More accurate electrodes will reduce leadtimes by eliminating the need for secondary processes such as polishing. Shorter leadtimes allow moldmakers to turn jobs over more quickly, driving higher volumes and better profits.
Several variables can help achieve top accuracy in electrode manufacturing, including machine tool, control system, CAM system, graphite quality and cutting tools.
Machine Tool
There are several factors on a machine tool that can significantly impact mold building accuracy. Machine tools with a rigid design and heavy-duty cast iron construction deliver the best surface finishes because of the cut consistency achieved at all accelerations.
Reliable fixtures will add accuracy in electrode manufacturing. A well-designed fixture provides the same level of repeatability as the machine tool and holds extremely tight tolerances for fine finishing.
Toolholders can have a direct impact on machining accuracy. Although shrink fit toolholders are more reliable for balance and run out, a pre-balanced collet holder will hold the tightest tolerances, helping achieve the finest finishes and maintain superior mold geometries.
Control System
Another important machine tool element is the quality of its control system. A machine’s controls must be capable of processing large amounts of information quickly. Machine tools from a single-source manufacturer will ensure all elements of the control system work together for maximum speed and accuracy.
Program look ahead also is crucial to the overall accuracy in electrode manufacturing. The program must have enough look ahead for the control to adjust acceleration/deceleration without hesitation. Without adequate look ahead, gouging or over-step can occur. Look ahead allows the program to adjust machining speed, helping achieve superior contour accuracy.
CAM System
Software used in moldmaking is often application focused, meaning it is specifically designed for applications such as cavity design, electrode design and mold machining.
For top accuracy in electrode manufacturing, the software should be able to streamline the creation of toolpaths for fast and efficient machining, while preserving the high tolerances required in moldmaking.
CAM systems will assess a design’s manufacturability by detecting surface overlaps, twists, holes and undercuts. This allows parting lines and surfaces to be easily modified for preservation of the mold design’s integrity.
Graphite Quality
It is critical that the proper graphite grade be selected to meet the final mold design criteria, particularly for thin ribs and when electrode wear and metal removal rates are a consideration.
The quality of the graphite used in moldmaking can affect electrode fabrication time, the number of electrodes needed to complete a job, EDM time, accuracy of the cavity and polishing time. Factors such as flexural strength, particle size and microstructure influence the level of detail and surface finish that can be expected in the sinker EDM operation.
Cutting Tools
For moldmaking applications that require such high precision, quality and tolerance of the cutting tool is extremely important. A high-quality cutter will have closer tolerances on the radii on both a ball cutter and a bull nose.
Ball cutters are very versatile and therefore a good choice in these applications. They can rough and finish a wide range of materials. The tool’s shape helps achieve deep cuts and absorbs vibration to preserve accuracy.
Bull nose cutters require fewer passes and deliver better surface finishes. Because very little pressure exists between the cutter and the graphite, these cutters work well in moldmaking by achieving thin walls and ribs and very fine stepovers.
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Once the design has been finalized, the information in the data file would need to be extended to include the spark gaps necessary to provide the offsets required in the machining software that will be used to cut the roughing and finishing electrodes. Inspection points would also have to be added, so that the measurement of the machined electrode could be automated to a large extent. In addition, setup sheets for the electrode’s manufacture and use should be produced automatically—either to a standard format or to a user-defined template.
when machining a large number of electrodes, it is inefficient to wait for toolpaths to be calculated on each individual project. To overcome this problem, applying batch processing to multiple projects is necessary. With this approach, processing the toolpaths starts as soon as the first project is loaded.
The automated generation of setup sheets would need to be possible for both the machining and application of the electrodes to ensure that all the data needed at each stage is readily available. A documentation pack, including the GA and detail sheets, would need to be generated in various formats—such as drawings, HTML files or Microsoft Excel spread sheets. All these options are important to allow easy communications between the various people involved in the design, manufacture and use of the electrodes.
Precision is one of the most important aspects of manufacturing. High precision creates high quality, high performance, exchangeability, reliability, and added value for industrial products. Over the past decades, remarkable advances have been achieved in the area of high-precision manufacturing technologies, where the form accuracy approaches the nanometer level and surface roughness the atomic level. These extremely high precision manufacturing technologies enable the development of high-performance optical elements, semiconductor substrates, biomedical parts, and so on, thereby enhancing the ability of human beings to explore the macro- and microscopic mysteries and potentialities of the natural world.
Graphite is unforgiving of surface finish defects, clearly reflecting even the smallest perturbations. As such, it is important to use a fine step over and small chordal deviation to optimize surface finish. This results in very large part programs, and given the high axis feedrates, often causes data starvation in the CNC. Careful consideration of the data processing capabilities of the CNC is necessary before releasing graphite-machining programs into production
Unlike hard die milling, the process design when machining graphite is less critical in the sense that a poorly designed process is less likely to lead to broken tools. But, a well-designed process is still imperative to produce high quality electrodes. While cutter breakage is less of a concern, the toolpath must be designed to avoid flaking and chipping of the surface. Entry and exit from the part should use a ramping approach, rather than a direct step into the surface. Sudden changes in chip load should also be avoided, again to prevent chipping of the workpiece. When developing a toolpath for electrode production, follow these guidelines to minimize overall machining time.
Using a single software solution throughout the electrode design and manufacturing process, seamless integration to NC and automatic programming for an EDM—can go a long way to smooth out the process and eliminate unnecessary steps, data transfers and reduce the probability of errors and repeat work.Integrated design and manufacturing electrode software should automate the electrode production process while providing complete control over all machining and burning parameters. In order to maintain flexibility, the system should allow the definition of both general and machine-specific parameters, electrodes and burning locations.
The principle of EDM for injection molding is that when the tool electrode and the workpiece are not in contact during processing, the workpiece is processed by electric erosion. Therefore, it seems that any conductive material can be called an electrode. However, electrodes of different materials have a great influence on the stability of EDM, productivity, and the quality of the molds to be processed. The characteristics of the electrode material selected in actual use are as follows.
1. The relative loss is small.
2. The process is stable.
3. High productivity.
4. Easy to manufacture and process.
5. Low-cost materials.
To meet the electrical processing requirements of molded parts.
Many injection molding companies in the application of the actual injection-molding electrode, the principle of selection of the tool electrode material is to take into account factors such as the economy, processability, processing accuracy, resistance toelectrode material electrical corrosion and the like in meeting the requirements of the process itself.
1. For workpieces with particularly complex and varied shapes, the dimensional accuracy of the workpiece is generally required, graphite or copper materials can be used.
2. For the processing of large cavities, it is desirable that the electrodes have a lighter weight and graphite should be used as the electrode.
3. When performing high-precision EDM machining, in order to ensure the dimensional
accuracy and shape accuracy of the tool electrode, materials with the lowest electrode loss, such as silver-tungsten alloy, copper-tungsten alloy, etc., must be selected, but their cost is high.
Graphite material has good machinability and cutting resistance is only 1/4 of copper. Under the correct processing conditions, the efficiency of milling graphite electrode is 2-6 times higher than that of copper electrode.
Graphite electrodes are easy to clear the angle, so the workpiece usually completed by multiple electrodes can be designed as a whole electrode to process. The unique particle structure of graphite material makes the electrode not burr after milling. For complex shape, it is not easy to remove burrs, which directly meets the requirements of use, eliminates the manual polishing process of electrodes, and avoids the shape change and size error caused by polishing.
It should be noted that because graphite is a dust accumulation, milling graphite will produce a large amount of dust, so milling machine tools must have a sealing and vacuum device. If WEDM is used to process graphite electrode, its processing performance is not as good as that of copper, and the cutting speed is about 40% slower than that of copper.
Problem: Moldmaker couldn’t machine graphite electrodes fast enough to meet needs of new sinker EDMs, so was sending work to outside companies to relieve production bottleneck.
Solution: Purchased one, and then another, OPS Ingersoll high-speed, five-axis hard-milling centers.
Results: Moldmaker could now mill much faster and more accurately that it brought all graphite electrode production back in-house and increased mold production rates.
among the eight EDM characteristics of graphite electrode, its advantages are obvious: the efficiency of milling electrode is significantly better than that of copper electrode; the efficiency of EDM is better than that of copper electrode; the weight of large electrode is light, and it is very suitable; the size stability is good, the thin electrode is not easy to deform; the milling electrode has no burr, and the automatic head.
Graphite electrodes achieve less than 1% wear concerning the depth of cut, during its function with aggressive machine parameters. Hence, at high amperages and longer-on times, graphite is preserved, and fewer electrodes and dressing are required.
Graphite provides an economical way to manufacture electrodes and creates a very stable EDM process with, in most cases, less wear than other electrode materials,
Graphite does have limitations in its ability to achieve ultrafine surface finishes, however. These electrodes typically can produce a surface finish down to 30 μinRa/0.7 μmRa, which is sufficient for most general applications.
the manufacturing capabilities require fast responses in workshops to cope with different demands in product geometrical features and qualities in fabricating moulds. Therefore, direct machining of complex mould features using simple shape machined electrodes with the electro-discharge machining (EDM) process, empowers the flexibility and the capacity of the enterprises and dramatically reduces lead times resulting in much more efficient production processes.
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