Permanent ferrite magnet production technology innovation
Advance the cost-effective of permanent ferrite magnet
“High performance, low cost, developing new applications of products as well as building industry chain should be a basic guideline for realizing sustainable and higher developments of permanent ferrite magnet, moreover, the whole magnetic industry in China. On the way to construct a leading manufacturing country in permanent ferrite magnet yield, we have made a great progress and we will make further illustrious achievement in future.” mentioned in the last dissertation of Sir Shuixiao He, a famous scientist in China who has made great contributions to technical-fruit application, industrialization manufacturing, technical innovation as well as academically exchange in the magnetic industry.
The manufacturing of permanent ferrite magnet has made a great achievement via the continuous efforts of many scientists and it will keep developing smoothly. Permanent ferrite magnet, AlNiCo alloy and rare earth are the main members of permanent magnetic family, which have found their own position through competition with each other. Permanent ferrite magnet has formed its predominance for high cost-effective, excellent chemical stability and high HCJ. Coming through the technology innovation in the second half of last century, we have established the main composition and the subsidiary for the material and found a solution of mixing composition rate by great effort. Especially in material selection, we use iron phosphor or fine powder instead of Fe2O3 as the basic composition for producing permanent ferrite magnet. It greatly lowers the cost for manufacturing permanent ferrite and lays the foundation for further becoming a high output country. Still, the frame of products is a bit unreasonable. Large gaps on aspects of product property, specification, types and strain ability still exist when competing to foreign countries, which will lead our enterprise to a disadvantage position. The major job currently is to advance product property under the contiue lowing cost condition, improve production technology for high-class magnets as well as production equipment to advance a new level of product quality and achieve a higher cost effective further. Moreover, this will also help to take up an advantage position in the ferrite market competition. Innovation products from TDK and other metal manufacturer are setting up new overtaking target for us.
The way to advance cost effective
There are two ways to improve product cost effective. One is to lower product cost and the other is to advance product performance. But normally the two ways are contradictory. Only by advancing technology, improving quality of products as well as raising product passing rate, the cost can be lowered indeed. So both ways oppose each other while complement each other. The point is to advance the manufacturing technology and management.
After technical advancing of several times and production making of many years, there basically comes up a phenomena that the first process are separately from the second flowing on industrialization manufacture of permanent ferrite magnet. The first process normally runs form mixing of material to pre-sintering of material, so called “material pre-sinter”, and the second process runs from coarse powdering, fine powdering, adding additive, forming, grinding, inspection and so on. Following comes further discussion on individual aspects.
2.1 Basic materials preparation
- Material proportion
- Materials mixing and milling
For strontium ferrite, the material component can be adjusted in a limited range based on index request of Br, HCJ and so on. Different kinds of materials are optional. For example, different brands of Fe2O3 can be chosen according to different ferrite property requirements. Iron phosphor is also optional to achieve the lowest cost.
Generally speaking, it is not easy to control component and particle size of using iron phosphor than using Fe2O3. However, many magnetic material workers still would spend much of time and energy on it due to the cheap price. Through hard and fruitfulworking, they have made great achievements.
Due to the unstable constitution of Fe2O3, Iron Phosphor, Phosphor powder or other widely used raw materials for ferrite, the value weight out is still hard be the basis on controll of production artwork. So physical analysis, chemical analysis and technology test are still necessary before feeding materials. Adjust the proportion of material and set rules of material-mixing based on the test result to make sure the production process stable and secure.
Before establishing the proportion, it needs a complete and equal mixing in order to achieve the anticipative shape by pre-sinter.
The complete and equal mixing of material mentioned above is hard. Many workers make a judgment for the quality of material based on the number of white point, which is very coarse and inaccurate. Only if the material is fine enough, SrCO3 and Fe2O3 can be mixed together according to specified ratio. So it is better to complete the mixing and milling in the same process. It would be best to mill material with a wet mixing by specified type ball mill, or plus additional processing by grinder to improve particle size distribution of the first materials. This is also an innovation of technology.
Of course, dry pressing can also be applied on producing materials of some special specifications. Although the product performance and the production environment are worse, it saves a lot of energy and cost.
- Generally there are two kinds of pre-sintering technology. One is using rotary kiln, the other is using 4 holes pushing plate coal-kiln. Both technologies need the compound of pre-sintering to be shaped as a milling and caking. It would be possible to lower cost of product further if omit this process.
- Bring SHS technology to permanent ferrite powdering process
SHS is a technology to synthesize ceramic powder by burning base material and corresponding metal powder under high temperature, which is wildly applied on material constructing and utility. The college and research institute are also extending wide study and have made some achievement on producing Mn-Zn ferrite.SHS proves to be quite suitable for producing ferrite by using Iron-Phosphor after analyzing strontium ferrite constitutes.
We hope to bring SHS high temperature synthesis technology to Sr ferrite synthesizing process, so that we can get magnetic powder of fully reaction, high synthetic, loose and easy to be shaped into single particle. The powder is helpful for establishing magnetic field and sintering.
Permanent ferrite magnet SHS technology flowchart:
Comparing to rotary kiln, SHS sintering and synthesizing furnace differs from working principle to equipment configuration, neither compound sorting machinery nor fine mill. All equipments need technical innovation. Only by using new equipment, powder quality can be ensured and SHS technology can be realized.
2.3 Pasty material preparation
High quality pasty material is required to obtain ideal orientation degree. It requires pasty material possessing low water content, high fluid and low precipitation as it is possible. As to magnetic powder, it is far from enough to describe its average particle distribution. It is necessary to know the character and distribution of particles and meet the required orthodox distribution. According to this point, the powdering artwork should be weighed up by the times to be cost on milling. Firstly it should be sorted before milling, which means to make it sort on process of milling. Separating powder can meet the requirement of particle, and rest would be milled further till meets the standard. But what should be paid more attention, such as the property of powder becomes worse as the particle come to a critical value.
It is necessary to adjust water content of pasty material and add more additive, which will help to increase the intensity of final product and establish magnetic field.
The basic index of raw product density, orientation of magnetic compound as well as water proportion have been standardized in forming process, but some details still need to be paid more attention to.
- The equivalence of the density and demagnetization and others where on process should be guaranteed. It is necessary to avoid making abrupt change as starts from point to line, even extended to the whole, or else, which may not only impact the property of products but get product to distortion and leaped.
- It should be carefully to choose material. High-destiny and high hardness material fits for large and complicate mould, meanwhile, it should be strong enough to demould smoothly and prevent from the imposing of strength.
- There have some strictly requirement about verticality and homocentricity. It is necessary to smoothly move and avoid to wrecking the raw product due to improper mould and demould.
- To decrease outer force to demould process, the flow should be decreased accordingly. For most products, backing mould instead of moving mould can ensure the short process of demould and stable operation.
It will help to decrease the crack of products by putting some additive into pasty material.
The press mould machinery with high filling capacity should be the best choice on further technology innovation.
- Drying of raw product
- It is different on sintering between permanent ferrite and soft Mn-Zn ferrite. The solidification reaction has been secondary for sintering permanent ferrite. Strictly, the fundamental material has completed it and formed the heteromorphism before pre-sinter. Workers usually make control of density according to the size of single particle for milling process. The raw products will be heated up and then lead to contracted and compacted under the temperature of pre-sinter, meanwhile compound will become close with assistance of mineralization additive and flux. For the rest unfinished M shape compound, it is necessary to continue the solidification reaction. So the sintering of M shape permanent ferrite is easier.
The passing rate of sintered product without damaging from outside pressure normally depends on its water content rate. Therefore, the products with large sizes normally need a drying process additionally before sinter.
It should be careful to control the proportion of water and working temperature on drying process to pervert from generating a cover, which not only slows drying down but also tends to cause the crack of finished product.
There are 2 kinds of sinter system including high auto sinter furnace and domestic exclusive 4-hole pressing coal furnace. Both of them have their own advantages and applications. They play an important role on domestic permanent magnet sintering.
For sintering large magnet, not only the electromagnetism property of magnet should be considered, but also the mechanism performance of magnet. So, the temperature curve and the circumstance for sintering them should be moderately adjusted. It is wrong point to slow heating process to advance magnet performance. Actually the quality of product mainly depends on the moderate temperature.