Shell mold casting or shell molding is a metal casting process
in manufacturing industry in which the mold is a thin hardened
shell of sand and thermosetting resin binder, backed up by some
other material. Shell molding was developed as a manufacturing
process during the mid-20th century in Germany. Shell mold
casting is particularly suitable for steel castings under 20
lbs; however almost any metal that can be cast in sand can be
cast with the shell molding process. Also much larger parts have
been manufactured with shell molding. Typical parts manufactured
in industry using the shell mold casting process include
cylinder heads, gears, bushings, connecting rods, camshafts and
The first step in the shell mold casting process is to
manufacture the shell mold. The sand we use for the shell
molding process is of a much smaller grain size than the typical
green sand mold. This fine grained sand is mixed with a
thermosetting resin binder. A special metal pattern is coated
with a parting agent, (typically silicone), which will latter
facilitate in the removal of the shell. The metal pattern is
then heated to a temperature of 350F-700F degrees, (175C-370C).
The sand mixture is then poured or blown over the hot casting
pattern. Due to the reaction of the thermosetting resin with the
hot metal pattern, a thin shell forms on the surface of the
pattern. The desired thickness of the shell is dependent upon
the strength requirements of the mold for the particular metal
casting application. A typical industrial manufacturing mold for
a shell molding casting process could be .3in (7.5mm) thick. The
thickness of the mold can be controlled by the length of time
the sand mixture is in contact with the metal casting pattern.
The excess "loose" sand is then removed, leaving the shell and
The shell and pattern are then placed in an oven for a short
period of time, (minutes), which causes the shell to harden onto
the casting pattern.
Once the baking phase of the manufacturing process is complete,
the hardened shell is separated from the casting pattern by way
of ejector pins built into the pattern. It is of note that this
manufacturing technique used to create the mold in the shell
molding process can also be employed to produce highly accurate
fine grained mold cores for other metal casting processes.
Two of these hardened shells, each representing half the mold
for the casting, are assembled together either by gluing or
The manufacture of the shell mold is now complete and ready for
the pouring of the metal casting. In many shell molding
processes, the shell mold is supported by sand or metal shot
during the casting process.
Properties and Considerations of Manufacturing by Shell Mold
The internal surface of the shell mold is very smooth and rigid.
This allows for an easy flow of the liquid metal through the
mold cavity during the pouring of the casting, giving castings a
very good surface finish. Shell mold casting enables the
manufacture of complex parts with thin sections and smaller
projections than green sand mold casting.
Manufacturing with the shell mold process also imparts high
dimensional accuracy. Tolerances of .010 inches (.25mm) are
possible. Further machining is usually unnecessary when casting
by this process.
Shell sand molds are less permeable than green sand molds and
binder may produce a large volume of gas as it contacts the
molten metal being poured for the casting. For these reasons,
shell molds should be well ventilated.
The expense of shell mold casting is increased by the cost of
the thermosetting resin binder, but decreased by the fact that
only a small percentage of sand is used compared to other sand
Shell mold casting processes are easily automated.
The special metal patterns needed for shell mold casting are
expensive, making it a less desirable process for short runs.
However, manufacturing by shell casting may be economical for
large batch production.
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