A mold is composed of many parts. Quality of parts directly affects quality of mold. Final quality of parts is guaranteed by finishing. Therefore, it is important to control finishing. In most domestic mold manufacturing companies, methods used in 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 specific production practice, there are many difficulties in operation, but there are still many effective experience methods worth learning.
Processing of mold parts can be roughly divided into three categories according to appearance and shape of parts: plates, special-shaped parts and shafts. Common process is roughly: rough machining-heat treatment (quenching, quenching and tempering)-fine grinding-electrical machining-fitter (surface treatment)-assembly processing.

 

In heat treatment process of parts, while obtaining required hardness of parts, it is also necessary to control internal stress to ensure dimensional stability of parts during processing. Different materials have different treatment methods. With development of mold industry in recent years, 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, severe stress, new material powder alloy steels, such as V10, ASP23, etc., can be used. Such materials have high thermal stability and good organization.
For parts made of Cr12MoV, quenching is performed after rough machining. After quenching, workpiece has a large residual stress, which is easy to cause cracks during finishing or work. After quenching, parts should be tempered while hot to eliminate quenching stress. Quenching temperature is controlled at 900-1020℃, then cooled to 200-220℃, air-cooled, and then quickly returned to furnace for tempering at 220℃. This method is called a primary hardening process, which can obtain higher strength and wear resistance. This effect is better for molds whose main failure mode is wear. Some workpieces with many corners and complex shapes are 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 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, quenching at 1050-1080℃, and then tempering at 490-520℃ for many times to obtain higher impact toughness and stability. It is very suitable for molds with chipping as main failure mode. Cost of powder alloy steel is relatively high, but its performance is good, and it is becoming a trend of widespread use.

There are three main types of machine tools used in grinding: surface grinders, internal and external cylindrical grinders, tool abrasives. During finishing grinding, grinding deformation and grinding cracks should be strictly controlled. Even very small cracks will be exposed in subsequent processing and use. Therefore, feed of fine grinding should be small, not large, coolant should be sufficient, parts with size tolerance within 0.01mm should be ground as much as possible. From calculation, it can be seen that for a 300mm long steel, when temperature difference is 3℃, material will change about 10.8μm, 10.8=1.2*3*3 (1.2μm/℃ per 100mm deformation), each finishing process needs to fully consider influence of this factor .
It is very important to choose right grinding wheel during fine grinding. For high vanadium and high molybdenum condition of mold steel, it is more suitable to choose GD single crystal corundum grinding wheel. When processing hard alloys and materials with high quenching hardness, organic binder diamond grinding wheel is preferred. Organic binder grinding wheel has good self-grinding properties, and roughness of ground workpiece can reach Ra=0.2μm. In recent years, with application of new materials, CBN grinding wheels, namely cubic boron nitride grinding wheels have shown very good processing results. It is better than other types of grinding wheels for finishing on CNC profile grinding, coordinate grinder, CNC internal and external cylindrical grinders. During grinding process, care should be taken to dress grinding wheel in time to keep grinding wheel sharp. When grinding wheel is passivated, it will rub and squeeze on the surface of workpiece, causing burns on the surface of workpiece and reducing its strength.
Most plate parts are processed by surface grinders. In process, a long and thin sheet part is often encountered, which is difficult to process. Because during processing, under action of magnetic adsorption, workpiece deforms and clings to surface of worktable. When workpiece is removed, workpiece will return to deformation. Thickness measurement is consistent, but parallelism cannot meet requirements. Solution can be a magnetic isolation grinding method. When grinding, contour block is used to cushion workpiece underneath, and four sides are blocked by stopper. When processing, small feed and multiple smooth knives are used. After one side is processed, it is not necessary to pad high block and directly absorb processing, which can improve grinding effect and meet requirements of parallelism.
Shaft parts have revolving surfaces, internal and external cylindrical grinders, tool grinders are widely used for processing. During processing, headstock and top are equivalent to bus bar. If there is a runout problem, processed workpiece will also have this problem, which will affect quality of parts. Therefore, headstock and top inspection work must be done before processing. When grinding inner hole, coolant should be poured to grinding contact position to facilitate smooth discharge of grinding. For processing thin-walled shaft parts, it is best to use a clamping technology table, clamping force should not be too large, otherwise it is easy to produce "inner triangle" deformation on the circumference of workpiece.

Modern mold factories cannot 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 electric spark.
Machining accuracy of slow-moving wire cutting can reach ±0.003mm, and roughness Ra0.2μm. When processing starts, check condition of machine tool, check water deionization degree, water temperature, verticality of wire, tension and other factors to ensure a good processing state. Wire cutting is removal of processing on a whole piece of material. It destroys original stress balance of workpiece and easily causes stress concentration, especially at the corners. Therefore, when R<0.2 (especially sharp corners), suggestions for improvement should be made to design department. Method to deal with stress concentration in processing can use principle of vector translation. Before finishing, leave a margin of about 1mm, pre-process rough shape, and then perform heat treatment to release processing stress before finishing to ensure thermal stability.
When processing punch, cutting position and path of wire should be carefully considered. The best effect is achieved by punching and threading. High-precision wire cutting, usually four cutting passes, can guarantee quality of parts. When processing taper dies, based on a fast and efficient standpoint, the first round of roughing straight edges, the second round of taper processing, then finishing straight edges, so that X segment vertical finishing is not required, only straight edge of cutting edge is finished, which saves time and cost.
EDM first needs to make electrodes. Electrodes can be coarse and fine. Precision machining electrode requires good shape conformity, and it is best 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 electrode has good overall performance, especially consumption during processing is obviously smaller than that of copper. With a sufficient amount of scouring fluid, it is very suitable for processing of difficult-to-process materials and finishing of parts with complex cross-sectional shapes. When making electrodes, you need to calculate amount of electrode gap and number of electrodes. When processing large areas or heavy electrodes, workpiece and electrode should be clamped firmly to ensure sufficient strength to prevent processing looseness. When performing deep step processing, pay attention to wear of electrode and arc discharge caused by poor drainage.

Tool marks and wear marks left on the surface of parts are place where stress is concentrated and source of crack propagation. Therefore, after processing, surface of parts needs to be strengthened, and hidden troubles of processing need to be dealt with by fitter. Some edges, acute angles, and orifices of workpiece are dulled and rounded. Generally, electro-processed surface will produce a metamorphic hardened layer of about 6-10μm. Color is off-white, hardened layer is brittle and contains residual stress. Hardened layer must be fully eliminated before use. Method is surface polishing and grinding to remove hardened layer.
In the process of grinding and electrical processing, workpiece will be magnetized to a certain extent, with weak magnetic force, and it is easy to attract some small things. Therefore, before assembly, workpiece should be demagnetized and surface should be cleaned with ethyl acetate. During assembly process, first refer to assembly drawing, find all parts, then list equipment sequence between parts, list items to be noted, and proceed to assemble mold. Assemble guide pin and guide sleeve first, then install mold base, male and female molds, adjust gaps, especially gap between male and female molds. After assembly is completed, mold inspection should be implemented and overall situation report should be written. For problems found, reverse thinking method can be used, that is, from back process to process, from finishing to roughing, checking one by one until crux is found and problem is solved.
Practice has proved that a good finishing process control can effectively reduce parts out of tolerance and scrap, effectively improve first-time success rate and service life of mold.