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News Release from: Electron Energy Corporation | Subject: Applications of polymer-bonded magnets
Edited by the Engineeringtalk Editorial
Team on 01 April 2002
Applications of polymer-bonded magnets
Applications of polymer-bonded magnets are becoming increasingly important in our daily lives, as Jinfang Liu explains.
are becoming increasingly important in our daily lives High performance bonded magnets are used in various electronic devices, office automation equipment, automotive components, hard disk drives, scanners, CDs, DVDs, sensors, magnetic bearings and other motor or motion related applications
This article was originally published on Engineeringtalk on 4 Jan 2002 at 8.00am (UK)
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The following are some advantages and disadvantages of polymer-bonded magnets.
A major advantage is that bonded magnets are made by net-shape or near net-shape manufacturing processes.
Tight tolerances can be held without secondary or finish machining which significantly reduces the production cost allowing for competitive pricing.
A broad selection of polymer binders and polymer additives gives the flexibility for production and meets the requirements of various applications.
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Isotropic bonded magnets can be easily magnetised into various and complex magnetisation patterns.
Fully automated production processes lead to more uniform magnetic properties and lower cost.
On the down side, polymer-bonded magnets have lower magnetic properties comparing to their sintered counterparts due to polymer dilution effect.
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The maximum operating temperature is limited, to some extent, by the temperature characteristics of polymer binders.
And there is a higher tooling cost for injection and extrusion moulding processes.
Polymer bonded magnets can be manufactured by compression moulding, extrusion, injection moulding or calendaring processes.
For compression moulding, the magnet powders coated with epoxies are compression moulded into required geometries and then cured at an appropriate temperature.
Bonded magnets manufactured by this process can be held to tight tolerances, which eliminates the need for secondary or finish machining.
For injection moulding, extrusion and calendaring processes, the magnet powders are first blended with polymers and polymer additives.
The mixtures are then intensively mixed together at certain temperatures by a compounder to form a "compound".
The compound can then be extruded, injection-moulded or calendared.
Typical binders for rigid bonded magnets include thermoplastics, thermosetting polymers, and in some cases, metallic binders.
As a design engineer or a purchasing manager, what do you need to know about bonded magnet in your decision-making process? The following are some suggestions.
First, calculate the flux density required for the application to determine whether bonded magnets will satisfy the design requirements.
A finite element analysis (FEA) is normally necessary although a good analytical calculation would be enough for some less critical applications.
If you do not have FEA software, you can work with the engineers at EEC for this service.
Secondly, determine what is the highest temperature the bonded magnets would be exposed to during their life span, including post-production assembly.
It is important to understand the maximum operating temperatures of various grades of bonded magnets and choose the appropriate grades.
It is always a good idea to leave room for error.
If the application is critical, it is recommended to run an FEA using the demagnetisation curves measured at the maximum service temperature of the bonded magnets.
Thirdly, bonded magnets have certain dimensional limitations.
The wall thickness of a ring magnet should always be greater than 0.040in.
The length of the magnets should normally be smaller than or comparable to its diameter.
The maximum length of bonded magnets should not exceed 1.25in.
The larger the cross-sectional area, the higher the tonnage needed to mould a part.
It is recommended to use the smallest cross-sectional area of the bonded magnets when possible.
Ask EEC about the feasibility of producing the bonded magnet parts of designed dimensions.
Fourthly, the tolerance of bonded magnets is normally +/-0.005in on the thickness, and +/-0.002in on the other dimensions, such as the outer and inner diameters for the ring magnets.
Tighter tolerance always means higher cost.
If tighter tolerances are necessary, these details can be addressed with the supplier.
Fifthly, if the application is multipolar, you need to describe where the flux is needed.
It is not always possible to magnetise a bonded magnet all the way through the dimension of interest.
Sixthly, try to determine the mechanical requirements if necessary.
Normally there is a tradeoff between the magnetic properties and mechanical properties.
Seventhly, bonded SmCo magnets have a better temperature coefficient and higher maximum operating temperature than that for bonded NdFeB magnets.
For bonded SmCo magnets, the thermal characteristics of polymer binders will determine the maximum operating temperature.
But for bonded NdFeB magnets, the thermal stability of the magnet powders will determine the maximum operating temperature.
Please also note that the geometry of the magnet and the magnetic circuitry will determine the working point of the bonded magnets, which in turn, will influence the maximum operating temperature.
Finally, an epoxy surface coating is always recommended for bonded NdFeB magnets.
For bonded SmCo magnets, surface coating is not necessary.
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