7.6.2 Selected expendable mold processes
Excerpt from the publication, Materials, Design and Manufacturing for Lightweight Vehicles by G.T. Kridli, et. al.Sand casting is the most commonly used expendable mold process. In this process, sand is bound together (as detailed in Table 7.8) and shaped to conform to the negative impression of the desired component. Molten metal is then poured into the sand mold; upon solidification, the metal has the geometry of the desired component.
Binder systems used for creating sand molds
|Green sand||Sand, bentonite clay, coal and water||Mixture is placed in a core box and squeezed against the pattern to form a mold. Mold can be used immediately for casting. For very large or heavy castings, mold is often baked/dried in an oven to improve mold strength and rigidity. Sand is usually physically removed after casting by shaking or hammering ("rapping") to fracture the mold into small sections.|
|Resin-bonded Air-set ("No-Bake" or "Cold box")||Sand and a resin system based upon esters, furans, phenolic urethanes, soldium silicates or phenolics||Sand and resin are blended to create a semi-solid mixture, which is quickly deposited into a core box containing the pattern. The resin reacts with the moisture in the air and hardens the mixture; the working time is a function of the type and amount of resin used. After hardening, the mold is removed from the core box and can be used immediately for casting. After casting, sand is removed either by physical methods (shaking/rapping) or via thermal operations that pyrolize the binder, leaving the sand free to flow out of the casting and be reclaimed.|
|Gas-cured ("Cold box")||Sand and a gas-reactive resin system based upon phenolic urethanes, acrylics, or sodium silicates gases include amines (TEA/DMEA), SO2, and CO2||Sand is first mixed with the gas-reactive resin system and deposited into a core box. The mixture instantly hardens by blowing a specific gas through the core box; thus, working time is flexible as it is; a function of the delay between molding and gassing. The mold can be used immediately for casting. Sand is removed either by physical methods (shaking/rapping) or via thermal operations that pyrolize the binder.|
|Heat-cured ("Hot box")||Sand and a heat-curable resin system based upon furans, phenolics, or urea formaldehydes||Sand is first mixed with the heat-reactive resin and deposited into a heated pattern (200–260 °C); working time is flexible as mixture is stable until heated. The mixture rapidly hardens once heated and is immediately available for use. Sand is usually removed using physical methods. Commonly used for 'shell molding' process|
Advantages of this process include the fact that nearly all cast alloys can be poured into sand, it is the only method for high-melting-point alloys, thin-wall sections (< 4 mm) can be obtained, castings can be made in a wide range of weights (grams to tons) and production volumes (one to millions), and it is the most easily scalable of the casting processes.
Some limitations of sand casting include the fact that high volume production requires a large investment in automated mold/core making and assembly equipment as well as floor space. Additionally, casting yields are often < 50%, requiring a larger amount of machining and finishing prior to use. Metallurgically, cooling rates in sand castings are often slow, especially in thick sections; this often yields lower mechanical properties and higher amounts of porosity. Further, sand issues are important considerations, including the consistency and quality of the sand, the volume of the sand used (up to 10 – 20x the weight of part), and sand removal, disposal, and/or recycling.
Sand casting is the process of choice for many large powertrain components, including blocks, heads, and intake manifold. Sand castings are less common in structural applications, due to their slower cooling rate and generally coarser micro-structure. However, in certain applications, such as the cast 356-T6 nodes for the Ford GT spaceframe, need for specific geometries at low production volumes requires the use of sand castings. Other expendable mold processes, such as investment casting and lost foam casting will not be discussed, since their use for structural components is limited to special, small-volume production components.Read more
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