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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.

●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.

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.

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