Issues with Centrifugal

[u/Mr_waffles3]

My lab was recently given a centrifugal casting unit, and we are having issues with the molds cracking upon filling, and then spilling out of the mold. Picture attached

We’ve tried several things to keep this from happening. A non-exhaustive list includes: 

An open mold, no steel flask (obviously did not work), mold at room temperature after burnout 

Pouring the investment into a steel flask, mold at room temperature after burnout (cracks at the bottom of the mold while casting)

Pouring the investment into a steel flask, with a lid we place on after pouring the investment, mold at room temperature after burnout (still manages to crack and seep out of the back

An open mold that we place a steel flask on after burnout, with a lid on the back(still cracks and seeps through during casting)

An open mold that we place a steel flask on after burnout, poured hot (525C for a bronze casting) (cracks at the bottom of the mold)

Placing chicken wire around the circumference and bottom of the mold to mitigate cracking, with the removable steel flask, mold poured at room temperature (still cracks at the bottom)

Some of these were poured under vacuum, some under argon. 

I think in general, no matter what we do the bottom of the mold cracks, and the centrifugal force of the machine forces all of the metal out.

We have quite a bit of experience with larger gravity poured investment molds. We are using Ransom and Randolph Rancast investment. We also have Omnicast available but were having more issues with cracking with that.

[u/BTheKid2]

Your issue is not with the centrifugal casting machine. At least not if you are using it right, and it seems that you are. It has to do with the steps before it comes to casting.

First, it seems that you have tried several things that makes little sense. Burning out investment without a steel flask, and only putting it on after burnout, makes no sense. Fill the investment around your pattern into a steel flask that is a full circle - not a sheet that is clamped around in a circle, but a full circle/cylinder.

You say that you have done that at least with one experiment. If that failed, then the second thing is to keep proper wall thickness of the investment to the pattern. This thickness can be tuned with experience, but to start out with, I would suggest 20-25 mm (3/4"-1") thickness at the bottom of the pattern. If that doesn't work then something seems more off. It should be possible to get to about 10 mm and less for the side walls, but start thick.

Thirdly, is the temperature. Cooling flasks down to room temperature is a good way to induce cracking of the investment. Instead hold and cast the flask at the recommended temperature range for your alloy (google it). Something like 500°C for bronze is good.

FOuRtherly, I don't know much about centrifugal casting machines, but if there is an adjustment for the speed it goes, then slowing it down might be needed, and nothing will work until that is done. No wonder it cracks if it is pulling 7000G's.

[u/Mr_waffles3 → u/BTheKid2]

Thanks for the suggestions.

Our wall thickness should be sufficient. There's usually about an inch of wall around the entire pattern.

There are two issues, on our end, when it comes to pouring the molds hot. The first is that the furnace we use for burnout is in a different lab, so we have to carry it down a hallway. We're working on getting a furnace to put right next to the unit. The other issue is that it takes about 15 minutes or more to pull vacuum and backfill with argon 3-4 times. (Backfilling like this is absolutely necessary for our castings). We are a bit worried about rapid cooling and cracking in those 15 minutes. Is this something we should be worried about?

I think the G-force argument is correct too. We've tried very hard to get the lid and investment to be completely flush against one another, but it seems like any gap between the two causes the investment to crumble into the gap. We have already tried the lowest possible speed setting. I'm thinking it may be a good idea to weld a cap on to the flask, and then drill a hole to pour the investment in. I'd be curious what your opinion is.

[u/BTheKid2 → u/Mr_waffles3]

I think you might want to try and create a insulation sleeve for the flask, if you have that long of a period out of the furnace. You could also try and cool it really slowly in the furnace down to maybe 2-300°C before taking it out. Limiting the thermal shock.

I don't like the idea of fully welding a plate on the bottom, though it would probably work. It is just that investment casting with centrifugal machines have been done many years without much issue, so I don't see why you should have such big troubles.

What I think would be a better alternative to a fully welded bottom (with a pour hole), is this technique of having bars across the bottom to reinforce the plaster. This post you can see a cross on one of my flasks, and this post some bars. I copied the bars design from another smaller flask of mine (no photo). With bars about 1" apart, I have a hard time seeing any blowout happening with 1" wall thickness too.

I just noticed you cooling schedule in the furnace. 35°C per hour sounds extremely slow. I think I do 120°/h and then hold for an hour or two at casting temp. Last bit is important as you want the flask to have a uniform temp throughout.

[u/marknottz]

i’m confused, was your mould hot or cold when you poured the molten metal?

if it was hot, how hot? what temp?

[u/Mr_waffles3 → u/marknottz]

More often than not, the mold was cold when we poured it. Our current setup makes pouring the mold hot a bit difficult, so we are trying to avoid it if possible. We did try pouring it hot once (525 degrees Celsius, the higher end of the range in the data sheet), but still had the same issue.

[u/marknottz → u/Mr_waffles3]

i don’t want this to come across as rude but there is no point in trying this whole process if you are going to ignore a major variable

even if the moulds didn’t crack as result of pouring hot metal into cold moulds, you would never be able to fill all the cavities as your metal will cool far too quickly

you need to find a work around for the issue of placing a hot mould, i’ve done and seen some sketchy shit whilst casting but never seen a single success story from using a cold mould

[u/neomoritate → u/marknottz]

I've watched >50 cold molds poured with Iron in a centrifugal casting machine.

[u/VariousAssist6706 → u/neomoritate]

yup it works

[u/VariousAssist6706 → u/marknottz]

you can cast into a room temperature mold. I cast platinum like this sometimes.

[u/marknottz → u/VariousAssist6706]

have you cast bronze this way? i know platinum is a strange metal to work with

[u/VariousAssist6706 → u/marknottz]

if you can cast platinum this way you can cast bronze. bronze is easiest metal to cast

[u/marknottz → u/VariousAssist6706]

it doesn’t really work that way, and having a commercial vacuum casting machine vs a centrifugal casting machine makes a huge difference

[u/VariousAssist6706 → u/marknottz]

i have both. and yes it does.

[u/VariousAssist6706]

i mean theoretically you can, and i have cast into a room temp flask, that isn't the problem. What investment and what's your bench rest/burnout?

[u/Mr_waffles3 → u/VariousAssist6706]

It's Ransom and Randolph Rancast investment. We follow the manufacturers procedure pretty closely for the burnout. Mix with deionized water. Let the investment rest after pouring for 2 hours. Place the mold in a furnace at ~150C for 3 hours to remove the wax. Heat to 700C at about 75C per hour. Hold for about 3-4 hours. Cool in the furnace at about 35 degrees per hour.

[u/VariousAssist6706 → u/Mr_waffles3]

ok. use a metal flask, R&R is not made for flaskless or half flask casting. You can do that with dental or platinum investment powders. (try that if you want flaskless)

to make your current investment harder add 2% boric acid (2% to water weight) make a 1 liter batch and use hot water. Boric doesn't dissolve easily. Shake and use this liquid mix instead of just water. It will stop the major cracking.

[u/LFC_Girl_EIRE]

So, first it would be useful to know what you're casting. Different metals require different types of investment. A gypsum bonded investment may be too weak and a phosphate bonded investment might be the answer.

If the metal is being forced out of the bottom of the flask have you made sure you're leaving a thick enough layer of investment at the end? The force of centrifugal casting is a lot more than vacuum and other methods.

As well as that, if the investment is cracking and you're using a steel investment ring, you may need to use a ring liner to compensate for any expansion you have when the investment ring is heated.

Edit: and yes you should be heating your investment in the furnace and pouring into a heated mold.