Page 310 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Summary 28

° Levitation melting involves magnetic suspension of the molten metal. An induc-
tion coil simultaneously heats a solid billet and stirs and confines the melt, thus
eliminating the need for a crucible (which could be a source of contamination
with oxide inclusions). The molten metal flows downward into an investment-
casting mold placed directly below the coil. Investment castings made with this
method are free of refractory inclusions and of gas porosity and have a uniform
fine-grained structure.

I |.9 Foundries and Foundry Automation

Casting operations usually are carried out in foundries (from the Latin fundere,
meaning melting and pouring). Although these operations traditionally have involved
much manual labor, modern foundries have automated and computer-integrated
facilities for all aspects of their operations. They produce a wide variety and sizes of
castings at high production rates, with good quality control and at low cost.
As outlined in Fig. 11.2, foundry operations initially involve two separate
groups of activities. The first group is pattern and mold making. Computer-
aided design and manufacturing (Chapter 38) and rapid-prototyping techniques
(Chapter 20) are now used to minimize trial and error and thus improve efficiency.
A variety of automated machinery is used to minimize labor costs, which can be
significant in the production of castings. The second group of activities is melting
the metals, controlling their composition and impurities, and pouring them into
molds.
The rest of the operations, such as pouring into molds carried along convey-
ors, shakeout, cleaning, heat treatment, and inspection, also are automated.
Automation minimizes labor, reduces the possibility of human error, increases the
production rate, and attains higher quality levels. Industrial robots (Section 37.6)
are now used extensively in foundry operations, such as cleaning, riser cutting,
mold venting, mold spraying, pouring, sorting, and inspection. Automatic storage
and retrieval systems for cores and patterns using automated guided vehicles
(Section 3715) are other operations.

SUMMARY

° Expendable-mold, permanent-pattern processes include sand, shell-mold, plaster-
mold, and ceramic-mold casting. These processes require the destruction of the
mold for each casting produced, but mold production is facilitated by a reusable
pattern.
° Expendable-mold, expendable-pattern processes include lost-foam and invest-
ment casting. In these processes, a pattern is consumed for each mold produced
and the mold is destroyed after each casting.
° Permanent-mold processes have molds or dies that can be used to produce a large
number of castings. Common permanent-mold processes include slush casting,
pressure casting, die casting, and centrifugal casting.
° The molds used in permanent-mold casting are made of metal or graphite and
are used repeatedly to produce a large number of parts. Because metals are
good heat conductors, but do not allow gases to escape, permanent molds fun-
damentally have different effects on casting than sand or other aggregate mold
materials.