Choosing the correct screw barrel material has a major influence on extrusion stability, wear resistance and equipment operating cost.
For many plastic processors, the choice is usually between a standard nitrided screw and barrel and a bimetallic screw barrel.
Both solutions can perform well, but they are designed for different processing conditions. A more expensive material is not automatically the better choice. The correct specification depends on the polymer, fillers, production capacity and expected service life.
This guide explains the main differences and helps buyers select a suitable screw and barrel for their extrusion equipment.
What Is a Nitrided Screw and Barrel?
A common material for extruder screws and barrels is 38CrMoAlA nitriding steel.
After machining, the components undergo gas nitriding treatment. This produces a hard surface layer while maintaining the toughness of the base material.
Typical specifications can include:
- Nitriding depth: approximately 0.5–0.8 mm
- Surface hardness: approximately HV900–1100
- Good surface wear resistance
- Good dimensional stability after heat treatment
Nitrided screw barrels are widely used for processing materials such as PP, PE, HDPE, LDPE and many general-purpose plastics.
For standard extrusion applications without severe abrasive fillers, they usually provide a good balance between performance and cost.
What Is a Bimetallic Screw Barrel?
A bimetallic screw barrel uses a wear-resistant alloy in the critical working area.
For a barrel, the inner surface can be lined or centrifugally cast with an alloy layer. For a screw, wear-resistant alloy can be applied to the flight surfaces or other high-wear areas depending on the design.
The purpose is simple: protect the areas exposed to the greatest friction and abrasion.
Depending on the alloy system and application, the working surface can reach approximately HRC 56–62 or higher.
This makes bimetallic construction particularly useful when processing abrasive compounds.
Bimetallic Screw Barrel vs Nitrided Screw Barrel
The biggest difference is not simply hardness. It is the ability of the working surface to resist long-term abrasion and corrosion under actual production conditions.
1. Wear Resistance
For standard PP or PE extrusion, a properly manufactured nitrided screw barrel can provide satisfactory service life.
However, wear increases significantly when the raw material contains abrasive fillers.
Typical examples include:
- Calcium carbonate
- Glass fiber
- Mineral fillers
- Recycled plastics containing impurities
- Highly filled masterbatch compounds
Under these conditions, a bimetallic solution normally offers better wear resistance.
2. Processing Glass-Fiber-Reinforced Plastics
Glass fiber can be extremely abrasive to screw flights and barrel surfaces.
As clearance between the screw and barrel increases, melt can flow backward through the enlarged gap. This can gradually reduce pumping efficiency and production capacity.
For applications containing approximately 20–30% glass fiber or more, we normally recommend evaluating a bimetallic or higher wear-resistant material solution instead of automatically selecting standard nitriding.
The final recommendation should still depend on the polymer, filler percentage and operating conditions.
3. Processing High CaCO3 Filler
Calcium-carbonate-filled compounds are common in pipe, profile, sheet and masterbatch production.
High filler percentages can accelerate wear substantially.
For low or moderate filler levels, nitrided components may still be economical. For continuously running extrusion lines processing highly filled compounds, a bimetallic barrel and wear-resistant screw can often reduce replacement frequency.
Does Bimetallic Always Mean Better?
No.
This is an important point when purchasing an extruder screw and barrel.
If a machine mainly processes clean virgin PE or PP with little abrasive filler, installing the most expensive alloy solution may not provide a proportional economic benefit.
A properly designed and heat-treated nitrided screw barrel may already provide the required service life.
The material should therefore be selected according to the application rather than simply choosing the highest hardness.
Screw Design Is Just as Important as Material
Material selection alone cannot guarantee good extrusion performance.
Screw geometry must match the polymer and production requirements.
Important parameters include:
- Screw diameter
- L/D ratio
- Compression ratio
- Feed section depth
- Compression section design
- Metering section geometry
- Mixing section
- Venting design
- Screw speed
- Required output
For example, a screw designed for PE film extrusion should not automatically be used for PVC, PA6 or highly filled recycling applications.
There is no truly universal screw design for every plastic.
When Should You Choose a Nitrided Screw Barrel?
A nitrided screw and barrel is usually worth considering when:
- Processing standard PP or PE
- Filler content is low
- Raw material is relatively clean
- Abrasion is moderate
- Production conditions are stable
- Initial investment cost is important
For many conventional plastic extrusion lines, this remains a practical and economical solution.
When Should You Choose a Bimetallic Screw Barrel?
Consider a bimetallic solution when processing:
- Glass-fiber-reinforced plastics
- High CaCO3 compounds
- Abrasive masterbatch
- Recycled materials with higher contamination
- Engineering plastics requiring improved wear resistance
- Continuous high-output production
The higher initial cost can be justified if longer service life reduces shutdowns and replacement frequency.
Information to Provide Before Ordering a Screw Barrel
A manufacturer should not recommend screw and barrel material based only on diameter.
Before requesting a quotation, provide as much of the following information as possible:
- Machine brand and model
- Screw diameter
- Complete screw and barrel drawing
- Polymer being processed
- Filler type and percentage
- Required output in kg/h
- Screw speed
- Existing screw barrel material
- Current service life
- Photos of worn components
If an original drawing is unavailable, detailed measurements or a physical sample can sometimes be used to manufacture replacement parts.
For complicated components, however, a complete technical drawing or physical sample is preferable.
FAQ
Is a bimetallic barrel better than a nitrided barrel?
For highly abrasive materials, a bimetallic barrel generally provides better wear resistance. For standard extrusion applications, a nitrided barrel may offer a more economical solution.
What material is commonly used for nitrided screw barrels?
38CrMoAlA is commonly used for screw and barrel manufacturing because it can achieve a hard nitrided surface while maintaining good core strength.
Is a bimetallic screw barrel suitable for recycled plastic?
Yes, especially when recycled material contains abrasive contamination or fillers. The exact alloy should be selected according to the material composition and processing conditions.
Can I replace only the screw instead of the complete screw and barrel?
Sometimes, but the barrel bore should be measured first. Installing a new screw inside a severely worn barrel may leave excessive clearance and prevent the new screw from achieving the expected performance.
How do I choose the correct screw barrel material?
Provide your polymer type, filler percentage, screw diameter, production capacity and operating conditions to the manufacturer. Material and screw geometry should then be selected together.
Need Help Selecting the Right Screw Barrel Material?
Choosing between a nitrided and bimetallic screw barrel should be based on your actual production conditions, not simply on price or hardness.
We manufacture replacement screws and barrels for plastic extrusion and injection molding machinery, including single screw, parallel twin screw and conical twin screw applications.
Send us your machine model, plastic material, filler percentage, screw diameter and technical drawing or measurement photos.
Our engineering team can evaluate your application and recommend a suitable screw design, barrel construction and wear-resistant material.
Contact us for a screw and barrel quotation for your extrusion or injection molding machine.