.

A properly manufactured nitrided screw and barrel may already provide satisfactory service life. Paying for a higher-grade alloy may not create enough additional value.

Now consider another extruder processing PP with a high percentage of CaCO3.

The operating environment is much more abrasive. In this case, improving the wear resistance of the screw and barrel may substantially reduce replacement frequency.

The raw material is therefore one of the first things we ask about when preparing a quotation.

What About Glass Fiber?

Glass fiber can cause severe abrasive wear.

If an extruder screw and barrel or injection molding screw continuously processes glass-fiber-reinforced material, wear can become concentrated on the screw flights and barrel bore.

The glass fiber percentage matters.

Processing a small percentage occasionally is different from continuously running highly glass-filled engineering plastics.

When requesting a quotation, tell the manufacturer:

• Polymer type
• Glass fiber percentage
• Production hours
• Existing screw material
• Current service life

This information is much more useful than simply asking for the “hardest screw.”

Is Bimetallic Always More Expensive?

Normally, yes.

The manufacturing process and alloy materials increase the initial cost.

However, purchase price should not be the only comparison.

For a production line running continuously, premature screw barrel wear can cause:

• Reduced output
• Unstable melt pressure
• Poor plasticizing
• Higher scrap rate
• Production downtime
• Frequent replacement costs

If a more wear-resistant component significantly extends service life, its total operating cost may actually be lower.

Can I Use a Bimetallic Barrel With a Nitrided Screw?

In some applications, different surface treatments or materials can be combined.

However, the screw and barrel should be considered as a working pair.

The manufacturer should evaluate the wear conditions, component hardness and mating surfaces rather than selecting each part independently.

Replacing only one component also requires checking the condition of the other.

For example, installing a new wear-resistant screw inside a severely worn barrel will not restore the original clearance.

How Do I Choose the Correct Screw Barrel Material?

Before choosing between nitrided and bimetallic construction, provide the manufacturer with the actual operating information.

Machine Information

Provide:

• Extruder or injection molding machine model
• Screw diameter
• L/D ratio
• Screw speed
• Motor power

Material Information

Tell us:

• Polymer type
• Virgin or recycled
• Filler type
• Filler percentage
• Glass fiber percentage, if applicable

Production Information

Also provide:

• Required output
• Daily operating hours
• Current screw barrel lifetime
• Main wear problem

These details allow a more appropriate material recommendation.

FAQ

Is a bimetallic screw barrel better than a nitrided screw barrel?

For highly abrasive materials, bimetallic construction can offer better wear resistance. For standard polymers with low abrasion, nitrided components may already provide a good balance of performance and cost.

Which screw barrel is suitable for recycled plastic?

It depends on contamination and filler content. Recycled plastics containing mineral fillers, sand or other abrasive contamination may benefit from improved wear resistance.

Is 38CrMoAlA suitable for extrusion screws?

Yes. 38CrMoAlA with nitriding treatment is widely used for many standard plastic extrusion applications.

Do I need bimetallic material for CaCO3?

The higher the CaCO3 content and production intensity, the more important wear resistance becomes. The actual percentage and production conditions should be evaluated before selecting the material.

Need a Replacement Screw and Barrel?

If you are unsure whether to choose nitrided or bimetallic material, send us your machine model, screw drawing, polymer, filler percentage, current output and photos of the existing components.

Zhoushan Juyuan Plastic Machinery manufactures customized extruder screw and barrel, injection molding screw barrel and twin screw components according to actual production conditions.

We prefer to understand the material and wear problem first, then recommend an appropriate screw barrel material instead of simply offering the most expensive option.

Send us your drawing and production information for a technical evaluation and quotation.