How do I verify the screw and barrel steel quality (38CrMoAlA vs Bimetallic) when purchasing a plastic extruder?
Jul 29, 2026
Core Material Difference: 38CrMoAlA Nitrided vs Bimetallic Screw & Barrel
Before conducting quality verification, clearly distinguish structural, performance and application gaps between the two mainstream configurations, as the verification standards vary greatly:
1.1 38CrMoAlA Nitrided Screw & Barrel
- Base material: Standard alloy structural steel 38CrMoAlA, strictly compliant with national GB and international ASTM alloy standards
- Hardening process: Whole furnace gas nitriding treatment, hardening layer thickness 0.4–0.7mm, surface hardness HV 900–1100
- Advantages: Balanced toughness and wear resistance, low cost, stable processing for pure PE, PP, PS unmodified virgin plastic
- Limitations: Thin hardened layer; rapid wear under high-filler, glass fiber, corrosive PCR recycled materials; service life 2–5 years
- Typical JBD application: PE/PEX water/gas pipe lines, PP hollow sheet extruders, unmodified sheet production equipment
1.2 Bimetallic Composite Screw & Barrel
- Base material: High-toughness alloy steel substrate (4140/40CrNiMoA), wear-resistant alloy layer metallurgically bonded on melt contact surfaces
- Hardening process: Barrel inner wall centrifugal casting alloy lining (1.5–3mm thickness); screw flight PTA plasma surfacing alloy layer (1.0–2.5mm thickness), hardness HRC 50–58
- Advantages: Ultra-thick wear-resistant layer, excellent anti-abrasion and anti-corrosion performance, ideal for glass-filled, calcium-filled, PVC, PCR waste plastic
- Limitations: Higher manufacturing cost
- Typical JBD application: Twin-screw PCR recycling lines, medical material compound extruders, WPC wood-plastic profile equipment
Core Contrast Table
2 Step 1: Pre-Order Document & Certification Verification (First Line of Defense)
Before signing sales contracts, buyers must request complete material traceability documents from suppliers like JBD to eliminate false labeling risks. Unverified small factories cannot provide full formal paperwork.
2.1 Mandatory Material Mill Test Certificate (MTC)
- For 38CrMoAlA screw/barrel: Demand original steel mill delivery certificate, including chemical composition (Cr, Mo, Al, C element content), mechanical tensile strength, yield strength data, heat treatment batch number
- For bimetallic parts: Dual certificates—substrate steel report + wear alloy layer composition report (nickel/cobalt/tungsten carbide alloy content)
- Red flag warning: Suppliers who refuse to provide original mill certificates, only supply blurred photos or self-made simple sheets are likely using substitute low-grade steel
2.2 Heat Treatment & Hardness Inspection Reports
Qualified manufacturers such as JBD conduct batch hardness testing for each screw/barrel furnace batch:
- Nitrided parts: Furnace sample hardness test report, nitriding depth detection data, meeting GB 11354 standard
- Bimetallic parts: Alloy surfacing metallographic inspection report, lining thickness ultrasonic test record
- All reports must bear factory QC official seals and unique batch numbers matching the ordered equipment
2.3 Written Contract Material Commitment Clause
Add binding clauses to sales contracts:
- Explicitly mark base steel grade (38CrMoAlA / bimetallic substrate alloy)
- Specify hardening layer thickness, surface hardness standard
- Agree compensation standards for material counterfeiting (full machine replacement + liquidated damages)Dunya (Weifang) Trading, JBD’s exclusive global trading partner, adds standardized material guarantee clauses to all export contracts for overseas buyers.
3 Step 2: On-Site Factory Material & Process Audit (Most Reliable Verification)
Arrange a factory inspection before production or mid-manufacturing to visually inspect raw steel stock and processing workshops, a standard service supported by JBD for global clients.
3.1 Raw Material Warehouse Inspection
- Check steel bar inventory labels: Genuine 38CrMoAlA steel bars carry factory steel grade marking on the surface; counterfeit low-carbon steel has no brand alloy labeling
- Verify bimetallic alloy casting raw materials: Confirm complete alloy powder storage records for centrifugal barrel lining and plasma surfacing
- Cross-check batch numbers between raw steel and the mill certificates provided earlier
3.2 Heat Treatment Workshop Inspection
- Nitriding furnace production records: Check furnace temperature, holding time, batch serial numbers matching ordered screws
- Bimetallic surfacing equipment: Inspect PTA plasma welding machines and centrifugal casting production lines; small counterfeit factories lack this professional equipment and only perform cheap single nitriding
3.3 QC Laboratory On-Site Testing Request
Ask the factory’s quality department to conduct instant non-destructive testing on semi-finished screw samples on site:
- XRF X-ray fluorescence rapid element screening: Distinguish whether base metal is standard 38CrMoAlA alloy or cheap carbon steel (non-destructive, no damage to parts)
- Portable hardness tester detection: Nitrided surface HV reading must reach 900+; bimetallic alloy layer HRC 50+
- Ultrasonic thickness gauge test: Measure wear-resistant layer depth directly on barrel inner wall and screw flights to verify bimetallic thickness commitment
4 Step 3: Pre-Delivery Sample & Full Machine Inspection Before Shipment
All JBD production lines complete 72-hour full-load test before delivery; buyers can arrange third-party inspection agencies to conduct component verification at the factory warehouse.
4.1 Visual Distinction Clues (Auxiliary Judgment)
- Genuine 38CrMoAlA nitrided screw: Uniform matte silver-gray surface, smooth flight edges, no rough welding traces
- Fake "bimetallic" single nitrided screw: Partial local polishing bright spots, no clear alloy boundary on screw flights
- Real bimetallic screw: Obvious uniform alloy surfacing layer on flight tops, consistent color difference between substrate and wear layer
4.2 Cut Sample Destructive Test (High-Certainty Option for Large Orders)
For bulk procurement of twin-screw recycling lines or large-diameter PE pipe extruders, negotiate to reserve a small same-batch screw test piece for cutting metallographic analysis:
- Nitrided sample cross-section: Thin 0.4–0.7mm hard surface layer visible under microscope
- Bimetallic sample cross-section: Thick 1.5–3mm independent alloy layer tightly bonded with substrate steel, no separation gapJBD retains batch test pieces for all screw/barrel batches for 3 years to support customer re-inspection at any time.
4.3 Full-Load Production Trial Validation
Operate the complete extruder with designated raw materials for 24–72 consecutive hours (consistent with JBD’s factory test standard):
- If marked 38CrMoAlA for pure PP/PE: Stable output, no abnormal melt black particles after long-run operation
- If marked bimetallic for glass-filled/PVC/PCR: No rapid output decline or scratch marks on melt after high-filler processing (signs of thin fake hard layers)
5 Step 4 Post-Delivery Long-Term Production Performance Verification
After equipment arrives at your factory, actual production wear conditions act as the final practical verification standard:
5.1 For 38CrMoAlA Nitrided Screw & Barrel
Processing clean virgin PE/PP sheet/pipe for 2–3 years only produces minor uniform wear, without obvious output drop. If serious barrel scratch, screw flight thinning occurs within 6 months, the base material must be counterfeit low-carbon steel without standard 38CrMoAlA nitriding.
5.2 For Bimetallic Composite Screw & Barrel
Continuous processing dirty PCR flakes, calcium-filled plastic or PVC for 3+ years maintains stable extrusion output. If severe abrasion and melt leakage appear within 1 year, the supplier falsely labeled single nitrided parts as bimetallic products.
6 Standardized Screw & Barrel Quality Control at Qingdao JBD Machinery
As a compliant full-line extrusion manufacturer, JBD implements closed-loop full material traceability to help clients avoid material fraud risks:
- Raw material incoming inspection: All 38CrMoAlA steel bars and bimetallic alloy materials pass XRF spectral testing and retain mill certificates
- Batch heat treatment tracking: Each furnace batch generates hardness, hardening layer thickness test reports archived permanently
- Optional third-party testing cooperation: Support customer-specified inspection institutions to conduct XRF, ultrasonic, metallographic testing on any screw/barrel batch before delivery
- Transparent material labeling on quotation & contracts: Clearly separate pricing for 38CrMoAlA nitrided and bimetallic configurations, no false high-grade material price markup
- Global after-sales material guarantee: If material inconsistency is confirmed via third-party testing, Dunya Trading coordinates free replacement of all screw/barrel sets and covers inspection costs
7 Common Supplier Fraud Tricks to Watch Out For
- Substitute cheap 45# carbon steel for standard 38CrMoAlA, only perform simple chrome plating instead of deep nitriding
- Single thin nitrided parts falsely marketed as bimetallic composite, raising prices by 40%–70%
- Provide mill certificates from different steel batches inconsistent with actual production components
- Refuse factory audit and on-site hardness/XRF testing requests with various excuses
- Omit material guarantee clauses in sales contracts to evade compensation liability once counterfeiting is exposed