The castings that
resulted also had fewer
flaws that needed weld
repair, saving KomTek
in Worcester, Mass.,
about $6.75 per casting
in labor.
The photopolymer
is used to make a 2-lb
investment casting
through the Quick-
Cast method. Normally,
foundries burn
out QuickCast patterns
in a furnace, cool
the ceramic shell that
remains, flush out ash,
and then reheat the
shell before pouring
in metal. In this case,
though, there were
concerns that cooling
might compromise
shell integrity. “We
can’t risk a shell failure
that could cause a
spill of molten metal,”
says foundry engineer
Chan Nguyen. Consequently,
they skipped
the normal ash removal
step and poured immediately
after burnout.
Skipping ash removal brings
the risk of casting flaws from molten
metal interacting with the ash.
However, it provides several benefits.
There’s no potential damage to
shell integrity from cooling (particularly
for fused-silica shells),
and processing time drops by several
hours.
For the first few hundred castings,
Express Pattern, Vernon
Hills, Ill., built QuickCast patterns
from a low-viscosity liquid photopolymer, 11120 WaterShed from
DSM Somos, Elgin, Ill. Virtually
all patterns were cast successfully,
but most had flaws from residual
ash in the shell. The flaws were
typically minor pitting or inclusions.
“We had to weld repair 60%
of the castings made from Water-
Shed patterns,” says foundry manager
Bob McQuade. Weld repair
involve welding the flawed area
and then grinding the weld flush,
which takes about 15 min. Consequently,
weld repair added 9 min of
labor for each part.
Express Pattern beta tested ProtoCast
AF 19120 resin, also from
DSM Somos. The antimony-free
stereolithography photopolymer
produced significantly less residual
ash during burnout. After successful
trials, KomTeK switched to ProtoCast
AF 19120 patterns and has
seen a 15% drop in weld repairs.
Repair now takes 2.25 min/part.
With foundry labor and overhead
rates at $60/hr, the switch to ProtoCast
patterns cut costs by $6.75/
casting. “Additionally,” adds Mc-
Quade, “there’s a significant drop
in the amount of smoke generated
during pattern burnout. Less
smoke help us maintain our ‘good
neighbor’ status.”