Page 72 - Shigley's Mechanical Engineering Design

bud29281_ch02_031-070.qxd 11/11/09 09:34 PM Page 47 Debd Hard Disk1:Desktop Folder:Temp Work:Satya 10/11/09:

Materials 47
2–9 Shell Molding

The shell-molding process employs a heated metal pattern, usually made of cast iron,
aluminum, or brass, which is placed in a shell-molding machine containing a mixture
of dry sand and thermosetting resin. The hot pattern melts the plastic, which, together
with the sand, forms a shell about 5 to 10 mm thick around the pattern. The shell is then
baked at from 400 to 700°F for a short time while still on the pattern. It is then stripped
from the pattern and placed in storage for use in casting.
In the next step the shells are assembled by clamping, bolting, or pasting; they are
placed in a backup material, such as steel shot; and the molten metal is poured into the
cavity. The thin shell permits the heat to be conducted away so that solidification takes
place rapidly. As solidification takes place, the plastic bond is burned and the mold col-
lapses. The permeability of the backup material allows the gases to escape and the cast-
ing to air-cool. All this aids in obtaining a fine-grain, stress-free casting.
Shell-mold castings feature a smooth surface, a draft that is quite small, and close
tolerances. In general, the rules governing sand casting also apply to shell-mold casting.

2–10 Investment Casting
Investment casting uses a pattern that may be made from wax, plastic, or other material.
After the mold is made, the pattern is melted out. Thus a mechanized method of casting a
great many patterns is necessary. The mold material is dependent upon the melting point
of the cast metal. Thus a plaster mold can be used for some materials while others would
require a ceramic mold. After the pattern is melted out, the mold is baked or fired; when
firing is completed, the molten metal may be poured into the hot mold and allowed to cool.
If a number of castings are to be made, then metal or permanent molds may be suit-
able. Such molds have the advantage that the surfaces are smooth, bright, and accurate,
so that little, if any, machining is required. Metal-mold castings are also known as die
castings and centrifugal castings.

2–11 Powder-Metallurgy Process
The powder-metallurgy process is a quantity-production process that uses powders from
a single metal, several metals, or a mixture of metals and nonmetals. It consists essen-
tially of mechanically mixing the powders, compacting them in dies at high pressures, and
heating the compacted part at a temperature less than the melting point of the major
ingredient. The particles are united into a single strong part similar to what would be
obtained by melting the same ingredients together. The advantages are (1) the elimina-
tion of scrap or waste material, (2) the elimination of machining operations, (3) the low
unit cost when mass-produced, and (4) the exact control of composition. Some of the dis-
advantages are (1) the high cost of dies, (2) the lower physical properties, (3) the higher
cost of materials, (4) the limitations on the design, and (5) the limited range of materials
that can be used. Parts commonly made by this process are oil-impregnated bearings,
incandescent lamp filaments, cemented-carbide tips for tools, and permanent magnets.
Some products can be made only by powder metallurgy: surgical implants, for example.
The structure is different from what can be obtained by melting the same ingredients.

2–12 Hot-Working Processes
By hot working are meant such processes as rolling, forging, hot extrusion, and hot
pressing, in which the metal is heated above its recrystallation temperature.