In our lab at the college, we’ve always mixed investment by “feel” to
teach students how to determine the appropriate consistency.
Basically, our approach has been to use about 75% of the flask’s
volume of room-temp water, then adding the investment and mixing
until it’s the consistency of yogurt/sour cream/pancake batter.
Vacuum for 3 minutes, pour into flask and vacuum flask another 3
minutes. The more experienced students can do this in their sleep
and know exactly the consistency to look for.
Needless to say, though, with beginners among us (and even some of
the old-timers), there has been a decent amount of waste. Since all
of the students chip in to pay for the investment, it only seems fair
to do what we can to level the playing field and keep “beginner
waste” under better control.
Hence the introduction this semester of “The Chart” - the official
mixing chart that comes with each batch of investment that tells you
how many pounds of investment you need to how many mL of water for
each size of flask. We have a good (and new) investment scale and
well-marked graduated beakers. The problem is that if you follow
the chart, you get a very THICK investment that sets up WAY too
quickly. It’s the consistency of a very thick pancake batter when
mixed and you can vacuum it for about 1.5 minutes at each step - at
the most - before it sets up. We had some of the most experienced
students do their flasks this way and follow the chart so that we
could test it and see what the results were.
SO. do any of you know any sources of GOOD and accurate investment
mixing charts? I’m reluctant to spend the time it would take to
thoroughly test and revise this one, but am tempted at this point to
hide all but 3 or 4 flask sizes and work out the proportions for them
and post them. However, if I can get a good chart, it would
definitely help me avoid that step and we could maintain our
flexibility.
Try asking the school for the charts that were provided by the
investment company (manufacturer) with the product, @ my college we
have manufacturer specific charts because most products differ
slightly in ‘recipe’ and need specific measurements and times.
Karen, the way to do this is to use the chart, if you like, to
determine the correct amount of powder for a given flask size. But
then, using a good gram scale, weigh that amount (or use a calculator
to convert the pounds to grams). Then, following the manufacturer’s
recommended water to powder ratio, multiply that weight in grams of
the investment (this is more accurate if you actually use a gram
scale to weigh the investment, at least the first time) by the ratio.
Normally, this will be a 40:100 ratio, so for a hundred grams of
investment, you’d use 40 grams of water. with water, grams equal
millileters, so you can use a graduated container, if you wish, but
just this first time, actually weigh it. Then also measure the
temperature of the water. Now, with this test batch, with the water
at the manufacturer’s recommended temp, and a carefully weighed
amount of powder and water, mix it up. Look to see what the
consistancy is. I’d be willing to bet on a couple things. First is
that your students are likely being trained and used to using too
much water, for a slightly thinner mix than they may be used to.
Second, I’ll bet the normal temp of the water they’re using (or the
investment powder, if stored in a warm place) is warmer than the
manufacturer recommends. The two factors, too much water, but too
hot, cancel out if you get it just right. This would result in your
setting time being about right. If, however, the batch of investment
you have, when tested this way with the correct water temperature,
reaches the gloss off stage before the specified time on the
manufacturer’s chart, then you may have a bad batch of investment,
or something else may be non-standard in your setup. That might also
explain (whatever it is) why your students get too fast a setup
when using the chart’s ratios. Also, take the time to actually check
the accuracy of the investment scale. Doesn’t have to be off by much
to create havok.
By the way, if you mix exactly a hundred grams of powder with 40
grams of water, mix it and carefully pour it all into a container of
known volume, so you then have a reference of the exact volume taken
up by that quantity of investment, then you’ve got an easy reference
point for calculating what’s needed for other volumes
May I suggest you go by a weight ratio? Weigh the powder and the
water. Most regular investments run around 40% water by weight, many
of the high speed dental investments like Doc’s or Supersonic run
about 25% binder by weight and 75% powder.
The investment makers can provide you with detailed instructions,
just adjust as you please and keep track of the results.
I learned to mix investment in El Comino Colledge. Here is what they
taught us.
1st. use distilled water.
have water at room temp.
Measure out water to investment at 40-100 ratio. Weigh the investment
on a gram scale Weigh the water on a gram scale.
This is the simplist method, I have been using this method for 30
years. Ine gram = 1cc I mix with a electric drill that is pluged into
a reostat to control the speed of the mix which is not fast. I use a
small paint mixing rod. Mix for 2 min, Pull a vac of 29R.75 for 2
min., pour investment into glask and pull another vaccumfor 2 min.
Make the vaccum pull when the investment reaches the Bugbble action
then make the timing count at that peak of the vaccum pull run 2 min
and pour in flask , let set for 30 min and scrape off excess
investment from top and pull the rubber sprue base off and clean any
excess investment that might be in the bottom of the flask .
I let the flasks set for a total time of 2 hrs and then load in oves
and turn on the oves. I run my oven on a 12 hr cycle. 300* 2 hr 700*
1 hr 1000* 1Hr,1100* 1 hr and 1200* 3hr ,1100* 2Hr, 1000* 2 hrs. Mow
it is ready to cast.
For casting stones I use Ransom and Randolf Ultra Vest inventment,or
R&R Solitaire Investment.
Use distilled warer and heat the water to 120*f this will make the
investment set up quiker and the prongs will be tight to the stones
after casting in sterling ot gold. I have used the Ultra vest for
stones but the catalog says R&R Solitaire is for stone investing My
Pick for Investment is Ranson & Randolf.
First I would say that you should always use the manufacturers
recommended mixing ratios as your starting point. Mixing by feel is
asking for problems.
Second if you have a good scale ( a balance or digital scale not a
spring type scale) weigh the water as well as the investment. Water
weighs 1 gram per CC of volume. It is way too easy to make errors
that are outside the range of permissible variation when using
graduated beakers. For example one pound of investment (454 grams)
would be mixed with 177 grams (CC) of water. A 5% error in the water
is only 22 CC or about a tablespoon and a half and this would put
it beyond the recommended mixing ratios. Just for an experiment fill
your beaker to what you think is 177 CC then add two tablespoons and
see how easy or hard it is to see this amount of error.
A typical investment to water ratio is 100 parts powder to 38 parts
water but you should verify what your investment manufacturer
suggests. If the standard ratio is too thick or thin then you
typically can change the ratio by up to 2% in either direction to
make it thicker or thinner. Beyond 2% you are running a danger of it
being too weak and fracturing.
A 2% variation is a fairly significant difference in thickness of
the slurry.
Hope this helps,
Jim
Jim Binnion
James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160
@James_Binnion
Member of the Better Business Bureau