Page 291 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

2 0 Chapter 11 Metal-Casting Processes and Equipment
The pattern may be made of wood or metal. After setting, the molds (ceramic
facings) are removed, dried, ignited to burn off volatile matter, and baked. The molds
are clamped firmly and used as all-ceramic molds. In the Shaw process, the ceramic fac-
ings are backed by fireclay (which resists high temperatures) to give strength to the
mold. The facings then are assembled into a complete mold, ready to be poured.
The high-temperature resistance of the refractory molding materials allows these
molds to be used for casting ferrous and other high-temperature alloys, stainless steels,
and tool steels. Although the process is somewhat expensive, the castings have good di-
mensional accuracy and surface finish over a wide range of sizes and intricate shapes.

ll.3 Expendable-mold, Expendable-pattern
Casting Processes

Evaporative-pattern and investment casting are sometimes referred to as expendable-
pattern casting processes or expendable mold-expendable pattern processes. They
are unique in that a mold and a pattern must be produced for each casting, whereas
the patterns in the processes described in the preceding section are reusable. Typical
applications are cylinder heads, engine blocks, crankshafts, brake components, man-
ifolds, and machine bases.

I I.3.l Evaporative-pattern Casting (Lost-foam Process)
The evaporative-pattern casting process uses a polystyrene pattern, which evapo-
rates upon contact with molten metal to form a cavity for the casting; this process is
also known as lost-foam casting and falls under the trade name full-mold process. It
has become one of the more important casting processes for ferrous and nonferrous
metals, particularly for the automotive industry.
In this process, polystyrene beads containing 5 to 8% pentane (a volatile hy-
drocarbon) are placed in a preheated die that is usually made of aluminum. The
polystyrene expands and takes the shape of the die cavity. Additional heat is applied
to fuse and bond the beads together. The die is then cooled and opened, and the
polystyrene pattern is removed. Complex patterns also may be made by bonding
various individual pattern sections using hot-melt adhesive (Section 32.4.1).
The pattern is coated with a water-based refractory slurry, dried, and placed in
a flask. The flask is then filled with loose, fine sand, which surrounds and supports
the pattern (Fig 1 1.1 1) and may be dried or mixed with bonding agents to give it ad-
ditional strength. The sand is compacted periodically, without removing the poly-
styrene pattern; then the molten metal is poured into the mold. The molten metal
vaporizes the pattern and fills the mold cavity, completely replacing the space previ-
ously occupied by the polystyrene. Any degradation products from the polystyrene
are vented into the surrounding sand.
The flow velocity of the molten metal in the mold depends on the rate of degra-
dation of the polymer. Studies have shown that the flow of the metal is basically lam-
inar, with Reynolds numbers in the range of 400 to 3000. The velocity of the molten
metal at the metal-polymer pattern front (interface) is in the range of 0.1 to 1.0 mfs
and can be controlled by producing patterns with cavities or hollow sections. Thus,
the velocity will increase as the molten metal crosses these hollow regions, similar to
pouring the metal into an empty cavity.
Because the polymer requires considerable energy to degrade, large thermal
gradients are present at the metal-polymer interface. In other words, the molten
metal cools faster than it would if it were poured directly into an empty cavity.
Consequently, fluidity is less than in sand casting. This has important effects on the