Resin Coated Sand: Shell Molding Process

A resin – coated sand is shaken onto a heated pattern plate,
which has been sprayed with a parting agent. The synthetic resin
binder used for Sand coating cures in the heat, immediately
creating a shell like layer around the pattern.

After a baking (procuring) period which determines the wall
thickness of the shell, unbonded sand is shaken off the pattern,
leaving a shell or resin bonded sand. Following an adequate final
curing period, during which heat is frequently also applied to the
outer surface of the layer, the shell may be separated from the
pattern by lifting pins, and removed. The cycle may be repeated
after the pattern has been cleaned and again treated with parting
agent.

Hollow cores are produced in a similar manner. The resin-coated
sand is shaken or shot into heated core boxes. After a few
seconds baking time, excess coated sand is discharged; the core
is then cured by heating it from the outside or from both sides,
and is stripped from the core box.

A mold is formed by two half shells, in which the cores must be
set before assembly. The half shells are best joined together by
adhesive bonding. The finished, free – standing molds may be
poured off either upright of lying flat; backfilling of shell molds
with steel shots or sand is also possible. After being poured off,
the mold collapses due to combustion of the resin binder.

The following process steps are involved:
1.        Heaping of Resin Coated Sand on the heated pattern
       of introduction into the core box.
2.        Formation of a shell during the procuring period.
3.        Removal of unbonded molding sand.
4.        Final curing of the shell from one or both sides.
5.        Separation of the shell from the pattern of core box.
6.        Stripping of the molded shell.
7.        Assembly of two half shells in a bonding press.
8.        Cleaning of the pattern or core box and application of
       parting agent.

The individual process steps permit a high level of mechanization
in shell mold production lines. This represents a great advantage
of the process. Additional benefits are high dimensional accuracy
and surface quality, low fettling costs and further advantages
including a low rate of reject castings.

The sand-resin mix can be recycled by burning off the resin at
high temperatures.


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