Page 467 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 17.3 Compaction of Metal Powders 447

TABLE l7.l
Compacting Pressures for Various Powders
Metal Pressure (MPa)
Aluminum 70-275
Brass 400-700
Bronze 200-275
Iron 350-800
Tantalum 70-140
Tungsten 70-140
Other materials
Aluminum oxide 1 10-140
Carbon 140-165
Cemented carbides 140-400
Ferrites 110-165

require less than 0.9 MN. For small tonnage, crank- or eccentric-type me-
chanical presses are used; for higher capacities, toggle or knuckle-joint
presses are employed (see Fig. 14.17b). Hydraulic presses (Fig. 17.12) with
capacities as high as 45 MN can be used for large parts. Press selection de-
pends on part size and the configuration, density requirements, and produc-
tion rate. However, the higher the pressing speed, the greater is the tendency
for the press to trap air in the die cavity and thus prevent proper com-
paction.

I7.3.2 lsostatic Pressing
Green compacts may be subjected to hydrostatic pressure in order to
achieve more uniform compaction and, hence, density.
In cold isostatic pressing (CIP), the metal powder is placed in a flexible
rubber mold typically made of neoprene rubber, urethane, polyvinyl chlo-
FIGURE l7.l2 A 7.3-MN mechanical
ride, or another elastomer (Fig. 17.13). The assembly then is pressurized
press for compacting metal powder
hydrostatically in a chamber, usually using water. The most common pres-
Source: Courtesy of Cincinnati
sure is 400 MPa, although pressures of up to 1000 MPa may be used. The
Incorporated.
Upper cover
Cover -- Pf€SSUl’€
_ Wire vessel
Pressure _ mesh basket Rubber
Pressing
Vessel _ Mold diaphragm
Meta' _ seal plate rubber mold
Forming
mandrel _ Rubber
lT1Old (bag) |:|uid rubber mold
Flwd _ - Powder Powder
Pressure Lower
Pressure ,_ Source outside cover
source Lower inside
COVGI'
(H)
FIGURE I1.I3 Schematic diagram of cold isostatic pressing. Pressure is applied isostatically inside
a high-pressure chamber (a) The wet bag process to form a cup-shaped part. The powder is enclosed
in a flexible container around a solid-core rod. (b) The dry bag process used to form a PM cylinder