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Analysis of mold finishing control

Update:03-06-2020
Summary:

1 Introduction A mold is composed of many parts. The qu […]

1 Introduction
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, many technical parameters such as part deformation, internal stress, shape tolerance and dimensional accuracy should be well controlled. In the specific production practice, the operation is more difficult, but there are still many effective empirical methods worth learning.

2. Process control of mold finishing
Processing of mold parts, a general guiding ideology is to adapt to different materials, different shapes, and different technical requirements. It has certain plasticity, and can achieve good processing results by controlling the processing.
According to the appearance and shape of the parts, the parts can be roughly divided into three categories: shafts, plates and shaped parts. The common process is roughly: roughing-heat treatment (quenching, quenching and tempering)-fine grinding-electric Processing-fitter (surface treatment)-assembly processing.

2.1, parts heat treatment
The heat treatment process of parts, while making 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 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, for some convex and concave molds with high working strength and severe stress, new material powder alloy steel can be used. , Such as V10, ASP23, etc., such materials have high thermal stability and good organization.
For parts made of Cr12MoV, quenching is performed after roughing. After quenching, there is a large residual stress in the workpiece, which is likely to cause cracking in finishing or work. After the part is quenched, it should be tempered while hot to eliminate the quenching stress. The quenching temperature is controlled at 900-1020°C, then cooled to 200-220°C and then air-cooled, and then quickly tempered at 220°C. This method is called a one-time hardening process, which can obtain higher strength and wear resistance. The mold, which is the main failure mode, has better effect. In the production, some workpieces with many corners and complicated shapes are encountered. Tempering is not enough to eliminate the quenching stress. Before finishing, stress relief annealing or multiple aging treatments are required to fully release the stress.
For powder alloy steel parts such as V10, APS23, etc., because they can withstand high temperature tempering, secondary hardening process can be used during quenching, 1050-1080 ℃ quenching, and then 490-520 ℃ high temperature tempering and carried out multiple times, can get more The high impact toughness and stability are very suitable for molds with chipping as the main failure mode. The cost of powder alloy steel is high, but its performance is good, and it is forming a trend of wide application.

2.2, Grinding of parts
There are three main types of machine tools used for grinding processing: surface grinders, internal and external cylindrical grinders and tool grinding tools. When finishing grinding, it is necessary to strictly control the deformation of grinding and the occurrence of grinding cracks. Even very small cracks will be revealed in subsequent processing and use. Therefore, the feed for fine grinding should be small, not large, the coolant should be sufficient, and the parts with dimensional tolerances within 0.01mm should be grinded at a constant temperature. It can be seen from the calculation that for a 300mm long steel part, when the temperature difference is 3℃, the material has a change of about 10.8μm, 10.8=1.2×3×3 (deformation amount per 100mm is 1.2μm/℃), each finishing process needs to fully consider this One factor.
It is very important to choose the proper grinding wheel for fine grinding. For the high vanadium and high molybdenum condition of the mold steel, it is more suitable to use GD single crystal corundum grinding wheel. When processing hard alloys and materials with high quenching hardness, organic bonding is preferred Abrasive diamond grinding wheels and organic binder grinding wheels have good self-grinding, and the roughness of the workpieces can reach Ra=0.2μm. In recent years, with the application of new materials, CBN grinding wheels, that is, cubic boron nitride grinding wheels have shown Very good processing effect, finishing on CNC molding grinding, coordinate grinding machine, CNC internal and external cylindrical grinding machine, the effect is better than other types of grinding wheels. During the grinding process, pay attention to dressing the grinding wheel in time to maintain the sharpness of the grinding wheel. When the grinding wheel is passivated, it will slip and squeeze on the workpiece surface, causing burns on the workpiece surface and reduced strength.
The processing of board parts is mostly processed by surface grinders. In the processing, a long and thin sheet part is often encountered. The processing of such parts is difficult. Because during processing, under the action of the magnetic force, the workpiece deforms and adheres to the surface of the table (see Figure 1). When the workpiece is removed, the workpiece will be deformed again, and the thickness measurement is consistent, but the parallelism is not up to Requirement, the solution can use the magnetic isolation grinding method (see Figure 2), when grinding with a constant height block under the workpiece, the four sides of the stop block to die, small infeed during processing, multi-light knife, after processing one side, can There is no need to pad high-density blocks and directly absorb the processing, which can improve the grinding effect and meet the parallelism requirements.
Shaft parts have a revolving surface, and its internal and external cylindrical grinding machines and tool grinding machines are widely used in their processing. In the process of processing, the headstock and the top are equivalent to the bus bar. If there is a jump problem, the processed workpiece will also cause this problem, which affects the quality of the parts. Therefore, the headstock and the top should be tested before processing. When grinding the inner hole, the coolant must be poured into the grinding contact position to facilitate the smooth discharge of the grinding. For processing thin-walled shaft parts, it is best to use a clamping process table. The clamping force should not be too large, otherwise it will easily produce "inner triangle" deformation on the circumference of the workpiece.

2.3, electric processing control
Modern mold factory can't lack electric machining. Electric machining can process all kinds of special-shaped and high-hardness parts. It is divided into two types: wire cutting and EDM.
The precision of slow wire cutting can reach ±0.003mm, and the roughness Ra=0.2μm. At the beginning of the process, first check the condition of the machine tool, check the deionization of water, water temperature, perpendicularity of the wire, tension and other factors to ensure a good processing state. Wire cutting is a process of removing a whole piece of material, which destroys the original stress balance of the workpiece, and it is easy to cause stress concentration, especially at the corner, so when R <0.2 (especially sharp corners), it should be The design department makes suggestions for improvement. 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 a rough shape, and then heat-treat, so that the processing stress is released before finishing to ensure thermal stability.
When processing the punch, the selection of the wire cutting position and path should be carefully considered. As shown in Figure 3, the left end of the workpiece is clamped. It is better to choose route ① than route ② during processing, because route ① the workpiece and the clamping part of the material are tightly connected and the processing is stable. If route ② is used, after the first pass, the workpiece Formed into a cantilever shape, the force is poor, which affects the subsequent processing. Route ③, using punching and threading processing, the best effect. High-precision wire cutting, usually four cutting passes, can ensure the quality of parts. When processing a die with a taper, see Figure 4, in a fast and efficient position, the first pass roughing the straight edge, the second side taper processing, and then finishing the straight edge, so that it is not necessary to perform X-section vertical For finishing, only the straight edge of the cutting edge is finished, which saves both time and cost.
Electro-discharge machining must first produce electrodes, which are divided into coarse and fine electrodes. The precision machining electrode requires good shape conformity, and it is best to use CNC machine tool to complete the processing. In the choice of electrode materials, the copper electrode is mainly used for general steel processing. The Cu-W alloy electrode has good overall performance, especially the consumption during processing is significantly smaller than that of red copper. With a sufficient amount of flushing fluid, it is suitable for the processing of difficult-to-machine materials and the finishing of complex cross-sectional shapes. Ag-W alloy electrode has better performance than Cu-W alloy electrode, but its price is high, resources are few, and it is generally less used. When making an electrode, it is necessary to calculate the amount of electrode gap and the number of electrodes. When processing a large area or heavy electrode, the workpiece and electrode must be clamped firmly to ensure sufficient strength to prevent loose processing. When performing deep-step processing, attention should be paid to the loss of the electrodes and the arc discharge caused by poor drainage.

2.4、Surface treatment and assembly
The surface of the part leaves knife marks and wear marks during the processing, which is the place where stress is concentrated and the source of crack propagation. Therefore, after the processing is completed, the surface of the part needs to be strengthened and polished by the fitter to deal with the hidden processing problems. Invert and blunt some edges, acute angles and orifices of the workpiece. Generally, the electro-machined surface will produce a metamorphic hardened layer of about 6-10 μm. The color is off-white. The hardened layer is brittle and has residual stress. Before use, the hardened layer must be fully eliminated by surface polishing and polishing to remove the hardened layer.
During grinding and electrical machining, the workpiece will have certain magnetization, with weak magnetic force, and it is very easy to attract 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 drawings, find the parts, then list the equipment order of each part, list all the precautions, and then start to assemble the mold. Generally, the guide column guide sleeve is installed, and then the mold is installed. Frame and convex and concave molds, and then make adjustments to the gaps, especially the gaps between the convex and concave molds, after the assembly is completed, mold testing should be carried out and the overall situation report should be written. For the problems found, you can use the reverse thinking method, that is, from the back process to the front process, from finishing to roughing, and check them one by one until you find the crux and solve the problem.

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

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