Somebody have given me this recipe of investment for detailed work.
I have never done casting therefore I have never tried it. May be
some of you may try and let us know how successful it is.
Investment for detailed work
1500 dwts of investment
700 ml of water 110B0 - 115 B0 Fah. temp
6 1/2 minute mix time
Bell jar vacuum (30 sec. after it boils)
Pour into flask
second vacuum (30 sec. to 1 min. after it boils slightly) thicker
Set up time 2 1/2 min. (put on a table - no movement)
I have read all the posts pertaining to mixing investment.
Conclusions:
Some individuals mix the investment for 1.5 minutes before
vacuuming. Other mix for several minutes. I was taught to vacuum
for 1 minute after the investment pancakes.
Some individuals mix by hand and other use electric mixers. I
imagine small amounts of investment would be easier to mix by hand.
Some use distilled water and others probably use tap water.
Some use hot water other use whatever temperature of the tap water
is.
Some individuals let the investment pancake (boil) then maintain the
vacuum for a short time and others maintain the vacuum for longer
periods. I always mix 5 pounds of investment. My mixing bowl
overflows when the investment pancakes so I have to release the
vacuum for an instant to allow the investment collapse.
Some individuals mix small amounts of investment in baggies. What a
great idea for smaller amounts of investment. I am not sure it would
work with 5 pounds of investment.
The amount of time flasks are vacuumed varies.
I invest in flasks from 2.5 inch diameter by 2 inches tall up to
flasks 6 inched in diameter by 7 inches tall. I pour anywhere from a
couple ounces of silver to 40 ounces at a shot. I treat all sizes of
flasks and weights of silver equally. The only difference between a
2.5 inch flask with 1 ounce of silver and a 6 inch flask with 40
ounces of silver is the amount of time I allow for the flask to cool
before quenching. I do not have any problems. After reading the
posts I have come to the conclusion that either the investment
process is somewhat forgiving or I am one extremely lucky caster. I
would not change my method and I am sure all casters who have cast
for a long period of time would not change their process. May we
never have failures!
Hmmm, looks something like Marc Robinson’s hot water recipe… I’ve
never had to use any special formula for casting highly detailed
work, which is 90% of what I do. Because all of my work is in 18k or
22k, I tend to use thick investment for strength, it vacuums just
fine. I suppose if you are casting in large flasks, it could make a
difference, or if you’re at a higher altitude (which is why Marc
thought up using hot water for investing).
The only problem with the “high altitude” recipies for investment is
that they all assume that because the dial on the vacuum chamber
doesn’t go up as far as at sea level, that the vacuum is less. And
for that reason they tweek with the recipe. In reality all that is
happening is the dial is recording that the altitude has less
pressure to be removed to get to full vacuum than at sea level. All
that is required is getting a full vacuum.
Longer times and hotter temperatures are important in cooking
because of the chemical changes that involve starches and sugars at
altitude are temperature dependant and at altitude the water boils at
a lower temperature. In investment it is only requisite that the
chemicals in it get wet and nothing else. At altitude a full vacuum
has just as little air left in the dome as at sea level but the dial
is only indicating the difference in pressure between the full vacuum
and the outside air, therefore the needle is lower at altitude. But
it is still full vacuum.
To get a good job of investing, that has maximum strength and
sufficient gas permeability after proper burnout it is best to follow
the insructions of the manufacturer for times and temperature. As
long as your vacuum jar will boil water at room temperature within 30
seconds of the time you close the valve on it, you have enough
vacuum. When the water is boiling, then mark the dial and that mark
indicates full vacuum. Face it, when the air is gone from the dome,
it’s gone. And what is outside the dome doesn’t make any difference.
Welcome to physics 101.
Lots of wierd formulas work because the investment is very tolerant
and will let us get away with a lot. But for the best results the
guys that cast hundreds of flasks per week and have near zero rejects
are the ones that know. If you check with them you will find
virtually all are using the mfg.'s recipe.
I’m not saying that other formulas are wrong or won’t work, I’m
saying that the mfg’s. have spent years of time and zillions of
dollars to get it right. Why waste all that testing to get results
that are less consistent and not any improvement in quality? Best,
Bob Lynn
I have to agree I have used R&R, Kerr satin cast 20 and Americast
(which I won’t do again) In New Mexico at 6800 feet alt. working
between Albuquerque and Santa Fe. I had 7 people working for me and
none were relegated to pulling bubbles off of cast pieces, I used the
standard times, and mixed as called for as close as possible.
Occasionally if casting very fine and small filigree pieces I would
go to a 41 or 42 to 1 mix ratio but stayed with 39 or 40 to one for
average size items, real heavy chunky men’s rings and buckles at as
low as 37 to 1 ratio. My castings were of high quality with few water
separation lines or bubbles,
I’ve been using a Sargent Welch 5.5 CFM vacuum on a 12 in side draw
bell jar and replaced my vacuum pad at least once a year, I also made
sure my lines had no leaks , If memory serves me I would draw approx
26.5 HG mercury I found that if you play with the mix you either get
poor green or fired strength, I don’t like dodging molten silver gold
or brass, during this time we were casting average of 12 4 x 6
flasks, some days we ran as many as 20 flasks, Altitude is a relative
thing, in truth what is minus 3 atmospheres at an altitude of 6200
ft, the mean altitude of Albuquerque, (which is over a mile high)for
the benefit of the whiners in Denver, may only be represented as 25.5
HB Mercury on a Gauge The same minus 3 atmospheres at sea level might
be represented as 28.9 or 30 HG Mercury. Here at an Altitude of
approx 600 feet above sea level my system draws 27.5 to as much as
28.5 on days of low barometric pressure. In my opinion altering the
mixing proportions given by the manufacturer is in the long run
asking for trouble. Whether this comes from porosity caused by gas
not being able to escape to impaired permeability, Investment
cracking and throwing metal out the sides or end of the flask, Vacuum
casting not being able to draw enough to sufficiently fill the
cavities, or watermarks,
The only reason to have entrapped air bubbles on a casting is
incorrect mixing technique, inefficient equipment (too small of a
Vacuum pump or wrong type) or a leak in the system,
I recently started using Akron Investment from Akron rubber, at
first it scared me because it seemed so thick when mixed to the
stated proportions, I was delighted when I broke out the flasks, (No
relation to the company but a satisfied customer) it is very unlikely
that any two batches of investment will be the same viscosity when
mixed any way since the quality and consistency of the raw materials
will change from batch to batch. The viscosity is of secondary
importance, the prime factor that manufacturers try and control with
the most accuracy is the Work time. It has to remain as consistent as
possible. So altering the water to investment ratio and the
temperature might work, but why not just do it right and get better
quality castings, this will lead to less rejects, castings that are
of a higher quality and less work finishing. Unless of course you
just like having to do extra work. The other thing to consider is
that investment does not have a particularly long shelf life. It will
absorb moisture every time the container is opened, I at one time
considered building a dehumidified storage area for investment, or
some way of vacuum sealing a container of aluminum or steel to keep
the extra moisture from interacting with the mixing.