发布时间2019-9-25
Theoretically speaking: at the beginning of pouring in precision casting, the molten metal flows smoothly into the cavity from the inner runner at the bottom layer. When the liquid level in the cavity rises to approach the inner runner of the second layer, it pours from the second layer inward. The channel flows into the cavity. In this way, the sprue of each layer works successively from bottom to top until it finally enters the cavity from the sprue of the uppermost layer. This not only enables the molten metal to fill the cavity smoothly from bottom to top, which is conducive to exhausting, but also realizes sequential solidification from bottom to top, so that the sprue cup and riser can fully play the role of feeding. Thereby reducing defects such as cold isolation, insufficient pouring, pores, shrinkage holes (loose), and oxidation inclusions. Here, the inner runners of each layer work successively from bottom to top, which is the key to ensuring good results of this type of gating system.
However, if the molten metal is introduced into the upper inner runner too early, the phenomenon of "chaotic injection" will occur, which not only disturbs the ideal temperature field distribution of the molten metal in the mold, but also causes the molten metal to collide with each other in the cavity, which greatly increases the The possibility of defects such as pores, inclusions and slag inclusions is increased. According to the process parameters and data provided in actual production, the simulation results often show the phenomenon of "random citation". This not only greatly reduces the effect of the step gating system, but also leads to the exact opposite effect.
So, how to improve this situation? This situation is mainly due to the improper design of the gating system. The correct gating system of this type should make the height difference h between the free liquid surface of the molten metal in the sprue and the free liquid surface in the cavity not exceed the height difference H between the two inner runners during the pouring process, that is, h<H . This is the key to ensure that the inner runners of each layer work successively from bottom to top, so as to prevent the phenomenon of "random injection". To this end, the gating system can be improved from the following aspects:
1. The pouring system is designed to be open, so that the cross-sectional area of the sprue is less than or equal to the cross-sectional area of the inner sprue in each layer, that is: F straight ≤ ΣF, so that the sprue is not filled with molten metal during pouring. At the same time, the upper inner runner is inclined upward by 20~30°. This method essentially increases the height difference H between the two-layer inner runners, which is more conducive to the realization of h<H.
2. Set the blocking section (the minimum section of the gating system) at the outlet of the pouring cup, and F straight ≤ ΣF. Make the gating system closed before the choke section; open after the choke section. The whole pouring system is closed-open. To ensure that the molten metal is filled in the pouring cup, but not in the sprue. In this way, the scumming function of the sprue cup can be exerted, and h<H can be ensured. If the upper inner runner is inclined upward by 20~30°, the effect will be better.
3. If conditions permit, a gating system with a main sprue with a distribution sprue (distributing liquid flow to each inner sprue, also known as "transition sprue") can be considered. Make the cross-sectional size of the distribution sprue larger than the main sprue, and the sum of the cross-sectional area of the inner sprue of each layer is less than or equal to the sectional area of the sprue, so as to ensure that the distribution sprue is not filled, and the inner sprue will flow from the bottom to the bottom. Layer-by-layer succession works.
4. No matter what kind of pouring system is used, the pouring speed needs to be properly controlled.
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