A metal billet is loaded into a container, and a ram compresses the material, forcing it to flow through one or more openings in a die at the opposite end of the container. As the ram approaches the die, a small portion of the billet remains that cannot be forced through the die opening. This extra portion, called the butt, is seperated from the product by cutting it just beyond the exit of the die.
One of the problems in forward (direct) extrusion is the significant friction that exists between the work surface and the walls of the container as the billet is forced to slide toward the die opening. This friction causes a substantial increase in the ram force required in direct extrusion. In hot extrusion, the friction problem is aggravated by the presence of an oxide layer on the surface of the billet. This oxide layer can cause defects in the extruded product. To address these problems, a dummy block is often used between the ram and the work billet. The diameter of the dummy block is slightly smaller than the billet diameter, so that a narrow ring of work metal (mostly the oxide layer) is left in the container, leaving the final product free of oxides.
(Groover M. P., Fundamentals of modern manufacturing: materials, processes and systems, p. 417)
Impact Forging
Press forging employs a slow-squeezing action that penetrates throughout the metal and produces a uniform metal flow.
In hammer or impact forging, metal flow is a response to the energy in the hammer-workpiece collision. If all the energy can be dissipated through flow of the surface layers of metal and absorption by the press foundation, the interior regions of the workpiece can go undeformed. Therefore, when the forging of large sections is required, press forging must be employed.
(Kutz M., Mechanical engineers' handbook: Manufacturing and management, p. 250)
Martensite
Martensite is a form of ferrite that is supersaturated with carbon. In other words, because of
the very fast cooling rate, the carbon atoms do not have time to diffuse from their interstitial positions in the bcc lattice to form cementite particles.
Steel products produced with an as-quenched martensitic microstructure are very
hard and brittle, e.g., a razor blade. Most martensitic products are tempered by heating to
temperatures between about 350 and 650 C. The tempering process allows some of the
carbon to diffuse and form as a carbide phase from the supersaturated iron lattice. This
softens the steel and provides some ductility. The degree of softening is determined by the
tempering temperature and the time at the tempering temperature. The higher the temperature
and the longer the time, the softer the steel. Most steels with martensite are used in the
quenched and tempered condition.
(Kutz M., Mechanical engineers' handbook: Materials and mechanical design, p. 14)
Teflon (PTFE)
Poly(tetrafluoroethylene) is a highly crystalline polymer which is produced by the polymerization of tetrafluoroethylene. PTFE offers very high heat resistance (up to 250 C), exceptional chemical resistance, and outstanding flame resistance. The broad chemical resistance includes strong acids and strong bases. In addition, PTFE has the lowest coefficient of friction of any polymer.
PTFE has a high melting point and extremely high melt viscosity and hence cannot be melt processed by normal techniques. Therefore PTFE has to be processed by unconventional techniques (PTFE powder is compacted to the desired shape and sintered).
Uses
Typically PTFE is used in applications requiring long-term performance in extremeservice
environments. PTFE applications include electrical (high-temperature, high performance wire and cable insulation, sockets, pins, connectors), mechanical (bushings,
rider rings, seals, bearing pads, valve seats, chemical resistance processing equipment and
pipe, nonlubricated bearings, pump parts, gaskets, and packings), nonstick coatings (home
cookware, tools, food-processing equipment), and miscellaneous (conveyor parts, packaging,
flame-retardant laminates).
(Kutz M., Mechanical engineers' handbook: Materials and mechanical design, p. 370)
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