The goldmsith I work for part-time is having a heck of a time
casting lately. Major, abnormal porosity that he couldn't figure out
and he've done a lot of casting. New equipment earlier this year,
and already the rubber gasket and seal on his vacuum casting machine
is dry and brittle. Upon doing some research, he's been casting too
hot. His flacks were at 1150-1200 degrees for yellow gold, a bit
lower for white. Now he istrying at 900 degrees for the flask temp
for 14kt yellow casting. I haven't heard how it went.
Any of you ever had problems with too hot flasks, too short burnout
cycles and abnormal porosity? He's rethinking the burnout cycle, the
flasktemp and even the metal temp.
I don't cast, I just clean up the castings. I'm not into casting but
will do it when necessary.
Joy
John4
#2
YES to most all of the (?)'s. Is he using the same source for the
casting material? Sometimes "they" change the alloy and don't tell
anyone. Different batch of investment Same thought if it is new.
Water source?
New equipment is often a learning curve to get it and your processes
in line with each other. To me the flask temp is rather high (1150 -
1200). How is the flask temp determined? Pyro on the new kiln? If
so, has "it" ever been checked for accuracy? Also the new one may be
correct in the temperature is is showing, but the old unit was lower
than the gauge showed so some corrections may be in order.
Too hot, too cold, flask or metal, "dirty investment" ALL can/will
create problems. Casting is not as haphazard as some folks think.
Good luck on him/her getting things figured out. Will make your work
much easier and faster.
John
I vacuum cast and use an electrolmelt to melt the metal. I pour
silver into a 830 degree flask and a 1000 degrees flask for 14K
gold.
I read someplace that you can tell if the mold/metal temp is correct
by looking at the top of the sprue. The temperature was correct if
the top is flat. If the top is concaved the temp combination was too
cool. If the top is bubbled up the temp combination was too hot.
I once cast sterling sterling into a 1000 degree mold following
someone's suggestion. The sprue blew up like a balloon.
I cut the sprue in half. The inside looked like a sponge.
The flask temperature stays. fairly constant during the
vacuum/electrolmelt process because the flask is removed from the
oven and metal is poured almost immediately. The flask temp during
centrifugal casting may cool considerably before themetal is melted
and poured.
John had it correct. Metal/mold temp, metal composition, dirty/ old
investment and water make up and temperature can all affect the
casting process. On top of all the different procedures the casting
gremlins may decide that they don't like you and screw up a
perfectly good process. LOL Good luck in correcting your casting.
Lee Epperson
