Magnets consists of objects or materials that can create a magnetic field and influence ferromagnetic materials in close proximity. A magnet typically features a pole pairing from which the magnetic field emanates.
In this guide, you are going to learn how to manufacture magnets.
Classification of Magnets
Before discussing the manufacturing process of magnets, it is important to note the different classification of magnets as follows:
1. Temporary Magnets
These are materials that become magnetic when there is magnetic field and cannot sustain its own. Includes: Soft iron and some of its alloys like permalloy (iron and nickel), nails, paper clips.
With these magnets, a wire wound on metal core creates magnetism when the current flows through the wire.
The magnetic field, whose strength is determined by the amount of current, is lost when current supply is cut.
These are special materials that do not require any external force or phenomenon to possess magnetic properties. As long as the required environmental conditions, they maintain magnetism for a very long time.
Some permanents magnets can only lose a fraction of their magnetic properties after 10 years.
Factors to Consider before Manufacturing Magnets
Before contemplating the manufacture of magnets, you need to consider these four critical factors that influence magnetic performance:
Curie Temperature (TC)
The Curie temperature defines the temperature beyond which the magnetic properties (permanent) of a material are lost. Usually, such material will experience a phenomenon called induced magnetism at this point. It is especially an important factor to consider wen making magnets for high temperature application.
Coercivity is a property that describes the material’s capacity to resist demagnetization when in the presence of a foreign magnetic field. In making permanent magnets, you require materials with high coercivity which is measured in ampere/meters.
Also known as residual magnetism, it describes the amount of magnetic flux left in a permanent magnet after the manufacturing process. Remanence is the measure of a permanent magnet’s magnetic field strength.
Maximum Energy Product (BHmax)
The SI units for this is kilojoules/cubic meter – kJ/m3.
Usually, the maximum energy product signifies a permanent magnet’s magnetic energy density. It is the product of the magnetic flux density’s maximum value (B) and its field strength (H).
Manufacturing Process of Magnets
Part A: Electromagnets
Electromagnets feature a magnetic core with wire wound across its length, which induces magnetism when subjected to a current flow. When making electro magnets, you will find:
- Iron as the core
- Copper metal forming the coil
Metal casting is the common process used in making electromagnets.
During magnet metal casting, you can use expandable and non-expandable moulds. As a result, you can make magnets with many designs and configurations.
In casting the electromagnetic core, the following steps are typical:
Step 1: Pattern Creation
The pattern determines the shape of your mold based on the desired shape of the magnetic core. A pattern has to be dimensionally accurate while capable of being removed from the mold cavity without damage.
A pattern should also provide shrinkage allowance and passage for the molten metal through sprues, gates and risers. You can use plastic, wood, metal or wax for your pattern material depending on the process and volume.
Step 2: Mold Making
Depending on your casting technique, you can have a reusable or non-reusable mold. Metal finds typical use in reusable molds whereas ceramics and sand are archetypical to non-reusable molds.
The molding process involves cast formation by the pattern imprint using ceramic powder or sand in a flask. After achieving the desired cast, you remove the pattern either by melting or separating the flask halves for depending on casting technique.
Step 3: Melting the Iron or Iron Alloy
Electromagnetic cores are formed from iron or its alloys combining elements such as cobalt, which offer the best electromagnetic properties. Heat the metal to molten state.
Step 4: Filling the Mold
Fill the mold cavity by pouring the liquefied metal from the crucible bearing in mind all the necessary safety precautions. The filling process should not be too slow (can start hardening before completely filling) or too quick (potentially damage the mold).
Step 5: Cast Removal
The removal of the cast does not occur until the molten metal cools and fully solidifies. For an expendable mold, you vibrate the assembly to initiate breaking of the mold and freeing up the cast.
In non-expandable mould, removing the ejector pins will help remove the cast magnet.
Step 6: Finishing
After removing the mold, you undertake finishing procedures such as cleaning by removing any remaining mold material. You also cut off the gates used to direct the molten metal to the mold cavity and smoothen out the edges.
Step 7: Making the Coil
Copper wire made from wire drawing copper rods through a series of dies finds use in forming the coil of an electromagnet. After retrieving your iron core from the mold, you wind the copper wire over it across its length.
Note that the number if copper wire turns influences the strength of the electromagnet alongside the amount of current.
Step 8: Providing Terminals
Terminals allow you to connect your electromagnet assembly to a current source which will initiate creation of the magnetic field. Both AC and DC currents find use in electromagnets depending on the application.
Parts B: Permanent Magnets
Permanent magnets finding current use are a result of years of research with unique metal combinations. Ceramics, rare earth elements (neodymium and samarium cobalt) and alnico are the major magnets of this type.
In manufacturing permanent magnets from rare earth elements, they’re coated/plated for protection stemming from their susceptibility to deterioration and brittleness.
1. Neodymium (NdFeB) Magnets
Neodymium is one of the best permanent magnets today. It contains neodymium, iron and boron.
Over the years, sintering remains a popular manufacturing technique for neodymium (NdFeB) magnets.
The sintering process of manufacturing neodymium magnets is practical and had been adopted by many magnet manufacturer.
It depends on powder metallurgy principles. Usually, this process has a lot of freedom – allowing you to manufacture any magnets grade.
However, the process can be summarized as follows:
Alloying and Strip casting
In the alloying process, neodymium, iron, boron and other minor elements are combined in the desired quantities. The alloying process serves to alter the end product’s micro structure, improve subsequent operations and enhance magnetic qualities.
The agglomeration is then melted and strip cast by heating in a furnace under vacuum conditions. A vacuum environment is utilized to prevent air form reacting with the melt thus tainting the alloy material.
The molten metal is pressured into a cooling drum where it is taken through a rapid cooling process. The result is the formation of tiny metal grains well placed for efficient processing in subsequent stages and final product quality.
Hydrogen Decrepitation and Milling
The strip casting process results in sheets of small grains that require to be powdered for the manufacture of magnets. Hydrogen decrepitation infuses hydrogen to further fragment the material allowing for simpler processing in further stages.
The smaller particles are reduced further into powder form via jet milling which employs a stream of highly pressured inert gas. The inert gas is cyclonic ensuring particles are distributed by size while preventing attachment to the vessel’s wall.
The pressing process occurs in two steps as follows:
Under the Action of Magnetic Field
Here, the powder is transferred to a mold in an automated press while still under the influence of inert gas. A pressing operation follows under a strong magnetic field resulting in an alignment of the particles.
The orientation of your magnetic field can be:
- Aligned with molded block or,
- Perpendicular to the molded block
The former option is typically selected to provide the highest anisotropy for the magnet.
Here, the block extracted from the mold is subjected to a great pressure while submerged in a cold isostatic press. The result is that any air holes within the block are eliminated resulting in a smaller block.
Figure showing process flow for NdFeB magnet:
You will heat the block in the furnace. During the process, the heating temperature must not exceed the melting point of the material.
The high temperature excites the material’s atoms augmenting the mechanical and magnetic properties of the block.
The orientation of the domains are in the direction assigned on isostatic pressing. The high temperature result in the achievement of required flux density reducing the metal blocks further but for the last time.
The sintering process causes heat stressing in the metal block requiring another heat treatment at reduced temperatures to eliminate the stresses. This involves subjecting the blocks to alternating high temperature-low temperature episodes for a given time and gradually brought to room temperature.
Machining and Surface Treatment
Machining processes such as cutting and grinding are essential in shaping the NdFeB as desired and with minimal waste. This is conducted by initiating close control and recycling waste material.
Due to the brittle nature of NdFeB and its low corrosion resistance, a surface treatment procedure like plating is necessary. Some of the plating options include nickel-copper-nickel, aluminum zinc and even epoxy.
Like any manufacturing process, product testing is vital to ensure close quality control and conformity to standards. For increased efficiency, carry out process evaluation at every stage of the process minimizing the costly risk of a flawed product.
In the magnetizing process, the NdFeB is enclosed within an electric coil powered to generate a brief but massive magnetic field. The material assumes the magnetic field once the electric coil is depowered creating a neodymium magnet.
2. Samarium Cobalt (SmCo) Magnets
Like neodymium, making samarium cobalt involves powder conversion. However, this manufacturing process differs in certain aspects for samarium cobalt.
In manufacturing samarium cobalt magnets, the process flows as follows:
Preparation of Alloy
The preparation of samarium cobalt alloy involves induction melting of samarium and cobalt metals in a vacuum or inert gas presence. The molten alloy material can then be cast in ingots for the next process or made into lumps.
Here, the samarium cobalt alloy lumps or ingots are reduced in size to powder form through a milling process. A powerful hammer mill is employed to crush these large solid alloy materials usually in the presence of nitrogen.
You can use jet milling where the disintegration of alloy material is by attrition caused by the high-speed collision of particles. Important parameters to consider during the milling [process are: the size of particles ant heir distribution, oxidation and crystalline defects.
Milling results in the reduction of alloy material to tiny monocrystalline particles while increasing the surface area for the sintering process. The lack of grain boundaries in the particles allows for only a single magnetization axis while influencing coercivity.
Alignment and Pressing
You achieve the highest possible magnet qualities by instigating a particle alignment prior to compaction.
As you induce the magnetic properties, ensure they are parallel to powder particles.
A die or isostatic press conducts compaction in a magnetic field presence parallel or perpendicular to the pressing direction. The magnetic field can take static or pulsed forms where a large homogenous field results in uniform and high standard alignment.
Alignment is a product of particle characteristics such as shape and size, magnetic field strength and compacting pressure. The pressure applied during pressing shouldn’t upset the particle distribution but instead be just enough to support handling.
Isostatic alignment is preferred to die pressing since it can compact powders in magnetic fields four times the latter’s. Consequently, particle alignment is enhanced and sustained throughout the press, with higher maximum energy product and remanence.
Sintering and Curing
When sintering samarium cobalt, you carry out the process in reducing atmospheres, inert gas presence or in vacuum conditions. Through sintering, grain growth is inhibited promoting final magnet qualities.
Sintering should be carried out at a constant temperature and for a defined period. This prevents the formation of air gaps which could be prone to oxidation and hamper the magnet’s usefulness over time.
A curing process succeeds the sintering process to cultivate coercivity in SmCo to optimum level and form properties of loop.
Sintering alters the SmCo magnet shape and size usually to the desired final product parameters as a result of shrinking. Therefore, it is necessary to undertake a few machining operations to achieve prescribed magnet features.
SmCo material is very brittle and highly vulnerable to chipping and breaking, making direct holding in chucks ill advised. Instead, unique adhesives secured to steel backing hold the magnet material in position when using standard machines for grinding.
While you can magnetize SmCo before assembly due to high coercivity and fair recoil permeability, its brittle nature suggest otherwise. You therefore find many SmCo materials magnetized during assembly by applying a magnetic field about two times its coercivity.
The testing process for SmCo magnets dwells in understanding the demagnetization curves of the magnets using data from a flux meter. You compare the values to those of a standard magnet and then classify them according to their flux densities.
These magnets consist of:
- Cobalt (Co)
- Aluminum (Al)
- Nickel (Ni)
Minor additions of refractory metals, iron and copper can be made to augment mechanical qualities.
Alnico magnets features outstanding qualities such as impressive thermal qualities like high working temperature and excellent corrosion resistance. However, they display low coercivity making them liable to demagnetization.
You can manufacture alnico by employing either of these process: casting or sintering as follows:
Casting alnico allows you to fabricate complex shapes and thus enabling wider application. The casting process for alnico generally encompasses the following steps:
The temperatures achieved in induction melting exceed 1750°C converting the solid nuggets of aluminum, nickel and cobalt into molten state. Due to aluminum’s low melting [point, it is added in the latter stages of the process.
The molten alnico is transferred to sand molds designed from pattern plates sharing the final magnet shape for casting. After filling the mold cavities with the molten material, they are left to cool and solidify before being broken out of the sand molds.
The alnico casts from the casting process have surface imperfections stemming from the casting process that need removal. Such flaws include sharp edges, burrs and the gates through which the molten material was conducted.
These blemishes are removed by sanding suing scrubbers, grinding by utilizing abrasive wheels and fettling using water.
Figure showing Casting Process for Alnico magnet:
The now alnico magnet material undergoes various heat treatments first a homogenization process at about 1250 °C. A brief annealing episode follows magnetic cooling for a quarter an hour at about 850 °C.
The annealing process is essential in developing a microstructure with anisotropic and spinodal qualities. Finally, the magnet is tempered for at most a day, with dropping temperatures reaching 550°C. This allows for the achievement of maximum coercivity and chemical separation of the spinodal components.
The machining process is necessary to alter the appearance of the magnets albeit at a small scale. Common machining operations include cutting and grinding which allow you to apply tight tolerance finishing on your magnets.
In testing your magnets, you need to consider the importance of the parameter being tested to your application and cost. The results of your test should be directly attributable to a magnetic functionality.
Some common test processes for your magnet include:
- Plotting a B-H curve by employing a permeameter offering a description of temperature related magnetic properties.
- Making estimations of remanence, coercivity and maximum energy product by deciphering total dipole moments from measurements of total flux.
- Using a gaussmeter and probe to measure flux densities.
While alnico magnets have good corrosion resistance, coatings can still be applied to enhance surface protection. Some of the common coatings applied are epoxy coatings, nickel and conversion coatings like chromates, iron and zinc.
In the magnetizing process, a solenoid is used to produce a magnetic field that aligns the alnico’s magnetic domains. A solenoid is essentially an electromagnet, but hollowed out to allow positioning of the material to be magnetized.
The parameters of the magnetizing field are important when executing a magnetizing process as they influence the alnico magnets’ properties.
The benefit of sintering alnico is an improvement in the mechanical qualities relative to casting while sacrificing magnetic capability. The sintering process is conducted in the following stages:
The sintering process begins with the aluminum nickel and cobalt elements being melted in specialized conditions. Induction melting combines the individual elements into a single alloy material after which it is strip cast and cooled into granules.
The objective of the milling process is to convert the granular matter of alnico into powder for the pressing process. A milling machine is used in which the particle are mechanically acted upon to disintegrate them into the desired particle sizes.
The alnico powder material from the milling process is guided into special dies designed as the magnet would appear. Here, the die press compacts them under great pressure of about five kilo bars into a compact solid.
Sintering involves close controlled heat treatment to temperatures just below the melting point in an oven. Air gaps are eliminated compacting the alnico material further and enhancing mechanical properties.
The alnico piece is removed from the oven and cooled in one of two ways: through isotropic cooling or anisotropic cooling. Anisotropic cooling is done in the presence of a magnetic field while isotropic cooling is done without a magnetic field.
Cooling under a magnetic field aligns the alnico’s particles in the desired direction of magnetic orientation. When you cool without a magnetic field, you can implement multipolar orientation during magnetization.
After the cooling process, the latter procedures of testing, coating and magnetizing follow in a similar manner to the casting method.
Ferrite magnets are affordable. Additionally, they have excellent thermal stability and corrosion resistance.
However, compared to rare earth magnets and alnico, they have the least magnetic qualities.
The flexibility of manufacturing ceramics and its compatibility with other materials allows for the manufacture of different ferrite magnets. Ferrites can also be bonded with polymer materials resulting in much weaker but flexible plastoferrites magnets.
The manufacturing process of ferrites via sintering takes the following form:
Sintering is based on powder metallurgy with the powder production process beginning with mixing ceramic compounds. Ferric oxide is a primary material alongside strontium carbonate or barium carbonate, not both.
Using appropriate ratios of the two materials is necessary to ensure the coercivity and remanence of the magnet are balanced. When you exceed the required ferric oxide amount, you instigate bulging grain growth while negating coercivity.
A carbonate excess works against the magnets remanence property. Introducing silica inhibits grain growth and favors calcination at higher temperatures.
The temperatures achieved during calcination range between 1250°C and 1300°C. Very low temperatures hamper completion of the solid state reaction while very high temperatures reduce coercivity and promote grain growth.
Figure showing Process flow for Sintered Anisotropic Ferrite magnet:
The ferrite material from calcination is milled to powder by adding water to form a slurry and enhance alignment and density. Wet milling achieves particles between 0.7 µm and 0.9 µm.
Smaller particles less than 0.7 µm create issues during compaction and sintering. On the other hand, larger particles greater than 0.9 µm lower coercivity.
The small particles from wet milling are transferred to dies in a press station with a strong magnetic field presence. The magnetic field is usually orthogonal to the direction of mechanical pressing.
The wet particles are compressed under great pressure ordinarily between 5-15 MPa. Camphor and polyvinyl alcohol can be added as binder to enhance compression and eliminated in the succeeding sintering process.
An alternative to wet pressing is dry pressing which can be executed either anisotropically or isotropically. However, the pressures required for compressing the powder into compact form are much higher ranging between 40 and 80 MPa.
The compacted ferrite material from the pressing process is sintered by subjecting to heat treatment ranging between 1150°C – 1200°C. The sintering temperature is generally lower than that used in calcination by roughly 100°C to prevent growth of grains.
Carefully curating and controlling the growth of considerably aligned grains over those less aligned improves the ferrite material’s magnetic texture. The powder particles are about a micrometer making them just small enough for the next stage.
You can add silicon dioxide, alumina, chromia or calcium oxide individually or in mixtures to improve densification. Sintering is prone to anisotropic shrinkage making it a requisite consideration in die design.
Sintered ceramics can be cut and ground to shape despite their incredible hardness and brittleness. Diamond and carbide tipped tooling such as grinders and saws can be used.
This is the ultimate step in the manufacturing process of a sintered anisotropic ferrite magnet. The ferrite material is subjected to a solenoid generated magnetic field up to three times the magnet’s desired coercivity.
Bonded magnets are formed by combining a hard magnetic material with a non-magnetic binder usually a polymer or elastomer. Bonded magnets can be rigid or flexible (albeit at lower magnetic strength) depending on the material and mixing ratio.
Rare earth elements such as neodymium and samarium cobalt alongside ferrites find use as hard magnetic material individually or in combinations. Common binders include polyester, thermoset epoxies and nylon for rigid bonded magnets, and vinyl and nitrile rubber for flexible magnets.
With bonded magnets, need for secondary operations is avoidable. This reduces the cost of production which can be lowered even further by presenting single operation assemblies.
However, bonded magnets have relatively low maximum operating temperature with poor thermal stability.
The manufacture of bonded magnets can be represented as follows:
Here you will need binder and magnetic powder as the primary materials. You can have bonded neodymium magnets, bonded SmCo magnets or bonded ferrite magnets depending on your application needs.
Mixing and Molding
The mixing process serves to provide a uniformly distributed strain of bonded material for the molding process where it is shaped. The molding process is central to the manufacture of bonded magnets with four processing options available as follows:
Neodymium and ferrite materials are commonly utilized as the magnetic material in injection molding. The material mix is subjected to high melting temperatures before injection into a specially designed mold cavity for cooling.
You can utilize multiple mold cavities when undertaking high volume production to increase efficiency. Injection molding allows you to make bonded magnets with intricate shapes since you can make uniquely shaped molds.
Compression bonding is synonymous with making bonded neodymium magnets. In this process, neodymium powder grains are first encapsulated by the liquefied binder during mixing. The coated powder mix then transfers to a press cavity where high pressures compact it into the desired shape.
A curing process at high temperatures (150°C-175°C) in an oven follows compression bonding to further harden and solidify the binder. The compression bonding process achieves greater flux densities for its bonded magnets compared to those fashioned from injection molding.
Calendering mostly employs ferrite powders for the magnetic material and flexible rubber for binder to make continuous sheets with magnetic properties. NdFeB finds limited use with a hybrid material combining NdFeB and a ferrite compound more common.
The compound mix containing rubber and magnetic material is conducted to successive heated rolls applying great compressive force. The heat and pressure combination transforms the compound mixture into sheets with great tension to varying lengths.
Extrusion shares the same raw components as those utilized in calendering i.e. ferrites, NdFeB, or NdFeB/ferrite hybrids with flexible elastomers. The abrasive nature of NdFeB and ferrite materials requires tooling to be coated with wear resistant material.
The mixed and heated bonded material is delivered to a hot die system under pressure via an extrusion screw. The extruded product which takes the die cavity’s cross sectional profile is either stacked or cut up in desired lengths.
The dimensional accuracy for the bonded materials in the highlighted processes is high making secondary operations unnecessary. However, you can still undertake finishing operations such as cutting and punching to extract laminates in the desired shape.
For rigid bonded magnetics such as neodymium, coating offers surface protection from corrosion while enhancing durability. Common coating materials applied in bonding are epoxy and nickel plating.
The magnetizing process for bonded magnet materials involves subjecting them to great magnetic fields for brief periods. This process can be executed by taking either anisotropic or isotropic paths to varying degrees of success.
An isotropic path for bonded magnet materials simplifies the manufacturing process since absence of residual magnetization allows for easier process handling.
Comparison between Sintered and Bonded Magnets
The following table highlights the differences between sintered and bonded magnets.
|Makes very strong magnets.
|The magnets are less strong.
|Relatively low mechanical strength compared to counterpart.
|High mechanical strength due to increased particle adhesion.
|Limited in the magnet shapes you can achieve.
|Capable of making very complex magnets shapes.
|Usually employs only magnetic material powder only.
|Requires use of magnetic material powder and a polymer binder.
|Have low dimensional accuracy requiring secondary processing.
|Have high dimensional accuracy due to one-time formation and no need for further processing.
|Incurs large material loses from secondary processes.
|The one-time component formation ensures low material losses are recorded.
|Can only be magnetized in the direction given during mold pressing.
|The isotropic nature of material allows magnetization in multi-polar arrangement.
|Maximum Energy Product (BHMAX)
|Exhibits larger BH values exceeding 50M.
|Record much lower BH values below 10M.
|Have high cost stemming from use of 100% magnetic material powder and need for secondary processing.
|Has lower cost resulting from low powder requirement and less need for machining.
Whether you want to make permanent or temporary magnets, it involve sequential process.
Ideally, the magnet manufacturing process will determine the quality and characteristics of the final product.
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