1. Introduction
a mold is composed of many mold parts. The quality of non-standard mold parts directly affects the quality of the mold, while the final quality of non-standard mold parts is guaranteed by finishing. Therefore, it is important to control the finishing.
in most domestic mold manufacturing enterprises, the methods adopted in the finishing stage are generally grinding, electric machining and fitter treatment. At this stage, it is necessary to control many technical parameters such as part deformation, internal stress, shape tolerance and dimensional accuracy. In the specific production practice, it is difficult to operate, but there are still many effective experience and methods worth learning from.
2. Process control of die finishing
the general guiding ideology for the processing of non-standard mold parts is to carry out adaptive processing for different materials, shapes and technical requirements. It has certain plasticity, and can achieve good processing results through the control of processing.
according to the different appearance and shape of parts, parts can be roughly divided into three categories: shaft, plate and special-shaped parts. The common process is roughly: rough machining - heat treatment (quenching and tempering) - fine grinding - electrical machining - fitter (surface treatment) - assembly processing.
2.1 heat treatment of parts
in the heat treatment process of parts, while making the parts obtain the required hardness, it is also necessary to control the internal stress to ensure the dimensional stability of parts during processing. Different materials have different treatment methods. With the development of the mold industry in recent years, the types of materials used have increased. In addition to Cr12, 40Cr, Cr12MoV and cemented carbide, new materials such as powder alloy steel V10 and asp23 can be used for some convex and concave dies with high working strength and severe stress, which have high thermal stability and good microstructure.
parts made of Cr12MoV shall be quenched after rough machining. After quenching, the workpiece has great residual stress, which is easy to cause cracking during finishing or work. After quenching, the parts shall be tempered while hot to eliminate quenching stress. The quenching temperature is controlled at 900-1020 ℃, then cooled to 200-220 ℃, discharged for air cooling, and then quickly returned to the furnace for tempering at 220 ℃. This method is called one-time hardening process, which can obtain high strength and wear resistance, and has a good effect on molds with wear as the main failure form. For some workpieces with many corners and complex shapes encountered in production, tempering is not enough to eliminate quenching stress. Stress relief annealing or multiple aging treatments are required before finishing to fully release the stress.
for V10, aps23 and other powder alloy steel parts, because they can withstand high-temperature tempering, secondary hardening process can be adopted during quenching, quenching at 1050-1080 ℃, and then tempering at 490-520 ℃ for many times. High impact toughness and stability can be obtained, which is very suitable for dies with blade collapse as the main failure form. Powder alloy steel has high cost, but its performance is good. It is becoming a widely used trend.
2.2 grinding of parts
there are three main types of machine tools used for grinding: surface grinder, internal and external grinder and tool grinder. During finishing grinding, the generation of grinding deformation and grinding cracks should be strictly controlled. Even very small cracks will be exposed in subsequent processing. Therefore, the feed of fine grinding should be small and not large, the coolant should be sufficient, and the parts with dimensional tolerance less than 0.01mm should be grinded at constant temperature as much as possible. According to the calculation, when the temperature difference of 300mm long steel parts is 3 ℃, the material has 10.8 μ M, 10.8=1.2 × three × 3 (1.2 deformation per 100mm μ M/ ℃), and the influence of this factor should be fully considered in each finishing process.
it is very important to select the appropriate grinding wheel for fine grinding. In view of the high vanadium and high molybdenum status of die steel, Gd single crystal corundum grinding wheel is more suitable. When machining cemented carbide and materials with high quenching hardness, diamond grinding wheel with organic binder is preferred. The organic binder grinding wheel has good self grinding performance, and the roughness of the workpiece can reach ra=0.2 μ m. In recent years, with the application of new materials, CBN grinding wheels, i.e. cubic boron nitride wheels, have shown very good machining results. The results of finishing machining on CNC forming grinding machines, coordinate grinding machines, CNC Internal and external grinding machines are better than other types of grinding wheels. During grinding, the grinding wheel shall be trimmed in time to maintain the sharpness of the grinding wheel. When the grinding wheel is passivated, it will slide and squeeze on the surface of the workpiece, causing burns on the surface of the workpiece and reducing the strength.
most of the plate parts are processed by surface grinding machines. In the process of processing, we often encounter a kind of long and thin sheet parts, which are difficult to process. Because the workpiece deforms and clings to the surface of the worktable under the adsorption of magnetic force during machining. When the workpiece is taken down, the workpiece will be deformed again, and the thickness measurement is consistent, but the parallelism cannot meet the requirements. The solution can be magnetic isolation grinding. During grinding, the contour block is placed under the workpiece, and the four side blocks are used to resist. During machining, small feed and multi light knife are used. After one side is machined, the contour block can not be placed, Direct adsorption processing can improve the grinding effect and meet the requirements of parallelism.
shaft parts have a rotating surface, which is widely processed by internal and external cylindrical grinding machines and tool grinding machines. In the process of machining, if the head frame and its equivalent bus bar have run out problems, the machined workpiece will also have this problem, which will affect the quality of parts. Therefore, the head frame and its equivalent bus bar shall be tested before machining. During inner hole grinding, the coolant shall be poured to the grinding contact position to facilitate the smooth discharge of grinding. For machining thin-walled shaft parts, it is better to use the clamping process table. The clamping force should not be too large, otherwise it is easy to produce "inner triangle" deformation on the circumference of the workpiece.
2.3 machining control
modern mold factories cannot lack EDM. EDM can process all kinds of special-shaped and high hardness parts. It can be divided into wire cutting and EDM.
the machining accuracy of slow wire cutting can reach ± 0.003mm, and the roughness ra=0.2 μ M. At the beginning of processing, check the condition of the machine tool, check the deionization degree of water, water temperature, perpendicularity of wire, tension and other factors to ensure a good processing state. Wire cutting is the removal of a whole piece of material, which destroys the original stress balance of the workpiece and easily causes stress concentration, especially at the corners. Therefore, when R ＜ 0.2 (especially at sharp corners), suggestions for improvement shall be made to the design department. The method to deal with stress concentration during machining can use the vector translation principle. Before finishing, leave an allowance of about 1mm, pre process the approximate shape, and then carry out heat treatment to release the machining stress before finishing, so as to ensure the thermal stability.
when machining punch, the cutting position and path of wire should be carefully considered. When the workpiece is clamped at the left end, it is better to select route ① than route ② during processing, because route ① is closely connected with the clamping part of the material, and the processing is stable. If route ② is adopted, the workpiece will be in a suspended wall shape after one pass of cutting, and the stress is poor, which will affect the subsequent passes of processing. Route ③: drilling and threading are adopted for better effect. In high-precision wire cutting, the number of times of cutting is usually four, which can ensure the quality of parts. When machining the female die with taper, roughen the straight edge, taper the second edge, and then finish the straight edge in a fast and efficient manner. In this way, it is not necessary to carry out the vertical finishing of section x, and only finish the straight edge of the cutting edge section, which saves time and cost.
electrodes should be made before EDM, which can be divided into coarse and fine electrodes. The shape of finish machined electrode shall conform to the requirements, and it is better to be machined with CNC machine tools. In terms of electrode material selection, red copper electrode is mainly used for general steel processing. Cu-W alloy electrode has good comprehensive performance. Especially, the consumption in the processing process is obviously smaller than that of red copper. With sufficient scouring liquid, it is very suitable for machining difficult materials and finishing machining of parts with complex cross-section shape.
Ag-W alloy electrode has better performance than Cu-W alloy electrode, but its price is high and resources are few, so it is generally less used. When making electrodes, it is necessary to calculate the clearance and number of electrodes. When machining large areas or heavy electrodes, the workpiece and electrode shall be firmly clamped to ensure sufficient strength and prevent loosening. During deep step machining, attention shall be paid to the loss of electrodes and arc discharge caused by poor liquid discharge.
2.4 surface treatment and assembly
the tool marks and grinding marks left on the part surface during machining are the places where the stress is concentrated and the source of crack propagation. Therefore, after machining, it is necessary to strengthen the part surface and polish it by fitter to eliminate the hidden dangers of machining. Some edges, acute angles and orifices of the workpiece shall be blunt and R-shaped. Generally, machined surfaces produce 6-10 μ The metamorphic hardening layer of about M is gray white in color. The hardening layer is brittle and has residual stress. Before use, the hardening layer shall be fully eliminated by surface polishing and polishing.
during grinding and electrical machining, the workpiece will be magnetized to a certain extent, with weak magnetic force, which is very easy to absorb some small things. Therefore, before assembly, demagnetize the workpiece and clean the surface with ethyl acetate. During the assembly process, first refer to the assembly drawing, find out all the parts, then list the equipment sequence between the parts, and list all the precautions, and then start to assemble the mold. Generally, the guide post guide sleeve is installed first, then the mold base and the male and female mold are installed, and then the clearance at each place, especially the clearance between the male and female mold, is assembled and adjusted. After the assembly is completed, the mold inspection shall be carried out and the overall situation report shall be written. For the problems found, the reverse thinking method can be adopted, that is, from the post process to the front process, from finish machining to rough machining, check one by one until the crux is found and the problem is solved.
3. Concluding remarks
practice has proved that good finishing process control can effectively reduce out of tolerance and scrap of parts, and effectively improve the one-time success rate and service life of the mold.
