CASTING AND FORGING
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Casting
Casting is pouring molten liquid into a mold where it is subsequently allowed to cool & solidify before ejection for use. Casting is used for producing parts to near-net (i.e. near-final) shape. The molds can range from very cheap sand molds to elaborately machined chromium-steel molds costing well into the 6 figure range. Since molten metal is poured into the mold, the final shape can be quite intricate and can contain both external and internal (i.e. hollow) features. Due to the fact that almost all metals shrink when cooled, the mold design can be a complex undertaking and the final surface finish and feature tolerances are typically low (i.e. casting produces rough surfaces and tolerances in range of ±0.060&#;). However, important features can be finish-machined after the casting is removed from the mold. The casting process is used for complex (i.e. hollow parts or parts with lots of complex contours) low volume parts as well as high volume production.
SAND CASTING
Sand casting typically uses two mold halves formed around wooden or metal part patterns. Once green (i.e. moist) sand is tightly compacted around the patterns, the mold halves are separated and the mold is fired to cure the sand into a solid mold. The mold halves are then brought back together, where the molten metal is poured inside, allowed to solidify and removed from the mold halves via vibratory methods. A new mold must be produced for each cast part.
Sand mold casting videos. The first video (left) is more traditional and the second video (right) shows the sand casting process used to make a metal caster wheel. Click for videos.
INVESTMENT CASTING
Investment casting (aka the lost wax process) begins with a wax pattern made by using a die; the patterns are mounted with others on a &#;tree&#; or assembly; the tree is covered with stucco (ceramic slurry); the wax is removed in a steam autoclave and the ceramic shell is fired to cure it completely; then the molten metal is poured into the shell mold and allowed to cool; and the final part is removed via vibratory methods. As in sand mold casting, new molds must be produced for each set of cast parts.
Investment casting videos. The first video (left) is more traditional and the second video (right) shows the sand casting process used to make a metal caster wheel. Click for videos.
DIE CASTING
Die casting uses an automated machine to inject molten metal under high pressure into intricately machined alloy steel mold halves. Once the molten metal cools and solidifies, the die halves are separated and the cast part removed by mechanical ejection pins or a robotic arm. The high pressure reduces porosity and improves part strength. This is the most advanced (and expensive) method of casting currently used today.
Die casting videos. The first video (left) is a short process animation / overview and the second video (right) explains the die casting process in detail. Click for videos.
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Forging
Forging is the repeated deformation of heated solid metal to refine the grain structure and improve part strength without adversely affecting the material&#;s ductility. Forging produces the strongest metals used in industry today. Forging is used for both low and high volume part production.
OPEN DIE FORGING
Open die forging refers to the use of hammers, punches and accessories to forge the metal into general shapes called forged billets for subsequent machining to final size.
Open die forging videos. The first video (left) shows a smaller workpiece and the second video (right) shows a larger one. Click for videos.
CLOSED DIE FORGING
Closed (or impression) die forging refers to the use of custom made, specialized dies with the desired final shape that enclose the heated metal during the forging process, forging the metal into a desired final shape. Closed die forging is generally used for mass production of identical parts requiring high strength, such as tools (i.e. wrenches, ratchets & sockets) or internal engine parts (e.g. connecting rods, pistons and valves).
Closed (aka impression) die forging videos. The first video (left) shows a wrench being forged and the second video (right) illustrates the process in more detail. Click for videos.
Relative Strengths
On a strength scale, typical extruded billets (the type of material used for project wheel hubs, motor mounts, and the majority of parts you will likely design in industry) are average, so they would receive a 7 out of 10 ranking on a strength scale. Cast materials are weaker because they can have voids and inclusions upon solidifying and their grain structure is randomly oriented, so they would receive a 4 out of 10 ranking on a strength scale). Forged materials are the strongest because the compressive stresses help align the microstructure in a favorable way, so they would receive a 10 out of 10 ranking on the strength scale.
Representation of grain structure for CAST material (left), BILLET material (center), and FORGE material (right). Notice how the microstructure of the forged material follows the part geometry, making it the strongest and toughest material of the three.
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