Vacuum Casting speed/pressure


#1

I made a stainless steel vacuum chamber out of some old parts at work and it gets a very good seal. My question is should I put vacuum on the casting before pouring or right after? I can pull about 25inHg of pressure in about 6 seconds. Or I can pull vacuum before and have that that pressure already moving through the investment casting.


#2

you have to consider what the vacuum is doing.

If you are vacuuming material before pouring it into the mold, then you are degassing the material.

If you are subjecting the mold to vacuum after filling it, then you are lowering the pressure on your casting material inside the mold- this will cause all the air bubble in the material, and trapped inside the mold to expand dramatically, increasing their buoyancy so they rise to the surface, burst and are expelled by the pump.

At that point, if you leave the mold under vacuum, you still have bubbles trapped inside the mold- they are just at very low pressure.

What you want to do is vacuum the mold, and then, while the material is still liquid, release the vacuum… this will slam 14 lbs of pressure back on the material and cause any remaining bubbles to be squeezed either microscopically small, or into solution.

It is essentially like casting under pressure- you are just degassing the material and reducing the pressure of the remaining bubbles before re-applying a full atmosphere.

To that end- a couple of cautions…

Number one- it takes much longer to evacuate a chamber than to simply slam 80 psi into one.
As a result, you must make sure you are casting materials that stay liquid Long enough to allow you to attain vacuum, hold it long enough for the air to boil out, and then release the vacuum.

For that reason, Most urethanes will not be suitable- they set too fast.
Most vacuum casting is done with epoxies or polyester resins that give you at least 15 minutes of working time.

The second caution is that materials place under vacuum will triple or quadruple in volume as the air in them expands. this will cause resin to boil OUT of the top of the mold… and bubble for a while… so if vacuum casting, you need to make sure your molds are designed to accommodate this overflow and hold the resin over the gate so that as the air bubbles out, it can flow back down into the mold.


#3

or are you talking about vacuum casting metal?


#4

Thank you for the quick response, yes I am referring to after the burnout cycle when you are about to pour molten metal in, sorry for the confusion


#5

For metal casting into investment molds, I don’t think that it will help much to pull a vacuum on a mold, open it to air again, and then pour metal at ambient pressure. Some metal vacuum casting methods work by pouring the metal into the mold while both the metal and the mold are under vacuum, or where ambient pressure metal is sucked/forced into a mold under vacuum. This can help fill the mold better because there is no air in the way of the molten metal flowing into the small features and extremities. If your chamber is homemade, it seems unlikely that you already have a mechanism to use that vacuum to suck the metal directly into your mold. Unless you’ve got some kind of really cool suction tube that is quite impervious to your molten metal :slight_smile:

If your metal is hot enough to stay liquid in your mold for a few seconds after pouring (also depends on how hot your mold is), then pulling a vacuum after you pour the metal may help pull gas bubbles to the surface, which could either reduce the porosity of your finished metal part by removing the gas entirely, or, like with resin casting, increase the porosity by expanding otherwise tiny bubbles that can’t escape the liquid before it solidifies. That probably depends most on what metal you’re casting and how hard it is to avoid gas porosity when casting it, though.

What aspect of your process/results are you hoping to improve by adding a vacuum step?


#6

I think you should pull the vacuum and then pour the metal. I am assuming the you are using flasks with a ring near the top which allows a seal between the top of the flask which is at atmospheric pressure and the bottom which is under vacuum. If you are not using these flasks, all bets are off.

Bill B


#7

Correct, I’m only pulling vacuum on the bottom and sides of the flask. So the top is exposed. Here is a link to the flask I’m using: 3" x 4" Perforated Flask With… https://www.amazon.com/dp/B01M9J913N?ref=ppx_pop_mob_ap_share


#8

You are doing it right.
You only want to pull gasses thru the investment and allow normal air pressure on the cup.
Generally, you hit the vacuum after you pour metal into the cup… if you vacuum before pouring, you are simply pulling cold air into the mold cavity, cooling the surface of the mold and more likely to get a cold shut off in some detailed or thin area.

Ideally the mold should be pretty hot when pouring. Fresh from the burnout if possible.

The vacuum not only helps pull any entrapped air out thru the investment… more importantly is pulls out the gasses that come out of the metal as it cools, reducing or eliminating porosity in the metal surface.


#9

I hadn’t thought of the cold air being pulled Through the casting and cooling it down, that’s a good point.


#10

In my experience, it takes too long to pull a vacuum on the flask bottom to do much good for a home system. The metal begins to solidify quickly when it meets the much cooler flask walls and then the vacuum assistance is compromised. If you are concerned about cold air, perhaps you could fashion a plate that covers the flask top and minimize air entry. I don’t think there is an issue anyway.
I instrumented a 3" x 5" flask with thermocouples at my last job and it takes a very long time to cool noticeably. Investment is a pretty good insulator. Your flasks will be smaller, but the effect will be smaller, but similar. Another thing this experiment showed was that there is a considerable temperature lag in the flask both going up and down. There is also a big gradient between the flask inside temperature and the outer wall area. The actual temperature indicated on a temperature readout was always way off if soak time was too short (hours in our case with a large flask). Soak time variation was the cause of many casting problems.
Having said all that, there are Youtube videos showing casters without the equipment we are used too just pouring the metal into a flask sitting on the ground and - apparently - getting good results. Go figure.


#11

That depends on your pump. And if your investment is piping hot from burnout.

You don’t need to pull a full vacuum. In fact, you won’t read a full vacuum until the metal has totally filled the cavity. All yo need is a differential between the pressure in the cavity, and ambient pressure on the cup.

The pressure difference acts like an Injector to push the metal into the mold, even when the pressure difference is only a few pounds per sq inch.

And you should be using a pump large and fast enough to evacuate a chamber the size of your investment in a few seconds.


#12

Another thing I struggle with is casting temperatures. My investment material says to finish the burnout at 1300f degrees. I’m casting with sterling silver and I read a lot of different opinions on what your investment temperature should be at when you pour. I would think hotter the better as long as your casting doesn’t breakdown and this website: https://www.unitedpmr.com/helpful_hints_for_investment_casting_pure_gold_silver.php says 1300f but I’ve heard as low as 800f.


#13

generally- you want the metal to stay fluid as long as possible.
In bronze casting we fire ceramic shell molds to about 300 degrees above the pour temperature of the bronze.
In the time it takes us to pull the molds out of the kiln and set them, and then get the crucible out of the furnace- they cool considerably. The bronze crucible itself come out almost white hot, but by the time we get it over the molds it has cooled to yellow orange.

you have to consider the temperature differential…pulling a 2,000 degree crucible out of a furnace into a hot summer day of 100 degrees is a steeper gradient than dipping your hot cup of coffee into liquid nitrogen.

You want your mold to be almost as hot as the metal you pour into it… but not necessarily hotter.
The metal will transfer heat to the mold but in doing so it will lose heat… you don’t want that transfer of heat drop the metal temp low enough to cause it to thicken or to solidify.


#14

Is there investment material out there that’s rated for 2000 degrees and still have high detail? I’m trying to make rings so detail is fairly important


#15

We use ceramic shell for high temp castings, up to stainless temps.

We’ve poured bronze into investment with no issue- around 1900 by the time the bronze hits the cup- but we don’t burnout investment higher than 1400.


#16

Sculptingman, you use the shell method of investing, but he is talking about flask casting. There is a huge difference. In shell casting, you pour the metal at very high mold temperatures. One reason for this is that usually - but there are exceptions - the shell method uses a gravity pour. Generally speaking, flask casting makes better parts if the metal is poured into a mold that is 200 deg or more lower than the high burnout temperature, 1350 deg in cases using gold, brass, etc. This has been my experience after many experiments and we have made many, many parts this way. This is counterintuitive, but the data and the results do not lie. You do not want your mold near the metal melting temperature. If you are using gypsum based investment as most do, at gold melting temperatures the investment has decomposed into a stinky, crumbling mess. Ask me how I know. Most people here use this type of investment.

When you take your shell out of the furnace, it is very hot and cools off seemingly quickly as you have stated. However, you are looking at the outside shell temperature not the inside. Investment makes a pretty good insulator. The surface area to volume ratio is much higher than flask casting. This makes the cooling of the shells much faster. Also, a flask still has a thick metal cylinder wrapped around it. I have mapped 3" x 5" flasks for temperature and there is often a several hundred degree gradient from inside to outside. It takes several hours soaking at peak temperature to reach equilibrium. The same effect happens when cooling down. After one minute, the inside of the flask has cooled less than 20 deg.

If you really want to at cast higher mold temperatures, use phosphate bonded investment, It is used for platinum, steel, etc. These metals melt at much higher temperatures. However, even these are burned out a lower temperatures, but can be taken higher. Most people’s burnout oven max out at 2,000 deg or so. I have taken flasks out of a furnace and standing square in front of a 200 deg furnace with flask grasping tongs is dangerous. You must be completely clothed in a fire resistant suit. I have been there. The elements in the furnace quickly burn out also. Not good if you are on a budget.

I say pull vacuum and pour as soon as the vacuum reaches its highest level. Hopefully, no more than 30 seconds.

Anyone really interested in flask casting should buy Murry Bovin’s book about casting https://www.amazon.com/Centrifugal-Lost-Wax-Jewelry-Casting/dp/B000MOLMGK/ref=sr_1_1?s=books&ie=UTF8&qid=1548259730&sr=1-1 . It is old school, but all the basics are there. This wheel has already been invented.

BBox


#17

You’re right- I defer to your greater experience in flask investments.

We only occasionally poured investments- and they were straight out of the burnout furnace.
As I explained earlier- in the time it takes to pull them out and get the crucible over them, they drop in temperature anywhere between 200 to 400 degrees depending on the ambient temp and how fast we get tongs on the crucible.

But my experience is with much larger volume casting, 1/8" thick on average, but at least 25 lbs of metal at a time.