A mold is composed of many parts. The quality of the parts directly affects the quality of the mold, and the final quality of the parts is guaranteed by finishing. Therefore, it is important to control the finishing. In most domestic mold manufacturing enterprises, the methods used in the finishing stage are generally grinding, electrical machining and fitter processing. 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 experiences and methods to be used for reference.
The processing of mold parts can be roughly divided into three categories according to the appearance and shape of the parts: plates, special-shaped parts and shafts. The common process is roughly: rough processing - heat treatment (quenching and tempering) - fine grinding - Electrical processing - fitter (surface treatment) - assembly processing.
one
Heat treatment of parts
In the heat treatment process of parts, it is necessary to control the internal stress while making the parts obtain the required hardness to ensure the dimensional stability of the parts during processing. Different materials have different treatment methods. With the development of the die industry in recent years, the types of materials used have increased. In addition to Cr12, 40Cr, Cr12MoV and hard alloy, new materials such as V10 and asp23 can be selected for some male and female dies with high working strength and severe stress, which have high
thermal stability and good microstructure.
For parts made of Cr12MoV, quenching treatment is carried out after rough machining. After quenching, there is a large residual stress in the workpiece, which is easy to cause cracking during finishing or work. After quenching, the parts should be tempered while hot to eliminate the quenching stress. The quenching temperature is controlled at 900-1020 ℃, and then cooled to 200-220 ℃ 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 is better for molds with wear as the main failure form. For some workpieces with many corners and complex shapes in production, tempering is not enough to eliminate quenching stress. Stress relief annealing or multiple aging treatments are required before finishing to fully release stress.
For V10, aps23 and other powder alloy steel parts, because they can withstand high-temperature tempering, the secondary hardening process can be adopted during quenching, quenching at 1050-1080 ℃ and then tempering at 490-520 ℃ for several times, which can obtain high impact toughness and stability, and is very suitable for dies with broken edges as the main failure form. Powder alloy steel has high cost, but its performance is good, and it is forming a trend of wide application.
two
Grinding of parts
There are three main types of machine tools used for grinding: surface grinder, internal and external grinder and tool grinder. Grinding deformation and grinding cracks should be strictly controlled during finishing grinding. 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 dimension tolerance less than 0.01mm should be grinded at constant temperature as much as possible. According to the calculation, when the temperature difference of 300 mm long steel is 3 ℃, the material is 10.8 μ M, 10.8 = 1.2 × three × 3 (deformation per 100mm: 1.2 μ M / ℃), the influence of this factor should be fully considered in each finishing process.
It is very important to select an appropriate grinding wheel during fine grinding. In view of the high vanadium and molybdenum content of die steel, Gd single crystal corundum grinding wheel is more suitable. When processing hard alloy and materials with high quenching hardness, the 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 wheel, i.e. cubic boron nitride wheel, has shown very good processing effect, and the effect is better than other types of grinding wheels when finishing on CNC forming grinding, coordinate grinding machine and CNC Internal and external grinding machine. During grinding, attention shall be paid to timely dressing the grinding wheel 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 grinder. In the process of processing, a long and thin thin plate part is often encountered, which is difficult to process. Because the workpiece deforms and clings to the surface of the workbench under the adsorption of the magnetic force during processing, 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 equal height block is placed under the workpiece, and the four side blocks are blocked. During processing, the small feed and multi light knife can be used. After the one side is processed, the equal height block can not be placed, Direct adsorption processing can improve the grinding effect and meet the requirements of parallelism.
The shaft parts have a rotary surface, and the internal and external grinding machines and tool grinding machines are widely used for their processing. During the processing, the head frame and the center are equivalent to the bus bar. If there is a runout problem, the processed workpiece will also have this problem, which will affect the quality of the parts. Therefore, the head frame and the center should be tested before processing. When grinding the inner hole, the coolant shall be poured into the grinding contact position sufficiently to facilitate the smooth discharge of the grinding. For processing thin-walled shaft parts, it is better to use the clamping process table, and the clamping force should not be too large, otherwise it is easy to produce "inner triangle" deformation on the circumference of the workpiece.
three
Electrical machining control
Modern mold factories cannot lack electrical machining. Electrical machining can process all kinds of special-shaped and high hardness parts. It can be divided into wire cutting and electrical discharge.
The machining accuracy of slow wire cutting can reach ± 0.003mm, and the roughness Ra0.2 μ m。 At the beginning of processing, check the condition of the machine tool, check the deionization degree of water, water temperature, verticality of wire, tension and other factors to ensure a good processing state. Wire cutting processing is to remove processing on a whole piece of material, which destroys the original stress balance of the workpiece and easily causes stress concentration, especially at the corner. Therefore, when R ＜ 0.2 (especially sharp corner), improvement suggestions shall be put forward to the design department. The vector translation principle can be used to deal with the stress concentration during machining. Before finishing, a margin of about 1mm shall be reserved, and the rough shape shall be pre processed, and then heat treatment shall be carried out to release the machining stress before finishing to ensure thermal stability.
When machining punch, the cutting position and path of wire should be carefully considered. The best effect is achieved by drilling and threading. High precision wire cutting processing, usually four times of cutting, can ensure the quality of parts. When machining the female die with taper, the first rough machining of straight edge, the second taper machining, and then the finishing machining of straight edge shall be carried out in a fast and efficient manner. In this way, the vertical finishing of section x is not required, and only the straight edge of the edge section is finished, which saves time and costs.
The electrode must be made before EDM. The electrode can be divided into coarse and fine. The shape of the finished electrode shall be in good conformity, and it is best to use CNC machine tool for machining. In terms of material selection of electrodes, red copper electrodes are mainly used for general steel processing. Cu-W alloy electrode has good comprehensive performance. Especially, the consumption during processing is obviously smaller than that of red copper. With sufficient scouring liquid, it is very suitable for machining difficult materials and finishing parts with complex section shapes. When making electrodes, it is necessary to calculate the gap amount and the number of electrodes. When large-area or heavy electrode processing is carried out, 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 drainage.
four
Surface treatment and assembly
The tool marks and grinding marks left on the surface of parts during machining are the places where the stress is concentrated and the source of crack propagation. Therefore, after the machining, the surface of the parts needs to be strengthened, and the machining hidden dangers need to be removed by bench grinding. Some edges, acute angles and orifices of the workpiece shall be blunted and R-shaped. In general, electro machined surfaces produce 6-10 μ The color of the modified hardened layer is off white. The hardened layer is brittle and has residual stress. Before use, the hardened layer shall be fully removed by surface polishing and grinding.
In the process of grinding and electrical machining, the workpiece will have certain magnetization and weak magnetic force, and it is very easy to absorb some small things. Therefore, before assembly, the workpiece must be demagnetized and the surface should be cleaned with ethyl acetate. During the assembly process, first refer to the assembly drawing to find out the parts, then list the equipment sequence between the parts, list the matters needing attention, and then start to assemble the mold. Generally, the guide post and guide sleeve are installed first, then the mold base and the male and female mold are installed, and then the clearance of each part, especially the clearance of 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 forward process, from finishing to rough processing, check one by one until the crux is found and the problem is solved.
Practice has proved that good finishing process control can effectively reduce the out of tolerance and scrapping of parts, and effectively improve the one-time success rate and service life of molds.
