Surface Preparation: Why Coating Matters Before Gluing (Ni-Cu-Ni vs. Zinc)
The bond strength between adhesive and magnet depends on the surface energy of the coating. Ni-Cu-Ni plating has a moderately high surface energy (38-42 dynes/cm), allowing good adhesion after proper cleaning. Zinc plating has lower surface energy (30-35 dynes/cm) and may have residual passivation oils, requiring aggressive cleaning.
Before gluing:
Degrease using isopropyl alcohol (IPA) or acetone. Do not use chlorinated solvents.
For Ni-Cu-Ni, light abrasion with 400-grit sandpaper increases bond strength by 20-30%.
For zinc coating, do not sand (removes coating). Use a zinc-compatible primer (e.g., Lord 7701).
For epoxy-coated magnets, no surface preparation is needed for gluing to metal – the epoxy surface is already adhesive-compatible.
We recommend Ni-Cu-Ni coated magnets for epoxy bonding applications requiring high reliability. Zinc-coated magnets should only be used with cyanoacrylate and non-critical assemblies.
Cyanoacrylate vs. Two-Part Epoxy for Industrial Applications
CA adhesives (e.g., Loctite 401, 406) cure rapidly via moisture. They are suitable for:
Small magnets (under 10g) bonded to clean metal with gap <0.1mm.
Room temperature applications (<60°C).
High-volume manual assembly where fast fixturing reduces cycle time.
CA limitations: poor impact resistance, brittle failure, degradation above 80°C, and gap sensitivity (above 0.2mm void, CA cannot fill).
Epoxy adhesives (e.g., 3M DP420, Loctite Hysol 9460, Master Bond EP21TDC-2) are two-part (resin + hardener). They are suitable for:
All magnet sizes, especially heavy or vibrating assemblies.
Temperature range -40°C to 120°C (standard) or to 180°C (high-temp epoxy).
Gap filling up to 0.5-1.0mm.
Automated dispensing (1:1 or 2:1 ratio cartridges).
Disadvantages: cure time 30 minutes to 24 hours (heat accelerates), requires mixing.
Comparison table:
| Property | Cyanoacrylate (CA, e.g., Loctite 401) | Two-part Epoxy (e.g., DP420) | High-Temp Epoxy (e.g., EP21TDC-2) |
|---|---|---|---|
| Typical cure time (full strength) | 24 hours (fix 15-30 sec) | 24 hours (fix 5-10 min) | 24 hours (fix 60 min at 25°C) |
| Heat cure acceleration | Not recommended | 60°C for 90 min | 100°C for 60 min |
| Max continuous temp (°C) | 70-80 | 110-120 | 170-180 |
| Lap shear strength (MPa, on Ni-Cu-Ni to steel) | 15-20 | 25-32 | 22-28 |
| Gap filling max (mm) | 0.1 | 0.5 | 1.0 |
| Viscosity (cP) | 100-500 (low) | 10,000-50,000 (thixotropic) | 5,000-20,000 |
| Resistance to vibration | Poor (brittle) | Good (flexible) | Good |
| Suitable for automated lines | Yes (micro-dispensers) | Yes (cartridge mixers) | Yes (meter-mix equipment) |
| Relative cost per unit bond | 1.0 (lowest) | 1.8-2.2 | 2.5-3.5 |
Gap Filling and Dispensing Methods for Automated Assembly Lines
For high-volume magnetic assembly production (e.g., thousands of rotor magnets per day), automated adhesive dispensing is required. We recommend:
For CA: Pneumatic micro-dispenser with 0.2-0.5mm needle tip. Apply 2-4 small dots (1-2mm diameter) per magnet. Avoid excess CA that can overflow and contaminate the magnet's active surface.
For epoxy: 1:1 or 2:1 cartridge with static mixer and pneumatic gun. For fully automated lines, use meter-mix dispensing system with positive displacement pumps.
Dispensing parameters:
Epoxy bead width: 1.5-2.5mm
Bead height after magnet placement: controlled by fixture to 0.1-0.3mm
Overlap: magnets should seat in a shallow pocket (depth 0.2-0.5mm) to control bond line thickness.
Cure profiles: we recommend forced air oven cure at 60-80°C for 60-90 minutes to achieve full strength before handling and shipping. Room temperature cure (24 hours) is acceptable but requires jigging to prevent magnet movement during gel phase.
For magnetic assembly manufacturers seeking adhesives and dispensing systems validated for Ni-Cu-Ni magnets, please visit our Magnetic Solutions homepage for process guidance.
To request adhesive sample kits (CA and epoxy) for testing on your specific magnet coating and metal substrate, contact our assembly engineering team. We provide bond strength test reports.
Frequently Asked Questions
Q: Can I use epoxy to glue magnets that will be used in a vacuum chamber (10^-6 mbar)?
A: Standard epoxy outgasses under vacuum. Use vacuum-grade epoxy (e.g., EPO-TEK H77) or low-outgassing CA. We provide outgassing test data per ASTM E595 (TML < 1.0%, CVCM < 0.1%).
Q: How long do I need to clamp the magnet after applying CA before releasing?
A: For CA, 15-30 seconds clamping is sufficient for initial hold. Full strength (shear) takes 24 hours. For epoxy with heat cure, clamp for the entire heat soak (e.g., 60 minutes at 60°C).
Q: What is the typical shelf life of epoxy cartridges?
A: Unopened, refrigerated (2-8°C): 12 months. Opened cartridge (with cap): use within 7 days. Do not freeze. CA shelf life: 6 months unopened at 20°C; 12 months refrigerated.
