Simple Solutions That Work! Issue 7

Basic Theory of Gating Design Gating design is actually very simple. One starts by estimating the Fill Time required for a casting. This may be based on experience, or on a calculation involving the weight poured, the alloy type and the critical section thickness. Knowing the Fill Time, weight and density of the casting, you can calculate the volumetric flow rate using the formula: The example shown following was created using the Gating Design Wizard, a part of the SOLIDCast simulation software. Much of the data input needed for a gating system calculation can be extracted from simulation models. For detailed descriptions of complete design of gating components and systems, see the AFS Handbook on Basic Principles of Gating. Gating Design starts with calculation of an Optimal Fill Time (OFT). To calculate an OFT, the following is required: Alloy Sensitivity This is specified with the slider bar at the top of the screen (Figure 1). This is the tendency of metals to form oxides during pouring. Alloys of low sensitivity may be poured more quickly. Alloys which are more sensitive should be poured more slowly to avoid turbulence which may form and entrain oxides as inclusions in the finished casting. Pour Weight This is the weight of casting(s) plus rigging. The wizard will extract the weight from the simulation files. If the model does not include risering, you may need to increase the displayed weight. The exact value of the weight is not highly critical, since the OFT formula uses the cube root of the weight to estimate fill time. Critical Section Thickness The thinnest section of the casting, which is most likely to misrun. You may also select a fill time rather than use the OFT calculation. Once fill time and weight have been established, we can begin figuring flow requirements. First, a mass flow rate is calculated. Then, using the metal density, this is converted to a volume flow rate. The next requirement is the height that the metal will drop. This is the effective height of the sprue. The velocity of the metal after falling through this height can be calculated from the following: DAVID C. SCHMIDT Vice President FINITE SOLUTIONS, INC. ARTICLE TAKEAWAYS: • Gating components are sized using Bernoulli’s Theorem and the Law of Continuity • Properly sized gating systems help to fill the mold smoothly • Gating system design can be automated using casting simulation tools THE BASICS OF HORIZONTAL GATING SYSTEM DESIGN Volume Flow rate = Fill Time Next, we consider how far the metal will fall when poured, which gives a metal velocity. Knowing the velocity and the volumetric flow rate, the cross-sectional area of flow required can be calculated. The flow area is adjusted for friction loss, and finally this area is apportioned so there is the desired rate of flow at all gates into the casting. It is also necessary to establish the “choke” point, which controls flow through the gating system. Figure 1. Alloy Selection and Optimal Fill Time (OFT) Calculation. 16