Of pellets & magnets


Powerful magnets have powerful roles, so to speak, in processing plastics pellets.
Standard ferrous magnets, “rare-earth” magnets, and eddy-current metal separators all use magnetic technology to keep contaminants out of the polymer-melt stream.
Furthermore, magnetic technology is debuting in a new role, that of monitoring the moisture levels in material being dried before it gets to the process machine and is molded into defective parts.
Most material contamination is undoubtedly caused by accidental or careless behavior, but even if it is malicious, the stuff has to come out. Otherwise, nuts, bolts, nails, metal shavings, paper clips, and probably a lot of other things that never occurred to me could get into the screw/barrel unit and wreak havoc.
Ferrous magnets usually consist mostly of iron with charges aligned to give them a magnetic field that will attract anything ferrous, namely iron-based. Conventional magnets are relatively simple, economical, and require practically no management except that one must periodically clear them of whatever junk they have attracted.
Rare-earth magnets are incredibly strong, more complicated to make, and more expensive, at least initially, than the ferrous magnets. To get some personal experience with rare-earth magnets, I went into a hardware store a while back looking for rare-earth magnets. I thought, well I have nothing to lose, other than to have a clerk look at me like I was out of my mind, but lo-and-behold, there were the magnets, in little thermoformed plastics packages. I got two packs at about $5 each. They made a terrific crack of noise when they pulled together, while still in the package, no less. I took them to a meeting of local writer/editor/graphic arts sorts, and demonstrated how strong they were. Then I asked for a volunteer to put a finger between the two magnets. No way. Nobody would touch it. I had tried it first, with no damage, so I knew I wasn’t putting anybody at risk. The magnets were demonstrably strong, however, and nobody was taking chances.
So what is this “rare earth” business about? The rare-earth magnets most commonly used in the plastics industry are a combination of neodymium (the “rare earth”) plus iron and boron. From the Periodic Table of the Elements, the combination is “NdFeB,” the abbreviation for neodymium iron boron. These magnets are available in hopper-magnet form from multiple specialists in magnets for plastics processing applications, as well as auxiliary equipment suppliers. The magnets are not rare and don’t need very much neodymium, but the neodymium itself is relatively rare as an element.
Neodymium is one of 17 elements, out of more than a hundred elements on the Table, labeled as “rare earths” by IUPAC, an international chemistry group headquartered in Zurich. Here they are, in alphabetical order: Cerium, Dysprosium, Erbium, Europium, Gadolinium, Holmium, Lanthanum, Lutetium, Neodymium, Praseodymium, Promethium, Samarium, Scandium, Terbium, Thulium, Ytterbium and Yttrium. These labels are mostly of Greek origin, named for a person, time, or place associated with their discovery. Neodymium is in there, somewhere near the middle. Dysprosium, Gadolinium and Samarium can also be used in “rare-earth magnets.” Another application for Gadolinium is computer memories.
Rare earth elements are mentioned fairly often in the consumer press these days, usually in the context of what China has and the rest of us don’t, namely the bulk of the world’s economically accessible supplies (via mining, for example) of the rare earths. It’s seems amazing that not even the market for industrial rare-earth magnets remains entirely free of geopolitical implications, and these days, that means China is in the mix somewhere. No reason for panic that I know of, but worth tracking on the Internet if the subject interests you.
Eddy-current metal-separation systems are used to extract non-ferrous metal contaminants from the material stream before they can get in the melt and cause trouble. Eddy-current technology is based on exposing a conductor to changing magnetic fields in order to attract or repulse non-ferrous metals. The technology has been using in the plastics industry for some time, but seemed to remain inconspicuous for whatever reasons. Now, however, Bunting Magnetics (Wichita, Kansas) for example, is marketing an eddy-current system with the words “eddy-current” conspicuously appearing the system itself. Such contaminants include such common nonferrous metals such as brass, copper, and aluminum that are not effectively removed by conventional or rare-earth magnets. A U.S. penny, for example, contains copper. These days it is only copper coated, containing about 2.5% copper and 97.5 % zinc, though it was historically 100% copper.
Magnetic moisture measurement, and now, magnetic technology is being used in a polymer drying/moisture measurement system which was introduced by Novatec (Baltimore, Maryland) at NPE 2012. Novatec publicly announced the system on this web site and also in print, stating Novatec is the exclusive North American dryer integrator for the BryScan 100 manufactured by BryAir Prokon in Switzerland (Patent Pending).
According to the BryAir Prokon web site, the moisture measurement technique is based upon the relatively high dielectric constant of water in comparison to the dielectric properties of other materials and includes the measurement of diamagnetic properties. In addition, measurement is accomplished by the excitation of free moisture molecules when introducing a controlled magnetic field, which is measured with a 32-bit dedicated microprocessor. Novatec has integrated this technology into what they call “MoistureMaster™ and is now available on all Novatec dryers.   Material moisture content is displayed on dryer touch screen controls and, if the moisture content starts to stray towards pre-set levels that are too high or too low (in the case of nylon), the dryer parameters are automatically adjusted to keep the pellet moisture content within these required levels.
So there you have it. Once thought of childhood toys, magnets and sophisticated magnetic technology are now an integral part of material handling for plastics processing.
Revised October 2012
The Plastics Technology Drying, Conveying and Extrsion Knowledge Centers are sponsored by Novatec, whose personnel check each section for technical accuracy. This article is not a statement of Novatec policy.