Cast iron lost foam

Discussion in 'Lost foam casting' started by ESC, Jan 30, 2019.

  1. oldironfarmer

    oldironfarmer Silver Banner Member

    Surface finish does look great. I can see the curved scratch from the pattern. Well done.

    What kind of optical pyrometer do you have?
     
  2. ESC

    ESC Silver Banner Member

    I use a Pyro optical with that has the additional Cast Iron scale.
     
  3. Mister ED

    Mister ED Copper Banner Member

    Very nice. Any thoughts on what led to the (apparently) great success this time vs your other recent results/trials?
     
  4. ESC

    ESC Silver Banner Member

    Thanks guys.
    A couple of thoughts on the success.
    Kelly had mentioned needing the higher permeability coating, so I only put one coat of mud on it and vented it.
    I compared the ball turner base which was cast uncoated and it had folds on the upper surface, but decent a finish on the lower when poured horizontally. Although it was just a test it should have been poured vertically. Gravity is our friend in lost foam. This applies to the very first ram guide too. Even though I was using a lower temperature and was too casual with my compaction, I think that one would have filled if I had used a more vertical orientation.
    The other thing I did for the first time was to hollow the sprue with the outer edges almost even with the sand. I poured very quickly and had very little flare which I think was because I was vaporizing the foam short of available oxygen. I'll take a shot of the top of the sprue, There is a lot less porosity than previous attempts.
     
    oldironfarmer and Gippeto like this.
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    In almost all the papers, commercial publications, and commercial videos, they contend the single most important thing is that coating permeability is properly matched to the metal being cast. There is too little and too much permeability. Higher temperature and density metals vaporize foam faster because of both the higher heat and higher sprue pressure. There will be a layer of liquid and gas between the metal front and the foam pattern and the higher sprue pressure will compress the gas gap, and that will in turn place the metal closer to the foam and accelerate vaporization.

    Too much permeability is supposed to create the potential for turbulence and unstable molds. You can't get higher permeability than no coating at all. It's then the mold media that is the limiting factor. Adding vacuum has the affect of making a less permeable coating more permeable.

    Your comments about no flame ring true and a very positive sign to me. The sooner you can get the molten metal front to lay down into a stable gas gap and quiescent flow, the better. This is my interest in the Bifilm work and why I developed the hollow foil sprue. However, the kush cup and foil funnel is a high aspirating design. I think the non aspirating benefits of offset pouring basins and corresponding sprue design may be the most valuable bit for lost foam. I intend to test whether hollow or foam filled sprues help achieve stable pouring conditions more quickly. It would sure be nice if you could get there with the sprue and gating system full of foam because that will allow a lot of design freedom and ease of construction. A hollow feed system will take more effort.

    I don't think all the well and sprue sizing rationale in the Bifilm theory thread apply to lost foam after the sprue and feeder system is full because the fill velocities are controlled by the vaporization rate of the foam and moreover the permeability of the coating.

    I have had very unfavorable results hollowing sprues and patterns. In my experience it only delays and disrupts the time necessary for the melt to settle into that stable flow behavior. Hollowing a foam pattern is very bad as it leads to localized mold collapse and oxygen availability. Keep up the good work ESC.

    Best,
    Kelly
     
  6. ESC

    ESC Silver Banner Member

    I agree hollowing the patterns and sprue is a no go. One of the reasons for the failure when I tried the NaSi core was the expansion and perhaps aid to combustion of the air between the core and the foam.
    What I ended up with in a 1" x 2" sprue was a 3/4" dimple about 1/2" deep. This left a thin wall that was just enough to keep the sand back. The upper portion was uncoated, and even part of the lower at the joint, but after that from a quick look the metal flowed evenly and filled every detail.
    When things go correctly I don't learn as much unless I can repeat it for the next pour. And for me, it will be a different shape. I will have to develop a 10 rule approach and adapt them as necessary for the two metals, aluminum and iron.
    I'm really supposed to be making door jambs for the Willys, but now I need to machine the stake pocket.
     
  7. ESC

    ESC Silver Banner Member

    I cut the sprue off. Nice iron. And this is the junction of the sprue extension and the pouring basin head. The mud shows on the lower sprue and the upper exhibits the over penetration of the sand. This sand was some softer Home Depot that I used to top off the pail. The rest is good Monterey Sand and holds up well to iron temperatures.

    IMG_20190310_095720.jpg

    IMG_20190310_095208.jpg

    IMG_20190310_095148.jpg


    Then I set it up in the mill to give me a base surface. It looks hard, but that's from dull inserts.

    IMG_20190310_100818.jpg

    Then over to the shaper to clean up the square hole. This interior had a minor flaw at one end , maybe a fold or inclusion, but the cutter went right through it and I think eliminated it. Some of the mud still shows on the side wall. I always leave a machinery allowance as we generally do with sand casting, but with the accuracy of lost foam castings I believe I could have cleaned the foam to the .875" dimension plus 1% for shrink and had wire brush cleanup.

    IMG_20190310_121026.jpg

    Here mocked up right where it is needed.

    IMG_20190310_130932.jpg
     

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