A common challenge in most casting processes is minimizing, or in some cases eliminating, filling-related defects such as entrained air and inclusions. For example, in high pressure die casting, entrained air can be moved out of the casting by proper placement of overflow or at least moved to areas of the casting where strength and aesthetics are not compromised. However, some castings such as high pressure pipes, bushings, and high-end jewelry like platinum rings require exceptionally low porosity, high strength, and near-perfect finish. In this blog, we will explore the three centrifugal casting processes – horizontal, vertical, and centrifuge – available in FLOW-3D CAST v5.1’s Centrifugal Casting Workspace and its unique features that allow casting engineers to create high quality castings.
Centrifugal casting processes use rapidly spinning molds to force molten metal outward from the rotation axis while relatively light defects drift out of the casting or at least to the center of the casting where they can be machined away. Two unique features in the centrifugal casting process workspace provide the ability to accurately and efficiently simulate a given design – cylindrical meshes and spinning mold model. Let’s start by looking at a typical horizontal centrifugal casting to see how these features are beneficial.
Horizontal Centrifugal Casting
Here’s an example of a horizontal mold used to cast a pipe. The mold is spun on rollers at 1000 rpm.
Molten metal is poured into the open end of the mold and falls under gravity until it is picked up by the spinning mold. The melt spreads out quickly into a thin sheet as it fills the mold. The end-on view in the video below shows how a rather coarse 150,000 cell, cylindrical mesh with fine radial resolution near the wall captures the flow accurately and efficiently. Since the heat transfer in the melt and mold are mostly radial, the fine radial resolution provided by the cylindrical mesh also contributes to the accuracy of the simulation.
In this filling simulation of a horizontal pipe casting, an end-on view of the filling at the left shows the cylindrical mesh used to resolve the flow. This simulation was run on 10 cores of a medium-level CPU (AMD 1950x) in 11 minutes! Even with this relatively coarse mesh resolution, a great deal of process knowledge can be obtained. Once a rough idea of the proper values for the process parameters such as pour rate, melt superheat, and initial mold temperature have been identified, higher mesh resolutions can be used to zero in on more exact values.
Vertical Centrifugal Casting
The next centrifugal casting process we’ll investigate is a vertical centrifugal casting. The vertical centrifugal casting process is ideal for large, symmetrical castings with a length similar or smaller than their diameter. Again, the spinning mold model in a cylindrical mesh is used to provide for an accurate representation of the filling characteristics. Various fill configurations such as using moving metal inputs can be easily studied. For example, metal can be introduced into the spinning mold through a sprue that moves vertically and/or horizontally to distribute the melt. In this video, the metal input brings molten metal into the spinning mold at the top of the mold to fill the flange initially and then moves downward as the filling continues.
In this simulation, a moving metal input is used to fill a vertical spinning mold rotating at 50 rpm. This simulation ran in 17 minutes on 10 cores of an AMD 1950X, which is quite remarkable considering the complexity of the flow. This is due to the efficiencies of the cylindrical meshing method and the spinning mold model. Detailed parametric studies can be carried out to identify an optimal process design with such efficiencies.
Once the filling is complete and the metal has become stable in the spinning mold, the simulation can be restarted in a solidification subprocess. In the solidification subprocess, the flow field is set to zero in a rotating mesh and only solidification is computed. Computing only the solidification allows for extraordinarily fast simulation times. This video shows solidification in a cross-section of the vertical casting. A 600 second simulation time is computed in less than 1 minute.
The final centrifugal casting process we’ll investigate is a centrifuge casting using an example of a 6-handle lever set.
A caster might wonder how various process parameters such as mold spin rate and spin-up profiles may have on the casting quality. For example, should the melt be poured into an already spinning mold or should the mold be spun up gradually so that entrained air isn’t generated? We can answer this question by comparing two spin-up profiles. On the left is a mold spun up from stationary to 10 rpms over 2 seconds while metal is poured into the cup. From 2 seconds to 3 seconds, the mold spin rate is ramped up to 50 rpms. On the right, the mold spins continuously at 50 rpms.
A comparison of entrained air in the melt with a ramped up spin rate (left) vs pouring into a mold spinning at a constant rate. The simulation indicates that it is best to spin up the mold gradually to allow the runners to fill before the maximum spin rate is applied. A rotating mesh is used to achieve high filling accuracy as well as fast simulation runtimes. Both simulations ran in about 4 hours on 12 cores of an AMD 2990WX.
This image shows the last frame of the simulation, illustrating that the air entrainment is reduced by slowly spinning up the mold.
A casting process design engineer can use the Centrifugal Casting Workspace to study a wide variety of process parameters in almost any centrifugal casting setup to achieve optimal casting quality in a reasonable amount of time.