How turbulence is formed in precision casting

author: kevin

2024-06-12

How turbulence is formed in precision casting

In theory, at the beginning of casting in precision casting, the molten metal flows smoothly into the mold cavity from the bottom inner runner. When the liquid level in the mold cavity rises to near the second inner runner, it flows into the mold cavity from the second inner runner. Like this, the inner runners of each layer work layer by layer from bottom to top, until they finally enter the mold cavity from the top inner runner. This not only allows the metal liquid to fill the mold cavity steadily from bottom to top, which is conducive to exhaust, but also enables sequential solidification from bottom to top, so that the sprue cup and riser can fully play a filling role. Thereby reducing defects such as cold shuts, insufficient pouring, porosity, shrinkage (looseness), and oxidation inclusions. Here, the pouring channels in each layer work layer by layer from bottom to top, which is the key to ensuring that such pouring systems achieve good results.

However, if the metal liquid is introduced into the upper inner runner too early, there will be a phenomenon of "random injection", which not only disrupts the ideal temperature distribution of the inner metal liquid in the mold, but also causes the metal liquid to collide with each other in the mold cavity, greatly increasing the possibility of defects such as pores, inclusions, and slag inclusions. According to the process parameters and data provided in actual production, simulation results often show the phenomenon of "random injection". This not only greatly reduces the effectiveness of the stepped pouring system, but also leads to completely opposite effects.

So, how can we improve this situation? This situation is mainly attributed to improper design of the pouring system. The correct pouring system should ensure that the height difference h between the free liquid level of the molten metal in the sprue and the free liquid level in the mold cavity during the pouring process does not exceed the height difference H between the two inner sprues, that is, h

1. Design the pouring system as an open system, so that the cross-sectional area of the sprue is less than or equal to the cross-sectional area of the sprue in each layer, that is, F is ≤ ∑ F, so that the sprue is not filled with molten metal during pouring. At the same time, tilt the upper inner pouring channel upwards by 20-30 °. Essentially, this method increases the height difference H between the two inner gates, which is more conducive to achieving h

2. Set the flow blocking section (the minimum section of the pouring system) at the outlet of the pouring cup, and ensure that F is within ∑ F. Close the pouring system before the flow blocking section; Open after the blocking section. The entire pouring system is closed and open. To ensure that the metal liquid is filled in the sprue cup, but not in the sprue. This can not only play the scum function of the sprue cup, but also ensure that h°, the effect will be better.

3. If conditions permit, a pouring system with a main sprue and a distribution sprue (distributing liquid flow to each inner sprue, also known as a "transition sprue") can be considered. Make the cross-sectional size of the allocated sprue larger than the main sprue, and ensure that the sum of the cross-sectional areas of each layer of the internal sprue is less than or equal to the cross-sectional area of the sprue, in order to ensure that the allocated sprue is not filled and that the internal sprue is replaced layer by layer from bottom to top.

4. Regardless of the type of pouring system used, the pouring speed needs to be controlled appropriately.

In fact, due to the increasing complexity of castings nowadays, the most commonly used system is the stepped pouring system. Therefore, we need to consider more at this time, otherwise, the things we design will deviate from the actual pouring situation, which is something we do not want to see.