Sand castings are produced in specialized factories called foundries, as shown in Figure 15-9. In sand casting,
a suitable bonding agent (usually clay) is mixed in with the sand. The mixture is moistened, typically with
water, but sometimes with other substances, to develop the strength and plasticity of the clay and to make
the aggregate suitable for molding. The sand is typically contained in a system of frames or mold boxes.
As we mentioned above, CNC (computer numerical control) is used to make hard tooling for injection
molding. This process starts with a block of material (such as wood or metal), and removes material with
drills, boring tools and lathes, controlled by a computer program that is designed to make that pattern.
The CNC system transforms a block of material into the precise desired model as shown in Figure 15-10.
CNC is a subtractive process which removes material, while 3D printing is an additive process that builds
material up to create the model.
CNC was a major advancement and improvement over non-computer machining by hand. In modern CNC
systems, the design of a mechanical part and its manufacturing program is highly automated. The process
is fascinating to watch in person; bits of materials are flying everywhere within the enclosed machines and
the precision of the remaining design is impressive.
FIGURE 159: A foundry worker pours molten metal into sand based casts.
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