Magnetic Particle Inspection (MPI) is one of the most reliable NDT methods for detecting surface and near-surface defects in ferromagnetic materials like steel and iron.
1. What is Magnetic Particle Inspection (MPI)?
Magnetic Particle Inspection (MPI) is a proven NDT technique that reveals surface-breaking and shallow subsurface discontinuities—such as cracks, seams, laps, and inclusions—in ferromagnetic components.
Key Advantage: Zero damage to parts, high sensitivity, and rapid inspection cycles.
2. How Does MPI Equipment Work? (Step-by-Step)
- Magnetization: The workpiece is energized using AC, DC, or HWDC via headstock, coil, or central conductor.
- Particle Application: Dry powder or wet fluorescent suspension is applied.
- Leakage Field Detection: Defects disrupt magnetic flux, creating localized poles that attract particles.
- Visual Indication: Defects appear as sharp lines under white light (visible particles) or glowing green under UV-A black light (fluorescent).
3. Types of MPI Machines: Which One Fits Your Workflow?
| Type | Best For | Mobility | Automation Level |
|---|---|---|---|
| Bench-Type MPI | Medium-large parts (gears, shafts) | Fixed | Manual to Full-Auto |
| Portable MPI Units | Field inspections, welds, pipelines | High | Manual |
| Custom MPI Systems | High-volume OEMs (bolts, bearings) | Fixed | Semi/Full-Auto |
4. AC vs DC vs HWDC: Magnetic Current Comparison
| Current | Surface Sensitivity | Subsurface Depth | Best Applications |
|---|---|---|---|
| AC | ★★★★★ | ★★ | Smooth surfaces, fatigue cracks |
| DC | ★★★ | ★★★★★ | Castings, forgings |
| HWDC | ★★★★ | ★★★★ | Automotive, railway axles |
5. Fluorescent vs Visible MPI: When to Use Each
- Fluorescent MPI
→ 3–5x higher sensitivity
→ Requires darkroom + UV-A (365 nm)
→ Ideal for aerospace, turbine blades - Visible (Color Contrast) MPI
→ No UV light needed
→ Faster field inspections
→ Suited for construction, welds
6. Core Components of a Professional MPI System
- Magnetizing Unit – Headstock/tailstock + longitudinal coil
- Power Supply – 4,000–12,000 Amp turns (digital control)
- UV-A Black Light – ≥1,000 µW/cm² at 15″
- Demagnetization – Automatic AC decay or reversing DC
- Wet Bench – 316L stainless tank + peristaltic pump
- Inspection Booth – LED white light + UV-safe enclosure
7. How to Select the Right MPI Machine (Buyer’s Checklist)
| Factor | Questions to Ask |
|---|---|
| Part Dimensions | Max length/weight? Complex geometry? |
| Defect Type | Surface only or subsurface? |
| Throughput | 100 pcs/day or 10,000? |
| Standards Compliance | ASTM E1444, ISO 9934, NADCAP, ASME? |
| Integration | PLC, SCADA, or Industry 4.0 ready? |
8. MPI Equipment Maintenance Schedule
| Interval | Task |
|---|---|
| Daily | Check bath concentration (0.1–0.3 ml/100 ml) |
| Weekly | Clean pump filters, inspect hoses |
| Monthly | Calibrate ammeter & UV meter |
| Annually | Full system audit + coil insulation test |
10. Why Choose NK NDT as Your MPI Machine Manufacturer?
- Global Certifications: ISO 9001
- Premium Components: Siemens PLC, Schneider contactors, Omron sensors
- Customized Solutions: Tailored MPI systems, designed for your workpiece size, volume, and standards.
- Affordable Pricing: Original Factory, delivering at least 30% better value than imported equipment while maintaining top performance.
- Industries Served: Automotive (crankshafts), Aerospace (landing gears), Railway (axles), Petrochemical (valves)
Ready to Upgrade Your NDT Line?
Contact NK NDT for a free virtual demo or custom MPI quote tailored to your parts.
