Investment Procedure for Lost Wax Casting

Follow the correct and safeest way to do an investment procedure for lost wax casting. Remember: 'Well begun is half done'

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Clean, fresh investment, water and investment at room temperature, de-ionized or distilled water or water that has been sitting overnight.

Follow the proportions suggested by the investment manufacturer. Usually 40cc of water to 100 grams of investment is good. Use a dust filter mask and gloves. Time yourself.

1. If investing by eye rather than using measurements then fill flask between 2/3 and 3/4 full of water, and pour it into the mixing bowl.
2. Sift the investment gently but fairly rapidly onto the water's surface and mix well with a spatula, smoothing out any lumps. Note the time. The investment procedure should be finished about nine to nine and a half minutes after beginning to mix.
3. As soon as the investment is smooth swirl the bowl in your hands so that the investment continually moves and any bubbles are removed. Don't whip in more bubbles. If desired use a vibrator on the investment to encourage bubbles to leave.
4. At about the five minute mark vacuum the investment for 1-2 minutes or until the investment boils for a moment or two. Too much boiling may damage the investment.
5. While the investment in the bowl is vacuuming pour debubblizer over the wax in the flask and then pour it back into the bottle again, draining the flask well. While it is not supposed to be necessary to use it with vacuum investing I have found debubblizer is of some help in aiding trapped bubbles to slide off the model.
6. Remove the investment from the vacuum bell and swirl it, then pour it into the tilted flask and rotate the flask while pouring to encourage bubbles to leave. Swirl the flask gently. This is when you realize why one puts tape around the top of the flask.
7. Assuming there are no air bubbles present in the wax vacuum the flask well. When finished this should put one at about the nine minute mark from starting. Remove the flask from the vacuum bell and if the investment is not yet reasonably hard gently swirl it until it thickens. This will discourage water tracking. When the investment seems thick and sluggish let it be. Glossing off should occur at about thirteen and a half minutes after beginning to invest. If it does not occur by fifteen minutes after starting then either the investment proportions were too watery or the investment has absorbed water from the air, is no longer fresh and therefore the ratio of water to investment was wrong. This can lead to water tracking and investment breakdown.
8. After the investment is hardened slightly (about 30 minutes from starting) remove the rubber sprue base with a slight twisting motion and clean off any excess investment from the outside of the flask. A small flask may be burned out immediately, a medium sized one should rest for a little while and a large one may dry overnight without damage. If one wishes to cast later one can store the flask in a sealed plastic bag until required for casting (I would suggest under a week).

A sample burnout schedule for a medium to small flask (oF)

TempratureTime0-400o9:00-10:-700o10:00-11:-900o11:00-12:-o12:00-1:-o1:00-2:-o2:00-3:o3:00-Cast when ready.

Note that the larger a flask the longer and more gentle the burnout should be. The flask should sit at the high temperature for some time. When the investment looks white you are ready to cast. Dark investment will work but the results may not be good.

When melting metal

Oxygen is your enemy Do all you can to limit oxygen access at high temperatures to your metal. Preheat the crucible intensely and use a slightly gassy flame. Roll your metal out as thin as it will get and roll it up into coils for melting. Keep the melt covered by the gassy flame at all times and consider adding a chunk of charcoal to the crucible to consume oxygen. Cast into an appropriate flask temperature. The shorter the melting time the less oxygen can be absorbed by the metal. Do not add to much oxygen to the flame to shorten melting time.

I have it sealed tightly, but suspect that it has still absorbed
moisture. I ended up with fins on edges of casting. I never had
this unless not measuring H2O and weighing investment. I am pretty
sure the issue is not metal or alloy. I'm stumped 

It it sets up properly and solidly, in an appropriate amount of
time, then likely the investment is still OK. You might try to seal
the container, and roll it around a bit, to make sure the mix of
componants is still uniform. Investment can sometimes settle out
unevenly (especially in shipping, I was told), so rolling it around
to make sure it&#;s a uniform mix, can help.

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Also, I prefer to weigh both the investment AND the water, rather
than measuring the water in some graduated container. Weighing is
more accurate. Pay attention to water temperature, and how it relates
to gloss off time. If it sets more slowly than it should, then
perhaps the investment isn&#;t so good any more.

Also, I&#;d suggest allowing the invested flask to set, quietly, for
at least two hours or more, before starting the burnout. That allows
it to reach full strength.

And skip the debubblizer. It can cause as many, if not more,
problems than it claims to solve. Also take care if using those
&#;orange oil&#; wax cleaners. If not totally dry and clean, those can
really screw up some investments. Personally, I don&#;t take a chance
with them any more. The platinum investments seem especially weakened
by that stuff, causing very rough surfaces if it&#;s at all still wet,
or even if there&#;s too much and fully dry.

And try to use a slow temperature rise during at least the first 400
degrees F of the burnout cycle. If you don&#;t have a programmable
burnout controller, set the furnace so it takes at least an hour or
so to get those first 400 degrees. Crystobalite undergoes a
structural change within that temperature range that causes an
expansion to take place. If the flask is rising in temp too fast,
this can be uneven, resulting in increased risk of cracking, which is
what it sounds like you&#;re getting. Also, don&#;t use a larger flask
size than you need. Smaller flasks are more forgiving of overly rapid
temperature changes.

And finally, if you&#;re still unsure whether the investment is good
or not, and don&#;t have the time or energy to fuss with
troubleshooting it, there&#;s the other simple solution. Get a new
container. fortunately, the stuff isn&#;t as pricey as metals have
become. Use the new for &#;important&#; flasks, and save the old stuff
for things that might not be as important to have them come out
perfect. In my shop, that might include things straight from rubber
molds, with little labor in the wax, so if it doesn&#;t cast, one just
shoots another wax from the mold and does it over&#;

Hope that helps.

Peter Rowe

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