Stan Trout, Spontaneous Materials
We continue with the fifth blog in this series, describing the many types of mistakes made with permanent magnets. Again, my intent is to help engineers in the future avoid the mistakes made in the past, and not to embarrass anyone. Everyone’s experience is different, so please feel free to add your insights in the comments section below.
The fifth group of mistakes on my list are:
- Using the wrong adhesive
- Measuring the wrong parameters
- Failing to consider recycling in the design stage
One of the things we should learn early in our experience with magnets is that generally they are mechanically fragile. We can’t use a simple nut and bolt to hold them in place, nor can we thread most permanent magnets. Even if we could, the hardware would likely get in the way of the magnetic flux that we are seeking to use from our magnet. So, we are left looking for other ways to permanently attach magnets in our devices.
The most common way to bond a magnet is to use an adhesive, for example a structural adhesive or a cyanoacrylate like Krazy Glue. There are many types of adhesives available today, and the choice will vary with the application, temperature range and environmental conditions that the device might see in its lifetime. The companies who make these adhesives also do a very good job, in my humble opinion, of helping engineers find the right adhesive for their specific application.
Doing a little homework and including some confirming tests should be sufficient to get things right with adhesives. Occasionally some designs slip through with inadequate engineering in this area. That oversight can lead to failure of the adhesive and usually failure of the device itself. It seems sad that a poor choice for a relatively inexpensive item can lead to premature failure of a device.
Today we need to add a new requirement for our adhesives. Not only must they perform their traditional function during the life of the product, but we must also consider how they will behave at the end of a product’s life, and plan accordingly. In other words, we need to think about our choice of adhesive and the ultimate recycling of our device. We would like our adhesives to hold well during the life of the product and then cease holding the magnet as the product enters the recycling stream. People use the phrase “liberating the magnets” to describe the work they must do to remove the magnets from the device. This is a very challenging step because the adhesive is still doing its original job, i.e. holding the magnet. We need some new adhesives, that can be “turned off” at the appropriate moment to free the magnets in a much easier way than the current methods. In particular, we would like to release the magnet in a way that does no harm to it, both magnetically and physically, something that we struggle with today. This challenge will require some research to develop, but I think it is possible because this problem is not unique to our industry. In addition, designers need to think about this part of the process in the early stage of the design process, otherwise having better adhesives will not be helpful. (This issue is also part of mistake #12.)
As I mentioned in an earlier blog, engineers often do not specify their magnets well, either saying too much or too little about what they actually need in their specifications. This problem can also extend to the testing stage, as a shipment of magnets comes in the door of a factory. Because the best magnetic measurements usually destroy the part and can take a considerable amount of time to perform, people look for faster and nondestructive ways to test magnets for incoming inspection. That is completely understandable. Simple testing methods can work well, provided the supplier and the customer have discussed and agreed, before the supplier starts shipping magnets, on the measurement that is going to be used and the acceptable results from such testing. To assume that everyone will get the same results from slapping a Hall probe on the surface of a magnet, or taking a Helmholtz coil measurement, without doing any planning well in advance is asking for trouble.
It is also important that the magnetic test plan is designed with the ultimate use of the magnet in mind. For example, testing after a heat soak is likely warranted if the magnets are expected to operate over a range of temperatures, or if exceptional long term stability is required. Testing after exposure to an adverse magnetic field may be proper for magnets used in motors, or again for exceptional long term stability.
Recycling of magnets and other rare earths became a hot topic while rare earth prices were high a few years ago. That is no surprise. Even though rare earth prices are lower these days, the interest in recycling continues because of several drivers. New environmental laws push manufacturers to be able to take back their products at the end of their useful lives. Having a recycling plan in place is a good hedge against future raw material shortages. Recycling is just good stewardship for materials in finite supply. Yet most of the products that we now try to recycle were never designed for this stage of the lifecycle. It may be a slight exaggeration, but I feel that we are almost behaving like archeologists, sifting through debris trying to find valuable things that we can recover. This goal is noble, but we need to get more serious about it. If we really want to recycle more successfully, we need to plan in advance for this eventuality, and design devices that can be taken apart and reclaimed with a minimum of fuss.
Twelve down, five to go.
About the Author
Dr. Stan Trout has more than 35 years’ experience in the permanent magnet and rare earth industries. Dr. Trout has a B.S. in Physics from Lafayette College and a Ph.D. in Metallurgy and Materials Science from the University of Pennsylvania. Stan is a contributing columnist for Magnetics Business & Technology magazine. Spontaneous Materials, his consultancy, provides practical solutions in magnetic materials, the rare earths, technical training and technical writing. He can be reached at firstname.lastname@example.org.