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Introduction
Considerations in Magnet Bonding
The Study
Test Method
Magnet Bonding Blockshear and Impact Study(pdf)
Technical Product and Ordering Information(pdf)
Introduction
Only a decade ago, it was common to use
spring steel clips or bolts to fix magnet segments into position. Both
materials were costly, difficult to automate, and required maintaining large
inventories. In addition, they involved
highly complex parts handling systems and intricate insertion methods. Neither
method prevented noise from
mechanical vibration, and both could loosen or shift, allowing corrosion to
occur between the magnet and assembly.
Today structural acrylic, cyanoacrylate, and epoxy adhesives are
available, providing tough, durable, environmentally resistant
joints and quick processing. Adhesives first found acceptance in motor magnet
bonding, where the
severest environments may be encountered. Now they are widely used to bond the
magnet assembly in a wide
range of applications, including loudspeakers, headphones, appliances, lifting
equipment, power generators,
disc drives, microphones, telephones, motors (starter, micro-, electro-, and
servo-motors), and measuring
instruments (volt, amp, speed-meters, watt-hour-meters). Adhesive-bonded magnets
provide a wealth
of benefits, including:
●High
shear strength and impact resistant joints
●Room
temperature and heat cure versions for easy processing
●Ultra-fast
fixture times for increased throughput
●Easily
automated assemblies to increase production efficiencies
●Solvent-free
formulations for worker and environmental safety
●Excellent
durability for improved product quality
●Prevents magnet
movement and absorbs shock and impact
Considerations in Magnet Bonding
Factors relating to the
magnet and the opposing substrate play a critical role in successful
magnet bonding. To achieve consistent bonding performance, mating surfaces must
be clean
and free from contamination, such as plating residues or lubricants. Magnets
must be
dust-free and formed or machined to ensure that gaps between the magnet and the
mating
substrate remain small, preferably less than .010 inches. Generally, minimizing
the gap
insures faster fixturing, stronger joints, and in the case of loudspeakers,
minimizes magnetic
energy losses.
For curved magnet segments, some
motor
manufacturers have moved from single radius
segments to tri-arc configuration.
This design helps reduce magnet movement in the fixture,
and can help reduce the gap due to tolerance differences
between the housing and the magnet.
Examination of the tri-arc bond joints shows that
the adhesive fixtures or solidifies quickly at the
two points of contact on the magnet surface.
The surface of the housing, which is typically made of a steel
base material, is also critical
to the success of the bonding design. Housing surfaces are often fabricated toresist corrosion
and
may be painted or coated. Conversion coating processes such as chromating,
phosphating,
as zinc phosphate or chromic acid anodizing are best for adhesive bonds. Coating
processes
such as galvanizing or yellow zinc dichromating leave weak surface layers and
can be problematic.
For these challenging surfaces, heat cure epoxies may provide the best solution.
On in-active surfaces, structural acrylics are used with primers to enhance cure
speed.
Solventless primers for dichromated surfaces are available, as well.
For motor manufacturers, the method used to fabricate the
housing often affects its ultimate
dimensional tolerance. Tighter control of housing dimensions can be used to
ensure small
bondline gaps. Drawn or extruded housings are typically the most stable from a
dimensional
standpoint. Magnet bonded housings are commonly roll-formed housings, and are
more
difficult to control dimensionally. When rolled housings are used, it is
important that the
seam is smooth and properly fitted. Misalignment or tabs that cause large gaps
are unacceptable.

The Study
In response to frequent
requests from magnet bonding manufacturers for a more
consistent test method to determine the long-term durability of adhesive-bonded
magnets,
Loctite designed a study. Adhesion behavior was analyzed using several Loctite¨
structural adhesives, including acrylic, epoxy, and cyanoacrylate chemistries.
Blockshear
and impact strength testing was conducted, with a goal to qualitatively and
quantitatively determine the adhesive durability, using standard test methods.

The selected base material
substrates were comprised of mild steel, E-coat, and yellow
zinc dichromate, and the magnet substrates included ferrite, alnico, and
neodymium.
All magnets were assembled and tested without magnetization. Blockshear testing
was
performed according to ASTM D4501 and impact testing to ASTM D950.

Test Method
Methods of testing
adhesive-bonded magnets varies widely by manufacturers, from
simple drop tests to a more elaborate slide rail apparatus fitted with a
stationary striker.
Blockshear values determine adhesive durability per ASTM D4501, as depicted in
Figure 1.
A more scientific test method exists in ASTM D950, shown in Figure 2. This
method
determines the comparative impact strength of adhesive bonds in shear, when
tested
on standard specimens under specified conditions.

Magnet Bonding Blockshear and
Impact Study
Technical Product and Ordering
Information
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