Reinforced working surfaces for longer service life and more stable extrusion in abrasive and corrosive formulations
Main use: Single screw • Parallel twin screw • Conical twin screw extruders
Featured formulations: Glass fiber • CaCO₃ / talc • WPC • Recycled material • Corrosive additives
Core purpose: Strengthening the working surface against wear and corrosion
Manufacturing approach: Custom-made according to machine geometry, L/D, formulation and operating conditions
Technical illustration of bimetal / alloy layer application zones on the screw flight and barrel inner surfaces
Key message: Bimetal is not a screw geometry on its own. In single, parallel or conical screw-barrel systems, it is the material and surface technology that reinforces the working surfaces subject to heavy wear and corrosion.
Glass fiber, calcium carbonate (CaCO₃), talc, mineral filler, wood flour, high recycled content and corrosive additives can cause faster dimensional loss on the screw flights and barrel inner surface compared to standard polymers. Bimetal solutions support service life and process stability by reinforcing the heavily worn surfaces with a suitable alloy system.
The right alloy for the right geometry, on the surfaces where wear concentrates.
Glass fiber, calcium carbonate (CaCO₃), talc, mineral filler, wood flour, high recycled content and corrosive additives can cause faster dimensional loss on the screw flights and barrel inner surface compared to standard polymers. Bimetal solutions support service life and process stability by reinforcing the heavily worn surfaces with a suitable alloy system.
Why Is Bimetal Treated Differently in Extrusion?
Extrusion is a continuous process. The screw-barrel assembly operates not just for a short cycle but over long periods under feeding, compression, mixing, melt conveying and pre-die pressure loads. This is why, when selecting bimetal, the question is not only "which alloy?" but also which surface and which process zone the alloy will be applied to. For single screws, the screw flight and barrel bore are the key surfaces; for parallel and conical twin-screw systems, the mating surfaces of the two screws and the twin-bore barrel geometry must also be evaluated together.
In extrusion, bimetal is not just a coating choice — it is about correctly reading where wear occurs along the process.
When Should a Bimetal Solution Be Considered?
- If abrasive components such as glass fiber, CaCO₃, talc, mineral filler or wood flour are used
- If foreign particles and variable formulation structure in recycled feedstock accelerate wear
- If PVC, CPVC, flame retardants or chemically aggressive additives cause corrosion
- If the screw diameter or barrel bore quickly falls outside tolerance
- If capacity loss, backflow, pressure fluctuation or melt quality deterioration is related to wear
- If unplanned downtime and frequent part replacement are increasing total production cost
Which Extruder Systems Can It Be Used In?
Single Screw Extruder
In PE, PP, PPR, film, pipe, profile, sheet, cable, regrind and suitable recycling applications, bimetal screws and/or barrels can be considered — especially as abrasive filler content or recycling ratio increases.
Parallel Twin Screw
In PVC / CPVC, highly filled formulations and lines where the machine design is parallel twin screw, bimetal surface solutions can be applied while preserving the mating geometry of the two screws and the twin-bore structure of the barrel.
Conical Twin Screw
In PVC pipe, profile, WPC, SPC and similar applications, the conical geometry with its changing inlet and outlet diameters is preserved. Bimetal application can also be evaluated on a project basis for standard machine sizes such as 51/105, 55/110, 65/132, 80/156, 92/188.
Reference Technical Ranges
- Single screw: Ø20–500 mm diameter range, L/D 15:1–55:1 (custom geometry depending on application)
- Parallel twin: approx. Ø45–350 mm, L/D varies by machine (center distance and mating are critical)
- Conical twin: approx. Ø25/50–185/340 mm, defined by dual diameter (standards such as 51/105, 65/132, 80/156, 92/188 are common)
These figures are general production/reference ranges; they do not represent a fixed stock or guaranteed standard capacity for MILON. Final production is determined based on the machine model, technical drawing and verified dimensions.
Why Is L/D Also Important for Bimetal Selection?
L/D is not merely a geometric ratio. A longer working section can affect the contact time between the raw material and the screw-barrel surfaces and the total wear area. Therefore, if two extruders of the same diameter differ in L/D, screw speed, capacity or formulation, their wear behavior can also differ. When selecting a bimetal solution, all process conditions should be considered, not just the screw diameter.
How Is Bimetal Applied to the Screw and Barrel?
For the barrel, the goal is to create a wear- and corrosion-resistant working layer on the inner surface in contact with the raw material while preserving the supporting body. Depending on the production design, the bimetal inner liner can be applied full-length, selectively in specific process zones, or suited to a twin-bore geometry.
For the screw, bimetal / hardfacing application is generally evaluated on the outer diameter of the flight, the crown surface and the side surfaces exposed to heavy contact. Critical surfaces can be reinforced using hardfacing methods such as PTA; the required outer diameter, profile and working tolerances are then restored through grinding / finishing. Manufacturing the entire screw body from a special alloy is not the same solution as reinforcing only the critical surfaces with hardfacing.
Alloy Approach by Wear Type
Dry filler and recycled particles in the feed zone; increased contact and mixing in the compression/transition zone; and higher pressure with prolonged surface contact in the metering zone can each create different wear characteristics. For extruders with degassing, the venting area should also be checked separately depending on the formulation. This is why the location of bimetal application should be determined based on the actual wear map of the old part and process data.
Alloy Options and Selection Logic
- Iron-based hard alloy — general wear resistance: an economical and common option; evaluated based on formulation and hardness needs
- Nickel-based alloy — PVC / CPVC and corrosive formulations: can be preferred in applications where corrosion resistance is important
- Cobalt-based alloy — high temperature / specialty engineering plastics: can be considered for high-temperature and heavy-duty operating conditions
- Tungsten-carbide-reinforced system — glass fiber, CaCO₃, talc, high mineral filler: aims to extend service life on surfaces with very high abrasion
Application Example by Formulation and Wear Type
- CaCO₃ / talc / mineral filler: flight crown and contact surfaces on the screw, bimetal inner liner on the barrel — an abrasion-focused hard alloy system can be considered
- Glass fiber: high-hardness hardfacing on the screw flights, wear-resistant inner layer on the barrel — a tungsten-carbide-reinforced or equivalent high-abrasion solution can be evaluated on a project basis
- PVC / CPVC / corrosive additives: corrosion-resistant hard surface (screw) and corrosion-resistant inner liner (barrel) — nickel-based or other suitable corrosion-resistant systems can be considered
- Recycled / variable feedstock: selective reinforcement based on the wear map (screw), full or selective inner liner (barrel) — foreign particles, filler ratio and process stability are evaluated together
Reference Technical Values
- Bimetal barrel alloy layer: 2–5 mm (general reference range)
- Bimetal / dual-alloy hardness: HRC 55–62 (general reference range)
- Nitriding layer depth: 0.5–0.8 mm (general reference range)
- Nitriding hardness: HV 950–1020 (general reference range)
- Surface roughness: Ra 0.4 µm (general reference range)
- Screw straightness: 0.015 mm (general reference range)
- Final selection: determined based on formulation, operating temperature, wear/corrosion type and a verified manufacturing process
The figures above are compiled from industry reference sources. They should not be regarded as MILON's fixed standard or guaranteed technical value; alloy type, layer thickness, hardness and dimensions should be verified on a project basis before quotation.
Why Is Wear Critical in Extrusion?
- As the working clearance between the screw outer diameter and the barrel bore increases, backflow can increase and capacity can drop
- Wear on the screw flights can affect melt conveying efficiency and pre-die pressure stability
- In parallel and conical twin-screw systems, not only the screw-barrel clearance but also the mating of the two screws relative to each other is critical
- Wear does not occur evenly across all zones; the feed, compression, melt conveying or degassing zones can wear at different rates
- High filler ratio, high screw speed, aggressive temperatures and long operating hours can accelerate wear
Bimetal or Nitrided?
- Virgin PE / PP, low abrasiveness: nitrided may be sufficient for most applications; bimetal is not always necessary
- CaCO₃ / talc / WPC / glass fiber: service life may be limited with nitrided; bimetal is a stronger candidate
- Corrosive PVC / CPVC / additives: corrosion risk is evaluated separately; nickel and similar alloy systems can provide an advantage
Information Needed to Determine the Right Solution
- Machine brand, model and extruder type: single screw / parallel twin / conical twin
- Screw diameter, or inlet-outlet diameters for conical systems; L/D ratio if available
- Total screw length, barrel length, flange and connection dimensions
- For parallel twin systems: center distance, rotation direction and gearbox output
- Raw material: PE, PP, PPR, PVC, CPVC, PA, PC, etc.
- Filler and additive ratios: CaCO₃, talc, glass fiber, wood flour, pigment, flame retardant, etc.
- Recycling ratio and contamination / foreign particle condition of the feedstock
- Current capacity, screw speed, operating temperature and motor load
- Wear zone, usage period, measurement report and photos of the existing part
Frequently Asked Questions
Is bimetal necessary for every extruder?
No. For unfilled, non-corrosive formulations with low abrasiveness, a nitrided screw-barrel can be technically and economically sufficient. Bimetal should be chosen when there is a genuine wear and process need.
Can only the barrel be made bimetal?
Yes. If wear is predominantly on the barrel inner surface, a bimetal barrel combined with a screw of suitable hardness can be considered. The screw-barrel hardness balance and working clearance must be checked together.
Is bimetal necessary for glass-fiber-filled production?
Glass fiber is abrasive, and a bimetal / hardfacing solution is a strong candidate in most applications. However, processing ready-made glass-fiber pellets is not the same process as adding glass fiber to the system during compounding; the final choice should be made based on the formulation and operating conditions.
Can the same bimetal barrel be used for single, parallel and conical screws?
No. The alloy technology may be similar, but the geometries are different. Each part must be manufactured according to the machine's original screw-barrel design.
Can production be based on an old part?
Yes. If no technical drawing is available, the existing screw and barrel can be measured for reverse engineering. When taking measurements from a worn part, the original tolerances must be reconstructed.
