or thin features are filled. Even if the metal arrives at the gate at a sufficient temperature, metal moving through thin sections at slow speeds can aid in the heat extraction of the shell and plunge the temperature of the advancing melt front sufficiently low to labor the flow and ultimately stop it. Conversely, increasing the velocity of the advancing metal front can be an effective way of filling out casting cavities. As mentioned prior, even at temperatures below the liquidus temperature, if the metal is moving a sufficiently high velocity, momentum will move the material through thin sections, provided there isn’t resistance to flow like the accumulation of local air pressure or capillary repulsions. One way to increase the velocity leading into the casting cavity would be to angle the gates to take advantage of the increase of velocity due to the falling stream. If the gates are horizontal the metal will need to move down and across to enter the cavity. Minimizing that 90° turn and allowing the metal to run downhill will help the metal fill out the cavity. Of course, dewaxing challenges must be considered. Another way to increase the


velocity at the gate is to decrease the cross sectional area of the gate. Given a constant flow rate decreasing the cross sectional area of a gate will increase the velocity through that gate due to continuity, which will be discussed further below.


In some cases, the speed of the


moving metal front doesn’t need to be changed, but the direction of the metal. Figure 2 shows metal filling an impeller casting on a 48 on tree. The image on the left is a still image of the velocity profile with vectors added to the image to show the direction of the moving metal. The image to the right shows the casting shape for clarity. In the image to the left, you can see higher velocities in the yellow and white colors as well as the lower velocities in the shades of blue. As the metal enters the casting cavity from a vertical position. It swirls in the center of the cavity until moving outwards towards the impeller vanes.


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Figure 3: Air must be evacuated and displaced by the advancing metal front. Failure for this to occur either restricts the metal from flow or penetrates into the melt.


The changes in direction the metal must undergo makes it difficult maintain a higher velocity as the metal reaches the impeller tips. Changing the flow pattern in the cavity will improve the ability to fill out the vanes. The problem with increasing the


velocity is turbulent flow. Turbulent flow, in this case referring to an erratic filling patter, can increase the surface area of the metal in contact with air and thus cooling the metal faster. Another challenge is as you increase the surface area in contact with air, oxide forming alloys can be damaged and inclusions will become a problem. So balancing an increase in velocity to fill out a cavity vs potentially inclusion formation need to be considered. A parameter related to velocity but should be interpreted on its on is the pouring rate. Pouring rate, which is the rate at


which the metal is delivered to the shell cavity and is typically measured in lbs per second. Metal delivery becomes important because a relatively constant flow rate needs to be supplied during the pouring process. Any interrupted pours or drastically changing pour rates can have a substantive effect on the ability to fill all castings on the tree. Sometimes pouring rate and velocity are used interchangeably in foundry settings,


which is not correct. It is possible to pour at a higher rate but achieve lower velocities if the cross sectional area you are pouring through increases. This fact is due to continuity, a fundamental physical phenomenon derived from the conservation of mass. However, when you keep the cross sectional areas the same and increase the pouring rate, you will in turn increase the velocity and so understanding those effects both with production tooling or during sampling where gate sizes and locations can change becomes important.


Cavity Air Permeability of the shell is important for determining the rate at which air can evacuate out of the casting cavities through the porous shell media. The lower the permeability of the shell the longer it will take for air to evacuate out of the cavity. As the metal flows throughout the cavity, it has to displace the air that fills the inside of the shell, unless you pour in a vacuum. The air moves out of the way of the advancing metal front trying to escape through the area of lease resistance. Figure 3 shows a depiction of this. The transparent material in the image is liquid metal and


Continued on pg 22 November 2022 ❘ 23


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