Investment Casting Platinum Alloys
Quenching: Many casters will quench the flasks in water 10 to 20
minutes after casting. Great care should be used when quenching
because the higher temperature flasks can generate steam and
scalding hot water causing burns when put in water. Some casters
prefer to wait until the flasks are a dull red color, hold them with
tongs and use a hammer to knock the castings and investment out of
the flasks by hitting the open ends of the invested flask, then
quenching the investment covered castings in water. The platinum
investment after burn out will resemble a fused concrete that is
very hard and tenacious. It will take a lot of work to remove the
majority of the investment from the castings by hammering and using
a high pressure water gun or grit blasting.
Investment removal: The residual investment on the casting can be
removed chemically by putting in a de-vesting solution consisting of
1 part Hydrofluoric acid to 3 parts water put in a covered, acid
resistant plastic container. Be sure to read all the health and
safety warnings before using Hydrofluoric acid as this is a very
dangerous chemical. Proper eye protection and heavy acid resistant
rubber gloves must be worn when using dangerous chemicals. Recently,
some proprietary caustic alkaline de-vesting solutions have come on
the market such as "J- Break" (Romanoff) that is put in a covered
stainless steel pan and used hot. The hot caustic de-vesting
solutions will remove the investment in 20 to 30 minutes. A homemade
caustic de-vesting solution can be made using the following
formulas;
Use Equal portions of the following liquid chemicals if available
from a Chemical Supplier in liquid form.
Mix Equal Amounts of the Liquid Chemicals
- 50% Sodium Hydroxide Solution (Caustic Soda Solution)
- 50% Potassium Hydroxide Solution (Caustic Potash Solution)
- or -
Mix the Dry Chemicals and Water
- Sodium Hydroxide – (Caustic Soda) - Pels or Flake form -
3.15 pounds
- Potassium Hydroxide – (Caustic Potash) – Pels or Flake form
- 3.15 pounds
- De-Ionized or Distilled Water - 1 gallon
Heavy chemical resistant rubber gloves and safety glasses must be
worn when handling strong caustic chemicals, contact with the skin
will cause severe burns. A large covered stainless steel pot is used
to hold the caustic solution and a hot plate is needed to keep the
solution hot ( 180 deg, F). A stainless steel mesh basket with a
handle (French fry basket) can be used to hold the castings during
de-vesting. When mixing the 50% Sodium & Potassium Hydroxide
solutions, equal portions of each liquid chemical are put in the
stainless steel pot and used hot. When mixing the dry Sodium and
Potassium Hydroxide with water, slowly add the dry chemicals to the
water in the stainless steel pot. When the dry chemicals are added
to the water, a large amount of heat will be created, carefully stir
the solution until mixed with a large stainless steel spoon. The
heated caustic solution will take about 20 to 30 minutes to remove
the investment. Do not use aluminum pots or utensils with caustic
solutions. The castings should be well rinsed after de-vesting. When
the solution slows down it should be replaced. To dispose of the
caustic solution mix it with hydrochloric acid to adjust the pH to
7, filter the solution before disposal. The waste liquid will be
sodium and potassium chlorides, salt water.
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