Rubber / Flexible Magnets

Specification

Material / binder

Sr- (occasionally Ba-) ferrite powder in NBR, CPE or PVC binder; ~85-90 wt% filler (~60-70 vol%), giving ~3.5-3.8 g/cm³

Grade range (BHmax)

0.6-1.6 MGOe (4.8-12.7 kJ/m³); isotropic ~0.6-1.0, anisotropic ~1.2-1.6

Br / Hcb

Br 1,500-2,700 G (0.15-0.27 T) — isotropic ~1,500-1,800 G, anisotropic ~2,300-2,700 G; Hcb 1,300-2,400 Oe; Hcj 2,500-3,500 Oe

Max operating temperature

Binder-limited, not magnet-limited (ferrite Curie point is ~450 °C): ~80-100 °C continuous for NBR-bonded grades, ~70-80 °C for PVC-bonded and laminated stock. Excursions above this degrade the elastomer — plasticiser loss, stiffening, curl — before any magnetic loss occurs

Magnetisation

Multipole on one face; standard pole pitch 1.5-2.5 mm (typ. 2.0 mm). Single-pole/axial available at ~40-50% of the multipole surface pull in contact, but it reaches usefully further from the surface

Standard sizes

Thickness 0.4-6.0 mm (0.75, 1.0, 1.5, 2.0 mm most common). Roll width 10-1,220 mm (620 mm is the common stock width); roll length up to 30 m. Sheets typ. 620 × 1,000 mm

Tolerance

Thickness ±0.1 mm or ±5%, whichever is greater; slit width ±0.3 mm; die-cut profile ±0.5 mm; length ±1% on rolls. Do not expect machined-part tolerances

Surface / coating options

Plain black, brown or grey; white or coloured PVC lamination; PET film; UV/latex/solvent printable topcoat; acrylic or rubber PSA on the reverse. Note these are cosmetic and functional — ferrite needs no corrosion protection

Typical applications

• Refrigerator, cabinet and shower-door gaskets — profile-extruded flexible ferrite in a PVC or NBR seal carrier
• Printed vehicle door signage, whiteboard sheets and promotional magnets on laminated white PVC stock
• Die-cut components for point-of-sale, sample boards, planning charts and toy/education kits
• Reed and Hall-sensor actuation — but only with single-pole (axially magnetised) stock at sub-millimetre to ~1 mm gaps. Standard multipole stock is not a sensor magnet: its field is gone before it reaches the sensor
• Magnetic window insulation strip, insect-screen closures and darkroom/blackout seals
• Low-force retention of light panels, guards and covers on ferrous sheet metal where repeated repositioning is needed

Engineering notes

• Holding force is very low and drops off fast. Expect roughly 15-25 g/cm² for 1.5 mm multipole stock in direct contact with clean 1 mm mild steel. Every 0.1 mm of paint, powder coat or air gap costs a meaningful fraction of that — at 0.5 mm gap you may be down to a quarter of the contact value. The multipole pattern makes the field decay exponentially with distance: one magnetic wavelength spans an N+S pair, so the decay length is pole pitch / π — about 0.64 mm at the standard 2.0 mm pitch. Beyond ~2-3 mm there is essentially nothing left. If your design has a gap, this material is the wrong material.
• The multipole face is directional and unmarked to the eye. Two strips only attract each other properly when their pole lines are parallel and aligned; rotate one by 90° and they nearly cancel. It also means the strip has a strong face and a near-dead face — always confirm orientation and pole pitch in the RFQ if the part mates with an existing counter-strip (e.g. a legacy fridge gasket).
• The binder ages, not the ferrite. Plasticiser migration in PVC-bonded grades causes shrinkage, curl, staining of painted surfaces and stiffening over years, accelerated by UV and heat. NBR-bonded grades are more dimensionally stable and better for gaskets but cost more and are less receptive to lamination. Outdoors on a vehicle, expect visible degradation and lifting within 1-3 years unless a PET/UV-stable laminate is specified.
• Anisotropic grades reach ~1.6 MGOe because calendering aligns the ferrite platelets with their easy axis normal to the sheet plane — which is the through-thickness direction you actually magnetise and use. The caveat is that no such gain exists in-plane, and the price premium over isotropic is real. Isotropic stock at ~0.6-1.0 MGOe is often the honest choice — the difference in a printed sheet application is not worth the cost, whereas in a gasket it can be.
• Ferrite has a strongly negative Br temperature coefficient (≈ -0.2%/°C) and a positive Hcj coefficient — meaning coercivity falls as the material gets colder, which is the opposite of the intuition most engineers carry over from NdFeB. Below about -20 °C the coercivity falls enough that irreversible loss becomes possible if the strip is separated and re-applied cold. Cold-chain and freezer-door applications should be specified deliberately, not assumed.