Simple Solutions That Work! Issue 7

Contact: DAVID C. SCHMIDT dave@finitesolutions.com V = √ (2gH) Where V = velocity g = acceleration of gravity H = height through which the liquid has fallen This formula is based on Bernoulli’s Theorem, which describes the energy in a system. Given the velocity and volumetric flow rate, the area of flow of the liquid metal can be calculated from the following: Volumetric Flow Rate Flow Area = Velocity Flow areas are adjusted for frictional losses. A square tapered sprue has an efficiency of around 74%; this means that an area calculated above must be increased by a factor of (1/0.74) or 1.351 to account for the energy losses. Flow through runner systems also loses energy through friction with the channel walls. This is compensated for by increasing the area of the downstream runner segments. Another entry is the gating ratio, which is the ratio of the area of flow at three different points: the sprue; the runner; and the gates. This is usually expressed as whole numbers, giving the ratio of the area of each of these points as S:R:G. If the gating ratio is 1:4:4 then the area of the runners will be 4 times that of the area at the base of the sprue, and the area of the gates will be equal to that of the runners. The “choke” is the location in the gating system with the minimum cross-sectional area. In a 1:4:4 system, the choke is at the bottom of the sprue. In a 4:8:3 system, the choke is at the gates. Figure 2 Illustrates the entry of data about the pattern layout: The Sprue Type establishes the efficiency factor to be applied to the area calculation for the sprue. The next calculation is the Effective Sprue Height (ESH). This is based on pattern dimensions by selecting the type of gating system, then entering appropriate dimensions. Note that, if the metal is poured directly into the sprue and not into a pouring basin, then the height of the ladle above the top of the mold should be added to the ESH, since this height will establish the metal velocity after falling to the bottom of the sprue. The Gating Ratio is entered next, a set of three numbers as described above. The Number of runners leading away from the base of the sprue and the Total number of gates fed from this sprue are entered. At this point, algebra takes over to perform the gating calculations and design the individual components. Figure 3 shows a typical calculation of Sprue Data, including the choke area and areas at the bottom and top of the sprue. Also shown are the Total Runner Area, Number of Runners and the Friction Loss Factor. A typical Friction Loss Factor is 5%. For runners which feed multiple gates, it is common practice to “step down” the runner, after each gate to equalize the flow. The amount each section is reduced is equal to the area of the preceding gate. A typical Runner and Gate Design are shown in Figure 4 : Note that the runner cross-section is reduced for each subsequent gate along a runner, and also that the area of both the runner and the gate have been increased by the friction loss factor to compensate for the energy loss associated with friction. You can keep selecting subsequent gates along the runner until all of the gates and all sections of this runner have been designed. Once this runner is finished, you can select the next runner to design and perform the same operations to design all gates and runner sections for this runner. This process continues until all runners and all gates have been defined. Figure 2. Selection of Sprue Type, Pattern Layout and Gating Ratio. Figure 3. Sprue Data. Figure 4. Runner and Gate Size Data. 17