3.8 shows a schematic illustration of a typical gating system for a gravity casting process.
Pouring cup design. At the top of the sprue, a pouring cup (another common name is pouring basin) is often used to minimize splash and turbulence as the metal flows into the sprue. The design of the pouring cup, for an optimal casting process, needs to be such that it can keep the sprue full of molten metal throughout the pour; also, if the level of molten metal is maintained in the pouring cup during pouring, then the dross will float and not enter the mold cavity. A constant level of molten metal in the pouring cup is important for a successful casting process because in some cases there is an automatic regulator of the molten metal going into pouring cup.
Fig. 3.8 Schematic illustration of a typical riser-gated casting.
Sprue and sprue base design. The sprue is a tapered vertical channel through which the liquid metal, from the pouring cup, enters a runner that leads into the mold cavity. The main goal in designing a sprue is to achieve the required flow rates. A sprue tapered to a smaller size at its bottom will create a choke (a restriction in the gating system that limits the flow rate of molten metal), which will help keep the sprue full of molten metal, but the choke will also increase the speed of the molten metal, which is undesirable. To solve this problem, the enginer needs to create an enlarged area at the bottom of the sprue, called a sprue base. The sprue base decreases the speed of the molten metal. There are two basic types of sprue bases: enlargement and well.
The general rules for designing an enlargement base are these:
•The diameter is roughly 2.5 times the width of the runner.
•The depth is equal to the depth of the runner.
The general rules for designing a well base are these:
•The depth of the wall base is twice that of the runner.
•A cross-section of the area of the base is five times the cross-sectional area of the sprue bottom.
The bottom of the sprue base should be flat, not round. If it is round, it could cause turbulence in the metal.
If sprue design is not used, there is a danger of turbulent filling of the gating system during pouring, as well as the possibility of having a nonuniform metalostatic pressure head. Turbulent filling can introduce oxides, which may produce defects. Filters are used in keeping oxides from the casting and make the flow more laminar.
Runner design. The runner is a horizontal distribution channel through which molten metal is guided from the sprue into the gate. The type of molding metal determines size and shape of the runner. A large runner allows better flow of the liquid metals; however, cooling takes longer. Thus, a runner is normally made smaller than the section of a casting and gradually made bigger while being tested. In any case, a runner should be basically smaller than the thickness area of the casting part. One runner is used for simple parts, but two runner systems can be specified for more complicated parts.
Gate design. The gate serves as the entrance to the cavity and should be designed to permit the mold cavity to fill easily. A cavity can have more than one gate. It is positioned at the thickest area of a part. A fillet needs to be used where a gate meets a casting. The gate length should be three to five times the gate diameter, depending on the metal being cast. The cross-section of the gate should be smaller than the runner cross-section.
Riser design. Risers are created in the gating system that will be filled with molten metal. Risers provide reservoirs of molten metal that can flow into the mold cavity to compensate for any material shrinkage that occurs during solidification. Any shrinkage should be in the risers and not in the final casting. The metal in the risers must stay liquid longer than the metal in the part being cast. There are two types of riser: open and blind. The blind riser is completely in the mold. This type of riser cools slower because it is not open to the air. The risers can be attached to the top or to the side of a part.
Risers may be attached in the runner instead of the casting. In this case the metal must flow through the riser prior to reaching the casting, and after the pouring is completed the metal in the riser will be hotter than the metal in the casting. It does not matter where the riser is located: the cross-sectional area of the gate just needs to be the same as the runner and the length no more than 0.5 the diameter of the riser. Theoretically, the best shape of the riser is spherical, but in practice the most useful (and easy shape to mold) is the cylinder. The height of the riser should be:
| h = (0.5 − 1.5)D | (3.1) |
where
| h | = | height of riser, m (in.) |
| D | = | diameter of cylinder, m (in.). |
If possible, the top of a blind riser should be spherical. This will help the metal stay molten longer. Risers, therefore, are not as useful for metals with low density, such as aluminum alloys, as they are for metals with a higher density, such as steel and cast iron.
3.3 COMPUTER MODELING OF CASTING PROCESSES
Computer modeling of metal casting processes has been very successful for companies around the world. Numerical simulation is widely used and accepted in manufacturing as a way to reduce hardware prototyping and to improve the parts design and manufacturing processes. The automotive industry and others are increasingly using computer simulations to attain their design objectives. As a result of rapid advances in computing technologies, three-dimensional (3-D) process modeling became practical during the last decade of the twentieth century. Today, many small- to midsized companies perform multiple simulations on a daily basis. Process modeling is no longer a luxury but a necessity for survival in the casting industry.
Cutting costs and reducing time to market are two of the most pressing issues in the foundry industry today. Development time can be very high in the conventional trial-and-error-based process design. In the current competitive environment, there is a need for foundry and casting units to develop the components and the process within quick response windows. Further, the costs of development also have to be kept low to be competitive. Casting process simulation helps engineers achieve these goals and is now widely used throughout the industry for process design and other various aspects of casting processes; tools such as simulation software systems help the designer to visualize the metal flow in the die cavity, the temperature variations, the solidification progress, and the evolution of defects such as shrinkage porosities, cold shuts, hot tears, and so on.
The application of casting simulation has been most beneficial toward avoidance of creating shrinkage scrap, improving cast metal yield, optimizing gating system design, optimizing mold filling, and finding the thermal fatigue life in permanent molds.
Many major foundries are using software based on powerful Finite Element analyses such as ProCAST, which covers a wide range of casting processes and alloys. However, several another commercial software programs are now available for modeling casting processes, such as Magma SOFT, Solidia, and AFS solid.
In the early centuries of casting, bronze and brass were preferred over cast iron as foundry materials. Iron was more difficult to cast due to its higher melting temperatures and workers lack of knowledge about its metallurgy. In the second part of the 16th century, the first cannons were cast from iron and after that a large demand for cast iron began to grow, first for military items and later for commercial products. Today, most commercial castings are made of alloys rather than pure metals. Pure metals like zinc, lead, tin, and copper are really good metals for casting but are used only for art, not for commercial products.