•Types of Materials 材料的类型
such as aluminum or magnesium have low densities, and are used in applications that require other metallic properties but also require low weight.
special properties: some ceramics are magnetic materials; some are piezoelectric materials; and a few special ceramics are superconductors at very low temperatures. Ceramics and glasses have one major drawback: they are brittle.
properties at higher molecular weights.
The following discussion applies principally to the heat treatment of ordinary commercial steels known as plain carbon steels. With this process the rate of cooling is the controlling factor, rapid cooling from above the critical range results in hard structure, whereas very slow cooling produces the opposite effect.
remaining austenite to pearlite. The resulting structure is a mixture of primary or pro-eutectoid ferrite (ferrite that formed above the eutectoid reaction) and regions of pearlite.
workpiece to be hardened to depths) of the steel and does not affect the hardness except in unhardened or partially hardened steels.
temperature causes coalescence of the carbides with continued decrease in hardness.
steel is heated slowly to a temperature just below the critical range and held there for a prolonged period of time, this structure will be obtained.
effectively resists corrosive action of water, saltwater spray, alkalies, crude oil, and natural gas.
parting line. The cope is then assembled to the drag, and the sand is poured on the cope half, covering the pattern, core and the gating system.
The mold is made by making a pattern using wax or some other material that can be melted away. This wax pattern is dipped in refractory slurry, which coats the wax pattern and forms a skin. This is dried and the process of dipping in the slurry and drying is repeated until a robust thickness is achieved.
Forging is an important hot-forming process. It is used in producing components of all shapes and sizes, from quite small items to large units weighing several tons.
making it uniform; so for heavy forgings, such as marine propeller shafts, an immensely powerful hydraulic press squeezes the metal with a force sometimes as great as 10,000 tonnes.
stress-risers in the forgings, as well as make the dies weak in service.
with adequate strength for handling during sintering, but do not produce very high strength sintered parts.
minimal machining is required but when all things are considered this route is often very cost-effective.
•Physical properties can be controlled
In 1956, Ankerwerk Nuremberg commercialized the modern reciprocating screw injection molding machine for thermoplastics. Today, over 50 machine manufacturers are listed in Modern Plastics Encyclopedia, offering machines to the U.S. market ranging from 2 to 6,000 tons clamping capacity.
softened by heat and of hardening on cooling, even after repeated cycling.
The latter also has an effect on processing since the life of the mold cavity is typically reduced from about 1,000,000 parts for plain resin parts to about 300,000 for glass-filled parts.
surface and the rest passing off on the back side of the chip, Fig.7.3c.
strap clamps and the like, whereas smaller workpieces are usually held by magnetic chucks.