Summary of the most popular die finishing

2022-07-26
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Summary of mold finishing

the finishing process of mold cavity is the last process of mold processing, and it is the most important link that directly affects the quality of mold. It accounts for about 30% ~ 40% of the total mold processing volume, so it is highly valued by experts at home and abroad. In China, although most of the processes of mold processing (turning, milling, planing, grinding, EDM, wire cutting, etc.) have been highly automated, most of the mold finishing processing still adopts manual processing, which has seriously affected the development of molds in China to a certain extent

the so-called finishing is a processing method to reduce the surface roughness of parts on the premise of ensuring the accuracy of the part surface. At present, the commonly used methods are: manual polishing, ultrasonic polishing, chemical and electrochemical polishing and so on. Among these methods, manual polishing is the most commonly used finishing method, because manual polishing is flexible and can process any complex cavity, but at the same time, this method has high labor intensity, low production efficiency and no guarantee of product quality. Although other methods have achieved good results in terms of product quality, processing efficiency and worker strength, due to the complexity, diversity and irregularity of the mold cavity, these processing tools are difficult to process along the contour line of the workpiece, and sometimes are limited by the space of these cavities. Therefore, many finishing methods can only be used in some fields, But it is difficult to popularize widely

problems existing in the surface finishing of mold cavity

most foreign mold manufacturers adopt the integration of mold design, processing and even assembly, that is, the integration of mold cad/cam/cae, using mold CAD software and reverse engineering to design; The virtual reality system is used to test the assembly mold, and the interference is found and adjusted in time. Only when there is no problem can the machining be carried out; In the process of machining, the machining center and cad/cam are used to integrate the whole machining process, that is, the workpiece can be machined once installed, so the accuracy of the workpiece can be guaranteed

nevertheless, the finishing of the surface of the mold cavity is still a world problem, mainly due to the following problems:

◆ the diversity and irregularity of the mold cavity. In many cases, the cavity surface of the die is a three-dimensional irregular free-form surface. Due to the different shapes of these surfaces, it brings great trouble to the motion path and feeding of the tool or abrasive tool during finishing. That is to say, modern NC machining technology is used to control the movement of cutting tools or abrasives, but it also brings great difficulties to the programming of NC program, so this is the fundamental reason why it is difficult to realize automation in die finishing

◆ poor adaptability and flexibility of tools or abrasives used for mold finishing. Due to the particularity of the mold surface, the cutting tool or abrasive tool for machining it is required to have a good self-adjusting ability, that is, the so-called self adaptability. It is necessary to change its own running path with the change of the machining contour shape. Of course, this refers to fine adjustment. This requires that the tools for machining the mold cavity have a certain range of plasticity, that is, flexibility

◆ the mold surface requires high precision and smoothness. This is also determined by the characteristics of the die itself. As a model for processing workpiece, the accuracy of the die directly determines the accuracy of the workpiece. It also plays a crucial role in its own life, corrosion resistance, wear resistance and whether the workpiece can be taken out of the die smoothly after processing. Even though some machining methods have high machining accuracy, when used in mold machining, it is difficult to ensure the original geometric tolerance of the workpiece while improving the surface finish of the mold, and the result is also very incompetent

at present, the precision machining methods of mold cavity in China are still two aspects of mechanical machining and electrical machining, and there is a trend that electrical machining is becoming more and more dominant; The other is the application of die cad/cam technology. However, due to the characteristics of the die itself, the shape is complex and difficult to standardize, so the development of cavity die cad/cam is not as fast and mature as that of stamping die and plastic die on cad/cam. Nevertheless, this is still a development direction of cavity mold processing method. Among these machining methods, Japan, as one of the most developed countries in carbon material industry, has developed rapidly in milling technology, grinding technology in mechanical machining and EDM Technology in electrical machining. The brief introduction is as follows

(1) the rise of milling technology - high speed milling

milling is an important processing method for cavity dies, especially suitable for the processing of medium and large forging dies. In recent years, milling has achieved rapid development, which is mainly reflected in the following aspects:

◆ high precision: the era that milling is an ordinary processing has passed. The positioning accuracy of the machine tool has increased from ± 12mm/800mm in the 1980s to ± 2 ~ 5mm/full stroke in the 1990s. The thermal balance structure of the precision machine tool and the cooling of the spindle are adopted to control the thermal deformation. The control resolution has been increased from 1mm to 0.2mm. In this way, the machining accuracy is increased from the original ± 10mm to ± 2 ~ 5mm, and the precision level can reach ± 1.5mm, making the milling machine enter the field of precision machine tools

◆ high speed machining efficiency: with the progress of cutting tools, motors, bearings and CNC systems, high-speed milling technology is rising rapidly. At present, the spindle speed has increased from 4000 ~ 6000r/min to 14200r/min. The cutting feed speed is increased to 1 ~ 6m/min, the rapid feed speed is increased from 8 ~ 12m/min to 30 ~ 40m/min, and the tool change time is reduced from 5 ~ 10s to 1 ~ 3S, which greatly improves the machining efficiency. Compared with ordinary machining methods, the machining efficiency of high-speed milling can be increased by 5 ~ 10 times

◆ high hardness of milling materials: high speed milling technology combined with new tools (cermet tools, PCBN tools, special cemented carbide tools, etc.) can process workpieces with hardness of 36 ~ 52hrc, and even 60HRC workpieces

The development of high-speed milling technology has promoted the progress of mold processing technology. In particular, it has injected new vitality into the manufacturing of medium and large-scale cavity molds in the automobile and household appliance industries

(2) EDM is facing new challenges

high speed milling technology has developed. As another important means of cavity mold machining, EDM is also quite perfect. However, as a machining system, it is indeed facing new challenges of high speed milling

◆ technological progress of EDM: due to the progress of related technologies such as micro precision machining pulse power supply, working fluid and mixed silicon powder machining process, the surface roughness of EDM can reach rmax0.6 ~ 0.8mm, and large-area machining can be carried out. And because the electrode damage can start the experiment after being checked by the instructor, the consumption is continuously reduced (at least 0.1%) and the machining allowance is accurately controlled, it can be said that EDM has entered the field of precision machining

◆ challenges faced by EDM: as high-speed milling can process materials with hardness of 36 ~ 52hrc, or even 60HRC, almost all cavity mold materials can be processed, changing the situation that only high hardness materials can be machined by EDM. The machining efficiency of high-speed milling is 4:1 compared with that of EDM, and some are even 7 ~ 8 times that of EDM, and the manufacturing of electrodes is saved. High speed milling also has certain machining accuracy and good surface roughness. Foreign countries believe that in the field of cavity mold machining, high-speed milling can replace EDM, which is not groundless. Because of this, in the application field, especially in the automotive industry, EDM has the danger of being squeezed out by high-speed milling. However, EDM has irreplaceable advantages in machining deep grooves, narrow slots, ribs, textures and so on. But on the whole. The application field of EDM in machining has narrowed, and part of the market has been occupied by other machining equipment, which will have a greater impact on the development of large EDM machine tools

◆ development strategy of EDM: EDM is a machining system formed for decades, and it is also developing continuously. In view of the development of milling technology, the "EDM milling" technology has recently been put forward to compete with it. In general, "EDM milling" aims to improve the efficiency of EDM forming and machining, using forming (graphite electrode) and using water as working fluid. Compared with using oil as working fluid, its machining efficiency is improved by 2 ~ 3 times. It is called "EDM milling" abroad, which represents a development direction. However, compared with high-speed milling, its overall machining efficiency still has a big gap. The high-speed rotating spindle is used to drive the rod (tubular) electrode to rotate, cooperate with the NC track movement and servo feed of the workbench and spindle, and its processing and forming method is similar to mechanical milling. This kind of "EDM milling" can store standard tube electrodes with different diameters in the electrode library and form them in the NC feed, which greatly simplifies the design, manufacturing and management of the electrodes. This is a new development strategy, but it also has the problem of low processing efficiency. It is expected that "EDM milling" will make new progress and compete with high-speed milling

with the selection of experimental scheme; With the rapid development of electronics, electrical appliances, communications, computers and other industries, the processing of precision, micro and complex molds is becoming more and more, and the market is becoming larger and larger. The processing of these molds is the advantage of EDM. Therefore, at the same time of competition, we should give full play to the advantages of EDM, that is, we should focus on the direction of precision, complex and micro mold processing, which is another important direction of EDM development

(3) grinding is still the main means of precision die machining.

grinding is a precision machining technology. So far, the grinding precision has been very high, up to 1 ~ 2mm, and the machined surface quality is also very good. The surface roughness is generally ra0.04 ~ 0.32 μ m. In addition, by grinding, the machined surface has no defects such as softening layer and metamorphic layer, so it is widely used in the machining of precision molds. With the increase of the types of grinding machines, such as coordinate grinding machines, forming grinding machines, light bending grinding machines and special mold processing grinding machines, especially the improvement of the degree of numerical control, the range of processing is becoming larger and larger, and the precision is becoming higher and higher. Not only can "At present, the polyurethane exterior wall insulation products produced by some small enterprises in the market process cold stamping dies, and can also process various cavity molds, such as forging molds, plastic molds, etc., so grinding is still the main means of precision mold processing.

the following will focus on the magnetic particle grinding technology.

the principle of magnetic particle grinding technology

magnetic grinding is to put magnetic abrasive in the magnetic field, which is formed under the action of magnetic force Magnetic particle brush: when the workpiece moves relative to the magnetic pole in the magnetic field, the magnetic particle brush will grind the surface of the workpiece. Because the magnetic particle brush has good adaptability and flexibility, it is very conducive to the machining of complex surfaces

characteristics of magnetic particle grinding

compared with other processing methods, magnetic particle grinding has the following characteristics:

◆ the workpiece does not contact with the magnetic pole, and the magnetic

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