In Low Pressure Die Casting processes, the liquid metal is moved from a furnace through a rising tube, typically ceramic, into a metal mould. To do that, the furnace is incrementally pressurized to control the height of the metallic fluid and to impose a final high compression once the part is completely filled. In this process risers are not necessary, reducing the trimming costs, and it is possible to obtain parts with a very good surface finishing and with very low porosity. Anyway, the flow rate of the metal and the die temperature should be accurately calculated to find the best compromise between the velocity of the process and quality of the cast part. In fact, a too fast flow rate could lead to a highly turbulent flow and to an excessive entrainment of air; on the other hand, a too slow filling (in combination with a low temperature of the die) could provoke an early solidification, preventing the complete filling of the part. Using FLOW-3D® CAST it has been possible to reproduce through an accurate simulation, the real process, in which a complete filling is not obtained. The flow in the rising pipe is not included in the setup, imposing the flow rate directly at the entrance in the metal mould, with its real temperature. Figure 1 shows the fluid at the half of the filling phase, coloured with the solid fraction. From this picture the early solidification can already be noticed, underlining the excessive cooling rate. Figure 1 - Solidification front Figure 2, instead, shows the final shape of the aluminium, compared with a picture of the real part. The solidification of the metal front creates a blockage that forces the still liquid metal to flow around it to the upper part of the casting, where it also solidifies generating a big hole in the final shape. Figure 2 - Early solidification defects The video, finally, shows the complete dynamic of the flow, underlining the phase of the early solidification and showing in details how the metal slows down and stops due to the increasing solid fraction. https://youtu.be/x-VaIm05q6s Video of the dynamic of the filling and solidification … [Read more...]