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Principles of pouring system and characteristics of liquid metal flow

Categories: ZCMIM NewsStars: 3StarsVisit: -Release time: 2019-10-12 08:35:00
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一. principles of gating system design

The pouring system is the channel through which the liquid metal flows into casting mold cavity.

Proper design of pouring system makes liquid metal fill cavity smoothly and reasonably, which is very important to guarantee the casting quality. Proper casting system design has a great influence on casting quality. About 30% of casting wastes are caused by improper casting system design.

In addition to the basic function of liquid alloy introduction, the correct casting system should have:

1. Make liquid alloy balance and fill mold cavity without impact on mold wall and sand core, without spatter and eddy, and without being involved in gas, and smoothly make the air and other gases out of the mold, so as to prevent excessive metal liquid oxidation and pores defects.

2. Prevent inclusions from entering mold cavity to avoid forming slag holes in casting.

3. Adjust the temperature difference between sand mold and casting part, control casting sequence, do not hinder casting shrinkage, reducing deformation and cracking of casting part.

4. It has a certain role of feeding shrink, which is generally feeding liquid shrinkage before inner runner solidified.

5. Make liquid alloy fill mold cavity at the shortest distance and the most suitable time, and have the proper liquid level rising speed on mold inner wall, so as to obtain complete and clear casting outline.The filling conditions of alloy flow in sand mold are shown in the figure below:

6. Mold filling flow should not be directly facing chiller and core support. Prevent the cooling effect of outer chillers from decreasing and surface melting, and prevent the core supports from softening and melting prematurely, which may cause casting wall thickness change.

7. On premise of casting quality, the gating system should be conducive to reducing riser volume. Its structure should be simple and the area and volume occupied in sand mold should be small, so as to facilitate manual operation, cleaning and save the consumption of metal liquid and molding sand, improve the utilization of effective area of sand mold.

二. Hydraulic characteristics of sand flow

When liquid metal flows in sand mold, it exhibits the following hydraulics characteristics:

1.Viscous fluid flow

The study of hydraulics usually deals with ideal fluids that are non-viscous, whereas liquid metals are viscous fluids. The viscosity of liquid metal is related to its composition and increases with the decrease of liquid metal temperature in process of flow. When in crystal, the liquid viscosity increases sharply, the flow velocity and flow pattern will change dramatically, if there are inclusions in liquid metal flow, the viscosity of fluid flow will increase further

2. Unstable flow

In process of mold filling, liquid metal temperature decreases continuously and mold temperature increases continuously, which makes the heat exchange between the two unstable. As the fluid flow temperature decreases, viscosity increases and the flow resistance also increases. In addition, with the increase and decrease of liquid flow pressure in mold filling process, the velocity and flow state of liquid metal are also changing constantly, so the liquid metal flows to fill mold are unstable.

3. Flow in porous pipe

Because the sand mold has certain pores, casting system and cavity in sand mold can be regarded as porous pipes and containers. When liquid metal flows in the "porous pipe", it is often not well attached to the wall of pipe. At this time, external gas may be sucked into liquid flow, forming pores or causing oxidation of metal liquid, forming oxidation slag.

4. Turbulent flow

It is proved by test and calculation in production practice that when liquid metal flows in pouring system, its Reynolds number is larger than the critical Reynolds number, and the alloy liquid is generally turbulent in pouring system. For some horizontally poured thin-wall castings or large thick castings, the liquid flow rises slowly and it is possible to get laminar flow.

5. Multiphase flow

In general, the alloy liquid always contains some small amount of solid phase impurities, liquid phase inclusions and bubbles.

During filling  process, grains and gas may be precipitated, so the alloy liquid is multiphase flow. There are intense thermal, mechanical and chemical effects between alloy liquid and mold.

In conclusion, the hydraulic properties of liquid metal are obviously different from that of ideal liquid. However, experimental research and production practice show that due to certain superheat of liquid metal during pouring, small length of pouring system and short filling time, no crystallization occurs on the runner wall during pouring, and viscosity change has no significant influence on the flow. Therefore, liquid metal filling process and pouring system design can be analyzed and calculated with the basic formula of hydromechanics.

Keywords in the article: metal liquid /pouring
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