The problem of investment breaking off and resulting in inclusions in
the castings is almost always the result of sloppy spruing. Be sure
thatt every piece is welded to the tree seamlessly, as well as the
weld of the tree to the sprue base. Do not melt a hole in the tree,
stick your piece in, and leave a blob of wax hanging there and a seam
around the sprue. Investment will fill these thin seams, and leave
small protrusions in the mold cavity which may break off when the melt
is poured. This will also lead to turbulence porosity. A smooth bead
run around each weld will ensure a smooth, unobstructed flow of metal.
Also be sure your oven is thoroughly cleaned after each use, as you
may pick up flakes of investment or other contaminents near your
button.
To counter other, more severe cracking problems such as flash, fins,
and blowouts, here is the mix I use:
*190mls 110F-120F water per pound of investment.
*Mix 6 1/2 minutes, vacuum bowl for about 30 seconds after the
surface begins to boil.
*Pour flasks, and vacuum 1 -2 minutes. Your investment should begin
to thicken substantially by the end of the cycle. Gloss off should
occur within 1 -2 minutes. NEVER move the flask when still liquid!!!
Allow them to sit for at least 10-15 minutes before GENTLY moving them
out of the way.
Your work/set time may depend on which brand you are using, room
temperature and humidity, etc. If your vacuum takes a long time to
get up to 29.9 inches, you may need to cut the mix time a bit. Test it
out on some dummy flasks first, or use some stock waxes.
Another major factor I have found pertaining to investment failure is
the freshness of the material. I HIGHLY recommend Orovest from
www.contenti.com. It is made in small fresh batches and turned over
about every 10 days. It also contains American crystobalite, quarried
here in VA, the premium grade of the material in the world. Some of
the larger manufacturers use inferior crystobalite, and if you are
buying through a distributor, you may have no way of knowing how old
the material is or how much moisture it has absorbed before you get
it. I have found this to be a major factor dealing with investment
problems during the ultra humid summers here in the Washington DC
area. Also, transfer any unused investment to a snap tight plastic
pail after opening.
I have a lot more tips, feel free to email me!
Mark Moretti
Fredericksburg, VA
@Mark751