Investment Casting Ideas, trials and failures.

From molding systems to gating, what goes on at the molding bench will make or break a casting.
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Jammer
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Investment Casting Ideas, trials and failures.

Post by Jammer »

We got into a long discussion on investments in another thread so I wanted to start a spot where we could have all the info in one place.

The investment we were talking about is a Plaster of Paris, Sand mixture, poured over a wax pattern and burned out. Rasper uses a 2 parts sand and one part POP, mixed to a soupy consistancy. My first attempt failed because I mixed the dry ingredients together first and then tried to add it to the water. I got about 2/3rds mixed in and it all set up hard. I was taking a small bronze casting class and the instructor recommended 1:1 ratio but he said just get close, it doesn't really matter. So I made may second batch a little closer to 2:1. Placed the Plaster in the water for a about a minute, just gently stirring. The more aggressive you stir, the faster it sets up. I also chilled the water with some ice to slow the reaction. Then I added the sand, mixed and poured into the mold. It worked well, I think I still had it a little too thick, ended up with a couple goobers on my casting from trapped bubbles.
What other tips can anyone think of? Does anyone add other things like Silica Flour, or saw dust? Do you think we could add something to help with higher temps for Bronze or even Cast Iron?
How do you estimate how much investment is needed to fill the mold? I either have to mix more or have enough left to fill another.
quando omni flunkus moritati 8-)
Rasper
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Re: Investment Casting Ideas, trials and failures.

Post by Rasper »

My molds are generally large enough that I have to mix and pour multiple batches. Given the cost of plaster and brick dust, I don't care how much I waste. I pay about 9 US dollars for a 40 kilo sack of plaster. My helper busts up old used bricks and roof tiles with a hammer.

One problem you have with just plaster and sand is that the sand settles to the bottom pretty quickly, before the plaster has had time to set. One advantage of the brick dust I now use is that it has some very fine particles as well as the coarser ones. The fine particles make a soup that keeps the coarser heavier particles from settling so fast. Silica flour mixed in with the sand would accomplish the same thing. Some lost wax casters have finely developed systems wherein they know exactly when their investment will begin setting and they time their pours accordingly. I think this is a bit much for sculpture, and with the investment materials we use. They use commercial investment mix such as R & R.

You get defects. Most are easy to fix. I cut off the lumps and such and make blind rivets to fill in the holes. If you're interested I will elaborate.

I like the 2 to 1 mix because it seems to be more gas permeable, and is plenty strong. I do reinforce it with a cylinder of rat wire just inside the outer circumference.

I brush on about an eighth inch of silica flour / plaster mix before I pour in the investment. That way bubbles don't matter and the it picks up all detail.

Richard
bunyip
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Re: Investment Casting Ideas, trials and failures.

Post by bunyip »

Jammer, if using Silica Flour handle with caution.Whilst dry any airborne dust particles can cause BIG PROBLEMS to the respiratory system. Note; Silicosis; lung fibrosis caused by inhalation of dust containing silica !!! I am not suggesting not to use silica ( I use it here) but take appropriate care. Mask up !!!Cheers Trev.
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Jammer
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Re: Investment Casting Ideas, trials and failures.

Post by Jammer »

Richard, I wanted to ask you about the bust you cast. Could the defect be from a different layer of plaster mix? I just thought it was strange for it to be in such a straight line across the peice and sprue. When you say "Rat Wire", is that like the 1" chicken wire or a smaller hardware cloth like 1/2" or 1/4"? We have smaller rats that could fit through the chicken wire. :P
As far as volume, one suggestion would be to have all the extra ingredients handy and not run all over trying to match the mix when you run short. :?

Thanks for the tip about Silica Flour, I wear a respirator when I mix Plaster, dusty stuff like Si flour or mess with ceramic blanket.
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Rasper
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Re: Investment Casting Ideas, trials and failures.

Post by Rasper »

I use 1/2 inch hardware cloth. I tried using plaster lath but the lath we have down here is a bit thin and after the burn out it was pretty weak.

I think that why the defect was in a line across the piece is that the steam reaction didn't begin until the level of metal in the mold reached the wet spot in the arm, which was halfway up the mold. It was a bottom feed investment. All was going well until the metal got to the damp area, and the path of least resistance was up the runner to the pouring cup. Not much steam went down as there was no place for it to escape.

One thing I did not consider in calculating my burn out time was that the 3/16 inch air space that completely encircled the core when the wax was melted out is a pretty good insulator. This is the first investment I have burned out with an enclosed core.

Richard
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Jammer
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Re: Investment Casting Ideas, trials and failures.

Post by Jammer »

Do you base the burnout time on the amount of wax, or the size of investment? I realize a soup can size won't take long and a 5 gallon sized investment is going to take a few days, I just wondered what basis is used to determine burn out time and temp.
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Rasper
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Re: Investment Casting Ideas, trials and failures.

Post by Rasper »

That's where experience comes in. I don't have a lot. The size of the investment is the most important factor, and it appears that whether or not there is a core matters as well. You are not going to hurt the investment with a long burn out, only your pocketbook. But a failure is much worse than buying another 20 kilos of propane. Down here that's about $16.50 USD. As long as you don't run your kiln up past 1450 F, so I've been told by a man with a lot of experience, you won't break down the plaster past an acceptable point. You want it to break down some. I keep my kiln at a maximum of 1200 F.

I have burned out a number of 5 gallon size investments for 36 hours with good success. The Queen was 15 inches diameter and 30 inch3s tall. That's 3 cubic feet, which is 22.5 gallons, over four times larger than a 5 gallon bucket.

Richard
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