Mold manufacturing finishing process

The mold is composed of many parts. The quality of parts directly affects the quality of the mold, and the final quality of parts is guaranteed by finishing. Therefore, it is important to control 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 specific production practice, it is more difficult to operate, but there are still many effective experience and methods worthy of reference.

The processing of mold parts can be roughly divided into three categories according to the different appearance shapes of 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 mold industry in recent years, the types of materials used have increased. In addition to Cr12, 40Cr, Cr12MoV and cemented carbide, new materials powder alloy steel, such as V10 and asp23, can be selected for some convex and concave dies with high working strength and severe stress. Such materials have high thermal stability and good microstructure.

For the parts made of Cr12MoV, quenching treatment shall be carried out after rough machining. There is a large residual stress in the workpiece after quenching, which is easy to lead to finish machining or cracking in work. After quenching, the parts shall 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 in production, tempering is not enough to eliminate quenching stress. Stress relief annealing or multiple aging treatment shall be carried out 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 tempering at 490-520 ℃ for many times, which can obtain high impact toughness and stability. It is very suitable for molds with blade collapse as the main failure form. Powder alloy steel has high cost, but its performance is good, which is forming a trend of wide application.

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 and use. 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 far as possible. According to the calculation, when the temperature difference of 300mm long steel is 3 ℃, the material is 10.8 μ M, 10.8 = 1.2 × three × 3 (1.2 deformation per 100mm) 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. For the high vanadium and high molybdenum condition 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 wheel, that is, cubic borax nitride wheel, has shown very good machining effect. It is better than other kinds of grinding wheels in NC 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 slip and squeeze on the workpiece surface, resulting in burn on the workpiece surface and reduced strength.

Most of the plate parts are processed by plane grinder. In the processing, we often encounter a kind of long and thin sheet parts, which is difficult to process. Because during processing, the workpiece will deform and cling to the surface of the worktable under the adsorption of magnetic force. When the workpiece is removed, the workpiece will return to deformation, and the thickness measurement is consistent, but the parallelism cannot meet the requirements. The solution can be magnetic isolation grinding method. During grinding, the contour block is placed under the workpiece, four blocks are blocked, small feed and multi light knife are used during processing, After machining a good side, it can be directly adsorbed without cushion and other high blocks, which 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 grinder and tool grinder. In the process of processing, the head frame and the equivalent bus bar, if there is a runout problem, the processed workpiece will also have this problem, which will affect the quality of parts. Therefore, the head frame and should be tested before processing. During inner hole grinding, the coolant shall be fully poured to the grinding contact position to facilitate the smooth discharge of grinding. For machining thin-walled shaft parts, the clamping process table shall be better adopted, and the clamping force shall not be too large, otherwise it is easy to produce "inner triangle" deformation on the circumference of the workpiece.

3. Electrical machining control

Modern mold factories cannot lack electric machining. Electric machining can process all kinds of special-shaped and high hardness parts. It is divided into wire cutting and electric spark.

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 is easy to cause 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 method to deal with stress concentration in 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 to ensure thermal stability.

When machining punch, the cutting position and path of wire should be carefully considered. Drilling and threading are adopted for better effect. High precision wire cutting, usually cutting times for four times, can ensure the quality of parts. When machining the female die with taper, rough machining the straight edge for the first time, taper machining for the second time, and then finish machining the straight edge. In this way, it is not necessary to carry out the vertical finishing of section x, but only finish machining the straight edge of the cutting edge section, which saves time and cost.

The electrode must be made before EDM. The electrode can be divided into coarse and fine. The shape of finish machining electrode shall be in good conformity and better processed with CNC machine tool. In terms of material selection of electrode, 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 the processing of difficult materials and the finishing processing of parts with complex cross-section shape. When making electrodes, it is necessary to calculate the gap 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 knife marks and grinding marks left on the part surface during processing are the places where the stress is concentrated and the source of crack propagation. Therefore, after processing, it is necessary to strengthen the part surface and polish it by fitter to eliminate the hidden dangers of processing. 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 grinding.　

In the process of grinding and electric machining, the workpiece will have a certain magnetization and 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 the parts, then list the equipment sequence between the parts, and list the matters needing attention, and then start to assemble the die. Generally, the guide post guide sleeve is installed first, and then the die base and male and female die are installed, and then the clearance of each part, especially the clearance of male and female die, is assembled and adjusted. After the assembly, the die inspection shall be carried out, Write an overall situation report. For the problems found, the reverse thinking method can be adopted, that is, from post process to forward process, from finish machining to rough machining, check them 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 one-time success rate and service life of molds.