Many defects take place during the process of moulds and dies manufacture. This paper will suggest some solutions to these problems.
1. The forging process.
High carbon steels and high alloy steels, such as Cr12MoV and W18Cr4V, are widely used in manufacturing of moulds and dies. Nevertheless, these steels have defects like composition segregation, uneven carbides, uneven microstructures, etc. Therefore, rational forging process should be used to shape work-blanks. With this method, sizes and specifications of steels can meet those of work-blanks, while micronstructures and performances of steels can be improved at the same time. Besides, due to the poor thermal conductivity of these forgings, heating process should be slow and well-distributed. One should take suitable forging ratior within the range of forging temperatures.
2. The cutting process.
Perfect cutting process of moulds
and dies can make sure precise fillet radius in transitional regions as well as smooth connection between arcs and straight lines. With poor machining process, three kinds of damages might happen to moulds and dies. First, unsuitable machining process might lead to sharp corners or undersize fillets. In this case, grave stress concentration will occur to moulds or dies in work. Second, rough surfaces of tooling after machining process might exist marks, fissures, incisions and other defects. These defects are stress concentration points as well as initiation of crack, fatigue crack or thermal fatigue crack. Third, if decarburized layers of work-blanks that produced during roolling or forging process were not cut completely and evenly, uneven hardening layers might occur during thermal treatment. Thus, the abrasive resistance of tooling will decrease.
3. The grinding process.
In general, moulds and dies should undergo grinding process after quenching and tempering to reduce the surface roughness. Faults like excessive grinding speed, undersized granularity of the grinding wheel, poor cooling conditions and others may shorten the service life of tooling. That is because, these faults may result in partial overheating of tooling surfaces and changes the microstructures of tooling. The surface of products may soften with abating hardness. Moreover, relatively high tensile stresses might occur. To take stress relief treatment after grinding process can effectively prevent from grinding cracks. There are many measures can be employed to keep off overheating or cracks.
4.The electrical discharge machining.
A mass of heat will be generated during this process. The temperature of regions for machining reaches about 10000℃. Due to the high temperature, structures within the region will certainly change. Surfaces of tooling will molten under high temperatures and cooling down in no time. Therefore, new solidified layers will take form on the surface. These new solidified layers are white and have many microscopic cracks. In order to prolong service life of moulds and dies, die casting manufacturers can take the following measures. First, adjusting the parameters. Second, using electrolytic method or mechanical grinding to remove the white layers on the surfaces, especially those microscopic cracks. Third,employing low temperature tempering for one time after electrical discharge machining to stabilize the abnormal layers and stop microscopic cracks from expansion.