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In high pressure die casting, porosity is created by solidifying melt, which occupies less volume than liquid and due to entrapped air, which can’t escape during cavity filling. The solution to avoiding entrapped air is to push it out of the mould. If this can’t be done, engineers try to reduce the amount of air in the cavity at begin of fill. This part of the process is called “using a vacuum”. Knowing that a perfect vacuum will not be achieved, reduced air pressure has to be sufficient to do the job. The simple evacuation principle is a hose connection between a vent at the mould and a vacuum tank. During mould preparation in the casting cycle a vacuum pump is used to pump as much air out of the tank as possible. A rest pressure below 1/10 of the atmospheric pressure should be reached. To achieve a similar under-pressure in the mould, the tank volume has to be at least 10 times bigger than the shot sleeve volume. The cavity volume does not count in the use of a cold chamber machine. It will be compensated by the melt volume, which is poured into the sleeve before the vacuum is pulled. This is how the sequence runs: The mould closes and is locked in the die casting machine. Melt is poured into the shot sleeve. The shot plunger moves and pushes the melt toward the mould. When the plunger covers the pouring hole the vacuum vent at the mould opens and the air is pulled out of mould, feeder system and sleeve. The melt is pushed into these cavities and will close the vacuum vent at the end of fill. The casting solidifies, the mould opens and the casting gets extracted. The mould is prepared for the next shot and closes. The cycle begins again. Although this is the basic principle, vacuum variations are found in the industry. Some systems evacuate during the slow shot phase only. If the under-pressure is not reached, the fast shot phase to fill the mould is not started. Other systems pull air until the cavity is completely filled. The vacuum shut off can be done by the melt hitting the vacuum vent or by an electric control unit closing the vent. While vacuum is crucial to produce high pressure die casting products that can be welded for lower quality castings, the use of vacuum is normally avoided. Pulling a vacuum requires additional equipment, which costs money, and has to be maintained and cleaned frequently. Advanced die casters do not use vacuum systems unless there is a pressing need to do so. Their first approach is to simulate the complete process by using state of the art simulation software like MAGMASOFT®. Based on these findings they are able to design a runner and overflow system that avoids the additional costs and efforts of using a vacuum system. Even if a vacuum system is required, simulation helps to determine the last points of cavity fill and the locations of the vacuum system connections. Under-pressure also changes the filling conditions in the cavity. Melt does not push the air out of the cavity and can fill with less resistance respectively faster into the cavity. A smaller machine can be used or a casting with thinner walls can be made. However the disadvantage of faster filling is the creation of cavitation and a higher die wearing in these fill paths. When moulds are made clearances between components can’t be avoided. The larger the mould and its components, the more critical it becomes. Keep in mind that the temperature in the mould increases unevenly, as does the temperature of the components. Parts with clearance could fit now or clearance could appear where the mould was tide fit before. In these cases it gets very complicated to create the required under-pressure without serious engineering designing a sealed tool. Again, running a simulation can help to cover all these aspects before cutting the steel for the mould. P.S.: What is a vacuum? I can’t remember, but I know I have it in my head.
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