In most mold manufacturing enterprises in China, 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 practices, operation is more difficult, but there are still many effective empirical methods that are worth learning from.

General guiding idea for processing of mold parts is to adapt to different materials, different shapes, and different technical requirements. It has a certain degree of plasticity and can achieve good processing results by controlling processing.
Depending on appearance and shape of parts, parts can be roughly divided into three categories: shafts, plates and special-shaped parts. Common process is roughly: rough machining-heat treatment (quenching, and tempering)-fine grinding-electrical machining-fitting (surface treatment) -assembly processing.

 

Heat treatment process of parts, while obtaining required hardness of part, also needs to control internal stress to ensure dimensional stability of part 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 hard alloys, for some convex and concave molds with high working strength and severe stress, new material powder alloy steel such as V10, ASP23 can be selected. Such materials have high thermal stability and good organization.
For parts made of Cr12MoV, quenching is performed after roughing. After quenching, workpiece has a large residual stress, which easily causes cracking during finishing or work. After quenching, part should be tempered while hot to eliminate quenching stress. Quenching temperature is controlled at 900-1020℃, cooled to 200-220℃ and air-cooled out of furnace, then quickly tempered at 220℃. This method is called primary hardening process, which can obtain higher strength and wear resistance, has a better effect on molds with wear as main failure mode. In production, some workpieces with many corners and complicated shapes are encountered. Tempering is not enough to eliminate quenching stress. Before finishing, stress relief annealing or multiple aging treatments are required to fully release stress.
For V10, APS23 and other powder alloy steel parts, because they can withstand high temperature tempering, a secondary hardening process can be used during quenching, quenching at 1050-1080℃, then tempering at 490-520℃ for many times. High impact toughness and stability are very suitable for molds with chipping as main failure mode. Cost of powder alloy steel is high, but its performance is good, and it is forming a widespread application trend.

There are three main types of machine tools used for honing grinding: surface grinding machines, internal and external cylindrical grinding machines, and tool grinding tools. During finish grinding, deformation of grinding and occurrence of grinding cracks must be strictly controlled. Even very small cracks will be revealed in subsequent processing and use. Therefore, feed for fine grinding must be small, not large, coolant must be sufficient, and parts with a dimensional tolerance within 0.01mm must be ground as constant as possible. From calculation, it can be known that when temperature difference of 300mm steel parts is 3 ℃, material has a change of about 10.8μm, 10.8 = 1.2*3*3 (1.2μm /℃ deformation per 100mm), and each finishing process must fully consider influence of this factor.
It is very important to choose a right grinding wheel for precision grinding. For high vanadium and high molybdenum conditions of mold steel, use of GD single crystal corundum grinding wheels is more suitable. When processing hard alloys and materials with high quenching hardness, diamond wheels with organic binders are preferred. Organic binder wheels have good self-grinding properties, and workpieces can be roughened up to Ra = 0.2 μm. In recent years, with application of new materials, CBN grinding wheels, also known as cubic boron nitride grinding wheels, have shown very good machining effect. It is better processed on CNC form grinding, coordinate grinding machines, CNC internal and external cylindrical grinding machines, and effect is better than other types of grinding wheels. During grinding process, pay attention to trim grinding wheel in time to keep grinding wheel sharp. When grinding wheel is passivated, it will slip and squeeze on the surface of workpiece, causing surface of workpiece to burn and reduce strength. 
Most of plate parts are processed by surface grinders. A long and thin plate part is often encountered in processing, and it is difficult to process such parts. Because during processing, workpiece is deformed under magnetic force and adheres to surface of table (see Figure 1). After workpiece is removed, workpiece will be deformed again, thickness measurement is consistent, but parallelism cannot be achieved. Solution can be magnetic isolation grinding method (see Figure 2). During grinding, contour block is placed under workpiece, and supported by four-sided stopper. During processing, tool is fed in small, multi-blade. After processing one side, it is not necessary to pad contour block, directly absorb and process, which can improve grinding effect and achieve parallelism requirement.
Stern shaft parts have a turning surface, their internal, external cylindrical grinders and tool grinders are widely used for processing. During processing, headstock and center are equivalent to bus bar. If there is a runout problem, this problem will also occur in processed workpiece, which will affect quality of part. Therefore, headstock and top inspection should be done before processing. When internal hole grinding is performed, cooling liquid should be fully poured to grinding contact position to facilitate smooth discharge of grinding. For processing thin-walled shaft parts, it is best to use a clamping process table. Clamping force should not be too large, otherwise it will easily cause "inner triangle" deformation on workpiece circumference.

 

In modern mold manufacturing enterprises, electrical machining is indispensable. Electrical machining can process various special-shaped and high-hardness parts. It is divided into two types: wire cutting and electric spark.
Precision of slow wire cutting can reach ±0.003mm, and roughness Ra = 0.2μm. At the beginning of processing, check condition of machine tool first, check deionization degree of water, water temperature, verticality of wire, tension and other factors to ensure a good processing state. Wire cutting is to process a whole piece of material. It destroys original stress balance of workpiece and easily causes stress concentration, especially at corners. Therefore, when R <0.2 (especially sharp corners), improvement suggestions should be made to design department. Method of processing stress concentration during processing can use vector translation principle. Before finishing, leave a margin of about 1mm, pre-machine rough shape, then perform heat treatment to allow processing stress to be released before finishing to ensure thermal stability.
When processing punch, position of wire cut and choice of path should be carefully considered. As shown in Figure 3, left end of workpiece is clamped. It is better to choose route ① than route ② during processing, because route ① is tightly connected to clamping part of material and processing is stable. If route ② is used, after the first pass, workpiece becomes cantilevered, and force is poor, which affects subsequent passes. Route ③, the best effect is achieved by punching and threading. High-precision wire cutting processing, usually four cutting passes and can guarantee quality of parts. When machining a die with a taper, as shown in Figure 4, in a fast and efficient position, first pass is roughing straight edges, second side is taper machining, and then finishing straight edges. This eliminates need to perform X-section vertical finishing and only straight edges of cutting edge, which saves time and costs.
Electro-discharge machining must first make electrodes, which are coarse and fine. Finished electrodes are required to have good shape conformance, and it is best to use CNC machining. In terms of electrode material selection, copper electrode is mainly used for general steel processing. Cu-W alloy electrode has good comprehensive performance, especially consumption is obviously smaller than that of copper in process of processing. With sufficient amount of scouring liquid, it is very suitable for processing of difficult-to-machine materials and complex processing of complex sections. Ag-W alloy electrode has better performance than Cu-W alloy electrode, but its price is high, resources are low, and it is generally less used. When making an electrode, you need to calculate gap between electrodes and number of electrodes. When processing large areas or heavy electrodes, workpiece and electrode must be clamped firmly to ensure sufficient strength to prevent processing from loosening. When performing deep step processing, pay attention to loss of electrodes and arc discharge caused by poor drainage.

Tool marks and abrasion marks on the surface of parts during machining are places where stress is concentrated and source of crack propagation. Therefore, after machining is completed, surface of part needs to be strengthened and polished by fitting to remove hidden processing hazards. Edges, acute angles, and orifices of workpiece are dulled and R-shaped. Generally, electro-machined surface will have a modified hardened layer of about 6-10 μm, which is grayish white, hardened layer is brittle and has residual stress. Before use, hardened layer should be fully eliminated. Method is to polish surface and remove hardened layer.
In process of grinding and electric processing, workpiece will be magnetized to a certain degree, with weak magnetic force, it is very easy to attract 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 order of each part, list all items that should be paid attention to, and then start to assemble mold. Generally, firstly install guide column and guide sleeve, then install mold base, convex and concave mold, and adjust gaps everywhere, especially clearance between convex and concave molds. After assembly is completed, mold inspection should be carried out and an overall condition report shall be written. For problems found, you can use reverse thinking method, that is, from post process to front process, from finishing to roughing, one by one to check, until crux of problem is found and problem is solved.
Practice has proved that good control of finishing process can effectively reduce part tolerances and scrap, and effectively improve first-time success rate and service life of mold.