Precision machining process of mold cavity is the last process of moulding process, and it is the most important part that directly affects quality of mold. It accounts for 30% to 40% of total moulding process volume, so it has attracted atttention of experts at home and abroad. In China, although most of moulding process (milling, planing, grinding, electric spark, wire cutting, etc.) have been highly automated in injection molding manufacturers, most of finishing of mold still uses manual processing which seriously affected development of china plastic injection molding to a certain extent.

 

So-called precision machining is a processing method that reduces surface roughness of part under premise of ensuring accuracy of part profile. Currently commonly used methods are: hand polishing, ultrasonic polishing, chemical and electrochemical polishing, and so on. Among these methods, hand polishing is the most commonly used finishing method, because manual polishing is flexible and can process any complicated mold cavity, but at the same time this method is labor intensive, production efficiency is low, and quality of product is not guaranteed. While other methods have a good effect, quality of product, efficiency of processing, and strength of workers have all been greatly improved. However, due to complexity, diversity, and irregularity of mold cavity, it is difficult for these processing tools to be completely processed along contour of workpiece and sometimes limited by space of these cavities, so many finishing methods can only be used in certain areas, but it is difficult to widely use.
Most injection molding manufacturers in foreign countries use integration of molding design, processing and even assembly, which is integration of mold CAD/CAM/CAE, using mold CAD software and reverse engineering for molding design; using virtual reality system for assembly test and finding interference and adjusting timely, processing is carried out without problems; in machining process, machining center and CAD/CAM are used to integrate entire machining process, that is, workpiece is machined once, so accuracy of workpiece can be guaranteed.
Despite this, problem of precision machining for mold cavity surface is still a world problem, mainly due to following problems:

In many cases, cavity surface of mold is a three-dimensional irregular free-form surface. Due to different shapes of these curved surfaces, this causes great trouble for movement track and feeding of tool or grinding tool during finishing. Even if use of modern CNC machining technology to control movement of tool or abrasive tool will bring great difficulties to preparation of NC program, so this is fundamental reason why it is difficult to automate mold finishing process.

Due to particularity of mold profile, tool or tool to be processed is required to have a good self-adjusting ability, that is so-called adaptability. It is necessary to change its own trajectory as shape of machining contour changes. This refers to fine tuning. This requires that tool for machining mold cavity has a range of plasticity, ie flexibility.

This is also determined by characteristics of mold itself. As a model of workpiece, precision of mold directly determines accuracy of workpiece. It also plays a vital role in its life, corrosion resistance, wear resistance and ability to smoothly remove workpiece from mold after moulding process. Even though some processing methods have high processing precision, it is difficult to ensure original shape tolerance of workpiece while improving surface finish of mold in moulding process.
At present, china plastic injection molding machining methods for mold cavity are still two major aspects of machining and electrical processing, and electrical processing is increasingly dominant. In addition, it is application of mold CAD/CAM technology. While due to characteristics of mold itself, shape is complicated and difficult to standardize, so development of mold cavity mold CAD/CAM is not as mature as die and plastic molding developed on CAD/CAM. Nevertheless, this is still a development direction of cavity moulding process method. Among these processing methods, a faster development is milling technology in machining, grinding technology and EDM processing technology in electric machining, which are respectively introduced as follows.

 

Milling is an important processing method for mold cavity, and is especially suitable for processing of medium and large forging dies. In recent years, milling has achieved rapid development, mainly in following aspects:

Era that people consider milling is ordinary processing has passed. Positioning accuracy of machine tool has been improved from ±12mm/800mm in the 1980s to ±2~5mm/full stroke in the 1990s. Thermal balance structure of precision machine tool and cooling of spindle are used to control thermal deformation. Control resolution has been increased from 1mm to 0.2mm. This improves machining accuracy from ±10mm to ±2~5mm and precision level to ±1.5mm, making milling machine enter field of precision machine tools.

With advancement of tools, motors, bearings, and CNC systems, high-speed milling technology has rapidly emerged. At present, spindle speed has been increased from 4000 to 6000 r/min to 14200 r/min, cutting feed rate has been increased to 1 to 6 m/min, rapid feed rate has been increased from 8 to 12 m/min to 30 to 40 m/min, tool change time is reduced from 5~10s to 1~3s, which greatly improved processing efficiency. High-speed milling can increase machining efficiency by 5 to 10 times compared to conventional machining.

High-speed milling technology combined with new tools (cermet tools, PCBN tools, special carbide tools, etc.) can process workpieces with a hardness of 36~52HRC, and even workpieces of 60HRC .
Development of high-speed milling technology has promoted progress of moulding process technology, especially for manufacture of medium and large cavity molds in automotive and home appliance industries.

Since development of high-speed milling technology, development of EDM as another important means of cavity moulding process is also perfect, but as a processing system, it is facing new challenges of high-speed milling.

Due to advancement of related technologies such as micro-precision processing pulse power supply, working fluid, and mixed silicon powder processing technology, surface roughness of EDM processing reaches Rmax 0.6-0.8mm, and can be made large area processing. And because electrode loss is continuously reduced (minimum 0.1%) and precision of machining allowance for micromachining, it can be said that EDM has entered field of precision machining.

Since high-speed milling can process materials with hardness of 36～52HRC or even 60HRC, almost all cavity mold materials can be processed, which changes situation that high-hardness materials only use electric machining. Machining efficiency of high-speed milling is 4:1 compared with efficiency of EDM, some are even 7 to 8 times that of EDM, electrode manufacturing is saved. High-speed milling also has a certain machining accuracy and good surface roughness. Foreign countries believe that high-speed milling can replace EDM in the field of cavity moulding process, which is not unfounded. Because of this, in the field of application, EDM process is in danger of being extruded by high-speed milling especially in automotive industry. However, EDM has its irreplaceable advantages in processing deep grooves, narrow slits, ribs and textures. But in general, EDM has been reduced in the field of processing applications, and some markets have been occupied by other processing equipment, especially for development of large EDM machines.

 

EDM is a processing system that has been formed for decades. It is also constantly developing. Comparing with milling technology, technology of “Electrical Discharge Milling” has recently appeared. In general, "Electrical Discharge Milling" is aimed at improving efficiency of EDM processing, using forming (graphite electrode), EDM with water as working fluid. Its’ efficiency is increased by 2 to 3 times compared with oil as working fluid, which is called "Electrical Discharge Milling" in foreign countries, which represents its development direction. However, compared with high-speed milling, there is still a big gap in overall processing efficiency. High-speed rotating spindle drives rod-shaped (tubular) electrode to rotate, cooperates with CNC trajectory movement of table, spindle and servo feed. This forming method is similar to mechanical milling and can store standard tube electrodes of different diameters in electrode bank and form them in CNC feed, which greatly simplifies design, manufacture and management of electrodes. This is a new development strategy, but there are also problems with low processing efficiency. It is expected that there will be new progress in "Electrical Discharge Milling" and fierce competition with high-speed milling.
With rapid development of electronics, electrical appliances, communications, computers and other industries, processing of precision, micro and complex molds is increasing, and market is getting bigger and bigger. Processing of these molds is the advantage of EDM. Therefore, at the same time of competition, advantages of EDM should be fully utilized, that is focus should be shifted to direction of precision, complex and micro-die processing, which is another important direction for development of EDM.

Grinding is a kind of precision machining technology. Up to now, grinding precision has been very high, up to 1~2mm, surface quality of processing is also very good, surface roughness is generally Ra0.04~0.32. mm, surface processed by grinding process has no defects such as a softening layer or a metamorphic layer, so it is widely used in processing of precision molding. With increase of types of grinding machines, such as coordinate grinding machines, forming grinding machines, optical bending machines and special-purpose machining grinding machines, especially improvement of numerical control degree, processing range is getting larger and larger, precision is getting higher and higher. Not only can it process cold die, but also can process various cavity molds, such as forging die, plastic molding, etc. Therefore, grinding is still main means of precision moulding process. The following focuses on magnetic particle grinding technology.

Magnetic grinding is to place a magnetic abrasive in a magnetic field. Magnetic abrasive forms a magnetic particle brush under action of magnetic force. When workpiece moves relative to magnetic pole in magnetic field, magnetic particle brush will grind surface of workpiece. Since formed magnetic particle brush has good adaptability and flexibility, it is very advantageous for processing complex profiles.

◆Workpiece is not in contact with magnetic pole, wear of magnetic pole is small, and shape error of magnetic pole has little influence on shape accuracy of machined surface.
◆Structure and shape of magnetic poles will affect distribution of magnetic field in processing area, thus affecting quality and processing efficiency of machined surface.
◆Magnetic abrasive brush has a certain rigidity, and at the same time has certain flexibility, which can be deformed with change of shape of machined surface, so it can process surface with extremely complicated shape.
◆Grinding pressure can be adjusted by changing excitation current, and grinding process is easier to control.
◆According to magnetic field force, abrasive is not easy to fly, durability of abrasive is high and can be used repeatedly, loss of abrasive is less, and working environment is relatively clean.
◆When diamond powder is used as abrasive, it can process super hard non-metal such as ceramic.
◆Processing equipment is simple and cost is low.

 

This technology has been studied abroad, and there are still few people studying in China. We believes that combination of this technology and numerical control technology will bring a new revolution to processing of mold cavity.
Basic characteristics of 21st century mold manufacturing industry are highly integrated, intelligent, flexible and networked. Goal is to improve product quality and production efficiency, shorten design and manufacturing cycles, reduce production costs, and maximize resilience of mold manufacturing industry to meet user needs. Specific performance is as follows:
◆Integrated technology;
◆Intelligent technology;
◆Application of network technology;
◆Multi-disciplinary multi-functional integrated product design technology;
◆Application of virtual reality and multimedia technology;
◆Reverse application of technology;
◆Rapid prototyping technology.