Simple Solutions That Work! Issue 10

System design starts with calculation of an Optimal Fill Time (OFT) (Fig 1). The following data is required: Alloy Sensitivity - This is specified with the slider bar at the top of the screen (Fig 1). This is the tendency of metals to form oxides during pouring. Low sensitivity alloys may be poured more quickly. Alloys which are more sensitive should be poured slowly to avoid turbulence which may form and entrain inclusions in the finished casting. Weight per Casting – This is the weight of one casting without gating. 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 thickness of the thinnest section of the casting, which is most likely to misrun. You may also enter your own Fill Time as an alternative to the OFT calculation. After calculating or entering the Fill Time, enter the Number of Castings per Mold and the Number of Gates per Casting . After this, select the following: Type of Gate Type of Sprue Pressurization Factor BASIC THEORY OF GATING DESIGN Vertical gating system design is quite simple. Step one is estimating the Fill Time required for a casting. This may be based on experience or on a calculation involving the pour weight, 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: 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 can be calculated. The flow area is adjusted for friction loss, and this area is apportioned so that there is the desired flow rate at each ingate. It is also necessary to establish the “choke” point, which controls flow through the gating system. The following example was created using the Gating Design Wizard, a part of the SOLIDCast simulation software. Much of the data input needed for gating system calculation can be extracted from simulation models. For a detailed description of vertical gating system design, see the AFS Handbook on Basic Principles of Gating, and papers by Roger Brown of Disamatic. DAVID C. SCHMIDT Vice President FINITE SOLUTIONS, INC. ARTICLE TAKEAWAYS: • Vertical Gating System components are sized using Bernoulli’s Theorem and the Law of Continuity • Properly-sized components will help to fill the mold smoothly • System calculations can be automated using casting simulation THE BASICS OF VERTICAL GATING SYSTEM DESIGN 32