I'm Back! Updates and a question

Discussion in 'Investment casting Block method' started by 0maha, Sep 20, 2021.

  1. 0maha

    0maha Silver

    Hung around here last year, and got some great info. Things moved slowly for a while (too many other projects going on), but they are heating up again.

    I think I have most of the process worked out, and I've done a few test pours.

    By and large, the results are far better than I expected at this point. Here's the one from yesterday, pretty much straight out of the block:



    That's cast in Everdur, and the parts are about 7" in diameter.

    (Btw, the red stuff is just some paint that rubbed off of the screen in the bottom of my blasting cabinet.)

    Having gotten this far, the next goal is to figure out how to improve the surface finish.

    Here's a closeup of the first one, after a little wire brushing and with a little Photoshop magic to exaggerate the texture:


    I need to figure out where that surface pitting is coming from, and how to eliminate it.

  2. 0maha

    0maha Silver

    ...my wax molds and sprues look like this:



    I fabricated these large, rectangular flasks:


    I'm using Rancast investment. Since the flasks are so big and it takes so much, I use the vacuum chamber I originally intended for the flasks as a mixing/vacuuming solution:


    The amount of investment I need for a flask is so large, I just mix it directly in that chamber, using a drywall mud stirrer and an electric drill. After mixing, I vacuum it down until it 'falls'.

    The flask itself goes in this big chamber that I made. I pour in the investment, then vacuum it again:


    Once it's solid, I pull off the lid then put it in a plain old household range, upside down, at 275 degrees for four hours. I use a cookie sheet underneath to catch the wax.

    Then it goes in the kiln. It's a simple pottery kiln to which I added a PID controller. The schedule is ramp up to 1350 F over five hours, hold at 1350 F for five hours, then drop to 1000 F and hold.

    I use a waste oil burner to melt the metal. Once it's ready I put the flask onto a vacuum table and pour:


  3. 0maha

    0maha Silver

    Should have mentioned, I am using Limerick wax from Ransom and Randolph.

    So here are some questions/ideas I've got:

    1) I don't have any way of measuring the temperature of the molten metal. Is that a problem? I just sort of go by eye and feel, stirring it with a steel rod. Do I need something to keep my temperature more exact? If so, what would that be?

    2) I was worried that the silicone sheet I used on the vacuum table would struggle to handle these temps, so I don't leave the flask on it any longer than necessary. I wonder if that's the problem right there. I put the flask on, turn on the vacuum pump, pour, then wait a moment or two and remove the flask. Looking at the silicone, so far it seems to be holding up fine. Think leaving the flask under vacuum for an additional 30 or 60 seconds would make a difference here?

    3) I wonder if my flasks are too narrow. As things stand, there is about 1/2" of investment between the mold and the wall of the flask at the narrowest point. Think that matters? I want to keep the flasks as small as I can, since the investment is so expensive.

    Anyway, that's where I'm at.

    Any thoughts?
  4. Petee716

    Petee716 Gold Banner Member

    Nice update Omaha. I got my eye candy for the day.
    I’m not a wax guy but I do work with everdur in sand. Some of the more experienced guys will chime in I’m sure, but I’m wondering if you might have had some residue left in your mold.
    To answer your question about temperature, yes, you need a pyrometer.
    After years of goofing around with cheaper methods I finally did what HT1 and others are advocating. I got it together last week and it’s a slam-dunk.

    0maha likes this.
  5. 0maha

    0maha Silver

    Thanks, Pete.

    Appreciate the pointer to the pyrometer thread. I think I'll go ahead and put one of those together.

    One thing I didn't mention but should have: This latest pour got interrupted. I had a problem with my foundry burner, and had to let the flask cool off. It wasn't until a couple of days later when my replacement burner part came in. I re-programmed the kiln to ramp up to 1000 F over four hours. My intention was to time the melt to correspond to the kiln coming up to temp, but my new burner ran so hot, my melt finished early. The kiln was at about 850 F when I poured.

    Not sure if that matters. What I've read is that the desired flask temperature for bronze is 800 F to 1000 F. My assumption is that the more fine detail you've got, the more toward the hotter end of that scale you want to be. Is that right?

    As for the residue in the mold, I wonder about that too. And I also wonder what if anything can be done about that.

    My plan for the next test goes like this:

    1) Increase the dewax temp from 250 to 350 degrees. I'm a little skittish about having the wax catch fire in my oven as it drips into the cookie sheet, but what the heck. I've got a fire extinguisher for a reason. The problem is my flask is so tall, I have to put the rack on the second to the bottom position, which leaves the rack holding the cookie sheet on the bottom position, which has it right above the heating element. I worry that I could get a hot spot that flashes over the wax.

    2) Reprogram the kiln a little. Right now, step #1 goes from 80 F (room temp, that is) to 1350 F over five hours. Since the flask is already going to be at 350 F when it comes out of the oven, I'll set step #1 to go from 80 F to 350 F over ten minutes, then make step #2 from 250 F to 1350 F over five hours. I doubt that will make much difference, but it occurs to me that the old program actually let the flask cool a little after melt out until the program caught up with it.

    3) Use the new pyrometer to ensure I'm pouring at exactly 2150 F, or at least as close as I can manage.

    4) Leave the flask under vacuum for at least 60 seconds.

    One idea I've got that I might try is to cut a piece of silicone mat to lay over the top of the flask immediately after the metal is poured. My idea is that I could be pulling air through the flask, top to bottom, and not actually introducing as much negative pressure in the mold cavity itself as I should. Don't know if that will work, but seems like something to try.

    Anyway, off to buy pyrometer parts.


  6. mytwhyt

    mytwhyt Copper

    Overall, the casting looks good because air is being pulled through the mold.. You could provide a way for the a vacuum to pull from the flat sides of the flask.. I was thinking of making a metal flask much like yours.. A cast for 2 plaques slightly smaller than yours.. My idea for adding vacuum from the sides was to be by adding that pathway to the flask before investing..
    This is also based on assumptions that vacuum is pulled from the bottom.. My addition to the flask was to be a wax grid tacked to the wall on each side of the flask.. An inch or two wide,from the base of the mold 2/3 of the way up.. Carve channel from the vacuum hole to the flask sides.. After the mold is drained of wax, turned sprue side up, the wax can drain from from the new vacuum cavity.. Any wax left will burn out.. Well, that's the way I saw it in my mind.. What do you think?
    Last edited: Sep 30, 2021
    0maha likes this.
  7. 0maha

    0maha Silver

    I really like that idea. Makes sense.

    I've got a couple of ideas going. First, I'm going to cut a piece of the silicone sheet to lay over the top of the flask after the pour. The theory is that the vacuum is pulling air all the way through the investment from the top, diminishing the negative pressure where I want it.

    Second, thanks to your suggestion, I'm going to try adding those wax channels. We'll see how that works.

    I'm also going to extend the time period of the vacuum. I'm confident now that the silicone can handle the temps. I'm going to try leaving it under vacuum for a full five minutes or so.

    If none of that works, I'm going to see if I can find a more powerful vacuum pump. I should add a gauge to the setup. I have no idea how much vacuum I'm getting. Would be interesting to have a number.

    Any ideas on a better pump? All I've got now is a HF AC pump.

  8. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    What does the bottom of your flask look like? Is there a plenum that exposes the entire cross section of the flask to vacuum when sitting on the vacuum table or is it just a small hole covered by investment? If the latter, that would create too much pressure drop to achieve any appreciable vacuum on the flask.

  9. 0maha

    0maha Silver

    The flask is just a four sided steel box. The entire bottom of the flask (in pouring orientation, that is) is open.

    When I fill the flask, the investment ends up about a half inch from the top. So when it's flipped over for pouring, there is about a half inch air pocket.
  10. mytwhyt

    mytwhyt Copper

    To achieve that space would be to pour the flask short, to within a 1/4" inch of the top, since the vacuum will be pulled from that end.. Any small vacuum pump that will pull a 29inch vacuum will work.. A good small pump will pull as deep a vacuum as a good big pump.. Just a lot slower..
    I faced the same problem when I built my first vacuum former.. A good small pump. was all I could afford. I bought a big 10 gal. air tank from Harbor Freight and used it to store the vacuum.. My 1.5 cubic feet a minute pump takes a couple minutes to empty it.. Plumb the tank into the vacuum table, start the pump and open the valve to the tank when you pore.. I think your pump and a tank, with a 1/4" investment recess space above the table might work without any added wax channels.. Can't be sure of that, just a thought.. It's kind of ridiculous that something we describe as nothing can be put and kept in a certain place.. If posting a pic in your thread is prohibited , I can delete it and post a link unrelated to casting..
    Haven't found a place for off topic threads..
    Picture016 vf.jpg
    Last edited: Oct 3, 2021
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  11. 0maha

    0maha Silver

    Latest test pour showed much improvement.


    That's right out of the investment, with a light hit of media blast.

    The porosity problem seems mostly solved. Not sure if it was pouring at a higher temperature* or from leaving the flask under vacuum longer, but all the details came out really well.

    The biggest problem now is the air bubbles. Can't seem to get rid of them. Next step is to fab up a vibrating base for the vacuum chamber. Hopefully the combination of vibration and vacuum gets it there.

    * Discussed this in the pyrometer thread. Using the new pyrometer, I realized I'd been a good 150 degrees F low on my pouring temps.
  12. mytwhyt

    mytwhyt Copper

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  13. mytwhyt

    mytwhyt Copper

  14. amber foundry

    amber foundry Copper

    Try washing the wax with methylated spirit before investing, also tip the flask as you fill to allow the air to be carried out from the corners and undercuts. I have discovered I gat better results mixing my investment a little bit wetter than the instructions suggest.
    0maha likes this.
  15. 0maha

    0maha Silver

    I'm not sure if it's the same thing chemically, but I mixed up some homebrew surfactant using JetDry (the dishwasher stuff). Gave the wax a misting of that prior to investing.

    The tipping might be the trick. These flasks are so big it makes handling them a bit of a chore.

    Starting the fabrication on the vibration table today. Haven't figured out all the details yet, but we'll see how it goes.
  16. mytwhyt

    mytwhyt Copper

    Went back and read the instructions on the bottle.. Besides washing or spraying, on the label it says to add 2 drops per 100 grams of investment.. Might want to try that on a smaller scale first.. Have never used it that way myself.. The debubbleizer I use is alcohol based..
  17. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I use Dawn dishwashing soap. Just a quarter size puddle in a few gallons of water than dip/spray the pattern. The common anti-spotting agent in dishwashing detergents is propylene glycol which is a decent surfactant.

  18. Zapins

    Zapins Gold Banner Member

    Very interesting I will have to try this.

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