Magnetic shields made of Mu-metal®, iron-silicon alloys, pure iron and other materials. The components are cut with a laser to meet the specifications and customer requirements and then are subject to thermal processing to provide them with magnetic properties as required.
Our offer also includes Mu-metal® shields in the form of deeply drawn canisters: round, square and rectangular. They come in various heights and with a variety of covers fitted both on the outside and on the inside. All shields can be customised to meet individual customer requirements. The available thickness ranges from 0.10 mm to 5.00 mm.
Mu-metal® is the trade name of an alloy containing 80% nickel, 4.5% molybdenum and iron. Other names include Permalloy, Hy Mu80, Magnifier 7904. The alloys feature very high magnetic permeability, which results in the highest possible damping factor.
Currently, we do not know any material that would block magnetic field without yielding to its attraction. The magnetic screen works by redirecting magnetic field to the area around it, therefore the material used to build the shield has to feature high permeability, i.e. ability to attract magnetic field lines.
The most commonly used alloys include Mu-metal®, Supra 50 and Supra 36 being selected depending on the magnetic field strength. If the magnetic strength for a particular material is too high, the material will be saturated, thus becoming ineffective. To prevent this phenomenon, multi-layer screens combining the above-mentioned alloys are used. The alloys should feature a low level of magnetic flux density to protect against permanent magnetisation.
The screening of radio frequencies occurs for frequency levels exceeding 100 kHz, while the materials used for this purpose include copper, aluminium or metallised plastics. These materials are used for radio frequency (RF) screening, because they are conductors and show low permeability. By contrast, magnetic screening occurs in the frequency range from 30 Hz to 300 Hz AC..
Direct current (DC) features a constant amperage and flow direction, in the same way as, for example, the field emitted by earth or the field generated by magnets and some motors. Alternative current (AC) is a current whose instantaneous values are subject to changes repeatable at a specific frequency. As a rule, such fields are generated by an electrical appliance operating at a frequency ranging from 50 Hz to 60 Hz. Magnetic screening is effective for the both current types.
The most effective shape is a sphere, however, it is hard to manufacture and to much extent, non-practical for a majority of applications. The second in terms of effectiveness is a cylinder that due to its structure provides an enhanced damping level. The next one is a cube whose corners have a large bend radius, thus minimising flux concentration. If possible, it is not recommended to use a flat sheet.
This is a material ability to absorb magnetic flux, the ratio of flux density to field forces. The higher the permeability, the better the damping effectiveness of a magnetic screen.
The magnetic field damping, shield factor (S) is a ratio of magnetic field strength outside a magnetic screen (Ha) and the resultant of magnetic field inside the shield, i.e. Ha/Hi (without units) or S = 20 × log (Ha/Hi) (Db). Depending on material permeability, the shape and the size of the screen, different formulae are used to calculate the magnetic field damping, in the majority of cases they are similar and are used for direct current (DC) fields.
Formula for a closed canister:
S = 4/3 × (Mu × d/D)
Mu – permeability (relative value)
d – material thickness
D – shield diameter
Formula for a hollow cylinder in a transverse magnetic field:
S = Mu × d/D
Formula for a cubic box:
S = 4/5 × (Mu × d/a)
a – side length
For multi-layer shields with air spaces provided by insulation spacers, the properties of particular layers are subject to multiplication, which results in obtaining excellent protective properties.
Formula for a double-layer shield:
S = S1 × (S2 × (2 × change in diameter/diameter))
Mu-metal® shows similarity to stainless steel, that is why external gas emission is at a minimum level.
Low temperatures affect Mu-metal®, i.e. magnetic flux density saturation remains at the same level while permeability decreases. At cryogenic temperatures it is recommended to use a special cryogenic Mu-metal®.
Thermal processing is required after forming to improve crystal structure and granularity. Without this stage magnetic properties and magnetic damping would be considerably reduced.
Yes. If the screen is impacted or if we are afraid of screen protective properties.
Yes. After welding it should be subject to full thermal processing.
Mu-metal® features very high permeability, but shows a relatively low saturation level. By contrast, Supra 50 features lower permeability, but its saturation level is higher. Supra 50 is placed closer to the source of strong magnetic field to protect Mu-metal® against saturation.