Process technology and quality control of mass pro

Abstract: the performance selection of soft ferrite in different applications is briefly analyzed. Combined with the influence of each manufacturing department on the core smashing performance, size, appearance quality, cost, etc., the process and technical requirements of each process and the control methods and control factors of product quality are clearly pointed out

key words: soft ferrite; volume production; Process technology; Quality control

0 preface

after more than 40 years of development, the process of soft magnetic ferrite has become increasingly mature and stable, and the differences between enterprises are narrowing day by day, such as formula and sintering process, and there is little technology. At present, the annual growth rate of China's plastic pipe market is 15% In the future, backward production capacity will be eliminated one after another, but the large-scale and stable production of high-quality soft ferrite will be more difficult. This is because there are many brands of soft magnetic ferrite materials, the shape and size of magnetic cores are different, users' requirements for quality are higher and higher, and the delivery time is greatly shortened. In order to control costs and reduce business risks, magnetic core manufacturers must greatly reduce inventory (which will shorten the production and technical preparation cycle to a certain extent), which brings many difficulties to mass production. The crux of mass production appears in various links such as material preparation, molding, sintering, grinding and subcontracting, and process technology and quality control should also be closely run through these links

1 basic ideas on the technological process and quality control of soft ferrite production the development of extruder industry still has a large market space. Think

soft ferrite is made of Fe, Zn, Mn or Ni oxides mixed in a certain proportion, pre burned, crushed, granulated, pressed, sintered and ground. Soft ferrite is divided into MnZn ferrite and NiZn ferrite. The output and dosage of MnZn ferrite are much larger than NiZn ferrite. This paper only briefly describes the mass production technology and quality control of MnZn ferrite

in the mass production of soft ferrite, it is very important to do a good job of technical quality control. It is one of the important ways to reduce production cost by strengthening technical quality control and improving product qualification rate. The quality control of soft ferrite mass production technology, in short, is to put the basic idea of prevention and control through the whole cattle production and management process from the selection of raw materials to the delivery of products

many process technicians believe that achieving excellent magnetic properties is the most important goal of technical quality control in the production of magnetic cores. In fact, this is a more one-sided understanding. According to the basic physical effects, a soft magnetic material cannot have all kinds of favorable magnetic properties at the same time, and some magnetic properties are always compromised, such as the highest permeability and the lowest power loss cannot be obtained at the same time. In actual production, some magnetic characteristics should be selectively guaranteed according to the different needs of users. For example, in the application of high inductance components, the high inductance factor al of the magnetic core should be emphatically guaranteed; In the application of power transformer, the requirement for power loss PC is higher; In the application of flyback transformer, the magnetic core needs to have high DC superposition characteristics. Therefore, in mass production, the excellent magnetic properties of soft ferrite is not the only important goal, which largely depends on its application. Other aspects such as mechanical properties, appearance quality, cost or delivery time are also important, and in some cases even more important. Adhering to the basic idea of giving priority to prevention and control, just from the control link of material characteristics, it is to select materials with different formulas or different sintering processes according to the differences in the use requirements of different users and different cores, solidify the process, standardize the management, realize "management process and operation standardization", simplify complex problems and repeat simple problems, In this way, errors can be avoided and the contradictory and unified relationship between product quality, production efficiency and product cost can be correctly handled

2 process technology and quality control of mass production of soft ferrite

each manufacturing process of soft ferrite has different effects on the characteristics, appearance quality, cost, delivery time, etc. of the magnetic core. Table 1 visually summarizes this correlation. Of course, the evaluation in Table 1 is also subjective, which may vary according to the status of enterprise process equipment and process matching, but it points out the key problems in the production and operation of the whole soft ferrite. Technical quality control is to serve production and operation. Therefore, it is necessary to work in combination with these practical problems

2.1 selection of raw materials and formula

one of the keys to improve the characteristics of soft ferrite is in the formula (including adding impurities in secondary ball milling), so we should focus on selecting the main formula, which requires high purity, less harmful impurities such as chlorine and acid, good chemical activity and fluidity, appropriate particle size distribution, and good matching of the specific surface area of the three main formulas. In terms of power ferrite, the current formula of domestic and foreign manufacturers is about: Fe203 (53 ~ 54) mol~a), MnO (35 ~ 40) M01%, ZnO (8 ~ 12) M01%. For example, for pc44, the formula taken by some manufacturers is: Fe203 53.3m01%, mn0 36.5m01%, zn010.2mol%. In order to promote solid-state reaction, facilitate melting, prevent grain growth, improve material properties and increase mechanical strength, some beneficial impurities, such as A1203, hfo3, nbo205, TiO2, V2O5, Cr2O3, Cao, ZrO, Pb3O4 and coo, should be added to the material formula with better performance price ratio, but the amount of addition should be controlled well, too much will be harmful

in this process, we should focus on preventing and controlling the purity of raw materials and ensuring the accuracy of formula weighing

2.2 material preparation

taking the typical oxide method material preparation (wet mixing) process as an example, this process includes primary sand (or ball) grinding, primary spray drying, pre burning, secondary ball (or sand) grinding, secondary spray drying and other processes. The main purpose of primary sand (or ball) grinding is to ensure that the main ingredients are mixed evenly. From the perspective of production efficiency, generally choose the sand grinding method, which takes only about 1H (ball milling takes about 6h). Material: Ball: water =1:2 ~ 2.5:o.6 ~ o.7, and use equal diameter steel balls. The purpose of one-time spray drying is to dry and granulate the evenly mixed raw materials to make them have a certain density, which is conducive to the solid-phase reaction and improve the pre firing efficiency, and reduce the wall sticking phenomenon of the pre fired medium materials in the kiln. During pre firing, it is necessary to determine the appropriate pre firing temperature and holding time of key materials for integrated circuits, advanced semiconductor materials and new display materials in the field of new generation information technology in Zhejiang Province according to the kiln body (rotary kiln has better effect) and raw materials used. This has a great impact on the shrinkage of green slabs, the fluidity of granular materials (secondary materials), the loose density and the selection of secondary sintering temperature curve. Generally, the pre firing temperature of soft ferrite is about 1000 ℃, and the effect of pre firing in rotary kiln is better

the pre fired billet is a partial ferrite material with many pores, defects and low density. It is crushed and ground by a ball mill to make it into a particle size conducive to compression molding. This process is customarily called secondary ball milling, and the ball milling time is about 12 ~ 16 h, so as to ensure that the average particle size of the granular material is 1 μ Below M. Impurities are usually added during secondary ball milling, and some adhesives such as polyvinyl alcohol (PVA) are added to meet the requirements of dry pressing

the last step of the material preparation process is to granulate in the spray dryer, and make the powder into particles with good fluidity, certain strength and viscosity, so as to facilitate molding. Dry press molding requires that the moisture content of granular material is 0.2% ~ o 4%, and the particle size is 100 ~ 350 μ M is appropriate, and it is best to have a normal distribution (150 ~ 300 μ M accounts for more than 80%. At the same time, the loose density is required to be ≥ 1.32g/cm3, which will reduce the layering of the formed blank and the "arch bridge phenomenon" when the granular material fills the mold cavity, so as to improve the strength and density uniformity of the formed blank. In this process, we should focus on the prevention and control of the average particle size, particle size and its distribution, loose density, water content, quality and addition of adhesive after secondary ball milling

2.3 molding

molding is one of the key processes in the manufacturing process of soft ferrite, and dry pressing molding is often used. The forming quality has a great influence on the geometric size, appearance and electromagnetic performance of the magnetic core. The uniformity of the density of the formed blank is particularly important. The uneven density of the blank will lead to defects such as cracking, delamination and deformation of the sintered products. These defects are common in can shaped, highly high arm thin EC, eel shaped and other magnetic cores. Molding is one of the most difficult processes in batch production. It has high requirements for particle size, fluidity, cohesiveness, mold, press, debugging personnel and operators. Improper operation of the press, poor characteristics of the granular material used, or inadequate mold design will cause uneven microstructure of the product and internal cracks. The fluidity of granular material determines the filling speed and filling effect of granular material in the mold cavity; The mold design should comprehensively consider the pressing direction, relevant dimensions, mold cavity height, female mold blank taper according to the product shape, blank shrinkage ratio, particle loading ratio, formability, etc

in this process, we should focus on preventing and controlling the selection and use of granular materials with different characteristics, the weight of green body, the bottom thickness of magnetic core and the uniformity of density, and strictly prevent the internal cracking or layering of product batches from flowing into the next and upper processes

2.4 firing

firing directly determines the final composition, phase distribution, grain size, compactness, size, appearance and properties of soft ferrite. The appropriate sintering temperature and sintering curve shall be determined according to the different sintering equipment used, the pre sintering temperature, the shrinkage of the pre sintering material, the type and addition proportion of the binder, the product performance requirements, shape and size, the weight and method of blank loading, etc. Generally speaking, in the heating stage (about from room temperature to 500 ℃), it is mainly the evaporation process of moisture, adhesive and lubricant in the blank. At this time, the temperature must be raised slowly to avoid blank cracking; After that, the billet gradually shrinks, and the heating rate can be appropriately increased, but the heating rate should be appropriate from 900 ℃ to 1200 ℃, because this stage of firing affects the size, uniformity, porosity and distribution of core grains; After reaching the maximum sintering temperature, there should be a holding section of about 3 ~ 5h; In the cooling stage, the cooling rate and oxygen content also have a great impact on the electromagnetic performance and qualification rate of products

in the firing process, we should focus on preventing product adhesion, deformation and cracking; Focus on the control of oxygen content, the change of kiln tail pressure and the consistency of product appearance, size and performance. According to the different requirements of users and products, standardize the process, and implement the standardized operation mode of Ding kiln, constant temperature, constant atmosphere, constant blank placement and regular dredging of rubber discharge pipes

2.5 grinding

only a few burned cores can be used directly, and most of them must be ground to obtain the mechanical size and appearance that meet the requirements of users. The grinding methods of magnetic cores usually include straight-line through type, disk through type and disk periodic type. The processing efficiency of the straight-line passing type is very high, but it needs to be padded with sand belt and steel belt, and the magnetic core is also moving relative to the table, so its accuracy is poor, the consistency of inductance is high, and users need to open air gaps in batches