Polyurethane (PU) Coating Process for Suspended Monorail Wheels
In suspended monorail systems, drive and load-bearing wheels operate under high-stress "inverted" conditions, simultaneously bearing the train's dead weight, passenger loads, and lateral curve forces. The Polyurethane (PU) Coating Process—precision casting high-elasticity polyurethane elastomers onto cast iron wheel cores—delivers high adhesion, low wear, and superior vibration dampening essential for reliable operation.
Step 1: Substrate Pre-treatment & Defect Detection
Differentiated cleaning for new vs. remanufactured wheel cores
New Cast Iron Cores: Ultrasonic degreasing with neutral industrial cleaning agents removes casting oils and oxide scale. This is followed by hot-air drying at 80–100°C to prevent moisture-induced bubbles during casting.
Remanufactured Cores: Specialized mechanical stripping equipment removes aged PU or rubber layers, followed by high-pressure rinsing. Non-Destructive Testing (NDT) is mandatory to confirm the core is free of fatigue cracks or structural deformation before re-coating.
Step 2: Surface Roughening & Chemical Activation
Establishing mechanical interlocking and interfacial chemical bonding
Grit Blasting: 80–120 mesh quartz sand is blasted at 0.4–0.6 MPa to achieve a surface roughness of Ra 1.5–2.0 μm. Clean compressed air then blows away residual dust.
Primer Application: A specialized polyurethane coupling primer is applied immediately to penetrate micro-cavities and form chemical anchors. It is flash-dried at room temperature for 30 minutes.
Step 3: Precision Casting & Stepped Curing
Strict control of mixing ratios, temperature, and coating uniformity
Proportioning & Mixing: Polyurethane prepolymer and curing agent are mixed at a precise ratio (typically 100:10–15), high-speed stirred, and degassed to prevent internal voids.
Mould Positioning & Casting: Preheated wheel cores are accurately positioned in specialized moulds to maintain a uniform coating thickness of 15–25 mm.
Two-Stage Thermal Curing:
Stage 1 (Pre-curing): Baked at 60°C for 2 hours to gel and set the coating shape.
Stage 2 (Final curing): Heated to 80°C for 4 hours to achieve full polymer cross-linking, maximizing mechanical strength and wear resistance. Cooled naturally before demoulding and edge trimming
Step 4: Comprehensive Quality Control (QC)
Ensuring full compliance with rail transit safety standards
Adhesion Testing: Cross-cut or peel testing verifies interfacial bonding strength between the PU layer and iron core, eliminating delamination risks.
Dimensional Accuracy: A Coordinate Measuring Machine (CMM) inspects wheel diameter and roundness within a tolerance of ±0.1 mm.
Dynamic Friction & Wear Testing: Simulated track tests measure the friction coefficient and accelerated wear life to ensure compliance with traction and braking specs.