Analysis on the finishing control of mold accessories

1. Introduction
A mold is composed of many mold parts. The quality of non-standard mold parts directly affects the quality of the mold, and the final quality of non-standard mold parts is guaranteed by finishing. Therefore, it is of great importance to control finishing.
In most domestic mold manufacturing companies, the methods used in the finishing stage are generally grinding, electrical machining and bench work. At this stage, many technical parameters such as part deformation, internal stress, shape tolerance and dimensional accuracy must be controlled. In specific production practice, there are many operational difficulties, but there are still many effective empirical methods worth learning from.

2. Process control of mold finishing
The processing of non-standard mold parts, a general guiding ideology is to perform adaptive processing for different materials, different shapes, and different technical requirements. It has a 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: shafts, plates and special-shaped parts. Their common process is roughly: rough machining - heat treatment (quenching, tempering) - fine grinding - electrical machining - benchwork (surface treatment) - assembly processing.
2.1 Heat treatment of parts
The heat treatment process of parts, while making the parts obtain the required hardness, also needs to control the internal stress to ensure the dimensional stability of the parts during processing. Different materials have different processing 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, for some convex and concave dies with high working intensity and harsh stress, new materials such as powder alloy steel, such as V10, ASP23, etc., can be selected. Such materials have high thermal stability and good organizational state.
For parts made of Cr12MoV, quenching treatment is performed after rough machining. After quenching, there is a large amount of residual stress in the workpiece, which is easy to cause cracking during fine machining or work. After quenching, the parts should be tempered while hot to eliminate quenching stress. The quenching temperature is controlled at 900-1020℃, then cooled to 200-220℃ and air-cooled, and then quickly returned to the furnace for tempering at 220℃. This method is called a one-time hardening process, which can obtain higher strength and wear resistance. It is more effective for molds with wear as the main failure form. In production, there are some workpieces with many corners and complex shapes. Tempering is not enough to eliminate quenching stress. Stress relief annealing or multiple aging treatments are required before fine machining to fully release stress.
For powder alloy steel parts such as V10 and APS23, because they can withstand high-temperature tempering, secondary hardening process can be used during quenching. Quenching at 1050-1080℃, and then tempering at 490-520℃ for multiple times can obtain higher impact toughness and stability, which is very suitable for molds with chipping as the main failure form. Powder alloy steel is expensive, but its performance is good and is forming a trend of widespread use.
2.2 Grinding of parts
There are three main types of machine tools used for grinding: surface grinders, internal and external cylindrical grinders, and tool grinders. During fine grinding, grinding deformation and grinding cracks must be strictly controlled. Even very small cracks will be revealed in subsequent processing and use. Therefore, the feed of fine grinding should be small, not large, the coolant should be sufficient, and parts with dimensional tolerances within 0.01mm should be ground at constant temperature as much as possible. It can be calculated that for a 300mm long steel piece, when the temperature difference is 3℃, the material changes by about 10.8μm, 10.8=1.2×3×3 (1.2μm/℃ deformation per 100mm), and each finishing process needs to fully consider the influence of this factor.
It is very important to choose the right grinding wheel for fine grinding. For the high vanadium and high molybdenum conditions of mold steel, it is more suitable to use GD single crystal corundum grinding wheel. When processing cemented carbide and materials with high quenching hardness, organic binder diamond grinding wheel is preferred. Organic binder grinding wheel has good self-grinding property, and the roughness of the ground workpiece can reach Ra=0.2μm. In recent years, with the application of new materials, CBN grinding wheel, that is, cubic boron nitride grinding wheel, has shown very good processing effect. It is better than other types of grinding wheel in fine processing on CNC forming grinding, coordinate grinding machine, CNC internal and external cylindrical grinding machine. During grinding, attention should be paid to timely dressing of the grinding wheel to keep it sharp. 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 its strength.
Most of the processing of plate parts is done by surface grinders. In the processing, a long and thin sheet part is often encountered, which is difficult to process. Because during processing, under the adsorption of magnetic force, the workpiece is deformed and close to the surface of the worktable. When the workpiece is removed, the workpiece will recover and deform. The thickness measurement is consistent, but the parallelism does not meet the requirements. The solution can be to use the magnetic isolation grinding method. When grinding, the equal height block is placed under the workpiece, and the four-sided block is used to stop it. During processing, the feed is small and the multi-light knife is used. After processing one side, there is no need to pad the equal height block. Direct adsorption processing can improve the grinding effect and meet the parallelism requirements.
Shaft parts have a rotating surface, and their processing widely uses internal and external cylindrical grinders and tool grinders. During the processing, the headstock and the are equivalent to the busbar. If there is a problem of beating, the workpiece processed will also have this problem, affecting the quality of the parts. Therefore, the headstock and the should be inspected before processing. When grinding the inner hole, the coolant should be fully poured to the grinding contact position to facilitate the smooth discharge of the grinding. When processing thin-walled shaft parts, it is better to use a 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 Electric machining control
Modern mold factories cannot lack electric machining. Electric machining can process various 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 Ra=0.2μm. At the beginning of processing, you should first 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 is to remove the processing on a whole piece of material. It destroys the original stress balance of the workpiece and can easily cause stress concentration, especially at the corners. Therefore, when R < 0.2 (especially sharp corners), improvement suggestions should be made to the design department. The method of dealing with stress concentration during processing can use the principle of vector translation. Before finishing, leave a margin of about 1mm, pre-process the general shape, and then perform heat treatment to release the processing stress before finishing to ensure thermal stability.
When processing the punch, the selection of the wire cutting position and path should be carefully considered. The left end of the workpiece is clamped. When processing, it is better to choose route ① than route ②, because the clamping part of the workpiece and the material of route ① is closely connected, and the processing is stable. If route ② is used, after the knife is fed, the workpiece becomes a cantilevered wall, the force is poor, and the subsequent processing is affected. Route ③, using punching and threading processing, the effect is better. High-precision wire cutting processing usually cuts four times to ensure the quality of parts. When machining a concave die with a taper, in the spirit of speed and efficiency, the straight edge is rough-machined first, the second edge is taper-machined, and then the straight edge is fine-machined. In this way, there is no need to perform vertical finishing in the X section, and only the straight edge of the cutting edge section is fine-machined, which saves both time and cost.
Electro-spark machining must first make electrodes, which are divided into rough and fine electrodes. The fine-machining electrode requires good shape conformity and is better to be processed by CNC machine tools. In terms of electrode material selection, copper electrodes are mainly used for general steel processing. Cu-W alloy electrodes have good comprehensive performance, especially the consumption during processing is significantly smaller than that of copper. With sufficient flushing fluid, they are very suitable for processing difficult-to-process materials and fine-machining of parts with complex cross-sectional shapes.
Ag-W alloy electrodes have better performance than Cu-W alloy electrodes, but they are expensive and have fewer resources, so they are generally less used. When making electrodes, it is necessary to calculate the gap between the electrodes and the number of electrodes. When processing large areas or heavy electrodes, the workpiece and the electrode should be firmly clamped to ensure sufficient strength to prevent loosening during processing. When deep step processing is performed, attention should be paid to the loss of electrodes and arc discharge caused by poor drainage.
2.4 Surface treatment and assembly
The knife marks and grinding marks left on the surface of the parts during processing are places where stress is concentrated and the source of crack expansion. Therefore, after the processing is completed, the parts need to be surface strengthened and polished by benchwork to eliminate processing hazards. Some edges, sharp angles, and openings of the workpiece are blunted and R-ized. Generally, the electro-processed surface will produce a metamorphic hardened layer of about 6-10μm, which is grayish white in color. The hardened layer is brittle and has residual stress. The hardened layer must be fully eliminated before use. The method is surface polishing and grinding to remove the hardened layer.
During the grinding and electro-processing process, the workpiece will be magnetized to a certain extent, with weak magnetic force, and it is very easy to attract some small things. Therefore, before assembly, the workpiece must be demagnetized and the surface must be cleaned with ethyl acetate. During the assembly process, first refer to the assembly drawing, find all the parts, then list the order of the parts to be equipped with each other, list the precautions, and then start to assemble the mold. Generally, the guide pins and guide sleeves are installed first, and then the mold frame and the male and female molds are installed. Then, the gaps at various places, especially the gaps between the male and female molds, are assembled and adjusted. After the assembly is completed, the mold should be inspected and an overall report should be written. For the problems found, the reverse thinking method can be used, that is, from the back process to the front process, from finishing to roughing, one by one, until the crux is found and the problem is solved.

3. Conclusion
Practice has proved that good finishing process control can effectively reduce the part tolerance and scrapping, and effectively improve the one-time success rate and service life of the mold.