I was facing a problem while setting casted jewelry. The prongs of
the collets kept breaking. Why does the metal get brittle after
casting? Also, can anyone tell me why rolled metal has more
strength than casted metal?
Rahul
India
The prongs of the collets kept breaking. Why does the metal get brittle after casting?
Really good question, Rahul!
What metal are you casting? !Are you using lost wax casting?
Centripetal (spin)? or vacuum casting? Do you recycle your injection
wax? What investment are you using? What is your burnout schedule?
How do you melt your metal? How do you protect the metal from oxygen
when pouring the cast? What is the temperature of the metal? What is
the temperature of the flask? Do all the prongs break? If only some
prongs break, where were those rings positioned in the flask during
casting? Is the surface texture of the cast ring identical to the
surface texture of the wax injection? Can you see the porosity in the
prongs before you bend them onto the stones? Are the prongs ever bent
before setting? (An employee once threw a load of rings in with a
load of clipped trees to tumble off the last bits of investment. The
heavy trees pounded the prongs down onto each ring. We carefully
pried them back up and were able to save about half the batch. But it
was still an expensive lesson. Could’a been worse. They were only
silver rings.)
Could you post some close-up pictures of the problem rings on a
website so we can see them?
There are several reasons that can cause the metal to be brittle.
Your metal may have been over heated. The flask may have been too
hot. The metal may have been contaminated by another metal (aluminum,
lead…). The volume of your model may cause the weakness and
porosity in the prongs. Lost wax casting is a system. If something is
not quite right in the system, it will cause problems further on down
the line.
These are the most obvious reasons to me. I’m sure that there must
be other potential causes which someone will describe.
Chuck in Asheville where Mother Nature seems to be deciding on
Spring!
Metal doesn’t get brittle after casting… the properties of cast
metal depend to a very large degree on several variables during the
casting procedure. I used to work for the inhouse casting area of a
large jewellery production company - when the jewellers complained
to us that the metal was brittle we had to look at what we had been
doing and make decisions on where to make changes. Very often the
culprit was the metal put into the cast. It is very rare for anyone
to put entirely “new” metal into a cast - sometimes we found that we
didn’t have enough new metal… or in some cases we just ditched all
of our old metal (sent it for refining) and started over with new
metal. Also, we found that sometimes it paid to adjust the sitting
time of the flasks before quenching.
18 White was a particularly difficult metal for us - it was often so
hard that the jewellers sent it back and said they couldn’t work
with it, it was also often brittle. In the end, we changed the
whole alloy for the metal to find a better balance between the
hardness of the metal and still get a good white colour.
If you are not doing your own casting you should report your
problems to the caster as they may not be aware them. Once they are
aware of the problem hopefully they will work at fixing it for you.
If not, I think it would be worth switching companies.
R.R. Jackson
Heat the casting to a red color and let it cool off by itself.