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, 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.
The processing of mold parts can be roughly divided into three types according to the appearance and shape of the parts: plate, special-shaped parts and shaft. The common process is roughly as follows: rough machining – heat treatment (quenching and tempering) – fine grinding – electrical machining – fitter (surface treatment) – assembly processing.
1. Part heat treatment
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.
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 ℃, 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 molds with blade failure as the main failure form.
Powder alloy steel has high cost, but its performance is good. It is becoming a widely used trend.
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 appropriate grinding wheels for fine grinding. In view of the high vanadium and high molybdenum status of die steel, Gd single crystal corundum grinding wheels are more suitable. When machining cemented carbide and materials with high quenching hardness, diamond grinding wheels with organic binder are preferred. The organic binder grinding wheels have 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 machine. In the process of processing, we often encounter a kind of long and thin sheet parts, which is 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 revolving surfaces, and internal and external cylindrical 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. 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.
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 best 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.
3. EDM control
Modern mold factories cannot lack EDM, which can process all kinds of abnormal and high hardness parts. It is divided into wire cutting and EDM.
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, 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. The best effect is achieved by drilling and threading. 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, the straight edge shall be roughened for the first time, the taper shall be machined for the second time, and then the straight edge shall be fine machined. In this way, it is not necessary to carry out the vertical fine machining of section x, and only the straight edge of the cutting edge section shall be fine machined, which saves time and cost.
The first step of EDM is to make electrodes, which can be divided into coarse and fine electrodes. The shape of finish machining electrode shall be in good conformity, and it is best to use CNC machine tool for machining. 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.
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.
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.
In the process of grinding and electrical machining, the workpiece will be magnetized to a certain extent, with weak magnetic force, and it 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.
Practice has proved that good finishing process control can effectively reduce out of tolerance and scrap of parts, and effectively improve the first-time success rate and service life of the die.
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About Huizhou Dj Molding Co., Ltd
Dj Mold is a China mold maker of plastic molds- injection mold, die casting moulds, plastic blow molding, rotational molding, medical plastic injection molding, two shot plastic injection molding, insert molding, overmolding, metal injection molding, micro injection molding, powder injection molding, ceramic injection molding, liquid injection molding, husky injection molding, household mold, casting mold, die mold tool, custom molds, china moulds, rapid prototyping tooling, plastic prototyping tooling, punch press tooling, die and tooling for mobile/ cell phone parts, automotive parts, vacuum cleaners, rechargeable tools, telephones, copiers, computers, multimedia speakers, and many other electronic products and household appliances. And also a plastic product manufacturer, mold manufacturer China– plastic parts, plastic water tank, plastic balls, plastic containers, plastic buckle, plastic anchor, plastic hanger, plastic spoon, plastic pipe fitting, plastic tumble, plastic tableware, plastic cups, plastic bottles, plastic tray, plastic cosmetic container, plastic case, plastic food container, plastic chairs, plastic caps, plastic cap closure, plastic tubes, plastic water pipes, plastic knobs, plastic tubing, plastic utility boxes, plastic racks and so on.
Person: James Yuan
Company: Huizhou Dj Molding Company Limited
Add: LongGang Village,LongXi Town,BoLuo County,HuiZhou City,GuangDong Province, China
Email: [email protected]