High quality permanent ferrite motor arc production and research

High-grade permanent ferrite material property researching files can be found everywhere. Indeed, we spend much time on this field. For the property of high-grade in mass production there still have some gap compared with imported products, although the mini products and the med-grade has equivalent to the technical level of foreign. According to the present condition of arc magnet manufacture and common issue, it is necessary to bring forward a question about how to improve the quality of product and share it with everyone.

Permanent ferrite arc magnet for motor appeared on early 70s of last century. As the market was small, plus the equipment is old that the property of magnet is under Y25, the output of product is low. There only have several small workshops for production, which yield only hundreds of units each year. But as long as the all kinds of motor vehicle appears including motor car, motorcycle, farming machinery, relevant DC motor promptly come up accordingly. All of these are leading to a large demand for segment of permanent ferrite magnet. As rough counting, the output of permanent ferrite segment magnet was near to 0.1 million tons in 2003, and gross production value reaches about RMB 2.5 billion. Although there have about 100 manufactory in this area, but there are only 3 capable to produce high-grade products (average rate of qualified production >95%), and basically the others manufacture products by old system and technical with small-scale. The standard product rate is less than 80%, of which even some are under 50%. How does it enable to compete with the products abroad through taking consumptive energy and high cost of equip caring, plus overstaffing as well as low standard product ration. It goes without making deep discussion into economic utility. As just I have been engaged on the research of magnetic material near 30years.It is possible that get 95% of the bulk products reach standard, even possible up to 98%.Herewith I release this article that share a idea for improving the rate of qualified product about permanent segment magnet with everyone. That is just for casting a stone to bring along jade. I hope trade personal can make further studying together with me for improving manufacturing technique.

High grade permanent ferrite arc magnet production material requirements

As far as we know, different property of magnet should be used on different area. Some might require High Hc or High Br, some might require both High Hc and High Br. All of these will decide the character & the magnet trace of motor. Segment magnet suppliers often choose material according to the property request from customs. Other details such as the brand of product, sinter-temperature, retention of products, characters of material, are always not paid enough attention to. But just for this factor, there may be some potential risks during the course of manufacturing high-grade segment for motor. Since the materials are supplied from different supplier, the rate for microelement or mol, pre-sinter temperature as well as additive would be different. So the activity of material and the temperature to physical solidifying and elongation of magnet will also be different. It is hard to separate it clearly to control products quality of artwork flow when the different material from different suppliers mixed up, which not only will tend to affect the property of product, but also can cause difficulty to confirm elongation of products. Some manufacturer would rather spend more energy for finish die-cut on the product than to calculate accurate dimension in the artwork drawing to cater the requirement of material. However, more die-cut will lead more flaws in the products, and the quality product rate will lower accordingly. Strictly speaking, the manufacture of high-grade segment magnet for motor just can choose a kind of material applying on process. Furthermore, they should require the supplier to ensure consistency of artwork and fixed rate on the composition of material. Any change to artwork and composition of material should achieve approval from demander and the process artwork should be proved scientific. Otherwise, this kind of material should not be passed.

Ferrite manufacturing equipment

The manufacturing equipment has finished several technical innovations since our country started to make permanent ferrite magnet from 1950s. First one is technical revolution on balling milling machinery, which has greatly shortened the course of balling milling from 120 hours to 20 hours or less. Second one comes to magnet sintering furnace, it greatly save the cost of process by coal furnace instead of electric system. Third one is current machinery innovation, which will help to improve the level of qualified products greatly. The equipment of processing high grade segment magnet abroad isn’t best, which just should be consider if it did meet the requirement of products process as follows.

Milling equipment

After many years’ experience on products process, internal milling machinery has become more and more perfect. Currently we mill the powder step by step, i.e. firstly it would be grinded to 5-7 um, then milling to 1.2-1.5um further, lastly go on it until it can meet the request of granular size. What’s most important is about ration of material, powder ball, water fundamentally. The material of different particle size needs different kind of material as well as ball size. After finishing above milling process on the material, not only the particle of material mixing should meet the typical value but also the distribution range of particle should be small with neutral PH.

Press molding machinery

The basic condition of forming is not only required mould the magnet to a regular shape of magnet, but also to reach the requirement of hardness and magnetic field. The results of forming will directly affect the final quality of products, so this process is important to reach high grade segment magnet by key machinery-press molding machinery. The press molding machinery   manufacturers in our country mainly locate in Nantong, Jiangshu province and Chengdu and Yibin, SiChuan province.

The raw product should possess the following features as high-grade segment magnet:

  1. density of raw product is high, which should over 3.15g/cm3
  2. finished on surface, both sides come up with crack and concavity like this fault
  3. there should be 30° angle between inner arc and outer
  4. the thickness should be within tolerance of 0.2mm (Cut depth of inner & outer arc should be kept within 0.60-0.80mm)
  5. direction of magnetization should be over 95%

To meet above-mentioned quality requirements of raw magnet, the molding system should posses following feature:

  1. press molding machinery should have enough pressure, and also it should be slowly on the course of pressing or decompress
  2. it should be equipped with automatic material mixing system, and the water rate of mixing material allows 1% tolerance upon to 38%, there should been placed a valve between the entry of material filling and mould that preventing the mixing material from flowing circum in cabin under pressure
  3. gap of concavo-convex moulds should be less than 0.01mm
  4. mould hardness (HRC) should be above 80, which also need to be processed with polish
  5. to ensure every finished segment magnet meet the tolerance requirement of product (irregular string and axis should be allowed) and avoid loss, it is necessary to make a sampling mould, and then make a firm mould if it is fine with sintered product
  6. there should be add a seal gasket on plunger, which prevent mixing material from leaking from gap of mould
  7. high current magnetizer, the coil of magnetizer should be processed with water-cool, and it should ensure negative voltage steady enough and pumping pipe to be unblocked

Ferrite magnet sintering furnace requirements

Sintering is a key step to reach high grade segment magnet, which should be processed with sintering at random. There just have a bit working temperature change for hearth of furnace, whose height is moderate between 160 to 180mm. The sintering products should be piled up at one level. It is best if can process material speedy but small (I ever operated a sinter furnace with single roller, whose furnace length up to 36m, speed to process for 3.5/h, rate of qualify products reach 99%), which can meet the high rate of qualify products.


Milling is the last process of producing magnet, which is critical for the quality of whole segment magnets. The previous process is standard, but if problem occurs to milling process, then all the previous efforts will be wasted. So, milling are also deems as worked as something like special care. Depth of die cut for inner arc can reach about 0.25mm, and that for outer arc also can reach about 0.35, both of them add up to 0.60mm, milling are running by auto grinder machinery. There have two processes—grinding and milling respectfully for inner arc and outer arc, Firstly, grinding for outer arc in depth of 0.25mm; secondly, grinding for inner arc in depth of 0.20mm; thirdly, milling for outer arc in depth of 0.10mm, fourthly, milling for inner arc in depth of 0.05mm. Grinding machinery should be smoothly placed at with a level. The gap of grind wheels is adjustable. The particle sizes are different by grinding and milling, outer arc face should be polished and the center of inner arc face should be die cut, which just need missing face area less than15% for the whole one. To be continuous treated by washing, drying, surface clearing, polish, lengthen arc, heighten axis as well as angle, plus hardness of magnet is high enough and distribution for magnetic filed is equal. The rate of qualified segment magnet will reach 98% by this way.

Management for production artwork

The production of high grade permanent segment magnet for motor requires strict conform to the parameter index for each process besides installed above-mentioned equipment necessary. Herein are parameter index for each process for trade professional.

Specification product process to high grade segment magnet for motor

Phase Testing program Parameter Instrument Frequency Inspector
1 Material Analysis        
1.1 Feasibility prove of material process artwork   Balling machinery, hydraulic press    
1.1.1 Magnetic property Br, Hcb, Hcj,(BH)max Hysteresigraph 5 pcs/batch IPC
1.1.2 Elongation Axially & diametrical Vernier caliper 10 pcs/batch IPC
1.1.3 Density g/cm3 Electronic Scale,beaker 5 pcs/batch IPC
2 Powdering inspecting standard   Balling/grinding machine    
2.1 Particle size after grinding um Average particle size analyzer One batch/time QC
2.2 Powder  size after milling um Average particle size analyzer One pot/time QC
2.3 Moisture of slurry % Auto moisture analyzer One pot/time QC
3 Testing standard for forming   Hydraulic pressure, vacuum pump, feeding system, magnetizer    
3.1 Molding condition Pressure from Main chamber and backup chamber, current of magnetization, time of pressing, molding speed, mold cycle, vacuum Make record on  spot 2H/time QC
3.2 Slurry Moisture Moisture analyzer Batch/time QC
3.3 Product features Thickness Vernier caliper



Density Electronic scale, beaker 2 times/cycle QC
3.4 Appearance No obervious crack inside or outside blank,no obervious magnetic field Visual

100% 5pcs/hour

Fade area on surface <20mm2 and depth ≤0.4mm Visual 100% 5pcs/hour QC/Operator
Viscidity: no powder to be absorbed Visual

100% 5pcs/hour


Capping: cap brink ≤0.3mm

Visual 100% 5pcs/hour Operator/QC
Area of overmold≤total area of inner arc, depth≤0.3mm Visual 100% 5pcs/hour Operator /QC
4 Sintering inspection standard   Sinter furnace    
4.1 Electro kiln Filling speed stopwatch, tape measure Timing IPC
Kiln temperature curve Plotting paper,ruler,marker 1time/quarter IPC
Sintering temperature Temperature modulator 1time/quarter IPC
4.2 Finished product size Length, width, height, thickness Vernier caliper 5pcs/2 hours QC
4.3 Surface quality of finished products Crack length max 1/5  surface allowed, width within 0.2mm, Cracks numbers less than 3 Visual 100%, 10pcs/batch, gauge Operator/QC
Fade area≤40mm3,depth≤0.3mm Visual 100%, 10pcs/batch, gauge Operator/QC
Overmold area≤5% total arc area Visual 100%, 10pcs/batch, gauge Operator/QC
4.4 Magnetic property Br,Hcb,Hcj,(BH)max Hysteresigraph 5pcs/batch QC
5 Fine milling inspect standard   Fine Milling machinery    
5.1 Finished product size Length, width, height(thickness) Vernier caliper,1000-feet 5pcs/30 mins,25pcs/batch Operator/QC
Inner and outer arc Projector, computer 2pcs/batch, 2pc/changing new grinder QC
5.2 Appearance

Fade area ≤2% total area, Depth≤0.4mm

Visual 100%, 5pcs/batch Operator/QC
6 Testing standard for clearing   Ultrasonic cleaner, dryer    
6.1 Appearance Magnetic powder residues and black spot are not allowed Visual

100%, 5pc/batch

7 Surface inspecting standard   Sorting table    
7.1 Crack Cracks length of back ≤1/5 back, Width≤0.2mm,depth≤0.5mm, Crack No.≤2 Visual 100% Operator
Cracks length of cutting≤1/5 axis height, width ≤0.2mm,depth≤1mm, Crack No.≤1 Visual 100% Operator
Cracks length of inner arc≤1/3 thickness, width ≤0.2mm,depth≤1mm, Crack No.≤2 Visual 100% Operator
Crack length of angle anywhere≤tatal1/5, width≤0.2mm,depth≤1mm,Crack No.≤1 Visual 100% Operator
Crack length of angle anywhere≤total 1/5,width≤0.2mm,depth≤0.5mm, Cracks No.≤1 Visual 100% Operator
Cracks length of side anywhere   ≤total 1/5, width≤0.2mm depth≤0.5mm, crack No.≤1 Visual 100% Operator
7.2 Fade for internal & external angle Fade area of internal angel face≤10mm2(measured by inner circumstance), also to external angle≤10mm2(measured by out circumstance), face area though thickness≤4mm2,(measured by inner circumstance),average fade depth≤1mm Contrast to prototype 100% Operator
7.3 Interior & exterior fade Area for interior& exterior fade≤20mm2,depth≤0.5mm Visual 100% Operator
7.4 Un-grind Outer circumstance unground  not allowed Visual 100% Operator
Inner circumstance unground not allowed, area unground≤15% total circumstance area, area unground without any defect Visual 100% Operator
7.5 Chamfering Chamfering angle (customer draw) Visual 100% Operator
Chamfer width (customer draw) Visual 100% Operator
7.6 Impurity No non-magnetic object stay on surface after grinding Visual 100% Operator
Only allow 2 air holes(1*1mm2), depth≤0.5mm Visual 100% Operator
7.7 Finish No indent allowed on finished surface Visual 100% Operator
7.8 Cleanliness No magnetic powder or black spot stay on surface Visual 100% Operator
7.9 Point 3 or more defects of each item exist on single magnet are not qualified      
8 Final inspect standard        
8.1 Sampling As “GB2828.88 sample sort and sheet bulks by bulks” Randomly once/batch QC
8.2 Property test Magnetic flux (based on prototype) fluxmeter, magnetic flux measuring coil 5pcs/batch QC
Central magnetic force central magnetic tester, magnetiizer, magnetizing coil, central bobbin, motor shell 10pcs/batch QC
8.3 Product size Length, width, height (thickness) Vernier caliper,1000-feet 25pcs/batch QC
8.4 Appearance Refer to (7.1—7.9) Visual once/batch QC

Since energy shortage, improving ferrite magnet product grade, avoiding repeat production, and increasing product profit have become core solutions to promote magnetic industry in China.