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WEDM technology of mould Nov 27, 2017

At present, many die are used in high-speed wire cutting machine cutting workpiece processing, surface roughness after Ra of more than 2.5 m, hardness distribution and stress state are very poor.

Line cutting machine, discharge current density as high as 10000A/mm2, the temperature up to 10000 DEG -12000 DEG, the rapid cooling medium liquid filling, the cutting surface hardness is only about 20HRC, while the internal hardened layer hardness is higher than 70HRC, followed by the heat affected zone, and then after the original hardness.

More seriously, the raw material is quenched because internal tensile stress is tensile stress produced by cutting the thermal stress, two kinds of results of stress superposition easily reach the strength limit of the material and produce micro cracks, thus greatly shorten the die life, therefore, line cutting can not as the final process punch.

At present, most of the mold processing enterprises are online cutting, with the grinding method to remove the surface layer of 20HRC gray layer (white layer), then assembled. Although it can get rid of the white layer hardness is low, but no change caused by line cutting stress state, even if the size of wire cutting after grinding allowance, but because of the high hard layer with high hardness (up to 70HRC), grinding grinding difficult, a large amount of damage to the part geometry, the produce line cutting the high hardness layer and can improve die life, because of its brittleness is the root cause of cracks, chipping.

After wire cutting grinding, the white layer and hard layer with low hardness can be removed, and the service life of the die can be improved. Because the thermal stress produced during grinding is also tensile stress, and the thermal stress produced by wire cutting will undoubtedly aggravate the damage of the die. If the low temperature aging treatment is carried out after grinding, the stress effect can be eliminated, the toughness of the die can be remarkably improved, and the service life of the die can be improved. Because the geometric shape of the majority of the die is using WEDM, so the grinding complex shape of the die must use expensive coordinate grinder and optical curve grinder, and the two kinds of equipment general wire cutting manufacturers do not have, so it is difficult to promote.

In high speed WEDM, working fluid is the medium of pulse discharge, which has great influence on machining. Working fluid generally requires a certain insulation performance, good washing performance, good cooling performance and no pollution to the environment, no harm to the human body. Working fluid configuration is also very important, too thick, too light, will cause broken wire, like water concentration ratio is within 1:10 to 1:20 range. When the cutting speed is high or the thickness of the workpiece is cut, the concentration is lower, so that the processing is relatively stable, and it is difficult to break the wire. But the concentration of the working fluid can not be too low, otherwise, the insulation performance of the working fluid will be reduced, the resistivity will be reduced, the cooling capacity will be enhanced, thereby reducing the lubrication of the workpiece, which is not conducive to chip removal, but also easy to break the wire.

In order to guarantee the working fluid channel without obstruction, working fluid flow size should also be properly controlled, usually in the cutting at the end of the workpiece to be cut due to sagging in the period of working fluid pressure, cut off the wire electrode is very easy to pinch off, at this time, should make the working fluid to maintain certain pressure and flow, in a timely manner to the corrosion of discharge, thereby preventing wire breaking phenomenon. Cutting aluminum, copper and other soft materials, especially in the case of thick cutting materials, in order to improve the processing efficiency and surface finish, reduce the phenomenon of broken wire, you can use some special cooling liquid, but also conducive to environmental protection.

In the process of high speed WEDM, in order to efficiently process the required parts, the operators also have certain requirements. The operator should correctly understand the technical requirements of the drawings. The working fluid should be replaced in time to maintain a certain degree of cleanliness, to ensure that the upper and lower nozzle is not blocked, flow appropriate. The electrode wire is calibrated vertically and the workpiece is clamped correctly. Reasonable selection of pulse power supply parameters, processing instability should adjust the frequency conversion feed speed. Switch function keys, not in the process of processing, so as not to switch contact bad cause short circuit or cause wire burn, and even broken wire. When the cutting is finished, the removal part will fall under the action of gravity, the electrode wire is easy to be broken, and the magnet can be fixed in time by magnet adsorption and splint clamping, so as to avoid breaking the wire.