Die Casting Mold Technology: Electrical Discharge Processing, Electrode design

Die Casting Mold Finishing

At present die-casting mold manufacturing products more and more, such as cars, motorcycles and many mechanical and electronic products of some parts die-casting, die-casting production products have penetrated various fields. Automotive die-casting parts such as clutch housing, transmission housing, four-cylinder body, three-cylinder body, filter, valve body, steering gear and luggage rack have accounted for more than 77% of the entire die-casting industry products. The demand for products produced by die-casting has surged, pushing the production to expand continuously and putting forward higher requirements for die-casting mold manufacturing, mold cavity processing (NC processing and electrical discharge processing).

The die casting mold is characterized by complex shape and structure, strict dimensional requirements and high precision. The machining of die casting mold cavity includes rough machining and finishing machining. Most die-casting mould cavity finishes by electrical discharge machining, and the number of electrodes for electrical discharge machining is large. It is very important to split the processing electrode, set the electrode and rough and finish machining the electrode.

Table of Contents

The root R of the mold cavity is too small. For R1mm, and R0.5mm, NC machining needs to replace the tool with a small diameter to do further local root clearing machining. As shown in Figure 1 and Figure 2, if NC machining cannot be achieved, sometimes it is necessary to make up partial electrode discharge machining to clear the root. The mold cavity is too high or too deep, the tool length of NC machining is restricted, and it can not process the mold cavity forming position. Eventually, all have to use electrical discharge machining. One of the main ways of die finishing is electrical discharge machining.

Discharge processing figure1

Figure 1

Discharge processing figure2

Figure 2

Die casting mold processing method

At present, die-casting mold processing, mainly NC high-speed cutting processing and ordinary machining, die-casting mold electrical discharge processing and wire cutting processing. As shown in Figure 3, die electrical discharge machining needs to do electrode design, purchase electrode material, process rough and fine electrode, manually repair electrode regarding sample parts, test electrode, and then electrical discharge machining die cavity. After discharge machining, the clampers should repair and polish the shape, remove the traces of discharge machining, and assemble and research the mold.

Figure 3

The basic principle of electrical discharge machining (EDM) is to discharge in a non-conductive medium between a graphite or copper electrode (anode) and steel (cathode). The discharge controls the erosion of the die. The negative electrode enters the steel during the operation to obtain the desired shape. The surface temperature of the steel in EDM is very high, which causes it to melt and evaporate.

The advantage of EDM is that what cannot be achieved by NC high-speed machining can be achieved by EDM. As long as the electrodes are split, EDM can very smoothly. The disadvantages of discharge machining are high cost and the long time to split and make electrodes. There are more steps to be processed and operated (see Figure 3), the amount of material consumed by electrodes is high, and the productivity is low.

Electrode design

Discharge machining is generally done two times, i.e. rough discharge machining and fine discharge machining, and the corresponding electrodes are rough and fine. The key to discharge machining is to split the electrode reasonably, and the design of electrode splitting should be beneficial to both electrode machining and discharge machining. The electrode design of die-casting mould electrical discharge machining mainly includes the following points.

Size of the electrode

It should select Electrode size reasonably and consider the rough and fine electrode processing cost. Such as automobile clutch shell, transmission shell electrode, extra large main body electrode (large electrode) coarse and fine electrode can use one, coarse electrode discharge processing finished, sinking processing for fine electrode, and then discharge finishing cavity. Suppose the customer’s product demand is great, making more than two sets simultaneously, or consider making a copy to update the mould. In that case, two large electrodes of coarse and fine can be set separately, and the height of the electrode material can be thickened and reused.

The fine electrode can be used as the coarse electrode of the same mold, and the original coarse electrode can be hollow and processed electrodes and reused as the fine electrode of the same mold. The small electrode size should be as small as possible to ensure that the electrode base size can ensure the discharge processing is fixed at the discharge processing moulding.

AIntegrated electrode

The integral electrode, also called one piece electrode, main electrode, or large electrode, can take mould cavity moulding outside edge set sprue, bridge part, etc… Figure 4 shows the main electrode with a small bump in the mold cavity on the bridge. Such as automobile die casting filter, valve body integral electrode with sprue, and bridge. The material of the overall electrode is generally graphite, which can choose from domestic or imported materials.

Integral electrodes figure 4

Figure 4

The key to the overall electrode design is to ensure smooth discharge processing and prevent carbon accumulation in the discharge process and damage to the mold cavity.

Local electrode

Local electrodes are also called small electrodes. Parts of the casting with strict local dimensional requirements, positions that the overall electrode cannot discharge, die casting positioning reference processing places, and positions that cannot be achieved by NC processing can be split into small local electrodes for discharge processing of the mould cavity. The partial split electrodes must check in 3D assembly to prevent interference in electrical discharge machining and avoid mistakes. As shown in Figure 5, the small local electrode is assembled in the cavity to do an electrode interference check. The electrode base should be offset from the molding place by 5-6mm, as shown in Figure 6, which can prevent the bottom of the electrode from machining with an R angle to avoid the interference of electric discharge machining and damage to the mould cavity. In the design of the electrode base, the left side is smaller than the molding place above to avoid electric discharge machining interference. Design small electrode material can choose graphite and copper material, general electric discharge processing cylindrical shape and text electrode using the copper electrode. Die-casting mold cavity size requirements are particularly strict, such as sealing grooves at the choice of copper electrodes to discharge fine electrodes.

Local electrodes figure5

Figure 5 

Local electrodes figure6

Figure 6

Tendon electrode

Many products such as motor shells, motorcycle cylinder bodies and radiators come with many tendons. The densely can split formed tendon electrodes into separate rows, which is conducive to the tendon electrode processing to clear the root, repair the shape and test to improve the processing quality. Electrode splitting to master certain skills, as shown in Figure 7a, the tendons are dense, NC processing electrode is difficult to achieve, can be split into Figure 7b and Figure 7c tendon electrode. It can avoid carbon accumulation in electrical discharge machining and ensure the smooth progress of electrical discharge machining.

Tendon electrodes

Figure 7a

Tendon electrodes -b

Figure 7b

Tendon electrodes--c

Figure 7c

Electrode setting

At the same time, mould cavity surface processing needs NC high-speed and electrical discharge processing. 3D figure (see Figure 8), the upper end of the line is NC processing, the lower end is electrical discharge processing, and the articulation must be careful not to have too big a segment difference. It should not set the articulation on the important size of the die casting, should pay special attention to splitting the electrode, and the articulation should be misaligned to avoid the important size of the die casting. Because the articulation place generally has obvious traces of high-speed and electrical discharge processing, the clampers will smooth the processing surface when repairing and polishing, resulting in size deviation.

Electrode settings

Figure 8

Electrode list

When splitting electrodes, we should establish an electrode list, marking electrode name, electrode material (graphite, copper), electrode quantity, electrode type (coarse electrode, fine electrode), electrode minus inch amount and electrode discharge position, etc. Establishing an electrode list is beneficial to electrode processing and discharge processing to avoid processing omission and lack of shape of the mold cavity.

Electrode reference

The electrode benchmark should be pointed out as the product benchmark. All electrode benchmarks must be unified for the product benchmark. The discharge machining benchmark is also the product benchmark consistent with the drawing benchmark. It is conducive to electrical discharge processing. Without conversion, it will not be wrong. It is very important to unify the electrode datum, and sholud pays special attention to splitting electrodes. In the production and processing of electrodes, the electrode benchmark and discharge machining benchmark are different, resulting in some mould cavity discharge machining errors, mold cavities directly scrap, and great losses.

Electrode splitting basis

① Splitting electrode by mold cavity requires accurate mold cavity design, mold finishing model, mold structure and die casting molding are correct. Splitting by mold cavity can directly see the sprue, slag bag and bridge of the mold cavity, with the connection, side extraction slide, etc. so that the electrodes can be integrated. The number of electrodes is reduced, and it is easy to check the electrode interference. As shown in Figure 5, it is easier to duplicate the surface of the mold cavity to build electrodes with fewer data.

② Splitting electrodes by die casting requires that the 3D model of die casting is accurate. The mold cavity should be assembled simultaneously when designing electrodes, copying surface to build electrodes in assembly, which involves many parts, a large amount of data, easy to miss the mold cavity structure shape, and a large number of split electrodes. The advantage of splitting electrodes by die casting is that while the mold cavity structure is designed, part of the electrodes at the mold forming can be finished splitting and the urgent electrodes can be finished processing. It can save time and ensure the production cycle. Figure 9 shows the mold cavity processing flow. You can see the importance of die casting mold electrode splitting.

Mold cavity processing flow

Figure 9

Electrode model

The electrode model built based on die casting cavity and die casting 3D must be associated with the main shape (die cavity, die casting 3D model). Don’t blindly set independent geometry and isolate the electrode model. Because of the mold design, customers often propose design changes to modify the die casting 3D and mold structure design. If the electrode model is correlated, modify the main body shape and size, and the regenerated electrode model will be adjusted and corrected automatically. Splitting the electrode again is unnecessary, which can save time and improve efficiency.

Electrode inspection

After the electrode is processed, it should be inspected by three coordinates and select some point inspection or scanning inspection on the electrode surface at the important size. The output data of CMM can be automatically transformed into the point data that Pro/E can read through the program and then return to the point inspection on the electrode model. It can determine whether the electrode contains a minus inch amount and whether the electrode processing model is correct, as shown in Figure 10 and Figure 11 for the electrode return point inspection point location. Using Pro/E to analyze the geometric deviation and select the inspection surface and the inspection point, all the deviation values of the inspection point on the surface can be obtained, as shown in Figure 12, and the maximum and minimum deviation values can be obtained.

Location of electrode return detection point

Figure 10

Location of electrode return detection point 2

Figure 11

Maximum and minimum deviation values

Figure 12

Electrode design software

It can use 3D modelling software Pro/E, UG, CATIA and SolidWorks. Pro/E software can quickly copy the surface, extend and elongate, cut and trim the type of split design electrode, do electrode interference check in mold cavity assembly, do return point detection, and ensure the electrode quality. Pro/E’s NC processing module can be applied to quickly generate an NC processing electrode program to integrate CAD/CAM of die casting mould. It can save mold processing time and improve labour efficiency.

Die cavity electrical discharge machining is the final stage of mold processing, and it is important to ensure accurate electrode design. The key to electrode design is the proper shape and unified benchmark to ensure the cavity is shaped right and correctly dimensioned after electrical discharge machining. Electrode design should be reasonable to prevent interference and avoid carbon accumulation in electrical discharge machining.

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