Hi Darcy, If you have cracking in your flask investment,It can usually
be caused by a few things…The first is the water /powder ratio…
When you use a postal scale or an investment scale to weigh your
investment, These scales can easily loose their accuracy by as much
as a pound. This will cause cracking , flaking and investment breakdown
. What I have done in the past , until I got a large digital scale…
is to use a small digital scale to weigh out a bunch of scrap nails
and assorted chunks of brass I had laying around into a plastic zip
lock bag… weigh out exactly what you need for investment weight of
your flasks.Now, Zip the bag shut and write the weight on the bag.
Use this bag to calibrate your investment scale so it is accurate
before weighing your investment… There is usually a small knob or
screw that can be turned to adjust the scale correctly.
You can also use the small ,or larger digital scale to weigh your
water as it is sometimes difficult to get the exact water amount
specifically when doing small flasks. 1 cc of water is equal to 1 gram
etc…
If you have too much water, you will have the possibilty of cracking,
striations ( weird build up) on your castings as well as blow outs .
If the item is heavy, use a 38/ 100 ratio, if they are light , use
40/100.
Now, another area where you get unwanted buildup on your castings can
be from the wax wash you use to dip your trees into . If it is a water
based wax wash, it takes a long time to dry… if it is not totally
dry, you will have investment breakdown ,striations where ever there
was excess water that you did not see.This can be seen on the inside
of borders on pieces and sometimes on the outside.long holes in waxes
tend to trap any wax wash you use , which is why some people have
areas where there is supposed to be a hole in a piece and it is
filled with a mixture of metal and investment which must be drilled
out after casting. The best wax wash to use is the alcohol based wash
/ debbublerizer sold by many casting supply houses.It normally dries
very quickly, but be careful as itcan accumulate in holes and recesses
and will take a little longer to dry… I put a fan next to my
dipped trees to help them dry quicker.
The other area that can cause cracking if all the above steps are
taken is if you are not using a digital control on your oven… If
you are using a gauge/panel meters that came mounted on your oven to
read the temperature of the oven, these panel meters can and usually
are highly inacurate… we have seen them to be as much as 200 oF out
of sync with our digital controls which are accurate.
If you have your oven set to go to 1350 oF as your high point ,With a
panel meter, It could actually be 1500 oF and you won’t know it! This
will cause your investment to show cracks as you have described.
If you have invested your flask, alowed it to sit for an hour, we
have burned out small flasks to 1350 in under 3 hours , dropped to
casting temp and cast without ever having a single flask go bad.
Remeber though , that we are using digital scales for investment and
water and the investing process is accurately timed out to do this
repetitively without fail. Doing this with inacurate equipment is very
risky. I hope this helps.
Daniel We do casting finishing for the trade