Are there any videos or tutorials online anywhere on how to attach
wax models to sprue trees?
No matter how careful I am about creating a fillet, my trees always
come out with lots of little crevices around the sprues that
investment hides in, making post-casting cleanup a lot harder than
it probably needs to be. I’ve looked online for a tutorial, book, or
video but haven’t found one anywhere. I’m hoping someone knows of
one that I couldn’t find.
Kathy, we could go on and on about this one…casting is not an
exact art.
But, for the basics always add a sprue to the object you are casting
at least as big around as the thickess part of the object and place
it at the heaviest part of the object as well if you can. Obviously
the blended wax should be smooth and depending on the size, shape,
and position on the tree you could get different results on each.
The general rule is the lighter weight objects at the top, medium in
the middle and heavier at the bottom.
To avoid this, cast all the same object. Usually you will have a few
bad ones at the top and the bottom. The sprue on the tree should
have 5 to 10% extra metal for the button. There are other things that
come into play, kind of machine you are casting with, heat of the
flask at casting time, heat of the metal going into the flask, how
long you let it cool before quench, etc. If you want to really learn
it you can go to any school for a casting class. I used to teach this
at GIA in Carlsbad, CA. You also have to make sure of the investment
mix ration of water to investment, the proper mixing time and setup
before popping the flask in the oven.
My inclination at this point to get back to your crevice situation is
that your metal is solidifying too fast which means the metal is not
hot enough or your flask is too cold. This area should be smooth
without porosity or cracks.
I don’t know of any tutorials or videos, but if the problem is caused
by holes in the wax created during the treeing process, try using a
little lower temp on your wax pen. Sprue wax has a lower melting temp
than carving or injection wax to allow it to flow out of the flask
before the model. If you are trying to use the same setting as for
patching models, it’s probably getting too hot and might be causing
little holes and low spots in the sprue because the wax shrinks as it
cools. A good wax pen is a necessity if you are doing much wax work.
I like the Kerr Master Touch. It has many different tips available,
and is infinitely adjustable, especially with a little finesse on the
foot pedal. A definite asset for wax working.
Always use the lowest temp you can for any project involving a wax
tool, especially when sprueing, just enough heat to make it flow and
no more. If it smokes at all while you’re working it, it’s way too
hot.
If the crevices you describe are caused by porosity in the tree /
sprue vicinity, Russ is right, you probably have a metal or flask
temperature problem, or maybe a tree construction or model
orientation problem that needs to be addressed as he describes. In
any event, the metal flow will only be as smooth as the wax allows
it to be.
Try using sticky wax to secure the pieces on the tree. It flows well
and tends to create a seamless fillet. I do reach carefully between
the sprued pieces with a fine-tipped wax pen, to smooth everything
to the tree trunk.
I use either 3/8 or 1/2 inch pink sprue rods. I take my wax pen and
have the heat at a temperature so I can poke the tip of the wax pen
into the sprue and then quickly stick the sprue for the model into
the melted wax. I can do 20 or 30 pieces for a 2 1/2 x 4 inch flask,
or about 50 to 75 pieces for a 3 x 7 flask. I do not seal the wax
around the sprue after sticking it onto the tree. The right temp
should keep the pink sprue wax hot enough to make a good bond, good
enough so pounding on the investment table will not dislodge them
when investing.
You have to be careful to not heat up the sprue rod to much all
around the sprue rod at one time or the rod will start bending to one
side. Occasionally I have a piece float to the top of the investment
while vacuuming, but far and few between, and this is production
casting, so 20% extra for safety in filling orders in sterling has
worked out well.