Machinery's Handbook, 31st Edition
1518 Design Considerations for Casting two gates from each runner, the total runner area should be 2 in 2 (1290 mm 2 ), so that each runner would be 1 in 2 (645 mm 2 ). The total gate area should be 2 in 2 (1290 mm 2 ), and there are four gates, so each gate would have an area of 0.5 in 2 (322 mm 2 ). The gate calculation only works this way if there are an equal number of gates on each runner; if not, divide the area of the runner by the number of gates on that runner to get the area of each gate. The area of the runners should be reduced just after a gate by an amount equal to the area of that gate. This ensures that each gate in the system will have the same flow of metal, even if it is farther from the sprue. The first bit of metal poured is most likely to be contaminated by air and sand entrapment. A runner extension can be used to prevent this metal from going into the mold cavity; this extension will have the same area as that of the last gate on that runner. Riser Design: Risers are important to ensure a flow of molten metal to the part being cast as it is starting to solidify. Basically, a riser is a vertical portion of the gating system (similar to a straight sprue) that stores the molten metal until it is needed by the casting. As molten metal solidifies, it shrinks. If it does not have a source of more molten metal to feed it as it shrinks, there will be defects in casting; a riser’s purpose is to provide that extra molten metal. This means the metal in the riser must stay liquid longer than metal in the part being cast. Without a riser, heavier parts of the casting will have shrinkage defects, either on the surface or internally. A riser may be required for every hot spot in a cast part. (In other words, the part of the casting that solidifies last, usually an area with a larger volume of metal.) The risers can be attached to the top or the side of a part. They also may be blind risers , which are completely contained in the mold and not open to the air; blind risers cool slower and thus will stay liquid longer. No matter where it is located, any gate that connects a riser to the casting must not be too small; however, it should be as short as possible (preferably half the diameter of the riser), else the material in the gate will solidify too soon and prevent metal in the riser from reaching the casting. Risers may be upstream from the casting in the runner/gate system. In this case, the metal must flow through the riser prior to reaching the casting. After the pour is com - pleted, the metal in the riser will be hotter than the metal in the casting. Risers also may be placed downstream, after the casting. Metal flows through the cast - ing to get to the riser, so the metal in the riser will be cooler than the metal in the casting. This could cause a problem if the metal in the casting feeds the riser as it cools. It is necessary for the metal in the riser to solidify last. Since the more surface area something has, the faster it cools, the surface area of the riser should be minimized for a given volume. The optimum shape for a riser would be a sphere, but that is not an easy shape to mold; next best is a cylinder, which is easier. Ideally, the cylinder’s height should be somewhere between one-half and one-and-a-half times the diameter. If possible, the bottom (and top if it is a blind riser) should be spherical or bowl-shaped to help the metal stay molten longer. Using Computer Modeling to Optimize Casting Processes In today’s global manufacturing environment, time is money. Casting buyers want foundries to deliver quality cast components with short lead times, regardless of the num- ber of steps in the component’s production cycle. To this end, foundries must take a greater role in the initial design of cast components, as well as gating/riser design. The one tool that has become indispensable to foundries when working with their customers to achieve an optimized casting design with minimal lead times is computer modeling. Using solid models, finite element and fatigue analysis, and casting process modeling, foundry work - ers can make numerous no-cost design revisions on the computer screen. For most foundries, computer modeling focuses on casting process modeling. This analysis ensures proper mold filling and solidification of the component. However, as foundries have begun to embrace casting process modeling, uses of this analytic tool have expanded.
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