Molds, as fundamental process equipment in industrial production, have their quality directly determining the precision, appearance, and production efficiency of the products. The quality of a mold, ultimately, depends on the machining quality of its core components – the mold parts. A qualified mold part requires a series of rigorous, systematic machining processes to complete. These interlinked processes work together to ensure the part’s dimensional accuracy, form accuracy, positional accuracy, and surface quality. This article systematically elaborates on the typical machining processes for mold parts.
I. Preparatory Work Before Machining
Before machining officially begins, thorough preparatory work is the foundation for ensuring the smooth progress of subsequent processes.
Drawing Analysis and Process Planning: This is the primary and crucial step. Technical personnel need to analyze the part drawing in detail, understanding its structural characteristics, dimensional tolerances, geometric tolerances, surface roughness requirements, and the material used. Based on this, a rational machining process route is developed, determining the entire processing flow from the blank to the finished product, including which processes to select, their sequence, and the equipment and tools used for each process.
Material Preparation: According to the drawing requirements, select the appropriate mold steel, such as P20, 718, H13, S136, etc. Large steel blocks are cut into blanks close to the final part’s external dimensions using methods like sawing or flame cutting, this process known as blanking. Leaving a reasonable machining allowance for subsequent processes is the key focus of this stage.
Heat treatment (preparatory): For some mold parts with higher requirements, preparatory heat treatment such as annealing or normalizing is carried out before rough machining. The purpose is to eliminate the internal stress of the material, reduce hardness, improve the machinability, and create favorable conditions for subsequent mechanical processing.
II. Conventional Machining Processes
Conventional machining is the main means of forming mold parts, mainly including turning, milling, planing, grinding, drilling, etc.
Rough Machining:
Purpose: To quickly and efficiently remove the majority of the machining allowance, bringing the part’s shape and size close to the finished product. This stage does not pursue high precision and smoothness; the core objective is efficiency.
Main Methods: Typically performed using milling machines (including gantry mills) or lathes. On CNC machines, large-diameter tools with high cutting depths and feed rates are used for powerful cutting to remove excess material in the shortest time.
Notes: After rough machining, significant residual stresses remain inside the part, and the surface is relatively rough.
Semi-Finish Machining:
Purpose: On the basis of rough machining, further remove the allowance uniformly, leaving a stable, even, and appropriate amount of machining allowance (usually about 0.5mm per side) for finish machining. Simultaneously, complete the machining of some non-critical features.
Main Methods: Also using milling machines and lathes, but with smaller cutting parameters than rough machining to achieve better surface quality and dimensional stability.
Heat Treatment (Final):
Final heat treatment processes, such as quenching, tempering, carburizing, nitriding, etc., are usually scheduled after semi-finish machining and before finish machining.
Purpose: To enable the part to acquire the required mechanical properties like hardness, wear resistance, strength, and toughness. For example, mold cores and cavity surfaces often require high hardness (HRC48-55 or even higher) to resist the erosion and wear of plastic or molten metal.
Precision Finish Machining:
Purpose: This is the key process to ensure the final dimensional accuracy and surface quality of the part. Through finish machining, all technical indicators of the part fully comply with the drawing requirements.
Main Methods:
Precision Milling/Turning: Uses high-precision CNC machines with small-diameter tools, high cutting speeds, and low feed rates for fine cutting. This can achieve high form accuracy and good surface finish.
Jig Grinding: For hole-type parts requiring extremely high roundness, cylindricity, and position tolerance (e.g., guide pin/guide bushing holes, pin holes), the jig grinder is an essential equipment. It can achieve micron-level machining accuracy.
Surface Grinding/Profile Grinding: Mainly used for machining part surfaces, right angles, complex profiles, etc. Particularly for parts that have become hard after heat treatment, grinding is the only effective finishing method. Optical profile grinders can produce extremely precise punches and formed parts.
III. Non-Traditional Machining Processes
For areas where conventional machining is difficult, such as high-hardness materials, complex cavities, narrow slots, and deep holes, non-traditional machining plays an irreplaceable role.
Electrical Discharge Machining (EDM):
Principle: Utilizes the instantaneous high temperature generated by pulsed spark discharges between a tool electrode (copper, graphite, etc.) and a workpiece electrode (mold steel), both submerged in a dielectric fluid, to erode the workpiece material.
Applications:
Sinker EDM: Uses a formed electrode to replicate complex cavities, surfaces, text, patterns, etc. It is particularly adept at machining hardened steels.
Wire EDM (WEDM): Uses a moving metal wire (molybdenum wire, copper wire) as the electrode to cut the workpiece through electrical discharge. It is mainly used for machining various complex vertical-walled profiles and holes, such as punches and dies for blanking molds, inserts, etc. Wire EDM is divided into high-speed (fast) and low-speed (slow) wire machines, with the latter offering higher precision and surface quality.
Precision Hole Machining:
Deep Hole Drilling: For deep holes with a large length-to-diameter ratio, such as cooling water channels, specialized deep hole drilling machines and drills are required.
Reaming/Honing: For holes requiring high precision and high surface quality, reaming or honing is used as the final finishing method after drilling or boring.
IV. Finishing and Surface Treatment
When mold parts have extremely high requirements for surface roughness (e.g., requiring a mirror finish to produce high-gloss plastic products), finishing processes are necessary.
Manual Polishing: The most mainstream method for mold polishing. Operators use tools such as oil stones, sandpaper, diamond compound of varying grits from coarse to fine, to manually grind and polish the cavity surface. This is a skill-intensive and time-consuming process, where the polishing quality largely depends on the operator’s experience and skill.
Ultrasonic Polishing: Uses ultrasonic vibration energy to cause abrasives immersed in the working fluid to grind and polish the workpiece surface. It is suitable for polishing complex narrow slots and small holes.
Surface Treatment: To further enhance the wear resistance, corrosion resistance, and release performance of the mold surface, surface treatment techniques such as chrome plating, titanium plating (PVD, CVD), and nitriding are employed to form a hard, smooth film on the part surface.
V. Inspection and Assembly
Inspection: Runs throughout the entire machining process. This includes incoming inspection of blanks, dimensional checks between processes, and comprehensive inspection of the final product. Tools used range from conventional measuring instruments like calipers and micrometers to high-precision inspection equipment such as height gauges, Coordinate Measuring Machines (CMM), optical projectors, and surface roughness testers. The CMM can quickly and accurately obtain the 3D geometric dimensions and geometric tolerances of a part, making it an indispensable inspection tool in modern mold manufacturing.
Assembly and Debugging: After all parts are machined and inspected, they are assembled by fitters. This includes fitting guide pins and bushings, installing slider and angle-lift mechanisms, and connecting cooling water channels. After assembly, trial molding is necessary, where actual production is carried out on an injection molding machine or die-casting machine to check the mold’s operation and the quality of the molded product. Final adjustments and corrections are made to the mold based on any issues identified.
VI.Conclusion
The machining of mold parts is a systematic engineering project that integrates multiple processes and technologies. From the initial blank to the final precision part installed in the mold base, it needs to undergo a series of rigorous processes including conventional machining, non-traditional machining, heat treatment, and finishing. Rational process planning, advanced machining equipment, skilled operational techniques, and strict quality control are the four pillars for manufacturing high-quality mold parts. A deep understanding and mastery of these machining processes is the core of improving mold manufacturing standards.
