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.
Kenneth Ferrell
www.shadras.com
