Production and properties of bonded Nd magnets

Production and properties of bonded Nd magnets 5.0

5

Introduction

Chapter 4, Production of rapidly solidified NdFeB magnetic powder, discussed the commercial production of NdFeB magnetic powder by the melt-spinning process. This chapter presents an overview of the production of bonded Nd magnets produced from these powders. There are four processing techniques that are currently used to produce bonded Nd magnets, which include compression molding, injection molding, extrusion, and calendaring. The process details and attributes of each of these methods are discussed and the magnetic and physical properties that are typically obtained for the various grades of magnetic powder using these processes are compared. Of these processes, compression and injection molding are by far the two most often commonly employed and are, therefore, presented in more detail. In addition to their magnetic properties, this chapter reviews the thermal aging properties of the various types of bonded Nd magnets, which are the magnetic losses that occur after long-term exposure to various temperatures, and the magnetization properties i.e., the magnetic field required to achieve a certain level of magnetization. Bonded Nd magnets are given a final coating to prevent corrosion and the different coating techniques that are used are also described. Finally, the quality assurance procedures and equipment used to test bonded Nd magnets are detailed. These magnets have found a natural application base where tough, complex shapes with highdimensional tolerance are required. The most common type of bonded Nd magnet produced is thin-walled rings, which are now commonly used in spindle and stepper motors used in a wide variety of computer peripheral, consumer electronic, office automation, and automotive products. Bonded Nd magnets were discovered and first developed at the General Motors Research Laboratories and first disclosed in Lee and Croat (1990, US Patent 4,920,361). Various aspects of the technology used to produce bonded Nd magnets by different processing techniques have been included in a number of review articles, including Ormerod (1989), Tattam et al. (1994), Ormerod and Constantinides (1997), Coey and O’Donnell (1997), Gutfleisch (2000), and Brown et al. (2002). There have also been several conferences devoted to bonded magnets and these have included papers on various technical aspects of bonded Nd magnets. The proceeding of these conference can be found in Hadjipanayis (2003) and Hadjipanayis (2002).

Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets. DOI: http://dx.doi.org/10.1016/B978-0-08-102225-2.00005-3 Copyright © 2018 Elsevier Ltd. All rights reserved.

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Compression-molded Nd magnets

5.1.1 The compression-molding process Compression molding is a technique in which the NdFeB magnetic powder is mixed or encapsulated with the smallest amount of resin possible and pressed into a green (uncured) compact with the highest practical pressure to achieve the highest possible density: the best compression-molded magnets have a density of B79% of the theoretical density of 7.65 g/cm3 for the NdFeB magnetic powder. The principle advantage of compression molding is that the metal-loading factor is the highest of any of the processing methods and magnets with high tolerance, except in the press direction, can be rapidly produced. This is particularly true of thin-walled, ringshaped magnets that are now commonly used in a variety of spindle and stepper motors. Ring-shaped magnets are almost impossible to produce from sintered Nd magnets because of warping or cracking during the sintering process. Although ring-shaped magnets can and are produced by the hot deformation of melt-spun ribbon, the back extrusion process used is more expensive and complicated compared to the compression-molding process. These hot-deformed ring magnets are discussed in Chapter 6, Hot-deformed NdFeB permanent magnets. The major disadvantage of compression molding is that complex shapes are difficult to produce. Also, the magnets are friable in the green state and are still somewhat brittle even after curing. The steps in the compression-molding process are shown in Fig. 5.1.

5.1.2 Powder encapsulation process The first step in the preparation of a bonded Nd magnet is the preparation of the melt-spun powder, which was described in Chapter 4, Production of rapidly solidified NdFeB magnetic powder. The magnetic and physical properties of the various grades of magnetic powder that are currently produced were reviewed in Powder production

Alloy is melt pun to produce magnetic powder

Powder encapsulation

Powder is encapsulated or coated with epoxy

Powder compaction

Powder is compacted into a green compact

Epoxy cure

Green compact is heated to cure the epoxy

Magnet debur Ultrasonic cleaning Painting Paint cure

Cured magnet is roto-finished to round the edges Deburred magnet is ultrasonically cleaned Magnet is spray painted Magnet is heated to cure the paint

Figure 5.1 The steps in the compression-molding manufacturing process that is most commonly used to produce bonded Nd permanent magnets.

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Section 4.4. Because the NdFeB magnetic powder can burn under some conditions, it is usually shipped to the customer with a particle size of ,400 μm, which is larger than desired for the production of the bonded magnet. Therefore, prior to blending with the resin, the powder is usually crushed to a smaller particle size using the same type of vibratory crusher shown and discussed in Fig. 4.27. The particle size of the crushed powder that is used for the bonded magnet can vary depending on the shape of the magnet being produced, with small parts and thinwalled rings typically using a powder with a smaller average particle size. Common practice, however, is to crush the powder to a particle size ,150 μm. The next step in the process is the mixing or blending of the bonding resin with the crushed magnetic powder, which is generally referred to as encapsulation. There are a number of different systems or methods that have been used to accomplish this (Doser and Floryan, 1976, US Patent 3,933,536; Yamashita et al., 1991, US Patent 4,981,635; Kawato and Tomioka, 1993, US Patent 5,256,326). The layout of one encapsulation system that gives consistently good results is shown in Fig. 5.2. A drawing showing the operation of this system is shown in Fig. 5.3. The main components consist of a mixing tank with a motor-driven tumbler, a pressurized epoxy solvent mixing tank, a vacuum pump, and sealable entrance and exit ports for loading the powder and removing the finished encapsulated product. To begin the process, a precise ratio of epoxy and solvent, usually acetone, is added and mixed in the epoxy solvent mixer tank and a precise quantity of magnetic powder is added to the encapsulation mixer tank. The encapsulation tank is then evacuated and back filled with argon. This is a precaution to prevent the possibility of a fire or explosion of the powder solvent mixture. The powder in the encapsulation tank is then gently tumbled while a timed amount of the epoxy solvent mixture is sprayed onto the powder. To prevent plugging of the ejector nozzles, the ratio of acetone to epoxy is kept .50%. The epoxy is typically a thermoset type with a cure temperature between 150 and 175 C. In some instances the crushed

Figure 5.2 Drawing of an encapsulation system used to epoxy coat NdFeB magnetic powder for the production of bonded Nd magnets. Source: Courtesy Richard Mulcavage.

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Figure 5.3 Drawing showing the operation of the epoxy encapsulation system.

powder is encapsulated by the powder manufacturer. If the cure temperature is too low, the epoxy can begin to cure in the shipping containers. The application of the spray takes place fairly slowly so that the epoxy is evenly distributed on the powder. Once the spray mixture has been applied for the proscribed amount of time, the vacuum pump is engaged and the solvent is drawn off as a vapor, leaving behind the dry encapsulated powder. The agitator continues to tumble the powder while the solvent is removed. The mixing tank usually has an outer water jacket through which hot water is circulated to speed up the removal of the solvent. Once the solvent has been completely removed, the powder is removed from the mixing tank into a powder tote. During the encapsulation process, there is some tendency for small clumps of powder to form, particularly from the powder having the smallest particle size. The final step in the process is to pass the encapsulated powder through a vibratory sieve similar to the one shown in Fig. 4.22, which breaks up these clumps as well as provides a powder with a consistent particle size distribution (PSD). The resulting product from a system of this type has been found to be uniformly coated and with good flowability. The amount of epoxy applied to the magnetic powder is typically 2.0 2.5 wt%. Many bonded Nd magnets are thin-walled rings that are used in spindle and stepper motors. To produce these rings, it is important for the powder to have good flow characteristics and quality assurance procedures include the measurement of flowability using a Carney flow cup, similar to the one shown in Fig. 4.30. The typical specification calls for 100 g of powder to flow through a 0.1 in. diameter orifice in the Carney flow cup in 12 15 seconds. Additional quality assurance procedures include the preparation of test samples, which are submitted to crush strength tests to confirm that the powder has been properly encapsulated.

5.1.3 The powder compaction process The next step in the production is the compaction of the epoxy encapsulated powder into a “green compact” having the desired magnet shape. This step is the most

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difficult and important part of the production process because virtually all bonded Nd magnets are produced to a net shape with high tolerance. There are a number of technical references that can be consulted that deal with the cold compaction of powder metals including Heckel (1960), Jones (1960), and Fleck (1995). Another source is the technical information provided in the Powder Metal Handbook, Hagonas Corporation (2013), and the several references on powder metal processing by the American Society of Metals (ASM), for example, Samuel and Newkirk (2015). While the subject of most of these sources is powder Fe, the techniques used to produce powdered Fe compacts are very similar to that for compacting NdFeB magnetic powder. Fig. 5.4 shows the press room of a typical bonded Nd factory. The presses that are used are standard powder metal compaction presses and can be either hydraulic or mechanical type. Fig. 5.5 shows a typical tool setup

Figure 5.4 Press room in a typical bonded Nd factory, which uses both mechanical and hydraulic presses to form the green compacts. Source: Courtesy IMT.

Figure 5.5 Typical tool setup for compression molding a bonded Nd ring magnets.

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for a ring magnet, which includes a tool steel die case with tungsten-carbide die insert. Tungsten carbide is required because of the extremely hard and abrasive nature of the magnetic powder. The insert is shrunk-fit into the die case by heating the die case and then inserting the carbide insert. After cooling, the insert becomes firmly affixed within the die case. The punches must also be tungsten carbide. The die is attached to a die platen, which can move relative to both the upper punch and the stationary lower punch. To produce a ring magnet, the press must have a core rod, which is also independently moveable. Because the remanence (Br) of the final magnet is proportional to the density achieved during the pressing operation, bonded Nd magnets are pressed at a very high pressure, typically 8 9 t/cm2 (800 900 MPa). To reduce friction between the powder and the die walls, some kind of lubricant is always used. One common lubricant is a mixture of fine Teflon powder mixed with isopropyl alcohol, which is automatically spayed or wiped onto the die wall between each part or several parts. The isopropyl alcohol evaporates rapidly leaving behind the inert Teflon as a lubricant. Because of the high pressures involved, friction between the die and the brittle NdFeB particles can grind the powder into a very fine dust, which has a tendency to lodge in the clearance between the die and the punches. To prevent or minimize this problem, the die/punch clearance must be extremely tight, generally 0.025 mm/mm. Even with this tight clearance, some powder tends to work its way into the gap over time. To produce a part with high precision, the tooling must be ground to a tolerance of 6 0.005 mm. Fig. 5.6 shows the pressing sequence for producing a bonded Nd magnet ring magnet. Fig. 5.6A shows the die being filled volumetrically using a standard powder feed shoe. Because there is very little lateral motion of the powder during compaction, it is critically important that the die be filled uniformly before the compaction cycle. If it is not filled uniformly, the final part will not have uniform density and not meet both magnetic and dimensional specifications for the part. In most instances,

Figure 5.6 The sequence of operations for compression molding of a bonded Nd ring magnet. (A) Filling, (B) compaction, and (C) ejection.

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the shoe is jogged back and forth or vibrated to facilitate filling the die. In some factories the portion of the shoe directly over the die is rotated as well as vibrated during the die fill operation. It goes without saying that a die with a narrow cross section will be more difficult to fill than one with a large cross section. Die fill is related to the particle size of the powder. It is a generally observed that the cross section of the die should be more than five to six times larger than the width of the largest powder fraction. For a powder whose largest fraction is 150 μm, the narrowest cross section should be no less than 1 mm. In practice, the wall thickness of most ring magnets are .1.5 mm. It is for this reason that the production of very small parts and thin-walled rings usually employs an encapsulated powder with a smaller particle size distribution. Fig. 5.6B shows the compaction step in which the feed shoe has been withdrawn and the upper punch lowered into position to compact the powder. To obtain the highest uniform density, the platen is lowered exactly one-half the distance that the upper punch travels during the compaction cycle. Using this technique, friction between the powder particles and the die wall during the compaction cycle is minimized and the highest and most uniform density is achieved. At the completion of the compaction step, the platen is again lowered until the finished green compact exits the die as shown in Fig. 5.6C. To achieve uniform magnetic properties, it is important that the green compacts have the proper weight and dimensions. In some factories, each compact is weighed and the weight recorded by a programmable controller (PLC). If the weight begins to vary from the specified weight, the PLC will shut down the press. Weighing of the magnet is usually carried out using a pick-and-place to lift the magnet onto the scale and from the scale to the pallet used for curing the epoxy. When the pallet is filled, it is either manually or automatically moved and loaded into an oven to cure the epoxy. For a typical bonded Nd pressing operation, the starting encapsulated powder has an apparent or tap density of about 2.6 g/cm3. Because magnetic remanence is directly proportional to density, compression-molded magnets are typically compacted at the highest practical compaction pressure. For the best grades of bonded Nd magnets, the powder is pressed to about 6.1 g/cm3 or about 79% of relative density, with the balance of the green compact consisting of the epoxy resin and some voids. An optical micrograph of the cross section of a compression-molded Nd magnet is shown in Fig. 5.7. The ribbon fragments are B30 μm thick and B1.5 mm width and, as would be expected, align with the width of the ribbon normal to the axis of the punch during compaction. This results in the closely packed microstructure that is observed in this image. However, to achieve 79% density still requires a very high pressure of nearly 8 9 tons/cm2. This is illustrated in Fig. 5.8, which displays the density versus compaction pressure for NdFeB-bonded magnets. Because the density that can be achieved is a function of the geometry of the green compact, these results are for a fairly simple round test magnet with a diameter of 10 mm and a thickness of 5 mm. To achieve this level of densification is quite difficult and can cause tooling problems. Higher pressure results in more rapid tool wear and the danger of breaking the tool. High pressure also results in more fine powder being pushed into the clearance between the die and punches, which results in more

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Figure 5.7 Optical micrograph of the cross section of a compression-molded Nd magnet showing the closely stacked fragments of melt-spun ribbon. 7 6

Density (g/cm3)

5 4 3 2 1 0 0

1

2

3

4

Compaction pressure

5

6

7

8

(tons/cm2)

Figure 5.8 Density versus compaction pressure curve for bonded Nd test magnets with a dimension of 10 mm diameter 3 5 mm height.

down-time to clean and refurbish the tooling. As might be expected, achieving uniform density becomes more pronounced as the height of the green compact is increased. For thin-walled rings, breakage from handling also increases dramatically. For these reasons, the height of ring magnets is usually limited to ,25 mm. During ejection, the friction between the green compact and the die wall can be so high that the pressure needed to eject the part is higher than the compaction pressure. This is why proper lubrication is so important. If the die is insufficiently lubricated, the green compact can actually weld itself to the die wall. Another consequence of the very high residual radial pressure is spring-back, sometimes referred to as elastic recovery. When the upper part of the die begins to protrude

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Figure 5.9 (A) Drawing showing the cracking that can occur in a green compact from spring-back unless the elastic energy is released gradually. (B) Drawing showing how a tapered die exit can.

from the top of the die, it will begin to expand to release the elastic pressure while the lower part of the part is still restrained by the die. The shear stresses in the part can lead to horizontal microcracks in the part, which are shown in Fig. 5.9A. In this drawing the size of the cracks are exaggerated for effect. One way to eliminate or reduce cracking is by releasing the stored elastic energy slowly, which can be accomplished by producing the die with a small taper at the opening and the edges broken with a chamfer, as shown in Fig. 5.9B. Spring-back is proportional to the pressure used to densify the part and is approximately 0.2% in compression-molded Nd magnets compacted at B8.0 tons/cm2. It goes without saying that the amount of spring-back experienced for a given part must be calculated into the dimension of the tooling to achieve the dimensional tolerance of the final part.

5.1.4 Curing the epoxy resin In a modern factory producing compression-bonded Nd magnets, the green compacts are pushed off the die with the powder filled shoe and then loaded onto pallets with a pick and place system. The primary reason for the pick-and-place is to reduce labor cost. However, the green compacts are extremely friable and can be easily broken. A properly designed pick and place systems can result in less overall breakage. Also, the green parts, particularly thin-walled rings, can distort if they are pushed together onto the pallet. A properly designed system can be easily programmed to position a variety of different-shaped parts in a variety of different positions on the pallet. The surface of the pallet must be flat because the shape green compacts, particularly thin-walled rings, can distort during the curing operation. Fig. 5.10 shows a photograph of a conventional conveyor type oven used to cure the epoxy. The system is typically heated to 175 C and the loaded pallets are conveyed through the oven using the chain conveyor. Air is circulated through the oven to maintain, to the extent possible, an even temperature distribution. Alternatively, the magnets can be placed on pallets and cured in a conventional box oven. Since there is a small loss in magnetic properties when cured in air, the box oven can be purged with nitrogen or even argon gas to prevent this small loss. The

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Figure 5.10 Photo of a conveyor type oven used to cure the epoxy in compression-bonded Nd magnets. Source: Courtesy IMT.

very best grades of bonded Nd magnets produced are sometimes cured in such a box oven-type operation. An important note here is that all resins or epoxies currently used to produce compression-molded bonded Nd magnets exhibit some expansion during their cure cycle. This is typically about 0.4%. This expansion combined with the spring-back or elastic release result in a total expansion of B0.6%, which must be accounted for when designing the tool, in order to achieve the desired dimensional tolerance in the finished part.

5.1.5 Deburring and cleaning The edges of the pressed and cured bonded magnet are usually quite sharp and usually have a small amount of flash, particularly if the tooling is worn from extended use. Prior to coating, the magnet must be rotofinished to remove these burrs and to ensure that any sharp edges are broken and rounded. Because of surface tension, any paint applied to the edges has a tendency to pull away, resulting in a bare magnet or a magnet with a thin coating. Fig. 5.11A shows a photograph of a rotofinish similar to one that is used for this purpose while Fig. 5.11B shows a depiction of the cross section of a compression-molded magnet before and after rotofinishing. The vibration of the rotofinish abrades away any flash and rounds the edges so that they will more readily accept a coating. The grinding media used is almost always stone. Rotofinishing is usually carried out wet, with a soap or surfactent to pull away the dust that is generated by the abrading action of the stone media. After rotofinishing the magnet must be carefully cleaned prior to coating. This is typically carried out in a multistage ultrsonic cleaner. A photo of a typical cleaner is shown in Fig. 5.12. A cleaner of this type has three or four stages into which baskets of rotofinished magnets are dipped. The movement of the baskets can be manual but is often carried our automatically. The first stage is washing with a soap of

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Figure 5.11 (A) Photograph of a rotofinish that is typically used to deburr the edges of compression-molded magnets after curing the epoxy and before coating with paint. (B) Drawing depicting the cross section of a compression-molded magnet before and after roto-finishing. Source: Courtesy IMT.

Figure 5.12 Photograph of a an ultrasonic cleaning system similar to the ones used to clean compression-molded Nd magnets prior to coating. The basket of magnets are moved automatically from one stage of the cleaning cycle to the next. Source: Courtesy Gulftech Enterprises.

surfactent added to the water. Following this washing, the basket of magnets are moved to the first rinse cycle, which is usually followed by a second rinse cycle. After the second rinse, the magnets are moved to a drying stage where the magnets are dried using warm filtered air. After carefully drying, the magnets are ready for the coating process. All of the separate stages have filtering systems to remove any particles that have been abraded from the magnet samples during the rotofinsihing process.

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Coating compression-molded Nd magnets

Bonded Nd magnets are prone of oxidation when exposed to moisture, particulary condensed moisture. Therefore, all compression-molded magnets must be coated before use. The paint also serves to seal the surface to prevent any possible loss of magnetic powder from the magnet. This is especially true for magnets used in spindle motors for hard disk drives (HDD), where any loose magnetic powder can severly interfere with the magnetic memory of the HDD. There are two primary methods used to coat these magnets. The most common is spray coating, usually with a phenolic thermoset epoxy. The second is E-coating, which electically applies a coating of paint while the part is dipped into a pool of the paint. In both methods a precise amount or layer of paint must be applied because an excessive layer of paint can intefere with the air gap in the motors that are the end use for many of these magnets. If too thin layer of paint is applied, the magnets will likely fail the rigorous paint-coating tests that are applied to these magnets to ensure that they do not fail in the field. Small compression-molded magnets also must be coated but present a special problems. Several techniques for coating very small magnets are also discussed in this section.

5.2.1 Spray coating Although conceptually simple, spray coating of compression-molded Nd magnets can be technically challenging. This is because there is always a tight specification on the amount or thickness of the coating and because the entire surface, especially any sharp edges, must be adequately coated. The most common way that spray coating is carried out is by first deburring and cleaning the parts as described earlier. The parts are then positioned on screen-type pallets, which are then fed through an automatic spray booth, which applies a precise amount of the coating. A screen pallet is used so that much of the paint that does not strike the parts can pass through into the water tank at the bottom of the paint booth. The invividual magnets must be set far enough apart so that do not shadow each other during the painting operation. A photgraph of a typical spray booth is shown in Fig. 5.13A and a photgraph of a variety of finished coated magnets is shown in Fig. 5.13B. The pallets are moved through the paint booth on a chain-driven conveyor and the amount of paint applied is set by adjusting the paint guns to discharge a specific amount of paint in a given time and using a precisely time speed for conveying the parts through the paint booth. A drawing showing the position of the rotating paint sprayers relative to the conveyor and the pallets is shown in Fig. 5.14. There are a number of configurations that are used including rotatation or oscillaton or a combination of both. As might be expeccted, the difficulty in painting the parts is ensuring that the sides of the magnets are completely and evenly coated during two passes through the paint booth. This requires that the incident angle of the paint strikes the parts be within certain limits. This is particularly true for tall ring magnets, where it is particularly difficult to completely coat the inner diameter of the

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Figure 5.13 (A) Photograph of a conveyor paint booth used to paint compression-bonded Nd magnets. (B) Photograph of various ring magnets after spray painting with a phenolic epoxy paint. Source: Courtesy IMT.

Figure 5.14 Drawing showing the operation of a paint booth as the pallets containing the bonded Nd magnets are conveyed through the paint booth. This paint booth has a water-wash system to remove paint particles from the exiting air stream.

ring. After the painting operation, the paint is cured by conveying the pallets through a conventional oven similar to the one used to cure the epoxy. However, for paint curing, the temperature at the inlet of the oven is set at a lower temperture to allow the solvent in the paint to flash-off or be removed more slowly. Rapid removal of the solvent can result in bubbling of the paint and poor paint quality. Because a single pass will not coat the bottom of the part, it is necessary to cure or nearly cure the paint, invert the parts onto a second pallet and then pass the parts again through the paint booth. Follow this second pass, the parts are again run through the cure oven to fully cure the paint.

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One of the technical challenges of painting-bonded magnets is the management of the air entering and exiting the paint booth. Because any dust particles adhering to the painted part can degrade the quality of the coating, it is very important that the amount and size of dust particles in the air in the paint room and paint booth be kept to an absolute minimum. This is particulary true of any airborn particles of magnetic powder, which can have a particularly deleterious effect on the quality of the coating. The air in the cure oven must also be free of airborn dust because particle of dust can attach to the painted parts before the paint is cured. For this reason, the air entering the paint room and the paint booth must be carefully filtered, typically through high efficiency particulate arrestance (HEPA) filters. For factories producing bonded Nd magnets for HDD, the painting operation is extrememly critical and is usually carried out at a location some distance from the pressing operation to minimize the possibility of magnetic dust in the air. It has been found that removing all magnetic dust from the air in a plant with encapsulation and pressing operations is almost an impossible task. For enviremntal and maintanance reasons, the air exiting the plant must also be free of paint particles. While dry filters can be used to remove the paint, this option is impractical for a plant in continuous, highvolume production, since the filters would result in both a high cost and serious maintainancce issue. Although the initial capital investment is higher, removal of the paint particles from the exiting air stream is typically accomplished by means of a water-wash paint booth, which uses a curtain of water to capture paint particles and deposit them in a sludge or waste reservoir. While there are a number of different designs that are used, the most common is shown in the drawing in Fig. 5.14. As displayed here, the paint booth employs a wall of running water on either side (only the right side is shown) of the conveyor, which collects the overspray and moves it to a water tank in the bottom of the spray booth. The waste paint is periodically skimmed and discarded. As demonstated, much of the overspray is immediately captured by the layer of water on either side of the conveyor. However, in this spray booth design, paint particles that are not captured here are drawn up into a centrifugal wash separator, which contains a series of water sprayers. Centrifugal force on the paint particles, brought about by the rapid change in the direction of the air, causes the particles to leave the air stream and enter the water sprays. The air then exits the centrifugal wash separator and passes down and through a water curtain at the bottom of the assembly. This curtain should remove any remainging paint particles before it exits the plant through an exhaust fan. In this way, paint particles are almost completely removed from the air before exiting the plant. The paint solvent exiting the plant can also be an environmental problem, which is alleviated by burning the organic solvent into a mixture of CO2 and water.

5.2.2 The E-coating process E-coating is also a common method used to coat compression-bonded Nd magnets. This process involves dipping electically charged magnets into an aqueous bathcontaining particles of an organic paint that have an opposite charge. During the

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coating process, the electrically charged particles in a conductive media migrate to the elctrode bearing the opposite charge under the influence of a DC voltage and are deposited on the conducting part. Coating can be either cathodic or anodic, depending on the charge on the part being coated. However, most magnet E-coating is cathodic in nature, with a positive charge on the magnet and the paint particles having a negative charge. The principle advatages of E-coating are that you get a durable, uniform coating over the entire part, even in difficult, hard to reach places that might be difficult with spray coating. Another advantage is that the coating thickness can be easily controlled by adjusting the current. As the coating becomes thicker, it becomes nonconducting and the coating stops. There is also better edge coverage than with spray painting because the paint is deposited electricly and is less subject to the effect of surface tension. The process can also be made highly automatic and there is a 95% of better utilization of the paint. This is in sharp contrast to spray painting, which is more labor intensive and there is significant overspray and loss of paint which, depending on the size of the parts, can be over 50%. Since the paint is water-based and nontoxic, there are less environmental considerations. However, the process still requires carefully filtered air to prevent dust particles from attaching themselves onto the surface of the parts between process steps. As with spray painting, this is particlularly true of dust from magnetic powder, which is always prevalent in bonded Nd factories. The disadvantages of E-coating are that the investment is higher than for spray coating and is only feasible for high-volume production. The process also typically takes more floor space than spray coating, especially for fully automated coveyor systems. The process is also considered to be more technically challenging because of the need to maintain a charge on the parts and the need to maintain an alkaline pH in the bath. Because hydrogen gas is liberated at the cathode, the ionic equilibrium around the part is constantly trying to change. If the bath pH becomes too alkaline, the process will stop coating properly. A flow diagram of the E-coating process is shown in Fig. 5.15. This drawing shows a continuous process in which the parts are carried from one process step to the next with a conveyor: the drawing is for illustration only and the scale is not intended to be accurate. Alternatively, Ecoating can be carried out as a batch process in which the magnets are attached or racked on tree-like assemblies and moved manually from one operation to the next. There are a number of steps in the process which can be seen by referring to Fig. 5.15. These steps include: Racking: Because the parts have to be elctrically charged, they must first be attached to a electical contacts, which are either part of the conveyor system or, when using a batch process, a tree-like array. Racking and unracking of magnet parts are almost always done manually and constitute the most labor intensive part of any magnet coating operation. Fig. 5.16A shows a drawing of the electical attachment similar to that used to rack magnets for E-coating and Fig. 5.16B shows a photograph of an array of arc magnets that have been coated using a batch process. The spring-loaded electical contacts can be seen at either end of the magnet. One problem with E-coating is that the electrical contact points on the parts must

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Figure 5.15 E-coating process used to coat compression-molded Nd magnets.

Figure 5.16 (A) Electrical contact used to E-coat-bonded Nd ring magnets. (B) Photograph of arc-shaped magnets that have been racked for batch E-coating. Source: Courtesy JBM Magnetics.

be touched up with paint and a small brush after the parts have been cured. This is also a labor intensive process. Pretreatment cleaning: This step is to produce a magnet substrate suitable for receiving the the coating and typically involves cleaning or dipping the magnets in

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an aqueous alkaline solution. As with spray painting, the magnets should be free of any dust particles, particulary magnetic particles. After the pretreatment, the parts are rinsed several times to remove any trace of the alkline solution and then allowed to drain and dry. Electrocoating: After the pretreatment and cleaning, the parts are conveyed or carried into the E-coat tank. Once immersed, the negative charge on the paint particle draws the paint to the part. The thickness of the coating can be controlled by the amount of voltage on the part and the process stops when the layer of paint reaches certain thickness and is not longer able to conduct electricity. The coating process is typically quite rapid, usually 1 2 minutes. Rinsing: After coating, the parts are taken through a series of rinses to remove any drag-out paint from the E-coat tank. This can be either by water spray or dipping. In many high-volume painting systems, the drag-off paint is separated from the rinse water by filtration and fed back into the paint tank. Theoretically, the process can be a closed loop system with almost no loss of paint. However, this operation does add more technical sophistication to the process and more chance of problems. Paint cure: After rinsing, the parts are conveyed through or placed in a oven to cure the paint. Any rinse water should be allowed to drain off prior to entering the cure oven. Curing of the paint usually takes place at a temperature of approximately 150 C for approximately 30 minutes. After the paint is cured, the contact points must be touched up so that the entire surface of the magnet is coating. This is done manually and constitutes another labor intensive operation in the production of bonded Nd magnets. Following the touch-up operation, the magnets are subjected to the quality assurance procedures discussed in the following sections.

5.2.3 Coating small magnets There are a large number of very small compression-molded Nd magnets produced, which also need to be coated. These magnets present a special coating problem because placing the magnets on pallets for spray coating or racking the magnets for E-coating is too labor intensive. Moreover, during a conventional spray-coating operation, the parts are frequently blown about by the air flowing through the paint booth or curing oven. However, there are a number of specialized techniques that are used to coat these small parts. Barrel spray coating: Coating small parts is often accomplished by a process called barrel or tumbler coating. This process involves loading a mixture of the parts and plastic beads, typically nylon beads, into a devise that looks somewhat similar to a small cement mixer. The mixture is then slowly tumbled while a fast cure epoxy paint is periodically sprayed onto the mixture. A typical cycle might include 5 10 seconds of spray followed by 30 seconds of drying. The tumbling is continuous. A depiction of this process is shown in Fig. 5.17. Filtered air, often warm filtered air, is blown into the interior of the barrel and removed through an exhaust outlet to carry away the solvent used in the paint. The air is usually only slightly warmed (50 70 C) to prevent the paint from drying too fast. At the

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Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets

Figure 5.17 Barrel or tumbler coater used to spray paint small-bonded Nd parts.

completion of the spray-painting cycle, the paint sprayer and air tubes are removed and the mixture poured from the tumbler and into a screen to separate the parts from the plastic beads. Fluidized bed coating: Although less common, fluidized bed coating can also be used to coat small-bonded Nd magnets. This techniques involved suspending and circulating the magnets in a stream of air while they are simultaneously sprayed with a coating. The most common form of this technique is the bottom spray Wurster process, which is shown in the drawing in Fig. 5.18. Air is blown through an array of holes in the the bottom plate to suspend the magnets while paint is sprayed from a nozzle at the center of the plate. To provide circulation, the air holes are larger around the spray head and around the outer circumferance of the bottom plate. There is also an inner cylinder, which produces a venturi effect in which the pressure and velocity of the air is dramaticlly reduced upon leaving the inner cylinder. This causes the solvent in the paint to rapidly evaporate. The highly organized circular air flow draws the dried magnets back into the bottom of the cylinder, where they are given a second coating. The process produces a smooth, highly uniform coating. The thickness of the coating is controlled by adjusting the paint spray level and by application time. This process is most commonly used in the pharmaceutical industry to coat pills, paticularly time-release medicines. Parylene or vapor coating: Small-bonded Nd magnets can also be vapor coated. The coating applied is typically parylene which, in the applied state, is an organic compound with a polymerized polycrystalline chain structure. The coating is applied by a process normally referred to as vapor deposition polymerization and is carried out in a vacum system. The steps in the process are shown in the drawing in Fig. 5.19A and the chemical change that occurs in the organic molecule is shown in Fig. 5.19B. The first step is to heat di-para-xlylene, usually referred to as the dimer because it has a double molecular structure, in a vaporizer, which operates at B150 C. The vaporized dimer then passed into a pyrolizer, where it is heated to B680 C. At this temperature the double molecule dimer breaks down

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Figure 5.18 Drawing showing the operation of a fuidized bed-coating system that can be used to coat small-bonded Nd magnets.

2) Pyrolize 680°C

3) Vaporize 150°C

CH2

1) Deposit 25°C

Pump

CH2 2 CH2

CH2

Cold trap

CH2

CH2

CH2

CH2

Di-para-xylylene (dimer)

Di-para-xylylene (monomer)

Poly-para-xylylene (polymer)

Figure 5.19 (A) Process flow diagram of a parylene coating system. (B) The chemical change in the di-para-xylylene during the coating process.

into a single molecule. This monomer vapor flows into the deposition chamber, where it deposits and is polymerized onto the magnets. The process does not involve line-of-sight coating and all exposed surfaces are uniformly impinged and coated with the parylene. However, most parylene-coating systems still include the

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Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets

means by which the parts are loaded onto trays, which are rotated. The parylene coating is very smooth, tough and pin-hole free and has a very low permeabilty to moisture. It is also chemically inert and with good dielectic properties. The rate of depostion is normally about 0.2 miles/hour and thickness can range from ,1 μm to .25 μm. As shown in Fig. 5.19A, the coating system necessarily includes a mechanical vacuum pump, preceeded by a cold trap to trap and prevent any of the monomer from entering the pump.

5.3

Quality control procedures for bonded Nd magnets

All compression-molded Nd magnets are given a number of quality assurance tests to ensure that all aspects of the magnet conform to specification. These checks typically include: Dimensional check: Foremost among the requirement of the finished coated magnet is that its dimensions conform to specification. The specifications can be very strict, particularly for thin-walled ring magnets used in micromotors. Dimensional checks are usually carried out today using one of the high-quality noncontact laser surface profilers that are readily available and can accurately measure a 2D or 3D surface with an accuracy of 0.01 μm or better. A photograph of one such instrument is shown in Fig. 5.20A. Acid test: This test is carried out by immersing a specified number of magnets into an aqueous solution containing 15 vol% HCl acid. Any pin-holes in the coating will almost immediately reveal themselves by the presence of bubbles on the surface of the magnet or a stream of bubbles resulting from reaction of the acid with the underlying NdFeB magnetic powder. Crush strength: The crush strength of bonded Nd magnets is determined by placing the magnet in a standard crush strength tester, such as the one shown in Fig. 5.20B. The crush strength, in grams of force, is measured on a dial as the arm is brought down to crush the magnet. This test is carried out after the epoxy resin

Figure 5.20 (A) A noncontact laser profile projector that is used to measure dimentions of bonded Nd magnet. (B) Instrument used to test the crush strenth of bonded Nd magnets. Source: (A) Courtesy IMT and (B) courtesy Imada Corporation.

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cure cycle to ensure that the epoxy has been completely cured. The crush strengh must pass a certain set value for a given part or geometry. Paint hardness and adhesion test: Paint hardness is usually tested by the pencil hardness test. Special pencils with various hardness from soft to very hard are pushed firmly against the paint at a 45-degree angle. The test starts with the softest tip and progressively moves to the harder pencils. The hardness of the paint is determined by the pencil that scrates through the paint surface. Paint adhesion is usually tested using the cross hatch adhesion test, which measures the resistance of a coating to separation from the substrate. The test is carried out by using a carbide blade to cut an X or a right angle lattice pattern into the coating, cutting all the way to the underlying magnet substrate. Pressure sensitive tape is then applied over the cut and pressed firmly in place. The tape is then rapidly removed by pealing back at a 180-degree angle. The adhesion of the coating is graded based on the percent of the paint surface that is removed with the tape. Solvent resistant test: This test is carried out to determine the resistance of the cured paint to attack by a harsh solvent. The solvent used is usually methyl-ethylketone, which is applied to cheese cloth and rubbed onto the surface of the coating. After a specified number of rubs, there should be no discoloration of the cloth indicating that paint has been disolved. If paint has been removed by this test, then the paint is not suitable for the application or has not been adequately cured. Humidity and salt corrosion test: There are two standard tests carried out on compression-molded magnets to test the quality of the coating. These include temperatrure tests and salt corrosion tests. Humidity tests are carried out in a chamber in which both the humidity and temperature can be varied. For bonded magnets the test conditions are typically set at 85% humidity and 85 C. The length of the test can vary depending on customer requirements, and can range from 100 to 1000 hours. At the completion of the test, the surface of the magnets are then examined with a microscope to check for any signs of blistering or corrosion. For magnets that have been properly coated, 100% should pass this humidity test after 100 hours of exposure. Another common test of the coating is the salt spray test, which is carried out by placing a specified number of magnets in a cabinet in which a 5% NaCl solution is atomized and directed onto the magnets. Technically, the agent is a fog rather than a spray. The cabinet temperarure is usually set at 35 C. The length of the test is usually 24 48 hours after which the parts are examined with a microscope to check for any blisters or other signs of corrosion. For well coated, bonded Nd magnets, 100% of the parts should pass this test without any visible sign of corrosion.

5.4

Properties of compression-molded Nd magnets

5.4.1 Magnetic properties of compression-molded Nd magnets The composition and properties of the various commercial grades of NdFeB magnetic powder that are currently available were discussed in Chapter 4, Production of rapidly solidified NdFeB magnetic powder (see, e.g., Tables 4.3 and 4.4). The properties of compression-molded Nd magnets produced from these powders are tabulated in Table 5.1. The properties shown are representative of magnets compressed to a

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Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets

Properties of compression-molded Nd permanent magnets with an average metal-loading factor of 79% and a density of B6.0 g/cm3

Table 5.1

metal-loading factor of B79% or a density at or close to 6.0 g/cm3, compared to a theoretical density of B7.6 g/cm3 for the NdFeB alloy. The balance of the volume in these magnets would include B12 vol% (B2.25 wt%) epoxy resin and B9% voids. An optical micrograph of a compression-molded magnet was shown in Fig. 5.7, where the voids that remain in these bonded Nd magnet, particularly between two the ends of two ribbon fragments, are evident. As would be expected, the properties tabulated here, particularly the remanence (Br) and energy product (BHmax), are highly dependent on the density and, therefore, the compaction pressure. The intrinsic (Hci) and inductive coercivity (Hc) are less dependent on density.

5.4.2 Temperature-dependent properties of compression-molded Nd magnets Engineers who design products using bonded Nd magnets must know how the magnetic properties change after both short- and long-term exposure to temperature. The unique properties of melt-spun NdFeB magnetic powders result from the formation of a very finely crystalline, two-phase microstructure, consisting of a major Nd2Fe14B intermetallic phase and a minor Nd-rich intergranular phase. It is the finely crystalline microstructure that gives these materials their unique-bonded magnet properties, particularly their thermal aging properties. Crushed melt-spun powder typically used to produce bonded magnets has a particle size of B100 mesh or B150 μm. However, the average diameter of the Nd2Fe14B crystallites in meltspun NdFeB powder is around 30 nm, or 0.03 μm. This combination of relatively coarse particles combined with a finely crystalline microstructure imparts high stability to the bonded Nd magnets. It is believed that any surface of NdFeB alloy

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exposed to oxygen and moisture will form an oxide layer, which destroys the local hard magnetic properties. This is the reason that the properties of bonded magnets produced by grinding a cast ingot into a fine powder rapidly deteriorate. If we assume a square ribbon flake having major dimensions of 150 μm and a thickness of 30 μm, this single flake would contain B3 billion separate Nd2Fe14B crystallites. If only the outer shell of the flake that is exposed to oxygen becomes magnetically soft, then only a relatively small volume of the grain is affected. As the particle size of the flake is reduced, the loss in magnetic properties would be expected to become more pronounced and this is what is observed experimentally. In contrast, powder produced from a cast ingot, with much larger Nd2Fe14B grains, would have a much higher volume of the grain exposed to ambient conditions and would, consequently, experience a much greater loss in magnetic properties. Again, this is what is observed experimentally. This is particularly true for the temperature stability of the bonded magnets. This is the reason that only a fine-grain material such as melt-spun NdFeB can be used to produce bonded Nd permanent magnets. The same would apply to Nd-Fe-B powder produced by the HDDR process. Although in this case, the crystallite size is somewhat larger (B0.3 0.4 μm) and the temperature stability is in general lower than for bonded Nd produced from melt-spun ribbon. Figs. 5.21 and 5.22 show demagnetization curves versus temperature for standard B and A grade powders, respectively. The data shown here include the change in both M and B (M 5 B H) with temperature. The change in Br is directly attributable to the change in the intrinsic magnetization with temperature that occurs in the Nd2Fe14B intermetallic compounds and which was discussed in Chapter 2, The Nd2Fe14B intermetallic compound (see Table 2.2 and Fig. 2.26). –0.5

–1.0

–2.0 8 7

5 4 25°C

3

75°C 125°C 50°C

–10

–9

–8

–7

2

100°C

–6

B or M (kG)

6

1

–5 –4 H (kOe)

–3

–2

–1

0

0

Figure 5.21 Demagnetization curves of compression-molded B grade powder as a function of temperature.

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Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets

–0.5

–1.0

–2.0 8.0 7.0

5.0 4.0 25°C

3.0

B or M (kG)

6.0

75°C 125°C

2.0

50°C 100°C

–16

–14

–12

–10

1.0 –8 H (kOe)

–6

–4

–2

0

0.0

Figure 5.22 Demagnetization curves of compression-molded A grade powder as a function of temperature.

The change in Hci with temperature, however, is more complicated. While it is well known that the origin of the coercivity is the magnetocrystalline anisotropy of this intermetallic phase, it is typically observed that Hci values in these materials are ,20% of the measured magnetocrystalline anisotropy (again see Table 2.2). This is because the coercivity of R-TM permanent magnets is always highly dependent on microstructure, which is usually a mixture or combination of several phases. From the curves shown in Figs. 5.21 and 5.22, the magnetic flux available at various temperatures can be calculated for a specific magnetic circuit. These curves also provide the temperature coefficient of Br and Hci that are also tabulated in Table 5.1. These two coefficients, best known as α and β, define the percentage change in Br and Hci with each degree change in temperature, respectively. The two powders whose demagnetization properties are shown in Figs. 5.21 and 5.22 are representative of the two major powder types, those with high Br and low Hci, including B, B1 , E, and E1 grade powders and those with lower Br but higher Hci, which include A, F, and O grade powders. This change in magnetic properties is a result of the variation in the Nd:Fe ratio of the alloys as discussed in Chapter 3, The properties of melt-spun NdFeB alloys (see, e.g., Fig. 3.8). A higher ratio results in lower volume % of the majority Nd2Fe14B intermetallic phase and a higher volume of the Nd-rich intergranular phase and, correspondingly, higher coercivity and lower Br. In contrast, a lower ratio results in the opposite phase distribution with a corresponding increase in Br and a lower Hci. In general, these two types of powders have quite different uses. B-type powders are most often used in smaller multipole brushless motors, where magnetization is difficult if the coercivity is too high. In contrast, A-type powders are usually employed for brush-type motors, where

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reverse fields and operating temperatures are higher. These high Hci powders are often used in automotive applications. Permeance coefficient: Included with the demagnetization curves shown in Figs. 5.21 and 5.22 are several load lines or permeance coefficients (Pc), which are the straight lines radiating out from the origin and intersecting points on the B demagnetization curve: the slope of these lines is equal to B/H and equal to the recoil permeability of the magnet. These intersection points are referred to as the operating point of the magnet. After magnetization, any magnets or magnetic circuit will operate at some point on the demagnetization curve, depending on its shape and thickness. The permeance coefficient is calculated using the geometry of the magnet. In general, a thicker magnet will operate higher on the demagnetization curve, for example, at a permeance coefficient of 22.0. As the magnet becomes thinner, its operating point will fall farther down the demagnetization curve, for example, at a permeance coefficient of 20.5. As an example, the permeance coefficient of a long cylindrical magnet is much higher than that of a coin-shaped magnet of the same diameter. The reason for this is that the demagnetization field of the long cylinder is much lower than the coin-shaped magnet. If the magnet is too thin, the magnet can actually self demagnetize. In an anisotropic magnet, this demagnetization can be easily explained as the operating point of the magnet exceeds the knee of the demagnetization curve and the magnetic dipole of certain grains start to reverse into the opposite direction. In isotropic-bonded Nd magnets, however, the concept of a knee is not well defined but the demagnetization process would be the same. As the aspect ratio of length to diameter is reduced, the magnet operates at a lower operating point and is more susceptible to demagnetization. The permeance coefficient is mathematically equal to Bd/Hd, where Bd is the intersection of a line running from the operating point to the B-axis or vertical axis and Hd is the intersection of a line running from the operating point to the H-axis or horizontal axis. Knowing the magnet type and the geometry of the magnetic circuit, the design engineer can calculate the load line or the operating point of the magnet. Knowing this, the values of Bd and Hc can be determined, which gives the amount of flux that will be available in a magnetic circuit. This flux is proportional to the amount of work that can be done by the magnet, keeping in mind that no work can be done except for the flux leaving the magnet.

5.4.3 Long-term thermal-aging properties In addition to the short-term changes in magnetic properties, represented by the demagnetization curves displayed in Figs. 5.21 and 5.22, magnet users also need to know how these curves change with long-term exposure to temperature. This is the thermal stability of the magnet and usually expressed as the percent loss in flux over a certain time period at a certain temperature. There are three types of losses that can occur as a consequence of exposure to temperature. These are: Reversible loss: The loss that occurs when a magnet is taken from room temperature to some elevated temperature but is recovered when the magnet is returned to room temperature. For NdFeB permanent magnets, this loss is simply the loss that

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Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets

results from the drop in magnetic properties with temperature that is shown in Figs. 5.21 and 5.22. Irreversible loss: The loss that occurs when a magnet is taken from room temperature to some elevated temperature for some period of time, but is not recovered when the magnet is returned to room temperature. This loss is sometimes referred to as recoverable-irreversible loss and results from partial demagnetization of the magnet by exposure to too high a temperature. This type of loss is recoverable by remagnetizing the magnet. The microstructure of these melt-spun materials has been shown to consist of very finely crystalline, isotropic grains of the Nd2Fe14B intermetallic phase. In the magnetized state, the moment lies along the easy axis direction most closely aligned with the direction of applied magnetic field. The magnetization and demagnetization process in these materials was discussed in Chapter 3, The properties of melt-spun NdFeB alloys (see Fig. 3.28). When the magnet is taken to an elevated temperature, some of these moments flip or reverse into the opposite direction. The reversal or demagnetization is easier because the coercivity of the material drops as the temperature is increased. While this type of loss can be recovered by remagnetizing, this is not a practical solution since it is generally not possible to remagnetize a magnet once it has been assembled into a component. Therefore, this type of loss must be factored into the design of the application. Structural loss: Irreversible losses that cannot be recovered by remagnetization are structural losses, which are permanent losses that result from degradation of the magnetic material, usually from excessive operating temperature, or corrosion resulting from improper coating. In NdFeB magnets, this is usually associated with slow conversion of Nd metal to Nd2O3 or Fe to Fe2O3 by reaction with oxygen and moisture. This reaction can be greatly accelerated by the presence of NaCl or other active metal salt, but the reaction products would be the same. Excessive structural loss will eventually lead to failure of the motor and application. These various losses are summarized in Fig. 5.23, which shows the change in the second quadrant demagnetization behavior as a hypothetical magnet is taken from 25 to 125 C for an extended period of time and then returned to 25 C. For greater clarity, only a portion of the second quadrant is shown. After the temperature excursion, the demagnetization curve of this magnet does not return to the original 25 C curve but rather to the dotted line. After remagnetization, the demagnetization behavior is the second dotted line. The difference between the two solid lines is the total loss, while the difference between the lower solid line at 125 C and the lower dotted line is the reversible loss, since this loss is recoverable when the magnet is returned to room temperature. After remagnetizing, the demagnetization behavior of the sample is the upper dotted line. The difference between the dotted line and the lower 125 C line is the irreversible loss, since this loss is recoverable by remagnetizing. In turn, the difference between the dotted line and the original 25 C data is the structural or permanent loss, since it was not recoverable by remagnetization. The amount of the various losses have been exaggerated for effect. Structural loss in compression-molded Nd magnets would normally represent only a small fraction of the total loss except for very long-term exposure at 125 C or higher. After a single short-term thermal excursion, the total loss would be expected to be almost entirely

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8 Ireversible loss

Structural loss

25°C 6

M (kG)

7

Total loss

5

Reversible loss 125°C

4 –4

–3

–2 H (kOe)

–1

0

Figure 5.23 The thermal losses that occur in a bonded Nd magnet when taken to an elevated temperature for some period of time and then returned to room temperature.

due to thermal demagnetization and, therefore, almost completely recoverable by remagnetization. As mentioned, the small amount of structural or permanent loss would most likely be due to some chemical change in the composition of the magnet, typically the conversion of Nd metal to Nd2O3 and Fe to Fe2O3. The reactivity of the NdFeB material becomes significantly more reactive as the temperature is increased. This is particularly true if moisture is present. It has been observed that the melt-spun powder is actually very stable in a pure dry oxygen atmosphere but becomes more reactive at elevated temperatures when a small amount of moisture is introduced. One common misperception is that the resin in compression or injectionmolded Nd magnets imparts some additional protection for the magnetic powder. However, this is not the case. Tests have shown that the bare magnetic powder is more stable against thermal losses than the bonded magnets under normal ambient conditions. However, this is not the case where condensed moisture or salt is present. In these cases, a coating is necessary to protect the magnetic powder. Because it is impractical to remagnetize a magnet once it is has been installed in an product, all magnet producers provide the total flux loss of the magnet after heat aging for a specified number of hours, usually 1000 hours or more. From this, the loss that would be expected to occur over the life of the product can be predicted. Figs. 5.24 and 5.25 display total flux loss at 100 and 125 C for bonded Nd magnets produced from various grades of NdFeB magnetic powder after thermal aging for 1000 hours. These data were taken from cylindrical-shaped magnets with a B/H 5 22. The aging data of the B, B1 , and G grade powders are very similar and are represented by a single curve. The same is true for E and E1 grade powders. While some of the losses shown in Figs. 5.24 and 5.25 would be structural loss, most would be irreversible loss due to thermal demagnetization. Therefore, it

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Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets

0

Time (hours) 400 600

200

800

1000

0 A –2 Total flux loss (%)

O B. B+. G –4 E. E+ –6 110°C B/H = –2 –8

–10

Figure 5.24 Total flux loss (%) at 100 C versus time for various compression-molded Nd produced from different grades of NdFeB magnetic powder.

0

Time (hours) 400 600

200

800

1000

0 F

Total flux loss (%)

–2 O –4 A

125°C B/H = –2

–6 B, B+, G –8

E. E+

–10

Figure 5.25 Total flux loss (%) at 125 C versus time for compression-molded Nd magnets produced from various grades of NdFeB magnetic powder.

is not surprising that these losses correlate with the intrinsic coercivity of the material. Fig. 5.26 plots the total flux loss (%) for bonded magnets prepared from the various grades of powder after aging for 1000 hours at 250 C versus the typical intrinsic coercivity found for each grade of material (see Table 5.1). There is a clear correlation between these values for the B, B1 , E, E1 , G, and O powders. The Hci values for the compression-bonded magnets produced from B, B1 , and G

Production and properties of bonded Nd magnets

5

7

209

Hci (kOe) 11

9

13

15

17

0

Total flux loss (%)

–2 125°C 1000 hours

F

–4

–6

O

A

G –8

E, E+ B, B+

–10

Figure 5.26 Total aging loss (%) versus intrinsic coercivity (Hci) for various grades of magnetic powder.

powders average 9.8 kOe and their average total loss is closely grouped at B7.5% after 1000 hours at 125 C. The corresponding Hci values for the E, E1 , and G magnets average 7.8 kOe and their total losses are also quite similar at B8.5% after 1000 hours at 125 C. The data for the A, F, and O powders are less clear, with the data for both the A and F powders well outside the trend line. The Hci values for the A, F, and O powders are 15.3, 12.3, and 12.5 kOe. The thermal aging loss for the A grade compression-molded magnets is higher than would be expected based on its comparatively high intrinsic coercivity of 15.3 kOe, probably due to a higher component of structural loss. The composition of each powder type is listed in Table 4.4, and shows that A grade powder has a rare earth content of 28.8 wt%, the highest of any of the powders types. Since structural loss in these materials is believed due to oxidation of the Nd-rich intergranular phase, it is possible that the higher overall rare earth composition makes the A grade powder more susceptible to this type of chemical change. In contrast, the total flux loss of the F powder (14 12) is ,2% after 1000 hours at 125 C, much lower than would be expected from its intrinsic coercivity of 12.3 kOe. This is believed due to the composition of this grade of magnetic powder, which includes 1.3 wt% niobium. This grade of magnetic was developed by Magnequench for use in high temperature, underhood automotive applications. Niobium is among a group of refractory metal additives that are sometimes referred to as “grain growth inhibitors.” It is believed that the niobium retards the growth of large grains and results in a finer, more uniform microstructure across the thickness of the melt-spun ribbon. In a comparative TEM study, Chen et al. (2004) found that melt-spun Nd12Fe80.5B6Nb1.5 ribbon did have a substantially finer average grain size than melt-spun Nd12Fe82B6 ribbon prepared

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Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets

under the same conditions. They reported that the melt-spinning and subsequent annealing processes expel the Nb from the Nd2Fe14B phase into the grain boundaries, where if form a Nb-rich phase with inhibits grain growth. Because irreversible losses due to demagnetization are greater if the demagnetizing field of the magnet is higher, the aging losses are sensitive to the shape of the magnet, which is represented by B/H, the permeance coefficient of the magnet. Therefore, the load line of the magnet must be included with the data. Fig. 5.27 compares the total flux loss of compression-molded B magnets, one with B/H 5 22 and one with B/H 5 25. As would be expected, the total flux loss for the magnet with B/H 5 22 is higher because its higher internal demagnetization field leads to more thermal demagnetization. Thermal aging test is typically carried out by first completely magnetizing the test samples in a magnetic field of .4 T. Magnetization is carried out using the same type of fixture that was shown in Fig. 4.14. However, in these samples, the magnetization direction is parallel to the axis of the magnetization coil as opposed to the perpendicular magnetization used for the VSM tests. Therefore, the sample is loaded into the center of the air-core solenoid from the top of the fixture. The full second quadrant demagnetization curve is then measured at room temperature using a standard hysteresis graph. The samples are then remagnetized. The flux of the samples are then measured using a Helmholtz coil attached to an integrated voltmeter. In this test the samples are positioned in the center of the Helmholtz coil and then smoothly extracted on a slide attachment. As the samples are extracted, the voltmeter measures the integrated current produced as the magnetic field of the test magnets cut the copper coils of the Helmholtz coil. A photo of a Helmholtz coil similar to those used in these tests is shown in Fig. 5.28A. An alternative procedure is to use the simple drop coil test shown in Fig. 5.28B, in which an

0

200

Time (hours) 400 600

800

1000

0

Total flux loss (%)

–2 125°C –4 B/H = –5 –6

–8

B/H = –2

–10

Figure 5.27 Total flux loss for compression-molded magnet produced from B grade powder with B/H 5 22 and B/H 5 25 when thermally aged at 125 C for up to 1000 hours.

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Figure 5.28 (A) Photograph of a Helmholtz coil test system similar to those commonly used to measure thermal flux loss in bonded Nd magnets. (B). Drawing of a drop-coil test which is also used to measure flux loss in bonded Nd magnets. Source: (A) Courtesy Lamba Scientific Systems.

embedded coil, with an ID slightly larger than the OD of the test magnet, is dropped over the test sample and then smoothly lifted off the sample. As with a VSM, both the Helmholtz coil and the drop coil are based on Faraday’s law, which stipulates that the movement of a magnetic field will induce and electric field in a conductor. Using either procedure, several tests are carried out to confirm that the results are repeatable. Both procedures can be used to provide good results. After this first flux test, the magnets are positioned on a nonmagnetic plate, such as Al, so as not to change the demagnetization field in the sample, which, as seen in Fig. 5.27, can change the total flux loss experienced by the samples. For the same reason, the magnets are also spaced apart on the plate so that the field from one magnet does not overlap with neighboring magnets. After a specified period of time, the magnets are removed from the oven, cooled and retested. The loss in recorded current is equal to the relative loss in magnetic remanence, which can be converted to absolute loss by referenced back to the original demagnetization curve obtained from the hysteresis graph. The process is then repeated for the next time increment of time. Of course, the magnets must not be remagnetized between tests. If the magnets are accidently remagnetizes, the data are meaningless and the test must be completely rerun.

5.4.4 Magnetizing-bonded Nd magnets Engineers designing products that use permanent magnets must consider the magnetic field necessary to magnetize the magnet. Rare earth-transition metal magnets in general require high-magnetizing fields and the isotropic nature of bonded Nd magnets makes magnetization even more difficult. This is a particular problem for applications such as small stepper motors that can have many narrowly spaced magnetic poles. Fig. 5.29 shows an example of a ring magnet, which has been magnetized with 16 separate magnetic poles and where the magnetic poles have been highlighted with magnetic paper. On several occasions, it has been mentioned that various lower Hci powders were developed to facilitate easier magnetization of

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Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets

Figure 5.29 Example of a bonded Nd ring magnet that has been magnetized with 16 separated poles, which are highlighted with magnetic paper. Source: Adapted from Croat, J. J., IEEE Trans. Magn., MAG 18 ( 6), 1442 (1982b). 100

E

Percent of saturation (%)

80

60

B

40 Q A 20

0 0

5

10 15 20 Magnetizing field (kOe)

25

30

Figure 5.30 Percent of magnetic saturation of Br for A, B, E, and Q grade powders versus magnetizing field. Source: Adapted from Brown et al., 2002. J. Magn. Magn. Mater. 248, 432.

multipole magnets for brushless motors. While these lower Hci powders did result in higher magnetization at lower magnetizing field, lowering Hci does not result in the ability to achieve complete magnetic saturation at lower magnetizing fields. Fig. 5.30 plots the increase in Br versus magnetizing field for E, B, and A grade powders, which have Hci values averaging 7.2, 9.6, and 15.3 kOe, respectively. These data show a clear correlation between the initial rise in Br with lower Hci

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powders. For the E grade powder, B90% of magnetic saturation can be achieved at a field level of only 15 kOe. In contrast, a field level of .20 kOe is required to achieve the same 90% of saturation with A grade powder. However, achieving higher levels of saturation becomes increasing difficult for all of the powder types and achieving 98% 100% saturation requires a field of B35 kOe in all cases. These lower Hci powders were known to facilitation magnetization of difficult magnetization situations. However, as indicated by the data in Fig. 5.30, it would seem that the percentage of magnetic saturation for most of these applications was significantly less than 100%. During the early commercialization of these NdFeB magnetic powders, an additional powder having very low coercivity was developed for applications where magnetization was extremely difficult. This material was designated Q grade powder and had an intrinsic coercivity of B4.0 kOe. As with all the powder grades, this powder was produced by simply varying the Nd:Fe ratio as is demonstrated in Fig. 3.8 in Chapter 3, which shows the demagnetization curves of Nd12x(Fe0.95B0.05)x alloys for varying amounts of x. The demagnetization curve of the Q grade powder was close to that of the x 5 0.9 alloy. The magnetization characteristics of the powder is compared with bonded Nd magnets prepared from A, B, and E powders in Fig. 5.30. Using an arbitrary magnetizing field of 10 kOe, this powder would provide B70% higher magnetization and hence, motor torque, than that provided by B grade powder, the only low coercivity powder developed at that time. An example of one of the applications that had very difficult magnetization problems is shown in the photo in Fig. 5.31, which shows very small motors with multipole rotors measuring only 2 mm in diameter. However, with the development of E grade (15 7) powder, the production of the Q grade powder, which was always limited in demand, was discontinued. As noted in Fig. 5.30, this E powder does provide easier magnetization, even though it has a higher coercivity, averaging 7.2 kOe. This is not understood, but may be because the microstructure of this material is no longer a mixture of the Nd2Fe14B intermetallic phase and the Nd-rich intergranular phase, but probably also contains α-Fe and Fe3B, similar to that for nanocomposite magnets.

Figure 5.31 Very small motors having special magnetizing problems.

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Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets

As noted above, the field required to achieve full saturation in these isotropic melt spun materials is very nearly the same despite the difference in the coercivity level. This feature of these materials is somewhat puzzling. As discussed in detail in Chapter 3, The properties of melt-spun NdFeB alloys (see Fig. 3.28), these isotropic materials magnetize along the c-axis direction of the Nd2Fe14B grains that lies closest to the magnetic field direction. Magnetization of a thermally demagnetized sample requires that the magnetic dipoles half of the grains reverse from one c-axis direction to its opposing direction by the movement of a domain wall across the grain. As shown in Fig. 6.30, the ease with which the magnetic dipoles are reversed clearly follows a distribution from those that are easily reversed at lowmagnetizing fields to very difficult to reverse at the highest magnetizing fields. As would be expected, this same feature is found in the melt-spun ribbon from which the bonded Nd magnets are prepared. Reasons for this behavior were discussed in Section 3.3, which discusses the coercivity mechanism in these fine-grained isotropic melt-spun NdFeB materials. Because of the characteristic of bonded Nd magnets that are shown in Fig. 5.30, these magnets are difficult to magnetize in production. Many bonded Nd magnets are ring magnets and magnetization is carried out in specially designed fixture, which consist of wedge-shaped sections of laminated electrical steel (very low Hci) surrounded by a coil would from heavy gauge Cu wire. The fixture is energized by a short pulse of current from a bank of capacitors, which are charged and then rapidly discharged. Laminated steel is necessary to prevent eddy current losses. As described by the Lorentz Force Law, there will be a large force attempting to push the conductors apart during the magnetization pulse. For this reason, the fixture must be stoutly constructed, including slotted lamination, which are firmly embedded in epoxy. The length of the pulse can be varied depending on the type of magnetic material. If the magnetic material is a good electrical conductor, eddy currents will be generated that oppose the applied field. For these types of materials, which include sintered Nd magnets, a longer pulse is required. A wide pulse ensures that all domains are exposed to an adequate magnetizing field strength. However, a wider pulse results in higher heat losses, so the width of the pulse results in higher losses due to heat. This is a particular problem in production magnetization, where heat buildup can result in the fixture after repeated use. Fortunately, bonded Nd magnets have low electrical conductivity because the magnetic particles in bonded Nd magnets are magnetically insulated from each other by the resin, which reduces the ability of the magnet to carry flux. Therefore, only a current pulse of short duration is required. However, this same magnetic isolation results in an increase in the required magnetization field, which is already quite high for these magnetically isotropic materials.

5.5

Injection-molded Nd magnets

Ejection molding is the second most popular means of producing bonded Nd magnets. This method involves mixing or compounding the magnetic powder with a

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resin, heating the material to a semimolten state and forcing the material into the cavity of a mold. There are a number of contemporary books on this subject that can be referred to, including those by Bryce (1996), Olmstead and Davis (2001), and Kazmer (2007). The major disadvantage of compression molding is that the loading factor of the NdFeB magnetic powder is lower, 66 67% for the best grades of injection-molded magnets versus 79% 80% for the best compression-molded magnets. This is because the compound becomes quite viscous with higher loading factors and cannot be driven under pressure into the molds. Another disadvantage is the sprue, which is a necessary feature of any injection-molding operation. Because of the high cost of the magnetic powder, it is essential that the sprue be recycled by regrinding into a grannulate, which is added to the fresh compound. However, experience has shown that there is always some small loss in the magnetic properties of the material when it is reprocessed. Overall, however, the injection-molding process has many advantages and injection-molded Nd magnets are becoming increasingly popular. The primary advantage of the process is that net-shaped parts with complex shapes and tight tolerances can be produced in high volumn. The injectionmolded magnets are also appreciably stronger than the compression-molded variety and parts with cross section ,1.0 mm can be produced. Because the magnets have a higher percentage of binder, no coating is required for most applications. In the event that coating is required, the same coating techniques discussed earlier for compression-molded magnets would apply equally well for injection-molded magnets. Overall, the injection-molding process lends itself to high-volume production and can also be used to insert or overmold the magnet into an assembly, which can significantly reduce fastening and assembly cost. Although the injection-molding tooling can be significantly more expensive than compression-molding tooling, the molds are usually multicavity and a number of parts can be produced in one cycle.

5.5.1 Compounding NdFeB magnetic powder Injection molding requires that a compound of the NdFeB magentic powder and resin be produced. As with the encapsulation process for compression-molded magnets, the first step in the compounding process is to crush the powder to the desired particle size. As a general rule, the powder used for injection molding has a smaller particle size distribution than used for compression-molded magnets, ,100 μm versus ,150 μm for most compression-molded magnets. To produce this particle size the magnetic powder is first crushed using the same type of vibratory crusher as that shown in Chapter 4, Production of rapidly solidified NdFeB magnetic powder (see Fig. 4.27). The next step in the process is the actual compounding, in which the NdFeB magnetic powder is blended with a suitable thermoplasticmolding material such as polyphenylene sulfide, Nylon 6, or Nylon 12. Some kind of anitoxidants is also frequently added to the mixture. The ease by witch these resins can be used depends on their softening or melting points. Nylon 6 has a melting point of B200 C and can be used in applicatons up to 180 C. In contrast, Nylon 12 had a melting point of B170 C and cannot be used in application much above 150 C. However, it is easier to compound and is beleived to be the most

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Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets

common material used to produce compound. One problem with all of these resins is that they absorb moisture and must be dried prior to the compounding step. Compounding is typically carried out in a twin-screw extruder with an attached granulator. A drawing of such a compounder is shown in Fig. 5.32. The NdFeB magnetic powder and resin are poured into the extruder, where the resin in softened and mixed or compounded by the kneading action of the two screws. Compounding typically takes place at a temperature of 150 170 C. The hot mixture is then extruded into thin rods, which are cooled and then chopped or granulated into pellets suitable for use for the injection molding process. Because the NdFeB powder is susceptible to oxidation, this process must be carried out in an oxygen and moisture free environment. This is usually accomplished by evacuation the feed hopper and backfilling with pure argon. The second type of compounder that is used is a Z-Blade blender, which is a heavy duty kneader that is ideal for mixing highly viscous mixers such as Nylon heavily loaded with magnetic powder. This type of mixer shown in Fig. 5.33A

Figure 5.32 Twin screw extruder used to compound NdFeB magnetic powder and to chop or granuate the compound into pellets suitable for injection molding.

Figure 5.33 (A) Photo of a Z-blade used to compound NdFeB magentic powder for injection-molded magnets. (B) Photo of compounded NdFeB magentic powder. Source: Courtesy Yuxiang Magnet Company.

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combines the action of two counter-rotating Z-blades with an extrusion screw placed in the lower part of the mix trough. The blade is tightly geared and moves the material in opposite direction, thereby providing excellent cross mixing of all raw materials. Heat is applied by heaters attached to the exterior of the mixing tank. During mixing, the extrusion screw is run in reverse, which imparts a third mixing action on the product, increasing mixing efficiency. When the product has finished the mixing cycle, discharge/extrusion is performed simply by running the extrusion screw in a forward direction. By changing the extrusion die heads, the product can be discharged as round or square strips, which can be chopped or granulated into compound for the injection-molding process. The mixing tank has a tight sealing lid so that the tank can be purged with argon to prevent oxidation of the magnetic powder during the compounding process. One advantage of this type of compounder is that it can be operated in small batches and is more amenable to processing the expensive NdFeB magnetic powder. A photograph of NdFeB magnetic powder after compounding into pellets is shown in Fig. 5.33B.

5.5.2 The injection-molding process A drawing showing the cross secion of an injection-molding machine is shown in Fig. 5.34. Modern injection-molding machines consist of two basic parts: the compound injection unit and the mold clamping unit containing the die. The compound is fed into the machine via a hopper, where it is heated to a plastic state. At this point the compound is still highly viscous because it is so heavily loaded with the magnetic powder. The injector unit is a reciprocating extruder, which acts as a ram to drive the plastic compound into the mold. As soon as the compound enters the mold and makes contact with the cooler surface of the die, it will begin to cool. The die is cooled with water and also includes small holes, which are large enough to allow air to escape as the compound is forced into the mold, but still too small for the viscous polmer metal compound to escape. Ram pressure is maintained to push additional compound into the mold to compensate for shrinkage that occurs upon cooling. A nonreturn valve at the tip of the screw prevents the compound from flowing backwards. At the completion of the molding cycle, the ram is retracted back to its starting position. Hence, the name reciprocating injector screws. The mold is in two parts and held together along its center line or parting line by the mold clamping unit. The nonstationary part of the die is attached to a movable platent, which opens to eject the parts. Once the molded material has cooled, the mold is opened and the parts are ejected. Once the part has been ejected, the cycle is repeated. Fig. 5.35 shows in more detail a conventional two plate mold, which consists of the two-halves fastened to the stationary and movable platens of the clamping unit. The molded assembly contaning the parts is ejected by means of ejection pins attached to the ejector plate and the sprue puller, which are, in turn, attached to a hydraulic ram. Also shown here is the injection screw, which drives the heated compound into the die and the nonreturn valve at the tip of the screw, which prevents the mixture from flowing back out of the mold. Fig. 5.36A shows

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Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets

Figure 5.34 Drawing showing the injection-molding process used to produce bonded Nd magnets.

Figure 5.35 Detail of the mold-clamping unit and die for an injection-molding machine.

Figure 5.36 (A) Drawing of a multicavity injection-molded assembly showing the sprue, runner, and gate. (B) Photograph of typical injection-molded magnets and magnet assemblies. Source: Courtesy Arnold Magnetics Technology.

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a drawing of a typical ejection-molded assembly, which includes the several magnets as well as the sprue, which is the channel running into the die from the injection unit, and the runners, which distribute the compound from the sprue channel to the separate magnet molds. At the entrance into the mold cavity is the gate, which is a design feature that facilitates breaking the part from the runner. These can be round, square, or rectangular in design. Of course, there are many different configurations that can and are used in multicavity injection-mold dies. As previously mentioned, one of the disadvantages of injection molding is that the sprue and runners must be recycled because of the high cost of the magnetic powder. Fig. 5.36B shows a photograph of a variety of injecction-molded Nd magnets and magnet assemblies. As mentioned, one of the distinct advantages of injection molding is the ability to insert mold the magnet into an assembly such as a rotor. Noted here, in particular, are the rotor assemblies, which can be produced by insert molding of the shaft and rotor core, resulting in a considerable saving in assembly and costs.

5.5.3 Properties of injection-molded Nd magnets Magnetic properties: The magnetic properties of commercial injection-molded Nd magnets produced from melt-spun NdFeB magnetic powder vary greatly depending on the loading factor of the compound: densities can range from 4.8 g/cm3 or 62% metal-loading factor to 5.2 g/cm2 or 67% metal-loading factor and, representing the very highest grades of injection-molded Nd magnets produced. The magnetic properties obtained for the various grades of NdFeB magnetic powder using a loading factor of 66% and a density of B5.1 g/cm3 are shown in Table 5.2. Table 5.2 Magnetic properties of injection-molded magnets prepared from different grades of melt-spun NdFeB magnetic powder, and the properties shown here are for magnets with a loading factor of B66%

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Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets

8 7 A compression

6

4 B compression

M (kG)

5 A injection

3 B injection

2 1 0 –16

–14

–12

–10

–8 H (kOe)

–6

–4

–2

0

Figure 5.37 A comparison of the demagnetization curves of compression- and injectionmolded magnets prepared from A and B grade powders.

Because of the lower-loading factor, the properties of these magnets are necessarily lower than for their compression-molded counterparts, which can be seen by comparison with the data in Table 5.1. This is clearly shown in Fig. 5.37, which compares the second quadrant demagnetization curves of compression-molded versus injection-molded magnets produced from A and B grade powders. The demagnetization curves of compression-molded magnets produced from B and A grade powders are shown in Figs. 5.21 and 5.22, respectively. Their much lower Br and second quadrant magnetization shown here is the single biggest disadvantage of injected molded Nd magnets. However, despite this, injection-molded Nd magnets are becoming increasing popular for the reasons noted earlier, particularly the ability of the injection-molding process to insert mold or overmold to produce assemblies. Temperature-dependent properties of injection-molded Nd magnets: Because both short- and long-term temperature-aging properties are related to the powder properties, both the short- and long-term temperature-aging properties of injectionmolded magnets are the same as for compression-molded magnets. The coefficients of Br(α) and Hci(β) shown in Table 5.2 are the same as for the compression-molded counterparts, which, in turn, were the same as for the raw powder. Likewise, the long-term thermal-aging properties are also the same or very nearly the same. As has been mentioned, the thermal stability of the raw, uncoated powder is higher than for either compression or injection-molded magnets. However, in a high moisture or salt environment, the resin would provide additional protection against structural losses due to corrosion. Coating injection-molded Nd magnets: Because of their higher loading factor, injection molded are often not coated. However, in the event that a coating is

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required, the coating techniques described in Section 5.2.2 for compression-molded magnets would apply equally well for injection-molded magnets.

5.6

Calendered and extruded Nd magnets

Calendering and extrusion are processes that are also used to produce both flexible and rigid magnets from melt-spun NdFeB magnetic powder. However, neither process is widely used at this time. Both calendering and extrusion use elastomers, which are long-chain molecular materials, as the binder. Elastomers are tough, elastic materials, which can be streched to twice their lenth under low stress and, after release, returns to their original shape. The two most common elaster used to make flexible magnets are Nitrile, which is an acrylonitrile butadiene rubber and with a service range of 250 to 150 C. The second commonly used elastomer is Hypalon, which is a chlorosulfonated polyethylene, and which has a somewhat lower service range of 230 to 80 C. Callendering is a rolling process for making continuous flexible sheets. The first step in the preocess is to compound the NdFeB magnetic powder with a suitable elastomer to from a granulated feed stock. This step is the same as that discussed earlier for injection-molded magnets (see Figs. 5.32 and 5.33). This compound is first loaded into a hopper and then fed through a series of heated rollers, as shown in the drawing in Fig. 5.38A. The rolls apply high compressive load to form a strip that typically ranges from as thin as 0.5 mm to as thick as 6 mm and of any length desired. During this rolling process the melt-spun ribbon flakes orient themselves along the major axis of the flakes of melt-spun ribbon. However, because the

Figure 5.38 (A) Calendering process used to produce flexible bonded magnets from NdFeB magnetic powder. (B). Photograph of a roll of flexible bonded Nd magnetic sheet produced by calendering. Source: (A) Adapted from Ormerod, J., and S. Constantinides, J. Appl. Phys. 81, 4816 (1997) and (B) courtesy Global Permanent Magnet Company.

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Rapidly Solidified Neodymium-Iron-Boron Permanent Magnets

melt-spun flakes are magnetically isotropic, there is no anisotropic alignment. The properties of extrusion-molded Nd magnets obviously depend on the metalloading factor of the compound. Becasue it is difficult to callender a highly loaded compound, magnetic properties are generally lower than for injection-molded Nd magnets. However, with careful processing, properies with Br . 6.0 kG and (BH)max . 6.5 MGOe can be achieved (Panchanathan and Davis, 2003). As with all bonded Nd magnets, the Hci values achieved depend on the coercivity of the powder grade used. The temperature coefficients of Br and Hci are the same as for the powder and the thermal aging properties are similar to compression or injection-molded magnets. A photgraph of a sheet of flexible bonded Nd magnet is shown in Fig. 5.38B. The parts must be cut from this sheet. There are many uses for these elastic magnets include various motors, sensors, and holding devices. Extrusion is also used to produce both flixible and rigid magnets from melt-spun NdFeB magentic powder. This drawing of this process is shown in Fig. 5.39. It is obvious that this process is very similar to injection molding, as can be seem by comparison with Figs. 5.34 and 5.35, which show cross section rendering of injection-molding machines. In this case, however, the binder can be either a thermoplastic-molding material such as Nylon 6 or Nylon 12 to produce rigid magnets, or an elastomer to produce flexible magnets. In either case the compound is fed into the molding machine, where it is heated to a plastic state and then driven with the screw through the heated die to produce the extrusion. Since the highly viscous compound is not being driven into a mold, loading factor can actually be higher than for injection-molding and magnetic properties as high as 8.0 MGOe have been reported, which are higher than those shown in Table 5.2 for injectionmolded magnets. However, there is less flexibility in the range of shapes that can be produced since the finished parts must be cut from a single longer extrusion. Simple shapes such as arcs and rods can be easily produced and the only wastage is the loss from cutting the individual segments. As with injection-molded magnets, extruded magnets are generally used in applications that do not require a coating. However, in the event that they do require a coating, then the coating processes

Figure 5.39 Drawing showing the extrusion process used to produce both flexible and rigidbonded Nd magnets.

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discussed earlier for compression-molded magnets would apply here equally well. As a general rule, if the applicaton sees condensed moisture or a corrosive envirement, for example, NaCl, then the magnet should be coated. Both short- and long-term temperature-aging properties are also similar to other grades of bonded Nd magnets.

References Brown, D.N., Ma, B.M., Chen, Z., 2002. J. Magn. Magn. Mater. 248, 432. Bryce, D.M., 1996. Plastic Injection Molding: Manufacturing Design Fundamentals. Society of Manufacturing Engineers, Dearborn, MI. Chen, Z., Wu, Y.Q., Kramer, M.J., Smith, B.R., Ma, B.-M., Huang, M.-Q., 2004. J. Magn. Magn. Mater. 268, 105. Doser, M., Floryan, D.E. Method od Making Magnets by Polymer-Coating Magnetic Powder, US Patent 3,933,536, issued 1976. Fleck, N.A., 1995. J. Mech. Phys. Solids. 43, 1409. Gutfleisch, O., 2000. J. Appl. Phys. 33, R157. Hadjipanayis, G.C. (Ed.) Bonded Magnets: Proceedings of the NATO Advanced Research Workshop, 2002. Hadjipanayis, G.C. (Ed.), 2003. Bonded Magnets. Kluwar Academic Press, Dordrecht. Heckel, R.W., 1960. Trans. Met. Soc. AIME. 221, 671. Jones, W.D., 1960. Fundamental Principles of Powder Metallurgy. Edward Arnold Ltd, London. Kawato, H., Tomioka, T. Method for Preparing Magnetic Powder Material, Process for Preparation of Resin Composition and Process for Producing a Powder Molded Product, US Patent 5,256,326, issued 1993. Kazmer, D., 2007. Injection Mold Design Engineering. Hanser Publications, Cincinnati, OH. Lee, R.W., Croat, J.J. Bonded Rare Earth Iron Magnets, US Patent 4,902,361, issued 1990. Ormerod, J., 1989. Powder Metallurgy. 32, 244. Panchanathan, V., Davis, H.A., Hadjipanayis, G.C. (Eds.), 2003. In: Bonded Magnets. Kluwar Academic Press, Netherlands, p. 13. Samuel, P.K., Newkirk, J.W. (Eds.), 2015. ASM Handbook, Vol. 7: Powder Metallurgy. ASM International, Russell Township, OH. Tattam, C., Williams, A.J., Hay, J.N., Harris, I.R., Tedstone, S.F., Ashraf, M.M., 1994. J. Appl. Phys. 76, 6831. Yamashita, F., Wada, M., Masahara H., Miyagawa, M. US Patent 4,981,635, issued 1991.

Selected Readings Coey, J.M.D., O’Donnell, K., 1997. New bonded magnet materials. J. Appl. Phys. 81, 4810. Olmstead B.A., and M.E. Davis, Practical Injection Molding, (Marcel Dekter, New York) (2001). Ormerod, J., Constantinides, S., 1997. Bonded permanent magnets: Current status and future opportunities. J. Appl. Phys. 81, 4816. Powder Metal Handbook, Hagonas Technical Information, Material and Powder Properties, Chapter 1, Production of Sintered Components, (2013).