Think This Process Will Work?

Discussion in 'Investment casting Block method' started by 0maha, Oct 2, 2020.

  1. 0maha

    0maha Silver

    I ordered the "big pot + vacuum pump" setup off of eBay. The dimensions are such that I'll need to make a minor tweak to my flask design so they fit in the pot, but other than that, I'm hoping (it hasn't arrived yet) my investment degas solution is in place.

    I'm also considering using the same setup to degas my RTV. Worth a try, anyway. I can't see how that could hurt anything, and it might help. We'll see.

    Regarding pulling a vacuum on the casting itself, I built a flask/chamber setup that looks like this:

    [​IMG]

    Here are the two pieces together. You can see the nipple I welded in to attach the vacuum pump:

    [​IMG]

    I'm doing a total do-over on that setup. I'm trying to solve five problems:

    1) The flask is HUGE and won't fit into any normal degas chamber.
    2) Because the flask is so big, I can only fit one at a time into the kiln.
    3) The flask is heavy, and only going to get heavier once it's full. I worry about how to handle it when its up to casting temperature.
    4) The flask will consume an insane amount of investment, needlessly running up the cost of each cycle.
    5) Attaching the vacuum port at the bottom was a bad idea. I can see a problem if I get a flask blowout and sucking a bunch of molten metal into the vacuum pump.

    I looked at the "solid flask + vacuum table with a single hole" idea. I don't have any experience, so its impossible for me to judge, but my gut tells me that a "flask full of holes + vacuum chamber" will give me a better result.

    My modified flask design looks something like this:

    [​IMG]
    I'm also going to make a new vacuum chamber sized to fit that flask. If everything works out the way I'm hoping it will, I'll still be able to make these two-up, while using considerably less investment and having a somewhat easier to handle flask.

    We'll see.

    I've also refined my mold making process a bit.

    Previously, I was using US Composites 74-30 RTV on both the front and the back, then applying a big schmear of plaster of paris to give it some rigidity:

    [​IMG]

    I decided to try switching to 75-60 for the back side of the mold. (The front side where all the detail is at is still 74-30 + PoP) The 75-60 is stiff enough that I don't need the PoP on the back. I made these little knob thingies that I hot glue to the back of the 3D print I'm molding:

    [​IMG]

    After the RTV sets up, I pull them out and use these little funnels I made (they have a nipple that's a snug fit into the holes in the RTV) to pour in the wax:

    [​IMG]

    The other thing I've learned in all this is that there's no need to paint on any wax before pouring it in. I just slush the whole thing. BUT the temperature of the wax is semi-critical. It has to be hot enough to flow, but not too hot. Too hot gives irregularities, but "just right" works out perfectly.

    The other thing the funnels do is allow me to sort of rock the mold around a bit while the wax is still liquid, making sure there aren't any air bubbles. They also provide a couple of inches of head pressure on the wax, but I'm not sure that's worth much of anything.

    The only downside of the funnels is they end up filled with hardened wax. I have to boil it out, which is another chore.
     
  2. Jason

    Jason Gold

    What's with the holes in the flask??? At the end of the day, it matters very little if you suck from just the bottom or if you have the ability to suck all around. It's what you have. Jewelry guys go on and on about which is better, but my experience it matters very little. You will hear some strong opinions on this one for sure!

    Wax is funny when it comes to new silicone. Yeah, painting isn't necessary. I find molds kinda season and usually the 2nd or 3rd pull will be a keeper. I start with my wax just about smoking! I'm talking HOT! I shut the heat off, dump it in, swish and dump out. Wait a minute and repeat. With the heat off, the wax slowly cools and this way it doesn't wash off what I just laid down. To much temp variation and your wax will have layers. Can become an issue with shell. You are running solid block, so just watch out for shrink creating voids and you'll be fine.
     
  3. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Nice job fabricating that flask and chamber Omaha. What are the ID/ OD, height, dimensions? So are you going to make a rectangular flask and chamber too? One trick you might consider to save on investment, fill the empty space in the flask with a cheap fire brick. It takes up volume and you fire it and can reuse it. I've done this with lost foam casting when I had large molds and not enough sand.

    Not a block investment guy Jason, but assuming investment would have relatively uniform porosity, your ability to evacuate the investment cavity should be a function of the exposed area of the investment and the distance through the investment the escaping gas needs to travel. Both of these factors affect pressure drop which is the absolute enemy of pulling high vacuum. More holes/area should mean the mold pumps down faster and ultimately achieves higher vacuum, both of which should be beneficial to a successful casting. Also, there be some leak rate of the flask/chamber and for a given vacuum pump, more pressure drop will require a bigger capacity pump for the same pump down rate and end state. -It aint rocket science pal.

    One thing to remember, it isn't a linear world and the volume you need to evacuate a volume is a cubic function (3rd power) for most geometries, so the for puny little volumes like jewelry, it probably makes little difference but as the volume of the investment mold goes up I'd have to think it makes a big difference in the vacuum source required.

    Best,
    Kelly
     
  4. Jason

    Jason Gold

    Kelly, you are way over thinking this suck thing. o_O
    It all depends on your equipment. Some guys spin, some suck. A solid flask works everywhere and a perforated flask requires this ancillary tank what our op here has built. If you are casting a tiny piece and only using a 1/3 of the flask, a perforated flask would be better IF you have the equipment. BUT, If you are utilizing 3/4 of the cast investment block as you should be doing, then it doesn't matter where you suck. It comes down to how much money in investment you want to pour. With a solid flask, you just don't fill it all the way up! With a flask with 50holes in the side, you get to tape up the mess. We pour some large trees with solid flasks under vacuum and there is zero difference. I couldn't tell you the last time we even bothered with a perforated flask. I think there is still one or two laying under the bench somewhere. When it's under vacuum, you see the gold and silver get sucked home in a blink of an eye. It happens REALLY fast!
     
  5. 0maha

    0maha Silver

    This is a very interesting discussion.

    I was assuming I wanted the "vacuum tank surrounding the flask" system, but if I can get away with a simple vacuum table pulling on the bottom of the flask, that would make life a whole lot easier. Easier to invest (no wrapping tape around the whole thing) and probably easier to handle hot (just plop on the table instead of having to carefully position a hot flask into the tank). Flasks easier to make. Flasks take up less room in the kiln.

    Seems like the obvious move to make is to try the vacuum table version first. If it works, great. If not, go back to Plan "A".

    Vacuum pot system just showed up. Most of the oil bled out of it during transit, so I only turned it on long enough to see if it worked, and it seems to be fine.

    [​IMG]

    Anyone know what this thing supposed to be used for? It came with the kit.

    [​IMG]
     
    Jason likes this.
  6. Jason

    Jason Gold

  7. 0maha

    0maha Silver

    Is vacuum pump oil the same as compressor oil? I've got a huge bottle of that stuff.
     
  8. Jason

    Jason Gold

    I dont think so, vacuum pump oil is very thin. It also resists water I think.
     
  9. rocco

    rocco Silver

    Last edited: Dec 2, 2020
  10. Jason

    Jason Gold

    Strange thing to include with an industrial type tool. It's for those industrial dehydrogenated environmentally friendly banana frisbee discs you shoot with a toy gun.o_O
     
  11. crazybillybob

    crazybillybob Silver Banner Member

    The liner is for the bottom of the pot. Keeps the item you're infusing with resin from sticking to the bottom (in theory). Remember these things are marketed to wood Elves and knife maker that want to stabilize rotten carbohydrate foam (wood). Or to folks that are casting resins. they're all messy.
     
  12. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    No. Vacuum pump oil will be formulated to have very low vapor pressure. Using regular oil will cause it to be consumed and worse, migrate toward your work. My pump was shipped dry with oil packaged in a separate bottle.

    Best,
    Kelly
     
  13. 0maha

    0maha Silver

    Learning every day.

    Yesterday, I learned that if you end up with a bunch of water in the bottom of your wax pot, and are impatient to have it boil off, setting the pot (at full temperature, half full with melted wax) in your new vacuum chamber is a bad idea. A very bad idea.

    Never know until you try, right?

    (Ok, truth is, I knew this was a bad idea when I tried it, but it seemed like it might be fun, so I gave it a shot.)

    Good news is it turns out the vacuum pot is induction compatible. Put a little water in it, put it on the burner, and steamed the wax right out.
     

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