Hot Mix Investment for High Altitude Casting
by Torry Hoover
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}
Weigh investment with accurate scales.
Measure water in graduated container.
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.
Always add the investment powder to the water .
G. Mix with electric mixer at high speed for 6 minutes.
Vacuum in mixing bowl for 20 seconds after boiling starts.
Pour investment into flasks.
Vacuum the investment in the flasks for 60 seconds after boiling
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.
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.
Remove sprue base. Gently load into oven. Load oven so that
flasks to be cast at highest temperatures are cast first.
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