Need help with this one....

Discussion in 'Investment casting Block method' started by TomL, Jun 16, 2024.

  1. TomL

    TomL Lead

    I have been doing a lot of brass and bronze into Glasscast 910 plaster, it is a very durable mold.
    I was asked to cast this aluminum carburetor, it was printed from polycast. it has a complete surround water jacket, double wall
    The square hole in the fill sprue is for the plaster to flow into the water jacket, no issues there.
    I am using 356 alloy, and getting failures I cant explain.
    I burnout to 1400f for 24 hr, then gently blow out the mold to endure there is no debris small plaster flakes .
    I poured at 1300f,and bottom fill. with a 1 inch fill sprue.
    The mold is at 1200 when I pour, since I am concerned about early freezeoff with aluminum.
    I was told by the Tech rep for R&R to lower my mold temp to 650f , that I am too the too high of heat causing gassing?( in Florida, lots of humidity)
    My other concern is how to clean out the plaster when I get a good cast, it will soften with vinegar, but the water jacket will be hard to clean.
    Wondering if I should try a lost foam?
    Thanks in advance for comments

    Attached Files:

  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Not an investment caster but when the question has been raised here for aluminum, the answer is always mold temp at 400-600F. The complaint is usually a surface finish issue due to surface interactions with mold with protracted exposure to the elevated temperature.....but I think you may have another issue as well.

    Maybe not. I had to look at the pictures a half dozen times before I could see what I think you were referring too. If it's that little square hole where the sprue meets the casting, where else is the internal core structurally supported by the mold? Just because it can be filled (or was it?) during vacuum assisted molding doesn't necessarily mean it's supported well enough to survive burn out or the pour. From the other defects, it looks to me that the water jacket core has broken and buoyantly shifted, probably during the pour

    That's always the dilemma. High pressure stream, water pick, ultrasonic cleaner......

    The core would still need to be adequately supported.

  3. TomL

    TomL Lead

    Those photo were from Pour 1, I have 3 3D prints, and am ready to go with #3
    In pour 1, the water jacket core is supported by 3 other openings for water fittings
    For pour 2 I changed the sprue and gate pattern per below photos to get rid of the high velocity from filling around the little square hole
    It fixed the erosion at that point but then the porosity bit me big time
    I felt I have enough ferro-static head pressure, but I will admit I did pause once while filling , and I dont know if that moment would have caused any issues.
    The outside of the part has great detail, and I dont see any evidence of a burnout problem, I forgot to mention , but the 3D print for the positive was with Polycast, and I gave it a prolonged burnout, and blew out the mold with 15psi of air to clear any plaster dust.
    I am getting ready to wax up the 3rd pattern, and we all know the cost of plaster........ I hope I can fix it this time
    The only thing I can do different is to lower the mold temp per comments.
    I had one vent that got knocked off while filling with plaster, and I will fix that by going to a larger dia this time.

    Attached Files:

  4. TomL

    TomL Lead

    I forgot to mention I tilted the pattern about 5 degrees to help with filling the second pour , - inconclusive
    Here are photos of the 3D pattern, all the small holes serve as support for the water jacket core, and I dont see any sign of core movement, just this porosity issue.
    In Pour 1, I think the mold was level, and when the core started to fill the erosion created the thin spot
    I did not have that problem in pour 2, just the huge porosity issue at the top of the fill.
    I plan to tilt the pattern again...didnt hurt.
    the black dashed lines on the pattern is me pondering how to split this into 2 parts, and TIG weld it back together......
    can you believe they made these by the thousands back in 1910 for Peerless Autos?

    Attached Files:

  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    In the first set of pictures the core looks to be filled with aluminum, but the investment and metal color are so close in the pictures it may just be investment that hasn't been removed. Even so, the sectioned picture of that water jacket looks paper thin. Surely you didn't core it that way.....did you? In the second attempt the sectioned pictures, the water jacket thickness appears more uniform.

    Between the multiple attempts, pictures, and jargon, it's a bit hard to follow. You say the first core was supported by the three openings and then you say you made changes and...

    Are "all the small holes" and "the three openings" one in the same? On the second set of pictures one (internal) side of the water jacket is still severely compromised.......doesn't look like a stable mold to me but......

    This is never good news for any casting method. It could be a contributing factor to the severe flaw if it's entrained air or evolving gas but the latter seems unlikely for the size and severity of the flaw.

    What are you using for a melting furnace and how are you measuring the pour temp?

    HT1 likes this.
  6. DavidF

    DavidF Administrator Staff Member Banner Member

    That's going to be a tough one to pull off using lost pla casting.
    Looks like your investment is failing and causing the issue. There's just not enough support for the water jacket core.
    If you are letting your mold cool off so you can blow it out could be one of the issues. Investment should not drop below 550F once it is burned out.
    Also, I'm not familiar with the investment your using, but you might want to try the Ransom and Randolph plasti cast investment. I've had good luck with it....
    But you might want to do this as a sand casting and just print patterns and core boxes. I think this is your best bet...
    Al2O3 likes this.
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Which is more than likely how they were originally made. The cores may have had some wire/chaplet reinforcements as well......pretty common in automotive castings with intricate water jackets.

    How thin is the water jacket? It looks to be circa 1/8". Whenever you get thinner than 1/4" in sand casting with binder (actually any method for that matter), it can become difficult to get leak tight castings, especially if you have a thin wall sandwiched between two cores. Those cores need to be well cured and often times vented and coated with a wash.......and you need to have very good (aluminum) melt practice or your sunk before you start. Achieving a watertight casting be the next challenge once you get a fully formed casting.

    I wouldn't give up on casting it. If you're finding it to be a challenge getting a fully formed and watertight casting, just wait until you try to TIG several of those castings together.

    Hang in there. This was seven years ago when I first started LF casting. My processes have improved quite a bit since but I've made a lot of them.

    Tubular Lost Foam | The Home Foundry
    Automotive Water Neck | The Home Foundry

  8. TomL

    TomL Lead

    water jacket wall is 1/8 inch thick and seems to have no issue on all sides except where it failed, the actual water space is about 1/4 " all around.
    I check the melt with a digital T/C, type K
    I do not cool off the mold to blow it out, and all I ever get is just a small puff of plaster dust.
    Glasscast 910 is R&R, rated to 1550f
    My furnace is LP fired with combustion air assist from my compressor at 1 psi....and I melt bronze with it regularly, It can melt the 356 aluminum in 30 minutes, about 6 lbs, I try to pour at 1270-1300f
    There are 2 large openings, they are for air/ fuel mix.
    The three smaller openings are for water and "should "support the plaster, they are about 1" dia, 1/2" dia,1/2"dis.
    The water jacket core in pour 1, and 2 is supported the same way,
    when I clean out what plaster I can, after the pour, I dont find any evidence of cracked or failed support of the core. for the water jacket gap.
    The 3 large openings then support the air path core
    The pattern has to have the 2 bolt flange up, or the plaster will trap air inside the air path
    The first set of photos from pour 1, the plaster core is still in the water jacket, except for photo xxxx221, I tried to remove some of it
    photo xxxx2737 shows a full core in the water jacket
    ...anyway, I will pour #3 in a few days.....getting ready again. you can see the original parts , in the first post, I named as original.
  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    If you more typically melt bronze which would favor a rich/reducing burner tune, you want the opposite, very lean/oxidizing tune for aluminum. A rich burn in Florida humidity is a sure recipe for gassy aluminum.

    HT1 likes this.
  10. DavidF

    DavidF Administrator Staff Member Banner Member

    Are you using any type of vacuum assist during the pour???

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