1. Introduction
A mold is assembled from numerous mold parts. The quality of non-standard mold parts directly affects the overall quality of the mold, and the final quality of these non-standard parts is guaranteed by finishing processes. Therefore, controlling the finishing stage is of great significance.
In most domestic mold manufacturing enterprises, the methods generally adopted in the finishing stage include grinding, electrical discharge machining (EDM), and benchwork fitting. During this stage, many technical parameters such as part deformation, internal stress, form tolerances, and dimensional accuracy must be controlled. In specific production practices, there are many operational difficulties, but there are still many effective empirical methods worth referencing.
2. Process Control of Mold Finishing
The general guiding principle for the processing of non-standard mold parts is adaptive processing based on different materials, shapes, and technical requirements. It possesses a certain degree of plasticity, and through the control of processing, excellent results can be achieved.
Depending on the appearance and shape of the parts, they can be roughly categorized into three types: shafts, plates, and irregular parts. Their common technological process is roughly: Roughing — Heat Treatment (Quenching, Tempering) — Precision Grinding — Electrical Machining (EDM/WEDM) — Benchwork (Surface Treatment) — Assembly.
2.1 Heat Treatment of Parts
The heat treatment process, while obtaining the required hardness for the part, also needs to control internal stress to ensure dimensional stability during processing. Different materials require 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, new powder metallurgy alloy steels such as V10 and ASP23 can be selected for punches and dies that operate under high intensity and harsh stress conditions. These materials possess high thermal stability and an excellent microstructure.
For parts made of Cr12MoV, quenching is performed after roughing. Significant residual stress exists in the workpiece after quenching, which can easily lead to cracking during finishing or operation. Parts should be tempered while still hot after quenching to eliminate quenching stress. The quenching temperature is controlled at 900-1020°C, then cooled to 200-220°C for air cooling, and then quickly returned to the furnace for tempering at 220°C. This method, known as the primary hardening process, can achieve high strength and wear resistance, and is effective for molds where wear is the primary failure mode. For workpieces with many corners or complex shapes, tempering alone may not be sufficient to eliminate quenching stress; stress-relief annealing or multiple aging treatments are required before finishing to fully release the stress.
For powder alloy steel parts such as V10 and ASP23, because they can withstand high-temperature tempering, a secondary hardening process can be used: quenching at 1050-1080°C, followed by multiple high-temperature temperings at 490-520°C. This achieves high impact toughness and stability, making it very suitable for molds where chipping is the primary failure mode. Although the cost of powder alloy steel is high, its excellent performance is making it a widely used trend.
2.2 Grinding of Parts
There are three main types of machine tools used for grinding: surface grinders, internal/external cylindrical grinders, and tool grinders. During precision grinding, the occurrence of grinding deformation and grinding cracks must be strictly controlled; even minute cracks will reveal themselves in subsequent processing or use. Therefore, the feed in precision grinding should be small, coolant should be sufficient, and parts with dimensional tolerances within 0.01mm should be ground at a constant temperature as much as possible. Calculation shows that for a 300mm long steel part, a temperature difference of 3°C leads to a change of approximately 10.8μm (deformation rate of 1.2μm/°C per 100mm). This factor must be fully considered in all finishing processes.
Choosing the appropriate grinding wheel is crucial. For mold steels with high vanadium and molybdenum content, GD single-crystal corundum wheels are suitable. When processing cemented carbide or materials with high quenched hardness, diamond wheels with organic binders are preferred. Organic binder wheels have good self-sharpening properties, achieving surface roughness up to Ra=0.2μm. In recent years, CBN (Cubic Boron Nitride) wheels have shown excellent results in CNC profile grinding, coordinate grinding, and CNC internal/external cylindrical grinding. During grinding, the wheel must be dressed in time to maintain sharpness. A passivated wheel will rub and squeeze the workpiece surface, causing surface burns and reduced strength.
Plate parts are mostly processed on surface grinders. Long and thin plate parts are difficult to process because they deform under magnetic adsorption, sticking tightly to the table surface. When removed, the part undergoes springback deformation; although the thickness measurement might be consistent, the parallelism fails to meet requirements. A solution is the magnetic isolation grinding method, where equal-height blocks are placed under the workpiece, and blocks on four sides are used to secure it. Use small feeds and multiple finishing passes. After one side is processed, the other side can be ground using direct suction without blocks, which improves parallelism.

Shaft parts have rotary surfaces and are widely processed using cylindrical and tool grinders. If the headstock or center has runout issues, the workpiece will inherit these problems. Therefore, the headstock and center must be inspected before processing. During internal hole grinding, coolant should be fully applied to the contact point to facilitate smooth chip discharge. For thin-walled shaft parts, it is best to use a clamping process tab, and the clamping force should not be excessive to avoid "inner triangle" deformation on the circumference.

2.3 Electrical Machining Control2.3 Electrical Machining Control
Modern mold factories cannot do without electrical machining, which can process various irregular and high-hardness parts. It is divided into Wire EDM (WEDM) and Sinker EDM.
Slow-speed Wire EDM accuracy can reach ±0.003mm with a roughness of Ra=0.2μm. Before starting, check the machine condition, water deionization level, water temperature, wire verticality, and tension. Wire EDM removes material from a whole block, destroying the original stress balance and causing stress concentration, especially at corners. Therefore, when R < 0.2 (especially sharp corners), improvement suggestions should be made to the design department. To handle stress concentration, the vector translation principle can be applied: leave a margin of about 1mm before finishing, pre-machine the rough shape, and then perform heat treatment to release processing stress before final finishing.
When machining punches, the wire entry position and path must be carefully considered. Clamping the left end and choosing path ① is better than path ② because path ① maintains a tight connection between the workpiece and the material. In path ②, the workpiece becomes a cantilever after the first pass, leading to poor stability. Path ③, using a pre-drilled hole for wire threading, yields the best results. High-precision wire cutting usually involves four passes. When machining tapered dies, for efficiency, perform the first pass as a rough straight cut, the second for the taper, and then finish the straight edge. This saves time and cost by focusing precision on the cutting edge.
Sinker EDM requires electrodes, categorized into rough and finish electrodes. Finish electrodes must have high shape fidelity, preferably machined by CNC. Regarding electrode materials, copper electrodes are used for general steel. Cu-W alloy electrodes have good comprehensive performance and significantly lower wear than copper, making them suitable for difficult materials and complex cross-sections when used with sufficient flushing fluid.
Ag-W alloy electrodes perform even better than Cu-W but are expensive and scarce. When making electrodes, calculate the discharge gap and the number of electrodes. For large-area or heavy electrodes, clamping must be firm. During deep-step machining, pay attention to electrode wear and arc discharge caused by poor fluid drainage.

2.4 Surface Treatment and Assembly
Tool marks and grinding marks on the part surface are areas of stress concentration and sources of crack propagation. Therefore, after processing, surface strengthening is needed. Benchwork polishing is used to eliminate these hazards. Edges, sharp corners, and hole openings should be blunted or radiused (R-finished). Generally, the EDM surface produces a recast layer (white layer) of about 6-10μm, which is brittle and contains residual stress. This layer must be fully removed by polishing and grinding before use.
During grinding and EDM, workpieces may become magnetized. They must be demagnetized and cleaned with ethyl acetate before assembly. During assembly, refer to the assembly drawing, identify all parts, and list the assembly sequence and precautions. Assembly usually starts with guide pillars and bushings, followed by the mold base and the punch/die. Then, adjust the gaps, especially between the punch and die. After assembly, perform mold testing and write a comprehensive report. If problems are found, use reverse thinking — from the final process back to the initial, from finishing to roughing — to identify and solve the core issue.

3. Conclusion
Practice has proven that good finishing process control can effectively reduce part out-of-tolerance and scrapping, effectively improving the initial success rate and service life of the mold.