Page 490 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

70 Chapter 18 Ceramics, Glasses, and Superconductors: Processing and Equipment

added to the mixture; these additives also act as lubricants. This process has the
same high production rates and close control of dimensional tolerances as does
powder metallurgy.
The pressing pressure ranges from 35 to 200 MPa. Modern presses used for dry
pressing are highly automated. Dies usually are made of carbides or of hardened steel.
They must have high wear resistance in order to withstand the abrasive ceramic par-
ticles; hence, they can be expensive.
Density can vary significantly in dry-pressed ceramics (as in PM compaction)
because of friction among the particles and at the mold walls. Density variations
cause warping during the firing, which is particularly severe for parts having high
length-to-diameter ratios. The recommended maximum ratio is 2:1. Several meth-
ods may be used to minimize density variations, including (a) proper design of tool-
ing, (b) vibratory pressing and impact forming (particularly for nuclear-reactor fuel
elements), and (c) isostatic pressing.

Wet Pressing. In wet pressing, the part is formed in a mold while under high pres-
sure in a hydraulic or mechanical press. This process generally is used to make intri-
cate shapes. Moisture content usually ranges from 10 to 15%. Production rates are
high; however, (a) part size is limited, (b) dimensional control is difficult to achieve
because of shrinkage during drying, and (c) tooling costs can be high.

lsostatic Pressing. Used extensively in powder metallurgy, isostatic pressing also is
used for ceramics in order to obtain a uniform density distribution throughout the
part during compaction. For example, automotive spark-plug insulators are made by
this method at room temperature, while silicon-nitride vanes for high-temperature
applications (see Fig. 8.1) are made by hot isostcztic pressing.

Iiggering. A series of steps is needed to make ceramic plates. First, clay slugs are ex-
truded and formed into a but over a plaster mold. Then they are jiggered on a rotat-
ing mold (see Fig. 18.5b). jiggering is a motion in which the clay bat is formed by
means of templates or rollers. The part then is dried and fired. The jiggering process
is confined to axisymmetric parts and has limited dimensional accuracy. The opera-
tion is automated for improved productivity.

Injection Molding. Injection molding is used extensively for the precision forming
of ceramics in high-technology applications, such as for rocket-engine components.
The raw material is mixed with a binder, such as a thermoplastic polymer (polypropy-
lene, low-density polyethylene, or ethylene vinyl acetate) or wax. The binder usually
is removed by pyrolysis (inducing chemical changes by heat); the part is then sintered
by firing.
The injection-molding process can produce thin sections [typically less than 10
to 15 mm thick] from most engineering ceramics, such as alumina, zirconia, silicon
nitride, silicon carbide, and sialon. Thicker sections require careful control of the
materials used and of the processing parameters in order to avoid defects, such as in-
ternal voids and cracks-especially those due to shrinkage.
Hot Pressing. In this process (also called pressure sintering), the pressure and the
heat are applied simultaneously, thereby reducing porosity and making the part
denser and stronger. Graphite commonly is used as a punch and die material, and
protective atmospheres usually are employed during pressing.
Hot isostatic pressing (Section 17.3.2) also may be used, particularly to im-
prove shape accuracy and the quality of high-technology ceramics, such as silicon
carbide and silicon nitride. Glass-encapsulated HIP processing has been shown to be
effective for this purpose.