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Hot Mix Investment for High Altitude Casting

Hot Mix Investment for High Altitude Casting
by Torry Hoover

  1. Calculate investment and water requirements. Use a ratio of
    water/powder of 44\100 for gypsum based investments, ie. 44g (or ml)
    water to 100g powder. For any flask of diameter (D) and height (H),
    calculate investment requirement as: Investment (grams) = 20 D^2H
    Water (ml) = .44 Investment (grams}

  2. Weigh investment with accurate scales.

  3. Measure water in graduated container.

  4. Investment should be room temperature of 78F (25C). Measure
    with accurate thermometer. Water should be 110F (44C). Mix hot and
    cold to achieve this temperature.

  5. Always add the investment powder to the water .

G. Mix with electric mixer at high speed for 6 minutes.

  1. Vacuum in mixing bowl for 20 seconds after boiling starts.

  2. Pour investment into flasks.

  3. Vacuum the investment in the flasks for 60 seconds after boiling

  4. Cap off flasks with investment to bottom edge of flask.
    Carefully move flasks to setting area and do not disturb while
    investment is setting up which should be less than 3 minutes. Mark
    identification numbers on investment to identify flask when cast.

  5. Then carefully remove sleeves. Place flask on floor with base
    down between feet. Use feet to hold collar and lift sleeve off-
    Remove any investment on collar seal surface and sides with wet rag.
    Remember, any investment on collar will prevent proper vacuum.
    Investment on collar surface will damage gaskets. Excess investment
    on the outside of the flask will fall into the vacuum chamber.

  6. Remove sprue base. Gently load into oven. Load oven so that
    flasks to be cast at highest temperatures are cast first.

  7. This procedure is especially useful at high altitudes where
    achieving good vacuum is difficult. It produces good results at all
    altitudes because it is less viscous than normal investment mix due
    to additional water and higher temperature. This gives it the
    capability of reproducing a better surface finish. The price paid for
    this is cleaning the bell jar because the hot mix boils much more.

Hoover and Strong

    3. Measure water in graduated container. 


Tech people from the companies that produce investment tell me to get
best results, weight water not use graduated container.

I did test this and measuring has a different result than weighing.

The problem for me, is with satin cast 20 vs prevest, satin cast sets
up here in Denver faster than prevest at the same ratios, water and
investment weighed. The solution for me was to use 43 to 100, instead
of ususal recommended 40 to 100, cold water from the tap (winter
water in Denver is cold), mix three minutes, vacuum 1 1/2 min after
boil, fill flask and vacuum 1 1/2 after boil, set aside where they
are not disturbed. I wait 2 hours, steam dewax, and into the kiln.
The mess from hot water investing was not worth doing it that way for
me. I get good results from what I do.

The problem I have, is no matter how careful I am with increasing
temp slowly, if I do not steam dewax, some sterling pieces have large
rough textured warts , and the gold pieces come out without the

Richard Hart

In our shop (elevation 7000 feet) we use an investment mixer which
pulls a vacuum prior to mixing the water with the investment. The
investment water mix remains under vacuum for the entire
mixing/pouring/vibrating time period. After mixing, the investment
is poured into the flask which is also under the vacuum. The
investment does not foam at all. No precautions for flask overflow
are necessary. The results are excellent, and a major improvement
over the traditional system. Also worth noting is that using an
investment mixer greatly reduces the exposure to airborne investment
dust, a major health hazard in many shops.

Our machine which is no longer sold by Rio Grande, is the Saint Louis
Mixer, single model, Cost us$1200, Mfg by the same company that
distributes Kerr products in Italy. A picture of their 3 flask model
is in the Kerr Satan Cast 20 mixing instruction booklet.

Cheers Martin