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
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 78°F (25°C). Measure with
accurate thermometer. Water should be 110°F (44°C). 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.
7. Vacuum in mixing bowl for 20 seconds after boiling starts.
8. Pour investment into flasks.
9. Vacuum the investment in the flasks for 60 seconds after boiling starts.
10. 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.
11. 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
12. Remove sprue base. Gently load into oven. Load oven so that flasks to be
cast at highest temperatures are cast first.
13. 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.
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